3C1

Hot vs Cold Working

"Hot" and "cold" refer to whether the material is pre-heated before forming - but all deformation generates heat, so even a cold-worked part may undergo some heating. The average temperature during forming determines the deformation mechanism. Key properties to understand processes - deformation geometry (plane strain, axisymmetric, 3D) - tool/workpiece interactions (friction, heat transfer) - temperature history (governs deformation mechanism, and microstructure evolution) Steady-state processes - (fixed deformation pattern through which material flows) Rolling hot or cold Extrusion usually hot Wire drawing cold Machining cold Non-steady state processes - (component geometry and deformation pattern continuously changes) Forging usually hot Sheet metal forming cold Hot working - high strain rate creep. T > 0.5T_m e_dot = 1-10^3 /s - T, e_dot strong influence on yield response - recrystallised, equiaxed grains; isotropic mechanical properties - must be worked in compression (dynamic softening: necking in tension) - poor surface finish (oxidation) and tolerance (differential thermal contraction) Cold working - plastic yielding. T < 0.3T_m e_dot = 1-10^5 /s - T, e_dot little influence on yield response - deformed elongated grains; anisotropic mechanical properties - modest forming in tension viable (work hardening suppresses necking) (but mostly compressive allowing much larger plastic strains) - good surface finish and tolerance Assumptions - cold deformation leads to work hardening - elastic strains negligible (except sheet forming, due to elastic springback in bending)

Fatigue failure of composites

(a) Due to external stress cycling Composites are full of stress concentrators and internal stresses. When subjected to alternating load, damage builds up in the form of fractured fibres and failure at the fibre-matrix interface. As a result, they show progressive degradation of mechanical properties. The elastic modulus falls, and the strength drops. Failure tends to be by the gradual linkage of many sub-critical cracks, rather than by the catastrophic fast fracture seen in metals. (b) Due to thermal cycling If composites are heated, the fibres and matrix expand by different amounts, often resulting in elastic or plastic deformation, sometimes accompanied by fracture. The following effects may be expected in composites which have suffered thermal cycling. Exactly which combination of effects is seen depends on the fibre-matrix mix, the lay-up sequence, and whether the temperatures involved are above the softening temperature of the matrix: (a) Build-up of internal stresses (b) Distortion of composite (particularly for multi-ply composites) (c) Fracture of fibres (reduction in effective fibre length; not for Kevlar which is tough) (d) Fracture of matrix between fibres (e) Fracture of fibre-matrix interface The effect of all of these factors (except possibly (b)) is a steady degradation in the mechanical properties of the composite. The tensile strength and the toughness will decrease (because of damage accumulation in the material), and the elastic modulus will also fall. Factor (e) in particular can lead to increased corrosion (environmental degradation) rates because of increased 'wicking' of liquid along the cracks at the fibre-matrix interface. Summary: There is progressive degradation of composite integrity in service. Monitoring the 'state of health' of a composite structure is essential in safety-critical structures (e.g. aeroplanes, bridges): the slow reduction in elastic modulus is often used as the diagnostic test

Thermal Residual Stresses: Polymer Injection Moulding

(a) The material close to the mould wall solidifies and contracts first, forming a 'skin'. The material in the centre is still molten. (b) This material then cools and shrinks. (c) This puts the surface regions into compression and the core into tension. This can lead to warping and distortion of the part. Note also that the different cooling rates may result in variations in crystallinity, with associated additional dimensional problems.

Two failure categories

- In-process scrap: expensive, but easily recycled, and not hazardous to public - Failure in-service: at best, expensive; at worst, serious loss of life, and litigation

Effect of tempering on steels

- Martensite is too brittle for use as a bulk microstructure in a component, so it is softened and toughened by tempering. - Tempering is reheating to a temperature below A1 to allow the supersaturated solution of carbon to precipitate as spheroidal Fe3C precipitates in a matrix of ferrite - A wide range of combinations of strength and toughness can be achieved by varying the temper temperature and time - In alloy steels, alloy carbides also form during tempering - this is called secondary hardening

Forging vs sand casting

- Mechanical properties of forgings typically superior to casting: no porosity, finer and more uniform microstructure. - Forging would use different Al alloy - potentially heat-treatable, and thus higher strength and lower wear rate than casting alloy. - Forging would give better dimensional accuracy and surface finish. - Forging would use a closed die with a solid billet, so an additional machining step needed (with additional cost) to make the central hole bored afterwards. - Potential reduction in cost per capstan, depending on batch size: forging favoured over sand casting if batch size is large

Centrifugal casting (permanent mould)

- a permanent-mold casting process in which hollow cylindrical parts are made by pouring molten metal into a rotating mold; various designs on the external surface may be cast but the internal surface remains cylindrical; because of density differences, lighter elements tend to collect on the casting's inner surface - used for axi-symmetric shapes - equivalent polymer process = rotational moulding

Parameters that affect mould fill rate in sand casting?

- alloy: determines viscosity and hence flow rate through channels; thermal properties affect conduction rate (hence onset of solidification); composition determines liquidus and solidus temperatures (from phase diagram), and hence onset of solidification (increase in viscosity, and narrowing of channels for flow) density affects the pressure difference and hence flow rate in gravity casting. process design and variables: number and location of ingates (govern flow distances); pour temperature and mould material + hence thermal properties (control heat flow and cooling rate of melt and hence onset of solidification): whether filling is under gravity or pressure (affecting viscous drag through channel of given size and roughness of mould walls); - part design: size and shape of casting - particularly the aspect ratio of channels through which the liquid metal flows (affecting flow velocity), but also affecting cooling rate (by conduction into mould)fc

Liquid metal embrittlement

- another type of SCC - favoured by tensile stress, like aqueous SCC - metal component in contact with liquid metal fails by intergranular fracture In some metal-liquid metal combinations, the molten metal is able to penetrate along grain boundaries forming cracks. It then diffuses a limited distance into the host metal. It reduces the bond strength at the crack tip, and so the fracture toughness falls dramatically. Because it is particularly easy for metal to diffuse along grain boundaries, this type of LME is nearly always intergranular. As with SCC, particular combinations are important, and prediction of effects is not easy. One of the clearer factors is that the liquid metal should not dissolve too readily in the solid metal (otherwise it will simply diffuse into the host metal, rather than forming a surface film at the crack tip). Common scenarios leading to LME • Exposure of welded carbon steel to molten metal (e.g. hot dip galvanising, tinning). Residual stresses in the weld can then cause initiation of cracks or propagation of existing cracks. • Brazing (Cu-Zn filler) and soldering (Sn-Pb filler) of structures which contain residual stresses • Process vessels containing liquid metal (e.g. baths for molten zinc made of carbon steel; baths for molten aluminium made of almost any metal; containment of mercury by carbon steels) • Accidental contact between liquid metal and susceptible alloy (e.g. mercury in contact with aluminium alloys in aircraft structure) • Overheating of components coated with a metal, above the melting point of the coating or plating (e.g. Cd - plated bolts made from high-chromium steel; galvanised steel bolts). Prevention - avoid overheating components - avoid exposure of components to liquid metal e.g. mercury in aircraft, gallium on aluminium

General features of SCC

- effects more severe in high-strength (usually also limited ductility) materials - fracture surfaces have appearance of brittle fracture - tensile stress, specific chemical required. Elimination of either removes problem - uncracked materials e.g. austenitic stainless steels may have a threshold sress below which SCC doesn't occur. Cracked material is always unsafe. - see image for SCC prone environments

Ferritic stainless steel

- enough Cr and other elements to stabilise BCC ferrite at all temperatures - solid solution strengthened cos heat treatment can't be used - high temp embrittlement a risk due to precipitated Cr carbides and nitrides, so loss of corrossion resistance likely. Hence, strong carbide and nitride forming elements such as Ti or Nb may be added to "stabilise" the steel (i.e. to clean up the free interstitial carbon). Ferritic steels have relatively low yield strength, and work hardening is limited. Good ductility requires very low levels of carbon and nitrogen.

Pressure die casting (permanent mould)

- externally applied pressure permits use of higher viscosity fluid, thinner sections, and minimises waste from runners, risers etc - often just called 'die casting' - equivalent polymer process is injection moulding (most common one) - limited to low-melting point alloys (because the dies must not distort or wear whilst making many thousands of castings) - susceptible to entrapped bubbles due to turbulence, which can be detrimental to properties. Common example: zinc die-casting alloy (a low-melting point alloy, Zn + 4Al, 1Cu, 0.05Mg), chosen for ease of processing and cheapness, rather than for good mechanical properties, e.g. toy models.

Effect on temperature on fracture toughness

- ferritic steels undergo transition from ductile at high temp to brittle behaviour at low temp - same transition also occurs with increasing strain rate (brittleness being encouraged under impact loading) - behaviour common in metals with bcc crystal structures: dislocation mobility falls as the temperature drops, and the yield stress rises rapidly; the stress at the crack tip then reaches a level at which cleavage between grains occurs in preference to ductile void formation and coalescence, with a lower dissipation of energy, and hence lower toughness. The fracture surface shows characteristic crystalline facets. The temperature at which the transition occurs depends on the specimen geometry and the strain rate. In thick plates (plane strain), the enhanced stress required for yielding at the crack tip leads to the ductile-brittle transition occurring at a higher temperature than in thin sheets. Austenitic steels (with an fcc structure, not bcc) remain ductile to very low temperatures. Effect usually described in terms of energy absorbed in impact tests - see attached Note that polymers also show ductile-brittle transitions (as shown here for nylon, a thermoplastic): here the transition is associated with the reducing mobility of the polymer chains below the glass transition temperature (Tg for nylon is ~50 °C) e.g. fracture of large welded steel structures

Martensitic Stainless Steel

- hardenable, they contain typically 0.6 wt% C - Carbon changes the Fe-Cr phase diagram, expanding the FCC γ field to high Cr content. - e.g. a 15% Cr, 0.6% C steel at 1000°C lies in the austenite field, and can thus be quenched and tempered. Martensitic stainless steels are characterised by high strength and acceptable toughness.

Microstructure of castings: Grain Structure

- heterogeneous nucleation occurs first on the cool mould walls, where small grains form the chill zone. As cooling continues, favourablyoriented nuclei grow inwards to form a columnar zone. The central part may be occupied by an equiaxed zone, heterogeneously nucleated within the melt. The proportions of the three structures depend on the alloy and the cooling conditions. Chill zone --> columnar zone occurs by competitive growth - those with their growth direction normal to the mould wall dominate Want equiaxed grains to form the major part of the casting: Finer grain size: improves strength. Reduce the extent of macrosegregation (see below). Interfaces between columnar grains contain high proportions of impurities/gas, due to lateral segregation. Fully columnar castings can contain interconnected porosity, particularly in mushy feeders where feeding liquid metal into these regions is difficult. This may lead, for example, to leaking in cast pipes. Nucleation sites for equiaxed grains? - oxide particles (or other impurities) entrapped during pouring - innoculants (grain refiners): small amounts of specific solid (fine powder) added just before pouring. Inoculants must have a very low contact angle to promote easy nucleation (the inoculant particles become coated in solid, overcoming the critical radius barrier). e.g. addition of 0.05 - 0.1% TiB2 is standard for cast Al alloys - dendrites formed from turbulence

All reasons for alloying steels

- increasing hardenability - increasing weldability - increasing cast properties (removes impurities e.g. O2 from a cast to prevent porosity) - increasing strength and toughness (see spreadsheet) - increasing corrosion resistance (e.g. Cr in stainless steel forming an impermeable outer layer) - increasing machinability (S increases machinability by encouraging inclusions which weaken metal and reduce chip size and cutting forces. see notes)

Form limitations for mould design

- it should have a single parting plane which allows removal of both parts of the mould - re-entrant corners can only be included at or near a parting plane (WHAT DOES THIS MEAN?) - parts must be slightly tapered with a draft angle to allow easy mould removal

Discussion of hardenability and weldability in steels?

- low hardenability gives high weldability - hardenability increases if grains grow too large - hence, microalloyed steels used (with small Ti alloy additions) to form stable alloyed carbides (TiC) which prevent grain growth and maintain high weldability

3 main stages of casting

- melt - transfer into mould - remove from mould

Properties that affect castability (ability to fill moulds without major defects)

- melt viscosity - freezing range between liquidus and solidus lines - desirability for lower melting point

Main manufacturing principles

- metals casting - wrought metal forming - moulding polymers - powder methods (metals, ceramics) - forming composites

DaF: Elastic stress concentration

- often given by local geometric features e.g. 1. hole in plate The local stress immediately beside a circular hole in a plate is 3 times the mean stress far from the hole. Sharp notches have much greater stress concentrations. The local stress at a distance r from the end of a sharp edge crack of length a is given by eqn. Local stress concentrations can lead to initiation of failure. They may be - pre-existing (e.g. design features, results of manufacturing processes) or - develop during service (e.g. by corrosion, fatigue crack growth). Some examples of pre-existing stress concentrators: holes for fasteners (e.g. rivets or bolts), notches, changes of section, porosity, welding defects e.g. 2. welds Welding generally involves a section change (even with butt welds between identical sections) which results in stress concentration. Examples of stress-concentrating geometry in welds: undercutting at the toe of the weld or abrupt convex profile. Eliminate by improved welding practice and design. Butt welds are often machined smooth ('dressed'). NB grinding direction is important as grinding leaves marks which can act as surface cracks. Examples: keyway in shaft; punched lettering; Comet Mk.1 Fast fracture of aircraft fuselage (Comet disaster): Case study in Engineering Materials 3, D R H Jones, Pergamon Press 1993 ISBN

Features of manufacturing process

- primary shaping (e.g. casting, moulding, AM) - secondary shaping/features (e.g. heat treat, machining) - joining (e.g. welding, riveting, friction bonds) - surface treatment (e.g. sanding, texturing, anodising)

Protection against corrosion

- protective coatings (e.g. paints, or the natural layer of chromium oxide which grows on a stainless steel) - reducing the potential of the metal to ensure it is the site only for the cathodic reaction (i.e. cathodic protection by an applied potential or by galvanising) - blocking just one of the reaction processes, by the use of an anodic or cathodic inhibitor. Inhibitors are chemicals added to the water (and so only applicable in closed systems where the presence of the inhibitor is acceptable, such as recirculating cooling or heating systems.) Cathodic inhibitors forming surface layers which inhibit the cathodic reaction, and are intrinsically safe. The rate of corrosion (corrosion being the anode reaction) falls steadily as the cathode area is reduced. If the concentration of inhibitor is insufficient to prevent corrosion completely then there will still be some corrosion, but the dissolution of metal is spread over the whole anode area and the rate of corrosion will be lower than in the absence of inhibitor (see (a) below) Anodic inhibitors very different (see (b) below). An example is sodium nitrite which forms a continuous protective film of iron oxide on anodic areas of the steel surface, acting as a barrier to further corrosion. The process of forming a protective film is called passivation. However, the inhibitor does this by encouraging oxidation of the steel, and unless the film is sufficiently thick and protective the corrosion rate of the steel is considerably greater than the corrosion rate with no inhibitor present at all. The concentration of the anodic inhibitor must therefore be kept above a critical level - if it falls below this level then rapid corrosion will result. If the anodic film is incomplete, then this corrosion will be localised in the unprotected regions, and lead to pitting. Anodic inhibition must therefore only be used when there is confidence that concentrations can be kept high enough, otherwise it can cause premature failure. e.g. localised corrosion leading to refinery failure: erosion-corrosion

Factors to consider when finalising alloy and joining process

- strength - for fusion welding, the key operating variables are power and speed, determining the thermal history (and thus size of the HAZ, and the loss of strength), and also influencing residual stress and distortion during assembly. These variables will also affect the production rate and cost. - using alternative thermal joining processes (e.g. friction stir welding) would change the size of HAZ and hardness profile, and residual stress etc. - using adhesives or mechanical joints would avoid loss of hardness and residual stress, but would require changes in joint geometry to give overlaps between the sheets, instead of a butt joint. Changing process also has production rate and cost implications. - as choice of process affects strength differently in each alloy, the choice of process will influence which alloy is selected to meet the loading requirements (as well as aspects such as corrosion resistance). This will also influence the production cost of the trailer

Polymers overview

- thermoplastics >90% of market Thermoplastics have: - long-chain molecules ~ 10^3-10^5 monomer units - strongly bonded (covalent) backbones - weak inter-chain bonds (van der Waals or hydrogen bonds) They may be amorphous (glassy ie with randomly oriented chains) or semi-crystalline (containing crystalline regions mixed with some amorphous component) e.g. semi-crystalline: o PE polyethylene 30% (LDPE 18%; HDPE 12%) o PP polypropylene 19% o PET polyethylene terephthalate 8% • amorphous o PVC poly(vinyl chloride) 10% o PS polystyrene 6% • The other 27% includes mixed plastics and thermosets.

Benefits of powder processing

1. High melting-point materials can be formed to final shape: ceramics (e.g. alumina, silicon nitride, zirconia); cermets (e.g. tungsten carbide/cobalt); metals (e.g. tool steels, carbon/alloy steels, bronzes, Al alloys, Ti alloys); polymers (e.g. PTFE, ultra-high MW polyethylene). Some of these cannot be processed in other ways 2. It is a near-net shape forming process (i.e. involves minimal final machining) with reasonably close tolerances and good surface finish e.g. small ceramic parts, metal gears, bearing bushes, connecting rods etc. 3. Material wastage is low (important for expensive materials) e.g. Powder route wastage <3%; Cast/forged parts wastage typically 60% 4. Achieves good dispersion of phases e.g. for reactive materials or of materials which cannot easily be mixed in the molten state. Fine particle distributions can be achieved. e.g. carbide particles in high-speed steel (alloy steels which retain high hardness at temperatures of up to 500º): 5. Avoids segregation effects which occur during casting (e.g. when densities of phases are very different) e.g. solid bearing alloys (mixture of hard particles in soft matrix) 6. Porosity can be controlled, either to achieve low porosity (essentially 'fully dense') or high porosity (up to 50%) for porous ('self-lubricating') bearings or filters. 7. Relatively cheap for large production runs (e.g. >10^4): materials costs are reduced by lack of wastage but plant and die costs can be high.

Combining fibres and matrix: Spray Lay-up

1. LEAST EXPENSIVE Chopped fibre (glass) and polyester resin + catalyst mixed in a hand-held gun and sprayed directly into the mould or on to the structure. Gives a random 2-D fibre array. Quality depends on operator skill. Advantages: Cheap, well-characterised, versatile. "Cheap and cheerful"; also quite foolproof. Disadvantages: Resin-rich laminates; only materials available are glass fibre and polyester; health hazards from styrene monomer. Good surface finish on one side (in contact with mould), rough on other. Poor dimensional accuracy and repeatability. Applications: Bathtubs, shower trays, small boats. Good for one-off jobs.

Design of Castings: Fluidity defects and solutions during mould filling

1. misrun - lack of fluidity 2. cold shut - streams of metal bring too cold to fuse 3. mould damage (and entrapped debris from mould): due to mould erosion near ingate - solve by reducing metal flow rate; 4. turbulence defects (entrapped bubbles and excess oxide) - ditto. SOLUTIONS: - Redesign running and gating systems (position, size and number of ingates and vents). - Increase fluidity by raising pouring temperature or preheating mould, or by changing alloy composition (lower freezing range).

Combining fibres and matrix: Wet Lay-up, Hand Lay-up

2. slightly more expensive Resins impregnated by hand (using rollers or brushes) into fibres (generally in the form of woven cloth). Only suitable for low-viscosity resins (may be warmed). Left to cure at room temperature. Roller used to spread resin and remove bubbles; gel coat allows release from mould and gives smooth surface finish. Advantages: Reasonably cheap, technique easily learned, versatile. Suitable for a wide range of fibres and resins. Good for one-off jobs. Disadvantages: Quality of composite very dependent on skill of operator. High fibre volume fractions difficult to achieve. Health hazards from low-viscosity resins and monomers. Applications: Wind-turbine blades, boats, architectural mouldings.

End use of polymer products + cost

40%: Packaging e.g. film, bottles, pots 20%: Construction e.g. building components, plumbing, gutters 6%: Cable/electrical insulation 10%: Transport e.g. automotive parts 4%: Household, leisure, sports 3%: Agriculture e.g. containers, crop protection 2%: Medical e.g. disposables, containers, implants 15%: Other roughly, most high-volume virgin polymers cost ~ £1100-1800 / tonne + fluctuate w/ oil price (Steel ~£200-400)

Investment casting overview (permanent pattern, expendable mould)

A hybrid process: here "permanent pattern" actually means a permanent mould is used to make an expendable pattern. This pattern is covered in an expandable ceramic/ refractory shell, in which the casting itself is produced. Advantages: Excellent accuracy and surface finish Disadvantages: Limited to small parts; labour intensive; more expensive

Advantages and disadvantages of fusion welding (to make continuous seam welds)

Advantages : continuous welds at edge of overlap seals the joint, preventing water ingress. Greater flexibility in orienting the joints in any direction. Disadvantages: more expensive, and requiring more clamping and support during assembly; cannot be disassembled for repair or at end-of-life. (Note that mild steel not expected to suffer from problems of HAZ softening or embrittlement or localised corrosion)

Advantages and disadvantages of sand casting (permanent pattern, expendable mould)

Advantages: -Complex 3D shapes can be made. -Cores can be used to produce hollow sections. -Automated processes are suitable for longer runs. -low material and equipment costs Disadvantages: -Due to poor surface finish, some machining is necessary. -Its not as accurate as die or investment casting -relatively high labour costs -not suitable for thin sections -"dirty" process Crib (i) dimensional accuracy and tolerance; [25%] (ii) mechanical properties; [20%] (iii) component cost. (i) Sand casting: The mould consists of sand with a small amount of resin binder packed around a re-usable pattern. This results in relatively poor dimensional accuracy and surface finish. The thin sections in the nozzle design would be vulnerable to sand particle defects and loss of dimensional accuracy due to erosion of the mould. This would be a limitation given the 2 mm minimum thickness for this component. Pressure die casting Permanent, metallic dies result in a better surface finish and tolerance compared to sand casting. The applied pressure would give better results for the thinner sections in the nozzle design. Both processes: The changes in cross-sectional thickness in the component would make it vulnerable to shrinkage defects. [5 marks] (ii) Sand casting May be susceptible to fluidity defects, such as misruns or cold shuts, which would affect the mechanical properties of the part. This would be a particular problem in the thinner sections. A suitable alloy would be needed with high fluidity - for example eutectic composition - which will also impact on properties. Pressure die casting Would be susceptible to porosity, as a result of turbulence. This would negatively affect properties. The applied pressure may give more flexibility over the choice of alloy composition, as fluidity will be less of a concern. [4 marks] (iii) Sand casting Low material and equipment costs, but high labour costs, which could be a disadvantage depending on batch size and required production rates. Pressure die casting Much higher equipment and setup costs, which mean a large economic batch size. The melting point of the Al alloy will affect the choice of material for the dies, to avoid excessive distortion and wear.

Effect of temperature rise in deformation processing

All of the plastic work is dissipated as heat. The workpiece temperature depends on: - thermal properties of the metal and tooling; - component geometry; - heat transfer conditions (to the tooling, coolant or atmosphere). Use attached eqn assumption How accurate is this approach? 1. The assumption of adiabatic heating will lead to an over-estimate, since there will be some heat loss to the tooling. 2. If all of the heating occurs in primary shear zones, through which all the material passes, the temperature rise will be reasonably uniform. But if significant heating occurs in secondary shear zones at the surface (i.e. due to friction on the tooling), a temperature gradient will be set up, and the peak temperature rise will be higher than the average value calculated. use sqrt(at) << d assumption then temperature gradients remain

Polymer alloys

Alloys can be produced by blending two or more polymers, for example: Vinyl chloride + vinyl acetate co-polymerised produce a polymer which is less brittle than PVC (chain irregularities lead to less dense packing; extra free volume allows more chain mobility). Styrene-acrylonitrile co-polymer reinforced with polystyrene-butadiene co-polymer, to make ABS. Styrene + butadiene to make High Impact Polystyrene (HIPS): polystyrene is brittle, polystyrene-butadiene 'precipitates' are rubbery giving higher toughness. These micrographs show a brittle epoxy resin (SU8) which has been toughened by inclusion of spheres of rubbery PAA which stretch and bridge across cracks.

Age hardening

Artificial hardness and yield stress rise to a peak in about 5-24 hours (the "T6 temper") and then fall. Natural slow rise to a plateau hardness over 1-28 days (the "T4 temper") Mechanism The shape of the ageing curve results from the interaction of a number of effects: (i) rapid initial fine-scale precipitation from supersaturated solid solution (SSSS). (ii) particle coarsening (i.e. steady decrease in the number of particles, with an increase in mean size and spacing), through one or more intermediate precipitates, eventually reaching the equilibrium phases. (iii) decrease in coherency (i.e. crystallographic matching) of the particle-matrix interface, as the particles coarsen and transform. (iv) transition from dislocations shearing the particles while they are small and coherent (the rising part of the curve), to dislocations bypassing the particles when they are well-spaced and incoherent (the falling part of the curve).

Powder manufacture

Atomisation - high pressure jets of water (water atomisation) or gas (gas atomisation) are drected at a stream of molten metal, causing it to break up into droplets and solidify Water atomization (fast quenching in a high heat-capacity medium) leads to irregularly shaped particles; gas atomized particles tend to be more spherical. For very reactive materials, inert gas or fluid may be used. Ceramic powders made by crushing or grinding (followed by sieving) - particles are very irregular

Combining fibres and matrix: Vacuum Bagging

C An extension of (b), but quality improved by using a vacuum to apply a uniform pressure to the moulding before and during curing. Heat and pressure can also be applied using an autoclave (pressurized oven) when making articles. The material may be supplied in the form of pre-preg (cloth plus uncured matrix resin in sheet or tape form) Advantages: High fibre contents, lower porosity, better process control. Disadvantages: More costly, greater operator skill needed. Applications: Wind turbine blades, large boat hulls, aircraft structures, racing car components. Can also be used for in-situ repairs, e.g. on boat hulls.

Porosity, vacuum degassing, micro/macro

Caused by dissolved gas cyuming out of solution during solidification. Gases may be absorbed from the atmosphere, or may be injected as part of prior processing - e.g. oxygen is injected in steel-making to burn off the excess carbon in pig iron (to the much lower level needed in the steel), leaving residual oxygen, CO and CO2 dissolved in the steel. Vacuum degassing, though expensive, improves casting quality - the metal is held under vacuum before casting. But as with other impurities, dissolved gas can also be rendered harmless by converting it into solid particles (e.g. oxides) during solidification. Example: 'killing' a steel means adding Al powder (or another oxide former), to produce Al2O3 particles, removing much of the oxygen from solution. Macroporosity: porosity on the scale of the casting is mainly relevant to bulk cast ingots, subsequently rolled to stock sizes e.g. Over 90% of steel in Britain is produced by continuous casting. The steel is vacuum degassed before casting, but macroporosity problems are avoided: most released gas escapes upwards through the liquid "sump" into which liquid steel is fed continuously. Ways of controlling: - Vacuum outgassing: stir melt at temperature under vacuum to draw out dissolved gas. - Add alloying elements that react with dissolved gases to form solid inclusions during solidification (e.g. "killing" steel with Al, to scavenge oxygen in the form of alumina). - Add inoculants to promote heterogeneous nucleation in the central region of the casting, trapping porosity over a large area of grain boundary over a wider volume of the casting. Microporosity As with solid impurities, it is preferable to distribute segregated gas as microporosity between the dendrite arms - but this is still detrimental to mechanical properties. A fine-scale dispersion of bubbles is a real "fingerprint" of castings when viewed under the microscope. e.g. Al-4% Cu (see attached) Left: as-cast - Dark regions are inter-dendritic regions and grain boundaries. The contrast arises from etching of impurities, and from porosity Right: homogenised Dendritic structure within grains no longer visible: impurities have redistributed. Dark spots in the grains are porosity trapped between dendrite arms. Microporosity remains after homogenisation: the pores are filled with gas, mostly in the form of molecules rather than single atoms, and the diffusion rate for molecules is very small (due to their size). Microporosity in ingots is removed by - hot rolling - but this is not a solution for near-net shape castings. - Hot isostatic pressing (HIPing) - costly, but provides marked improvement in mechanical properties. Easiest for Al and Mg alloys, where HIPing temperature required is comparatively low (so relatively economical).

Additives in polymers

Changes properties. Problem posed is when trying to recycle polymers - additives become impurities 1. Processing additives - Stabilisers (to prevent degradation at high T) decomposition or other reaction) - Lubricants - Viscosity reduction 2. Flexibilisers - Plasticisers (e.g. PVC plasticized by small molecules such as tricresyl phosphate TCP) 3. Anti-ageing additives - Antioxidants - Ultraviolet stabilisers (e.g. carbon in rubber) 4. Surface property modifiers - Antistatic agents 5. Coatings - Metallising - Ceramic surface (powder bonded onto polymer) - Barrier layers (e.g. gas-impermeable film on some packaging films and bottles) 6. Optical property modifiers Pigments and dyes 7. Fire retardants - Ignition inhibitors - Self-extinguishing additives - Smoke suppressants 8. Foaming agents - Blowing agents. For making polymer foams (closed cell or open cell) 9. Fillers - Increase rigidity (elastic modulus) - Increase creep resistance (e.g. glass fiber in PTFE) - 'Bulk out' expensive polymer with cheap inert substance

Evaporative mould casting (permanent pattern, expendable mould)

Closely related variant of investment, using a polystyrene foam pattern. Advantages: High accuracy and surface finish (especially with small-bead polystyrene); lighter patterns than wax, so suitable for large parts. Disadvantages: Labour again relatively high

Maps of aqueous corrosion behaviour: Pourbaix diagrams

Corrosion behaviour depends on ion concentration, pH etc. - Some metals form protective (passivating) coatings by reaction with the corrosive medium, so that the reaction is stifled and the metal does not apparently corrode. It is not in fact immune, but only a small amount of corrosion takes place until layers have built up so the system is 'safe'. - In other regimes, the coating dissolves, and the metal starts to corrode rapidly. It can be useful to represent the behaviour in aqueous solutions on a 'map'. Pourbaix diagrams are used to plot the electrochemical potential against the pH of the solution. The Pourbaix diagrams can be altered dramatically by the presence of certain ions. e.g. stainless steel in aerated water shows a very large passive region because of the formation of a stable protective Cr2O3 layer. In the presence of chloride ions (e.g. as present in sea water) the film breaks down as a soluble complex chromium chloride forms, and no passive region is found. The diagrams below show the reactions for iron, titanium and aluminium. N.B. there is no rate on these diagrams Immunity - range of pH and potential where corrosion of metal is thermodynamically impossible Corrosion implies that there is a thermodynamic driving force tending to dissolve metal as ions Passivation shows that there is a driving force to form a stable film (e.g. oxide or hydroxide) on the metal surface, but this may or may not form an effective barrier to further corrosion

DaF: Wet corrosion of metals 1

Corrosion in metals occurs when metal reacts to form a corrosion product (often an oxide). The product has inferior properties to the metal, so causes problems. e.g. • Lower mechanical strength; • Electrical insulator rather than conductor. Key features of wet corrosion: • Promoted by electrochemical couples. • Often occurs more rapidly in acids. • Can be prevented by formation of protective layers on metals (e.g. Cr2O3 on stainless steel). For corrosion to occur, we need two reactions, anodic (liberating electrons) and cathodic (consuming electrons) which transfer electrons between different chemical species. a) bimetallic corrosion: two dissimilar metals in contact under damp conditions REVISION b) Differential aeration in steels (leading to crevice corrosion; differential concentration corrosion) Corrosion of steel or iron in water in the presence of oxygen takes place by the reactions (as above): Fe = Fe2+ + 2 e- (Anode) O2 + 2H2O + 4e- = 4(OH- ) (Cathode) The two reactions occur at different regions of the steel surface, depending on the local oxygen concentration, and electrons are transported between the two through the metal. The oxygen levels are usually highest close to the surface of the water, and the lowest oxygen levels are deep inside cracks and crevices. This means that steels are particularly liable to form deep cracks or pits as a result of the presence of water, because once a crack has formed corrosion (the anodic process) will be concentrated at the growing tip of the crack where the oxygen concentration is lowest. Rust forms by combination of the Fe2+ ions with the OH- ions, and this may take place somewhere between the anode and the cathode. Rust has lower density than the metal from which it forms, and is associated with expansion which can wedge cracks apart. Once a paint layer has broken and corrosion has begun, rust can lift paint off, creating 'bubbles'. Steel is often used with structural materials, concrete or brick, and this wedging action from rusting of the steel can actually result in failure of structures.

Stress corrosion cracking (SCC)

Cracks propagating at stress well below the normal failure stress with little macroscopic plastic deformation, even in a material which is normally ductile, in certain types of environment - transgranular - intergranular Common cause of industrial failures (e.g. boilet explosions, hinkley point steam turbine). Liable to unexpected failures mainly because of range of possible effects and specificity of the metal/environment combinations Essential ingredients • a susceptible material • sufficient tensile stress (which can be residual stress) • a specific environment Mechanisms: Active path dissolution Active path dissolution involves rapid corrosion along a narrow path (such as a grain boundary) with the rest of the material being passive. An example is the process of weld decay in unstabilized stainless steel discussed earlier. Cracks will be intergranular. Film-induced cleavage In film induced cleavage, a brittle surface film (e.g. an oxide) on a ductile metal cracks, and the crack then propagates a short distance into the metal (~ 1 µm) before it is blunted. The brittle film then reforms by corrosion at the crack tip, and the process repeats. Hydrogen embrittlement separate card

Combining fibres and matrix: Filament Winding

D Generally used for hollow (circular or oval sectioned) components, though large curved sheets can also be made by carving these up after winding. Continuous fibres ('continuous rovings' in the figure below) are passed through a resin bath before being wound onto a mandrel in a variety of orientations. Advantages: Can be very fast and economical. Resin content carefully controlled. Composite structures can be designed precisely to support the anticipated stresses. Disadvantages: Limited to convex components. Fibres cannot be laid exactly along the length of a component. Fibre feeding mechanism and mandrel can be expensive. Suitable for low-viscosity resins only. Applications: Chemical storage tanks and pipes, boat masts, wind turbine blades, gas cylinders, other pressure vessels.

Casting defects, influences on formation and physical origins

Defects include porosity, internal or surface cracks Depends on: - alloy composition - design of the mould and gating system - melt pressure and temperature - shape details of the casting.

Deformation processes (forming)

Deformation processing (or forming) is the shaping of material in the solid state Processes don't compete with each other but compete with other processes e.g. - forging vs. casting vs. powder processing - extrusion vs. welded assembly of plate Why carry out? 1. Geometry: forming long, thin-walled shapes (i.e. high aspect ratio, difficult to cast). TECHNICAL 2. Low waste: forming processes mostly "near net shape". ENVIRONMENTAL 3. Tolerance and surface finish: usually good (and can be corrected by machining) 4. Microstructure: cast microstructures usually coarse - need to be refined by deformation (and heat treatment) to enhance properties. QUALITY 5. Energy and cost efficient: temperatures below melting. COST Disadvantages 1. High forces and complex control systems are required: can be high capital cost, with expensive high strength steel tooling. 2. Multiple stages (including machining) often needed due to physical limits on achievable shape changes and complexity. 3. Metals work-harden with cold deformation and often require intermediate annealing to enable further deformation

Powder processing: compaction - design of die

Design of the die/punch is very important. Friction between the powder and the die can lead to a non-uniform density distribution which results in strength variation More uniform distribution is achieved by • better lubrication between powder and die • multiple punches, e.g. with punches moving at both top and bottom of die. Get highest compaction close to moving punch, so average compaction increased and compaction variation is reduced. Compacts of non-uniform shape will have a complex and non-uniform density distribution unless multiple punches are used to give constant compression ratios. The design of presses and dies to achieve optimum filling and uniform compaction can be very complex

Dynamic recrystallisation and recovery

Dynamic = during firming Static = during annealing (after forming) Carbon steels (left) undergo dynamic recrystallisation - new grains forming, growing, work hardening, and recrystallising in a continuous cycle: flow stress peaks then falls Al alloys (right) undergo dynamic recovery - dislocation accumulation (work-hardening) balanced by dislocation annihilation (recovery): flow stress reaches a constant steady-state. Dynamic recovery and recrystallisation depend on both the strain-rate and the temperature (i.e. diffusion-controlled thermally activated processes), see figures.

Risk of cracking when electroplating

Electroplating may lead to hydrogen embrittlement (from atomic hydrogen diffusing in from the electrolyte). High strength alloy steel is particularly vulnerable, and the sharp corner may initiate cracking due to residual stresses from the heat treatment (for which air cooling is again the better choice). Precautions: reduce the stress-concentration at the corner, if possible; heat treat to remove the hydrogen; use a lower strength alloy, if possible

What does enamelling do?

Enamelling protects the cast iron from corrosion, gives a smooth finish (easier to clean, and with an aesthetic shine), and enables colouring and branding. Different colours inside and out are possible. Disadvantages are the additional complexity and cost of another process step (more complex for two colours); the brittleness of the enamel (glass) can lead to chipping damage in use.

Polymer processing: Extrusion

Extrusion is the most important processing method for thermoplastics. Granules of polymer are fed into the hopper. The screw turns and drags the polymer along the extruder. Heat from friction with barrel walls, and also external heat source. The polymer granules are melted and compacted, and the screw movement also mixes and homogenises the melt. The molten polymer leaves the extruder through a shaped die which defines the geometry of the product (e.g. thread, sheet, film, pipe, rod, or more complex sections such as channels for constructing windows/doors). Pressures: High (typically 100 MPa). Temperature: Above Tm for a crystalline polymer Two or more polymers can be extruded together (co-extrusion), so that the product has a multi-layer structure. Extrusion can be used for a wide variety of products, as discussed below

Extrusion definition

Extrusion uses compressive loading to force a billet (usually hot) through a die to make a shaped, prismatic section. With soft metals (e.g. hot aluminium) very large reductions in area can be made in a single step

Design of continuous fibre composite structures: Lay-up orientation

For critical applications (requiring optimum strength and stiffness), continuous fibre composites are used (more expensive than short-fibre). Composite can be designed to accommodate loads by positioning the fibres along directions of maximum stress. For simple structures, the required fibre distributions can be obtained by laying up sheets (laminae or plies) of pre-preg. More complex structures may use filament winding or tape laying. Because the material deforms anisotropically, internal stresses are created in a composite as it is loaded Example: 0°-90° two-ply laminate under uniaxial stress. Terminology means: a stack of two layers or laminae each of which contains uniaxial long fibres. The top layer here has fibres at 0˚ to stress axis; the lower layer has fibres at 90˚ to stress axis. These are laid-up and bonded to form a single block of composite. If we load the composite as shown, then the two laminae experience different strains. The top one is stiff and has low strain. The lower one, loaded normal to the fibres, has high strain. However, the two laminae are bonded together, so they exert stresses on each other. The results are: • Strain of the composite block is intermediate between the strains for the two laminae loaded in isolation; • High stress at the interface between the laminae (implications for delamination failure; environmental degradation by 'wicking' of liquid such as water between laminae) • Composite block will suffer out-of-plane distortions, as shown below (mainly because of Poisson ratio effects). Typical double curvature ('anticlastic') distortion: An important principle in composite design is to make sure that these elastic distortions are minimised by making multi-ply composites symmetric, giving balanced layup. e.g. instead of 0°-90° , use 0°-90°-0° by introducing a third ply at the bottom

Mechanical properties of PMCs: elastic modulus

For good properties, need strong, high-modulus fibres to which load is transferred from the weaker, low-modulus matrix. Load transfer happens via shear stress at the fibre-matrix interface. Bonding at the interface is therefore important. Elastic modulus - continuous or long-fibre composite highest modulus (and strength) // to fibre direction. - short fibres or particles have intermediate values - hence fibre orientation critical!

Upper Bound Analysis

Forging ez pz Extrusion Dead metal zone (DMZ) formed in square dies, leading to intense shearing and heating Machining - neglect friction (giving primary shear zone) In practical machining, there is a secondary shear zone associated with friction on the rake face of tool (neglected above). This can be included by adding a term for the power dissipation on the tool, e.g. assuming sticking friction over the length of the tool face, with a relative velocity given by the chip exit velocity.

Polymer properties

From architecture See photo Crystallinity (caused by slow cooling) Crystallinity leads to loss of optical transparency and shrinkage, as crystalline regions are more dense than amorphous regions. To avoid crystallinity, increase hold on time, increase cooling rate or increase injection pressure. Alternatively, use a different polymer with a lower crystallinity. - also causes volume reduction - thick sections cool slower than thin sections, giving higher degree of crystallinity, causing shape distortion - amorphous polymers have a glass transition temperature. 100% crystalline just has a melting temperature - optically transparent polymers usually amorphous because crystallites larger than wavelength of light scatter light! Chain Alignment (by cold drawing of fibres or types) The strength and elastic modulus of polymers increase significantly in the direction of chain alignment by making more use of covalent bonds. Extrusion processes align the molecules by forcing them through small inlets in tension by winding around a drum/using rollers. However, if the chains coil up once the mould is removed, this can cause the part to swell - melt swell. To avoid swelling either cool quickly as it leaves die or redesign the mould to compensate, or tension product whilst still hot (using draw down to reduce melt swell)

Constitutional Supercooling of Alloys

From phase diagram, higher the concentration of the liquid, the lower its solidification temperature Temperature has same form as concentration curve but inverted Consider again the steady-state distribution of composition. From the phase diagram, the higher the concentration of the liquid, the lower its solidification temperature. This temperature therefore has essentially the same form of the concentration curve, but inverted (see figure). Now superimpose the actual temperature gradient in the casting (decreasing from right to left, the direction of heat conduction). The temperature is equal to the local solidification temperature at the interface, but there are then two scenarios, depending on the gradient in the temperature (dT/dx) compared to the gradient in the local solidification temperature (dTm/dx). Case (i): (dT/dx) > or ≈ (dTm/dx): T in liquid everywhere above its (local) melting point; solidification is stable at the interface (leading to a planar solidification front or "cellular" growth of a few grains). Case (ii): (dT/dx) < (dTm/dx): in a region ahead of the interface, T in the liquid is below its (local) melting point: this is known as constitutional supercooling, since it is a gradient in composition (coupled with a thermal gradient) that leads to the supercooling. Here solidification is unstable, leading to dendrite formation.

Polymer extrusion: extrusion methods

Granules After the polymer is synthesised (usually from oil), it is passed on to processors in the form of granules. The polymer is extruded through multi-hole dies to form laces 3-6mm in diameter. The laces are cooled and chopped into regular granules - used as feedstock for other processes perhaps after blending with additives/fillers. Filament, thread and fibre Extrusion through a circular die, but extrudate is stretched to align polymer chains, see section 3.2(b) Thick film/sheet Several related processes for making the thicker grades of film, sheet (which may be corrugated) or multilayer films in which different properties of polymers can be combined. Extrusion of one or more polymers through a long slit can be followed by quenching: - With air jets or - On to cooled metal rolls Product may then be stacked (panels) or rolled (sheet). Air-cooled surfaces are rough; smooth surfaces made by cooling in contact with a metal surface. Even so, smooth surfaces are not guaranteed, as the slower cooling rate of the interior results in compressive stresses in the surface layer which can give a wrinkled surface) Coating A high-volume use is wire or cable insulation (e.g. with PVC or nylon for electrical purposes) With a different coating geometry, sheets of fabric or paper can be coated with polymer (often LDPE). Examples include waterproof fabric, flooring, polymer-coated cardboard for food packaging Pipes and hollow sections The melt flow is divided and then exits the die. The torpedo helps the polymer to divide and flow round the mandrel. The extrudate may remain as a pipe, or it may be immediately turned into some other shape. The most common way of dividing the melt uses a floating mandrel, held in place by a 'spider' of fine wires. The melt welds up after passing the wires, so there should be no damage line in the final article. For simple sections, the die may be made with a solid central part, and the melt flows around it. Paulsontraining.com In this illustration, the hollow polymer pipe (called a parison in this application) is used for extrusion blow moulding to make hollow products such as bottle, drum or barrel (described below). Extrusion blow moulding Bottles can be made directly from the extruded polymer in a continuous process, as illustrated above. Fast (cycle time ~10 s) and automated, avoids need for reheating (cf. stretch blow moulding). A tube of polymer is extruded vertically from an annular die. A mould is clamped around the tube while it is still hot, and the tube is inflated with compressed air until it fills the mould. After cooling it is removed from the mould and cut from the remaining tube. Widely used for containers e.g. milk bottles, barrels, drums. Material usage not optimized: wall thickness is variable; not much molecular alignment (compare stretch blow moulding 2.4) Film Blowing Most polymer film is made by blowing, which allows biaxial alignment of the polymer molecules in the film. Molten polymer is extruded through an annular die and inflated, providing circumferential hoop stresses and resulting in molecular alignment in this direction. Tensile stress is provided by pulling rolls, resulting in molecular alignment is this direction as well. The alignment increases the film strength, so it can be made thinner. Typically, a polymer film 1 mm thick extruded from the die is reduced to 0.1 - 0.2 mm, using a blow-up ratio (final bubble diameter/die diameter) of 3-5. The film is cooled by external air jets. Co-extrusion allows multilayer films to be produced (e.g. for food packaging). The bubble is usually split and flattened onto a coiling drum, and the film is used for plastic bags or sheeting. Align polymer chains axially by tension of windup rolls Align polymer chains in hoop direction by expanding the bubble. Gives biaxial chain alignment so film has higher strength than if chains are random

Dimensional changes on solidifcation of cast iron

Grey cast irons have low shrinkage Graphite is a low density phase, so the formation of graphite counteracts the contraction of the iron on solidification. For near-eutectic compositions of grey cast iron there is effectively zero volume change on solidification - useful for producing castings directly to final size and shape

Rotational moulding

Hollow mould is charged with thermoplastic granules, then heated and rotated. The granules adhere together to form the component (but do not fully melt). Typically used for PE or PP parts. Cheap: does not need expensive equipment; moulds made from aluminium sheet so easily made and customised; can be large. External heat provided by hot gas. There is no molecular alignment. Product hollow, but with poor control of wall thickness. Typical products: Traffic cones, tanks, barrels, canoes, pallets, toys.

Homogeneous heat treatment

Homogenisation is a prolonged "soak" of a casting at high temperature in the singlephase field (e.g. 24 hours at 530-580oC for Al alloys, with melting point c.630o C). This requires bulk diffusion of the components that are distributed inhomogeneously. In most cases the alloying additions form substitutional solid solutions (rather than interstitial), and are therefore inherently slow to diffuse - hence high temperatures and long hold times are needed. The degree of homogenisation can be estimated using the rule-of-thumb diffusion equation: x2 = Dt where diffusion of an element takes place over a characteristic distance x in time t, with a diffusion rate D = Do exp (-Q/RT) (cf. Materials Databook). The practical limits to temperature and time set a limit on the length-scale that can reasonably be homogenised (grain-scale and dendrite segregation can usually be remedied, but not macro-segregation)

Polymer joining methods: assessment

Hot plate welding: melts the interface before butting parts together - only suitable for thermoplastics (HDPE to itself, not thermosetting GFRP or dissimilar joints). Also requires good control of alignment in butt or T-joints in 10mm bars, and cannot be disassembled (note that shelter is temporary). Adhesive bonding: could join either material and dissimilar joints, but poorly suited to butt or T-joints with 10mm bars (low adhesion area, and not suited to lap configuration in this thickness). Cannot be disassembled (note that shelter is temporary). Press-fitting shaped Al joint pieces: the best solution - can handle either material in any combination, and can easily be disassembled (good for temporary shelter, and for separating the materials for final disposal/recycling). An external Al joint piece would also protect the ends of the GFRP and reduce the risk of damage and splitting (as used in many flexible tent poles

Dendrites

In constitutional supercooling, there is a greater driving force for solidification ahead of the solid-liquid interface than at the interface itself. Unstable growth occurs as follows: if a small part of the solid interface spontaneously extends into the surrounding liquid, it can continue growing rapidly into the melt, leading to long thin crystals. The same behaviour may then be repeated on the sides of these crystals, giving secondary side arms. The distinctive crystal shape formed during solidification is known as a dendrite. Once the arms of a given dendrite meet, they form a single grain (since all arms of the dendrite have a common crystallographic orientation), and their characteristic structure is not visible - the final structure only reveals the grain boundaries where the crystal orientation changes.

Future of Composites

Initially 'craft' industry - very labour-intensive, manual ops Automotive industry - could composites replace sheet steel? Advantages: Corrosion and fatigue resistant; lightweight; fewer parts Disadvantages: Pressed steel panels can be made in a few seconds; composite panels typically take hours; Composites can't (easily) be recycled at end-of-life; automotive industry carries requirement for recyclability [small volumes, so industry hasn't developed]; Joints more difficult with composites. A further general problem with composites is damage detection, particularly for delamination. This is particularly difficult with CFRP, because the carbon fibre is opaque to light so internal flaws or damage cannot be detected by optical techniques. GFRP is a bit easier (unless, as often happens, the surfaces are coated in an opaque layer). Reliable damage detection is essential for safety-critical applications such as in aircraft. Ultrasonic techniques are commonly used. Wind Turbine Industry Turbine blades are now up to 60m long (25 tonnes weight) and the industry is growing. Blades are made from GFRP and CFRP; annual material usage is currently about 0.5Mt and projected to double within 5 years. Manufacture is increasingly automated. Common technologies include extensions of filament winding: Automated Tape Layup (ATL) and Automated Fibre Placement (AFP). Prepregs are used for some parts: automated cutting is used to produce 'kits' of laminae, assembled using automated pick-and-place positioning. Finishing of blades can be automated (e.g. removing 'flash' and rough edges, polishing). The first generation of wind turbines has reached end-of-life, and structures are being dismantled and replaced. There is currently no good end-of-life destiny for wind turbine blades. Most, internationally, are sent to landfill. The volume of blades is projected to increase exponentially over the coming years, reaching 500k by 2029.

Polymer process variables

Injection Pressure: Higher injection pressure gives better surface detail and precision, and helps to reduce shrinkage, so very useful for semi-crystalline polymers. Hold-On Time: Helps reduce shrinkage in crystalline regions, however, it will also increase cycle time and reduce production rate if increased. Mould Temperature: Higher mould temperature gives slower cooling, which results in higher crystallinity in semi-crystalline polymers and therefore more shrinkage and higher stiffness. There will also be a loss of molecular alignment. Molecular Alignment: Molecular alignment occurs through constriction of the stream of molten polymer, e.g. in extrusion. To increase alignment in the final product, design the process so that the molten polymer flows through constrictions and so that the cooling rate is high so that alignment is not lost after flow. Residual Stress: A result of differential cooling, can lead to shrinkage, cracking and failure. The surface of thicker sections of polymer will be intension, whilst the inner sections will be in compression.

Laser Hardening

Laser hardening involves tracking a defocussed laser spot over the surface to produce a thermal cycle to a depth of order 1mm, with the peak temperature needing to be in the austenite region (e.g. 800-1000o C) in order to self-quench to hard martensite. Normalised microstructures can achieve uniform martensite at the surface where the peak temperatures and times are high enough for the carbon to fully re-distribute from former pearlite regions to former ferrite regions. But this is not the case where the temperature only just reaches the austenite field, giving mixed ferrite/martensite on quenching. Quenched/tempered microstructures have a fine-scale ferrite/iron carbide structure to being with, and transform to austenite easily, giving a uniform martensite layer below the surface

Failure of non-metals: degredation of polymers

Like metals, specific material/environment combinations e.g. fracture and crazing in polymer components (a) Photo-degradation Very common. Polymers are susceptible to damage from light, particularly from energetic UV photons (e.g. in sunlight) which cause breakage of covalent bonds. May lead to three distinct effects: reduction in polymer chain length; depolymerisation (i.e. chain breakage leading to monomer formation); or increase in cross-linking. Often some discolouration. Permanent damage; mechanical properties affected in various ways specific to the damage type. Polymers can be made UV-resistant either by surface coatings or by introducing scattering features into the structure (crystalline regions of the polymer, or white pigment particles), or absorption features (coloured or black pigment particles). (b) Oxidising atmospheres (e.g. air, various fluids, ozone (= O3)). Similar effects to photodegradation. Permanent damage and changes to mechanical properties. (c) High temperatures Reversible effects: softening of thermoplastics. Irreversible effects: increase in crystallinity; covalent bond damage as above; discolouration; charring (due to oxidation or chemical decomposition = 'pyrolysis'). (d) Solvent damage Some small solvent molecules can penetrate between the polymer chains, reducing the elastic modulus (plasticisation), and swelling the material. For example nylon can absorb >5% water, with a volume expansion of about the same amount. This effect is reversible. Irreversible effects: leaching of additives, or in some cases of polymer (e.g. a problem with thermosets such as epoxy if they have been incompletely cured); chemical reactions with polymer (e) Environmental Stress Cracking (ESC) This is the polymer equivalent of SCC in metals. Polymers suffer premature failure under stresses below the conventional design stress in certain environments. Amorphous polymers are particularly susceptible. Early stages of ESC can lead to the formation of multiple very fine cracks: 'crazing' The stress is often provided by residual stresses arising from the manufacturing process, e.g.: • Uneven shrinkage in an injection moulding leading to internal stresses and often distortion; • Parts which have been subjected to elastic stresses during joining, or assembly Weld-lines within injection mouldings (where molten streams of polymer meet) are often sites of accelerated failure. The environments responsible for ESC vary between different polymers, and as with metals there can be some unexpected reactions with specific polymer-environment combinations.

Austenitic Stainless Steel

MOST COMMON - contain sufficient austenite-stabilising elements, such as Ni, to retain austenite down to room temperature. - To minimise susceptibility to "sensitisation" during welding, "L-grade" alloys with especially low C contents are used, e.g. "304L". Austenitic stainless steels are not hardenable by quenching and tempering, but the high solute content gives a reasonable yield stress and strong work hardening, combined with very high ductility and toughness. On cooling below room temperature, martensitic and ferritic steels are characterised by a transition in toughness, from tough to brittle behaviour. The austenitic stainless steels do not exhibit a toughness transition - they retain toughness at all temperatures, and are the alloys of choice for cryogenic applications (storage of liquefied gas).

Combining matrix and fibres: Extruded short-fibre composites

MOST EXPENSIVE PMCs containing short fibres in a thermoplastic matrix (e.g. glass-fibre filled nylon or polypropylene) can be extruded using standard polymer technology (screw extrusion), and also injection moulded. Advantages: High toughness; thermoplastic matrix gives potential for recyclability. Disadvantages: Fibres increase the melt viscosity. This may place limits on maximum volume fraction of fibre. 7 C Y Barlow November 2019 Note: The extrusion process leads to significant alignment of the fibres as well as aligning the polymer molecules, so these composites have anisotropic modulus and strength. This can provide welcome strengthening in the direction of maximum stress in service, but there is associated weakening in the transverse directions. Applications: Currently comparatively limited, but applications in automotive and defense industries are increasing. Generally not used for primary load-bearing parts.

Machining definition

Machining is a plastic deformation process, used for refining shape or finish, and adding features (holes, threads etc.). Machining processes use a hard tool to selectively remove a softer material. Most components undergo some machining. Numerically controlled machines give high reproducibility and accuracy. There is a wide range of machining processes: turning, shaping, milling, drilling, tapping, grinding. The underlying mechanism is largely the same: plastic deformation of a thin surface layer or groove of material. Large shear strains are imposed on the chip and strain-rates are high (103 s -1 ) - this can cause temperature rises of the order of 1000C in steels. Machining vs casting crib Machining a wrought alloy extrusion: (i) - Improved dimensional accuracy and surface finish. - Fewer defects, greater repeatability. - Better mechanical properties of wrought alloys versus cast.

Segregation

Macro-scale A columnar zone through the whole casting leads to inhomogeneity on the scale of the casting. As noted earlier, impurity accumulations in the centre of the casting lead to a plane of weakness. In square sections (e.g. as in continuous casting), the columnar grains impinge along the diagonals, again giving planes of weakness (see figure above). It is expensive (or impossible) to remove the impurities before casting. One solution is additional alloying, to trap the impurities in a harmless form throughout the casting. Example: all C steels contain sulphur as an impurity; the addition of Mn leads to the formation of a dispersion of MnS particles thoughout the casting, rather than letting the S segregate to the grain boundaries, forming brittle FeS. A second solution is to promote the formation of the equiaxed zone, trapping impurities in more dilute concentrations over a large area of grain boundary: grain-scale segregation. Micro-scale/grain-scale (100um - several mm) Equiaxed and columnar grains will have some centre-to-edge segregation over distances of the order of the grain size (100µm up to several mm). Similarly, columnar grains will show compositional variation across their width: the interior of the grains being purer than the grain boundaries Dendrite (~mm) Segregation also occurs between the dendrites arms (primary and secondary). The dendrite arm spacing refers to the length scale of the secondary dendrite arms (typically a few µm). It is often cited as a key length-scale of a cast microstructure (i.e. the scale of inhomogeneity that can be influenced by heat treatment - see below). Grain-scale/dendrite-scale is preferable to macro-segregation, but high solute/impurity contents at grain boundaries can still cause problems: localized corrosion, or coarse precipitation reducing toughness or ductility.

Roles of various elements in tool steels

Manganese, Mn: primarily to react with S during solifidification to avoid formation of brittle FeS on grain boundaries (risking embrittlement); residual Mn in solution also contributes to increasing hardenability. Chromium, Cr: increases hardenability, and corrosion resistance Tungsten, W: increases hardenability, and provides high temperature precipitation hardening (through the carbide WC), and provides solid solution hardening

Mechanical properties of PMCs: Delamination

Many composite structures are made from layers of fibre+matrix pre-preg. Layers may be sheets of uniaxial fibres (i.e. all fibres in same direction), or may be woven cloth. Bonding between the layers is achieved by resin alone, so strength and modulus are low in this direction (often called "secondary properties"). Stress normal to the layers is liable to cause cracking and delamination (see section 4.2)

Failure in non-metals: corrosion of glass and ceramics

Many glasses contain sodium as a network modifier (breaks up silicate -O-SiO-Si-O- network, so reduces softening temperatures and promotes a wide temperature range in which glass can be worked). If the glass is in contact with moisture (particularly in an acid environment), the sodium atoms which terminate the silicate chains can leach out and be replaced by (smaller) hydrogen atoms. This 'corrosion' is associated with shrinkage of the surface layers of the glass. The glass is put into tension and surface cracks form. This leads to the phenomenon of 'static fatigue' when glass under stress suddenly breaks under static load. (n.b. the name is misleading: no cyclic stress is involved.) Cracks form at the surface and grow slowly under tension in the moist environment. The material at the crack tips, which is under higher stress because of the stress concentration, is particularly susceptible to corrosion. Once one crack reaches critical length, fast fracture takes place.

Introduction to composites and PMCs

Many natural materials (including wood and bone) are composites. Composite materials typically contain a stiff and strong 'reinforcing phase' dispersed as particles or fibres within a continuous 'matrix' which transmits loads to the reinforcing phase. A composite material often offers attractive mechanical properties combined with low density - with advantages in many applications. Focus on PMCs (polymer matrix composites). Usually reinforced with long or short fibres

Microbial corrosion

Micro-organisms (generally bacteria) can be responsible for corrosion of metals by various different mechanisms. For example Desulfovibrio desulfuricans thrives under damp or wet anaerobic (i.e. no oxygen) conditions. It produces sulphide ions which accelerate anodic dissolution and stress corrosion cracking. Microbial corrosion can occur in many places, including waste water and sewage handling, oil pipelines, aviation and fuel pipes and tanks. It is common in container and tanker ships

Design of castings: Form Freedom

Most castings involve separation of two mould parts perpendicular to a parting plane. This may be the casting mould itself (as in permanent mould casting), or separation of an expendable mould to recover the permanent pattern (as in sand casting). This imposes shape limits on castings: - it should have a single parting plane which allows removal of both parts of the mould - re-entrant corners can only be included at or near a parting plane - parts must be slightly tapered with a draft angle to allow easy mould removal e.g. capstan parting plane crib (a) (i) The parting plane needs to cut the capstan through the centre on its vertical axis - in practice, it would be cast on its side, with a horizontal parting plane. To make the central hole, a cylindrical core would be located on the parting plane, and supported in the mould, i.e. using a core that is longer than the capstan, as shown in the sketch. The LH design is preferable due to: - the taper angle on the flanges makes it easier to remove the pattern from the sand mould, without damaging the mould; - the radius on the corners improves metal flow, avoiding turbulence and mould damage; - the stress concentration at the corners is lower, reducing the risk of hot tearing during cooling.

Casting Alloys: advantages, properties

Often close to eutectic comp. - lower melting temp (cheaper, lower heating cost, faster production rate) - low soilidification ("freezing") range (high fluidity/castability, easier to feed, lower porosity, lower shrinkage defects) Properties Eutectics are two-phase material with a high volume fraction of a fine-scale hard second phase - which suggests potential for precipitation hardening. However, these second phases often non-metallic and brittle (e.g. Si in Al casting alloys). This has implications for strength and toughness - these properties depend on the morphology (i.e. size and shape) of the second phases formed in the casting. This is sensitive to whether the second phase is metallic or non-metallic. Metals - solidify easily from liquid cos low solid-liquid interface energy - surface relatively "rough" - many available nucleation sites => rounded, blobby solid particles Non-Metals (e.g. graphite, silicon) have a higher surface energy with the liquid, so prefer to form an atomically smooth interface. Growth is easiest by adding atoms to steps in the interface, so growth is much faster parallel to the interface than perpendicular to it. Overall this is slower than forming a solid metal phase, and leads to elongated, angular particle shapes, such as needles or flakes. This particle morphology is not good news for toughness - the brittle phases behave like cracks. The solution is to modify the growth by poisoning: atoms of an alloy addition which halt the growth of steps, leading to smaller more rounded particles of the brittle second phase. Examples see photo and handout 3 casting for more!

Physical Vapor Deposition (PVD) of TiN

PVD is conducted under vacuum at typically 200-400 o C, adding a TiN layer to the surface. At these temperatures there is no change in the underlying steel microstructure. The wear properties are largely controlled by the hard TiN layer, but a quenched/tempered microstructure underneath will provide greater wear resistance overall.

Permanent mould vs permanent pattern mould

Permanent mould: cavity machined in metal mould (or formed round a pattern in a ceramic mould) and mould used repeatedly - e.g. die casting. Permanent pattern: pattern (often wooden) used to shape cavity in a mould made from a material that can be broken up after casting; pattern removed prior to casting and re-used, e.g. sand casting. Large parts: permanent pattern (cheaper to make pattern than large cavity in metal mould, and low production rate unimportant as batch sizes low). Large production runs: permanent mould (higher mould cost justified, and higher production rate also needed).

Equilibrium analysis for deformation processes

Plane strain forging Analysis not required Force required for plane strain forging depends on - material parameters: yield stress, Y - process operating conditions: friction between die and workpiece, m - design parameters: the geometry of the slab - depth D, height h, width w Note in particular the sensitivity to the aspect ratio, w/h: - the high value of p/Y sets a limit on how thin a component can be forged - the thin flash formed at the edge in impression die forging accounts for a significant share of the total load Sheet and wire drawing 1. This analysis used the Tresca criterion - using von Mises instead, replace Y with 2Y/3 (identical to the forging example) 2. Note that σdraw may not exceed the yield stress Y (or the material fails in tension at the exit). This gives a maximum draw ratio: 3. A back tension σback may also be applied at the inlet. This has no effect on the initial analysis but changes one of the boundary conditions: at the inlet, h=hi , σ1 = -σback . So the draw stress is increased by the back tension, reducing the maximum draw ratio. But increased tension along the strip reduces the pressure needed to reach yield - hence back tension reduces wear of the die 4. The same analysis can be applied to plane strain extrusion, simply by another change in boundary conditions: at the inlet, σ = + σext (compression), and at the outlet σ = 0. The inlet material must then be enclosed, but the compressive stress is not limited by the yield stress - so greater reductions in section are possible. 5. The analysis can readily be extended to include the effect of friction on the die (giving additional terms in the equilibrium equations) - typical reduction in area per pass = 10-40% - work hardening + intermeiate annealing Rolling - analogy with forging - contact patch relatively small compared to radius, therefore reduction per pass is small! SO - can approximate deforming volume to a rectangular block - assume low friction coefficient Control rolling load? - reduce yield stress Y (i.e. hot rolling) - reduce friction m (but note that some friction is required to draw in the strip) - make small reductions per pass hi - ho (and tandem roll) - reduce roll radius R small radius rolls will bend, so larger backing rolls are used to reinforce the work rolls in a cluster mill design. Hydraulic jacks apply loads, continuously adjusted, to the roll stack. This ensures that a uniform material thickness (or gauge) is produced, both across and along the strip. Effects of tension at the inlet (back tension) and outlet (front tension) - reducing the pressure needed to cause yielding (as in sheet/wire drawing) - reducing the magnitude of the friction hill (and shifting the neutral plane) - reducing the rolling load, torque and power The analysis with forward/back tension follows the forging analogy, but with a change in boundary conditions at entry and exit. 1. If a sufficiently large back tension is applied, the neutral point can be shifted to the outlet, and the rolls begin to slip. This can be used experimentally to estimate the friction coefficient u. 2. In multi-stand tandem rolling, the forward tension on one stand = the back tension on the next. Why must this tension be carefully controlled? Assumptions - radius large compared to thickness and width of contact patch; - friction constant and independent of slip velocity between roll and strip; - assume p and σx are principal stresses (i.e. neglecting effect of friction on principal stresses).

Powder processing overview

Powder routes are used to make 3-D components from metals, ceramics and some polymers. Mechanical properties can be excellent, and the process can be cost effective: it is a near-net shape process so material wastage is small, and energy requirements can be modest. Powder routes are used for many specialized applications (e.g. manufacture of superconducting materials), but also some high-volume applications (e.g. steel parts in the automotive industry). What is powder processing? Material in fine powder form (particle size generally ranging from 50 to 500 µm, though sometimes as small as 1 µm for some specialist applications) is pressed into the required shape and then heated to bond the particles together by interdiffusion to form components, which generally require very little further processing. Occasionally (in the case of some ductile metals) the compacted powder is used as source material for further processing (e.g. extrusion, forging etc). Powder processing normally involves cold compaction followed by a high temperature sintering stage, in which heat is applied with or without pressure.

Why choose an austenitic stainless steel containing Ti or Nb for welding?

Problem: "weld decay" in stainless steel - precipitation of chromium carbide on grain boundaries in HAZ during welding, so boundary regions become susceptible to corrosion. Solution: Ti and Nb are strong carbide formers, scavenging any carbon in the stainless steel, removing the possibility of forming chromium carbides

Function of post-weld hammer peening of a weld

Problem: fatigue crack initiation from surface, particularly in the presence of residual stresses from welding. Solution: surface peening plastically deforms a surface layer, eliminating weld stresses at the surface, and leaving a layer in a state of compressive residual stress (due to constrained plastic deformation), enhancing fatigue life.

Function of post weld grinding of a butt weld

Problem: stress concentrations due to shape of weld metal protruding above plate surface, risking fatigue crack initiation from edge of weld. Solution: grinding weld to make the surface flush and eliminate stress concentration (though even this needs care to align machining marks in least damaging orientation to minimise risk of crack initiation). Also improves appearance

DaF: Residual Stresses

Residual stresses are common in manufactured components - often caused by plastic deformation, thermal expansion or contraction, or phase changes with associated volume or shape change. In any component containing residual stress, there must be compressive stress to balance the tensile stress. Residual stress due to deformation Localized plastic deformation is likely to leave residual stresses. If a region of the material is caused to yield (e.g. by rolling, forging, machining), there will be elastic 'spring-back' after the external force is removed which induces residual elastic stresses Shot peening exploits this effect in a beneficial way. Shot peening involves the impact of small hard iron, steel or ceramic shot (balls ~0.5 - 2 mm in diameter) on to the surface of a metal component, and is used to create a layer of compressive residual stress. It is often used to enhance the fatigue life of highly stressed components. (a) The shot (which does not itself deform much) strikes the surface of the metal. Under load, the material immediately under the shot flows plastically, yielding in tension. The material below the plastic region remains elastic. (b) The shot rebounds. The elastically deformed material relaxes, causing 'springback' as it tries to return to its original shape. It cannot recover completely because under plastic deformation the surface material has work hardened and the elastic stresses in the lower regions are insufficient to cause further plastic flow. The result is a permanent indentation in the surface, with a compressive residual stress close to the surface, balanced by tensile residual stress deeper into the material. (c) Multiple impacts of shot all over the surface produce a near-surface region containing compressive residual stress. This enhances the fatigue life of the component by reducing the tensile stress acting on short surface cracks, so making it more difficult for them to propagate.

Rolling

Rolling is a steady-state, continuous process for forming long prismatic shapes. Reversing mill: material passes backwards and forwards through the same mill stand, which is incrementally closed before each pass. Used for "breakdown" rolling of ingot/thick slab, and for section rolling. Tandem mill: several mill stands (typically four) in connected series. Used for strip/sheet/foil, i.e. stock material that is thin enough to be coiled after rolling. The strip accelerates considerably as its thickness is reduced (due to conservation of volume).

Hot Isostatic Pressing (HIP)

Similar to cold isostatic pressing, but powder is canned in a metal container to provide shape and subjected simultaneously to high temperature (e.g. 2000ºC for SiC) as well as high hydrostatic pressure (using gas, generally argon). HiPing allows very low final porosity. The short process time (minutes, rather than the hours needed for conventional sintering) reduces grain growth problems. However, the high gas pressure (typically 10 MPa = 100 bar) mean that the process is expensive (large pressure-vessels, operating at high T). HIPing provides components with good mechanical properties (low porosity), but dimensional accuracy is low. The improvement in ductility and tensile strength can be very significant. Driving force: contact pressure leading to creep deformation

Liquid phase sintering

Sintering can be speeded up dramatically if a liquid phase is present which can be drawn (by capillary action) into the spaces between the particles. Example: • sintering of alumina + 1% MgO which reacts to form a low meltingpoint glass which bonds the alumina grains together (e.g. for sparkplug insulator). A disadvantage is that the high-temperature strength of the material is reduced.

Mechanical Properties of PMCs: Strength and Toughness

Strength and toughness Continuous brittle fibre (e.g. carbon, glass) composites: - maximum strength is proportional to the volume fraction of fibres. - if load increased enough to exceed fibre fracture stress, then fibres break up into shorter lengths. Fracture resisted by fibre pullout (from friction between fibre and matrix)

Effect of alloying in steel

Substitutional alloying elements such as Mn, Ni, Cr, Mo diffuse slowly in iron, and delay the diffusional transformations from austenite to ferrite and pearlite. The CCT curves move to the right, enabling martensite to form with lower cooling rates. The C-curves for pearlite and bainite formation also separate in low alloys steels

Mechanical Properties of PMCs: Fracture mechanisms for brittle fibres

Tension - cracking generally starts with a break in a fibre (more brittle than the matrix) Toughness is increased if cracks running normal to fibres can be blunted (a) Brittle fibre has cracked. High elastic stresses in matrix at ends of crack. There are two ways to blunt the cracks, illustrated in (b) and (c): (b) Ductile matrix: cracks can be blunted by plastic deformation (maybe shear yielding) (c) If the fibre-matrix interface is weak cracks can be diverted to run along the fibre. (But if it is too weak we lose the load transfer properties. Fibre-matrix bond strength must be carefully controlled) (d) If fibre-matrix bond is too strong, the crack is not blunted and propagates through both fibre and matrix. Leads to low toughness (i.e. brittleness) Compression Composites tend to have inferior properties. Fibres buckle and fail by kinking at a much lower stress than in tension. Carbon fibres crush easily, so it is particularly important that CRFP (carbon fibre reinforced polymer) is used only in tension.

Powder processing route: 1. BLENDING

The basic metal or ceramic powder is blended with various additives. • Steel components are made from iron powder mixed with carbon (graphite - up to 1%) and often copper (1 - 4%). The copper strengthens the steel and prevents shrinkage during sintering. Other alloying elements can be used and alloy PM steels are easily made by mixing appropriate powders. • Non-ferrous alloys (e.g. bronzes, titanium alloys, aluminium alloys) can also be blended as powders. • Ceramic powders usually incorporate additives as sintering aids and to inhibit grain growth The mix also includes a lubricant/binder (e.g. stearic acid, zinc stearate) which performs several functions: • Powder flows more easily, so it fills the mould better • Die friction is reduced, so more uniform product density is achieved, part is ejected without cracking and die life is increased.

Design against failure introduction

The component will fail by fracture if the criterion is satisfied - i.e if - the applied stress rises to the critical level; or - the crack grows and reaches the critical length; or - the fracture toughness falls. Failure is often a consequence of local conditions (e.g. stress, material fracture toughness) being different from the design expectations. In some cases more than one effect plays a role.

Design of joints in composite structures

The difficulty of producing joints in fibre composites is a serious limitation to their use. In long-fibre composites, the strength normal to the fibres (and normal to the plies) is low, possibly leading to delamination. Standard joints (for joining metals, for example) tend to rely on tensile stresses in this direction for holding the joint together, so cannot be used. Compression joints cannot be used easily either, because composites crush easily. Solutions: Try to avoid joints altogether (but may still not eliminate out-of-plane stress) Use compressive joints, but with bulky couplings to minimise stresses Adopt designs from woodworking technology for strong adhesively bonded joints Delamination is likely to originate where there are out-of-plane stresses, such as joints in example image When composite sections need to change (e.g. 6 plies down to 2, as below), there will inevitably be out-of-plane stresses. In the example on the left, the stress concentrations in the plies leads to delamination at the free surface. The effect can be reduced by keeping a continuous composite layer on the surface, ensuring that stress maxima are internal. This has two benefits: - interior delamination will not provide channels for wicking of fluids in from the exterior - the continuous surface layers help to maintain integrity (Note: still need to ensure that layup in both thick and thin sections is balanced) It's important to be able to detect interior damage in composites: ultrasonic testing is the main tool, and can be used on existing structures (e.g. checking an aircraft wing).

Hydrogen embrittlement

The loss or reduction of ductility of a metal alloy (often steel) as a result of the diffusion of atomic hydrogen into the material. - special case of SCC - occurs when hydrogen atoms diffuse towards regions of high hydrostatic tension (e.g. just ahead of crack tip) - hydrogen atoms are very small and can diffuse rapidly in some metals - lowers fracture toughness - growth rates rapid up to 1mm/s - austenitic steels immune, but H2 diffuses rapidly in ferritic iron - Caused by hydrogen in high-strength alloy or plain carbon steels (principally). - Atomic hydrogen is necessary, usually generated chemically or electrochemically ('nascent hydrogen'); see section 3.1. Typically, damp/wet conditions in conjunction with electric currents or even small amounts of corrosion; e.g. pickling; electroplating; MMA (Manual Metal Arc) welding with damp electrodes. - Under tensile stress (applied stress or residual stress) failure occurs by brittle fracture. The fracture is often initially intergranular Preventing hydrogen cracking - Use a lower-strength alloy, which is less susceptible. Steels with yield stress lower than about 700 MPa are generally resistant to hydrogen cracking. - Avoid service conditions (e.g. wet corrosion) which promote hydrogen absorption - Heat component after treatments such as electroplating to remove any dissolved hydrogen (e.g. 150-200 oC for 1 - 2 hours). - Reduce residual stresses. - Reduce stress concentrators (e.g. reduce notch severity) e.g. electroplated steel coupling, wheel bolts

Powder processing: 2. COMPACTION

The powder is pressed into a shaped mould/die at high pressure: e.g. 350-800 MPa for steels, 70-275 MPa for aluminium, 100-150 MPa for alumina. Presses need capacities (maximum force) up to hundreds of tonnes force (several MN) to achieve these pressures. The press capacity usually limits the size of the part that can be formed (typ. <~1 kg steel). The component is now called a 'green compact' with the correct shape, but very little strength (typically 10-20 MPa). Since it is so weak it must be handled carefully, but it is very easy to machine if necessary ('green machining' to produce features which cannot be produced by pressing - e.g. transverse holes, slots). For ceramic materials the linear dimensions of the green compact will be 15-20% greater than those of the finished part because of the remaining porosity (up to 50%). With metals the powder particles themselves deform during compaction and the green compact has a similar density to the final product (up to 95% of bulk metal); there is little or no shrinkage on sintering. The mechanical properties of the final product (after sintering) depend critically on the homogeneity of the compact. If a compact contains a range of densities, each region will contract to a different extent on sintering. This means that not only will it have different mechanical properties in different regions but even more important the product will contain internal stresses. This causes problems in both metals and ceramics.

Thermal Residual Stresses: Thermal 'toughening' of glass

The surface of hot glass is rapidly cooled by air jets to 'freeze in' the hightemperature, low density glass structure. The interior cools more slowly and shrinks, putting the surface into compression. Kc is unchanged, but surface cracks are stabilised. Welding involves intense local heating and thus causes residual stresses which can easily reach the yield stress of the material. Example: Distribution of residual stresses across a butt weld in a plate: tensile stress in the weld balanced by compressive stresses in the surrounding metal:

PMC matrix materials

Thermosets By far the most common matrix for PMCs. They are cheaper than thermoplastics, and the higher stiffness is advantageous. In order of increasing performance, and cost: Polyesters - Unsaturated polyesters will cross-link in the presence of accelerators to form a rigid structure. - Some restrictions on their use as styrene can be released during curing. - Low performance so generally only used with cheaper reinforcement such as glass. Vinyl Esters - Much better mechanical properties and environmental resistance than polyesters. - More expensive. Epoxy Resin Usually made by mixing two liquid components resin + 'hardener' . Polymer cures by forming chemical cross-links between polymer chains. Good mechanical properties (strength, toughness, high modulus), good environmental resistance. May be more than twice as expensive as polyesters or vinyl esters. Used principally with carbon, glass and aramid fibres. Thermoplastics Advantages over thermosets: - Short-fibre composites can be made by conventional thermoplastic processing - Greater potential for high-grade recycling - Tougher matrix - Parts can be joined by welding (using heat and pressure) BUT elastic modulus lower than thermosets e.g. PEEK (poly-ether-ether ketone); PES (poly-ether-sulphone): High cost, high performance, can withstand relatively high temperatures (around 300ºC); used with carbon fibre or aramid. Polypropylene (low cost, low performance) with chopped glass fibre, for low-load automotive applications. Nylon with chopped glass fibre: electrical, automotive parts

DaF: Thermal Residual Stresses

These residual stresses are likely to develop whenever pieces of material of any sort are heated locally or unevenly, or whenever there are different cooling rates (e.g. surface or interior). (a) The top surface of a metal slab is heated. The hot region expands. (b) Because its yield stress is reduced when it is hot, it can deform plastically to accommodate the expansion. The elastic stress in the hot region to fall to a low value. (c) As the hot region cools, it contracts, but its yield stress has now gone back to its original higher value so it cannot deform plastically. It thus exerts a force on the neighbouring cold region. The final tensile residual stress in the region that had been heated can reach the yield stress, and is balanced by compressive stress elsewhere. Thermal residual stresses can arise in hot working of metals where different regions of the part cool at different rates - e.g. in an I-beam the central web may cool faster than the (thicker) flanges, and there may also be a contribution to residual stress from the deformation. The stresses from these factors superpose, so the combined stress may exceed the yield or fracture stress. Example: rolled steel I-beam

Design of Castings: Turbulence defects

Turbulence may arise close to where metal is poured into mould, or within mould if abrupt changes in section a) air entrapped, giving large porosity. Liquid metal surface oxidises - turbulent flow leads to more entrapped oxide. e.g. pressure die casting has turbulence and porosity as metal enters mould quickly. Shit mech properties for such castings b) sand casting - mould damaged by rapid metal flow at ingates, leading to sand particle defects in the casting, and causing loss of dimensional accuracy of the casting

Types of damage in CFRP and GFRP over time

Types of damage: - environmental degradation by wicking of moisture: delamination and loss of stiffness: affects GFRP, not CFRP - leaching (surface dissolution) of glass fibres, degrading properties - matrix swelling by absorption of water, degrading properties or stresses causing cracking - interply stresses from cooling after forming due to differential contraction (greater in CFRP), leading to delamination - microbuckling in compression (especially CFRP) - sub-critical transverse ply cracking and delamination, increased by cyclic loading (leading to loss of stiffness, strength)

Reinforcing PMC fibres

Typically 8-12um diameter Glass e.g. E-glass (electrical glass, the commonest general-purpose glass for composites). Relatively inexpensive. Density ρ 1/3 that of steel; specific modulus (E/ρ) similar; specific strength (σf/ρ) 10x greater. May be used as single or multi-filaments, or woven into cloth Often chopped to form short fibres. Standard polymer forming technologies such as extrusion may be used, particularly if the matrix is thermoplastic (e.g. nylon + chopped glass fibre for electrical plugs and sockets). Carbon fibre Very expensive. High strength; very high specific modulus (E/ρ); high resistance to corrosion, creep and fatigue. Impact resistance less than glass or aramid; poor in compression. Aramid (a polymer) e.g. Kevlar (Applications: bullet-proof vests; containment for failed gas turbine blades in aero engines) Very expensive. Yellow fibres containing highly aligned polymer chains. High strength, low density. Extremely tough; excellent impact resistance. Can be UV sensitive. Other fibres Polyester: low density, good toughness, low modulus, low cost Polyethylene: highly oriented fibres of ultra high molecular weight polymer (e.g. Dyneema). Very high tensile strength, low density. Difficult to bond to matrix, and expensive. Ceramic: e.g. silicon carbide monofilament. Very high strength and modulus, very expensive. For ease of handling, continuous fibres are often formed into 'rovings' or 'tows' (bundles of parallel fibres) which can also be woven to form cloth and other types of textile pre-forms. These are inflitrated with the matrix polymer in liquid form and cured to form composite

Powder processing: 3. SINTERING (Pressureless sintering)

Typically a continuous process: the time at highest temperature can vary from 10-45 minutes (for steels and copper alloys) to several hours (for high Tm metals and ceramics). The furnace atmosphere is closely controlled (e.g. to prevent oxidation or decarburization of steels). The green compact is heated to a temperature well above 0.5 Tm (typically 0.7 to 0.9 Tm) so that rapid diffusion can occur. Typical sintering temperatures: Iron/steel 1000-1200 ºC; Al alloys 600 ºC; copper, bronze 750-900 ºC; tungsten carbide/cobalt 1500 ºC; tungsten 2350 ºC; alumina 1600 ºC. Driving force for sintering: diffusion along composition gradients and reduction in particle surface area. Particles bond together, and interfaces between particles become grain boundaries. (Vapour and/or liquid phase transport can also occur.) Mechanism: atoms diffuse to fill the pores. Diffusion occurs along different paths: bulk (through the lattice), dislocation core, surface (along surface of unsintered particles), grain boundary (along the boundaries between the particles, once fused). Total rate is the sum of all of these, though one mechanism tends to be dominant at a particular temperature and particle or pore size. Rate: see eqn Stages in sintering: 1. Plastically deformed contacts (in metals) 2. Cohesion of particles by formation of bridges. Porosity is interconnected. Note: sintering can halted at this stage to create low-density material permeable to gases and liquids. 3. Particles grow competitively by diffusion across interparticle interfaces, leading to grain growth 4. Porosity becomes spherical and shrinks. Porosity becomes discrete. - Sintering rate is inversely related to (particle size) : (driving force from surface energy, diffusion distances). Fine compacts sinter faster, and as grains grow, the rate falls. • Very difficult to get 100% densification by sintering alone. • Pore size and spacing directly related to original particle size. • Gases contained within pores cannot diffuse (molecules too big). • Many advantages in keeping sintering times as short as possible. (Porosity is usually expressed indirectly in terms of density) For metals, if the sintering process is followed by mechanical working (e.g. hot forging, extrusion etc.) then the porosity can be completely removed and maximum mechanical properties are obtained. The process of forging and sintering can be combined into a single process ('sinter forging').

Cold isostatic pressing

Uniaxial (i.e. 1-D) pressing is restricted to simple shapes. For more complex shapes (especially for ceramics; e.g. spark plug body) isostatic pressing is needed to form a green compact with a uniform density distribution. The powder is contained in a rubber mould (n.b. no heating is used at this point), and pressure is applied by external fluid or gas. - more expensive than uniaxial pressing, and - dimensional accuracy low (products are often machined before sintering). + the need for lubricants/binders is reduced or removed.

Thermoforming

Used for a wide range of thermoplastic parts in simple shapes: heated sheet deforms onto a mould. Various methods can be used to achieve this (vacuum, air pressure, moving die) Examples: housing for appliances, automotive panels; also used for packaging: cups, trays etc.

Stretch blow moulding

Used for hollow containers. An injection-moulded pre-form is heated and stretched longitudinally (giving axial alignment of polymer chains) before also being inflated (chain alignment in the hoop direction). This biaxial alignment gives higher strength and stiffness so the polymer walls of the bottle can be made thinner. More uniform wall thickness than extrusion blow moulding. Process control is critical to optimize the properties. Example: PET bottles for fizzy drinks: low permeability to carbon dioxide.

Metal Injection Moulding (MIM) and Powder Injection Moulding (PIM)

Used for producing small, high-precision, low porosity components from metal or ceramic powder, in large quantities. These processes overcome the restrictions in shape/complexity of uniaxial pressing. The process is exactly the same for MIM (metal powder) and PIM (ceramic powder). Uses conventional polymer injection moulding technology with a blended polymer-metal or polymer-ceramic feedstock to produce the initial green compacts. These are therefore highly-filled polymer-metal or polymerceramic composites. Typical MIM process uses very fine metal powder (1 - 20 µm) with specially designed polymer binders. The volume fraction of binder is 30-50%. Binder removal (debinding) is a critical stage involving: • heating the green compact - taking several hours, or • chemical decomposition by e.g. gaseous nitric acid - quicker, or • dissolving the binder with a solvent The debinding process converts the 'green' part (as-moulded, containing the polymer binder) to a 'brown' part (with no binder, but still not sintered, so containing high porosity) which is then sintered. During sintering, the part typically shrinks 50% by volume (15-20% linearly). Care is therefore needed to retain shapes while dimensions change dramatically. Advantages of PIM, MIM: • low die wear rates; complex shapes (variations in wall thickness, moulded-in decoration) with high dimensional tolerances can be made; low and uniformly distributed porosity means products take high surface polish and have excellent mechanical properties. Particularly useful for high-volume production. MIM and PIM are typically used for small thin-section complex parts produced in high volumes e.g. small gears, disc drive parts, watches, camera parts, surgical instruments, spectacle frames and luxury goods. For some metals it competes with investment casting (lost-wax casting). Maximum section thickness is ~5 mm - limited by the binder removal process.

Gravity die casting

Variant process using gravity feed (as in sand casting) but with permanent mould in two separable parts, as in pressure die casting

Injection moulding

Very versatile process (can also incorporate inserts of metal, other polymers - 'overmoulding'). Molten polymer is forced into a shaped metal mould at high speed and pressure. It cools and solidifies rapidly, and the mould is then opened to release the article. Very short cycle times can be achieved (~seconds - but determined by the cooling time). The moulded items have some waste polymer in the form of sprues attached to them (remnants from the feeding system to get the polymer into the mould). These are often cut off immediately after the item is removed from the mould. The operation generally uses a screw extruder to mix and melt the polymer, but the process differs from the continuous extrusion processes discussed in 2.2 in a critical respect. The molten polymer accumulates in a chamber in front of the screw, and only once the chamber is full is the whole charge ejected into the mould cavity. This is achieved by the whole screw moving forwards, acting as a hydraulic ram. Once the polymer in the mould is solid, the screw is drawn back so that the molten polymer can be collected again. The process needs to be well-controlled to deliver high quality products. Important variables Injection pressure: The polymer shrinks as it cools and crystallizes. High pressures allow some compensation. Component thickness: Increased thickness reduces cooling rates, so increases shrinkage. This is especially important in semicrystalline polymers, as the longer cooling time allows more crystal nucleation and growth. Crystalline regions have higher density than amorphous regions, so the shrinkage is increased. Hold-on times: Time that the die is under pressure. Longer time allows for counterbalancing of shrinkage (as long as the polymer is still molten at gate). Mould temperature: Moulds are generally water-cooled to minimize cycle time. Increasing mould temperature reduces cooling rate, increasing crystallisation. Implications for shrinkage.

Processing defects

Weld lines When two polymer streams meet (e.g. in injection moulding), lines of weakness can form. These are the result not only from incomplete fusion (as in metals - 'cold shuts') but also from molecular alignment variations. Weld lines have low local density, strength and stiffness, and are susceptible to solvent-induced cracking. Sink marks at section variations Injection mouldings often suffer from sink marks at section changes, or where ribs meet a structural member, caused by slower cooling at this point. To avoid, keep ribs small and slender; avoid abrupt section changes and consider local cooling rates in design of parts and moulds. Examples of good and bad design: see attached

DaF: Welds (and crack growth around them)

Welds are frequently associated with failures, combining multiple factors: - Local tensile stress raised as a result of both thermal residual stress and stress concentrations; - Material properties changed in HAZ and in weld region; - Weld defects (e.g. inclusions, impurities, porosity) There are frequently cracks in and around welds (and perhaps slag inclusions which also act like cracks). The fatigue life of welded components is therefore controlled by crack growth rather than by crack initiation. Cracks in welds generally result from a combination of: • temperature gradients causing thermal stresses • variations in composition in the weld metal/HAZ giving differences in contraction • segregation during solidification • hydrogen embrittlement (see section 3.3) • inability of the weld metal to contract during cooling (similar to hot tearing of castings) Some measures to minimize cracks and residual stresses are: • modify design of joint to minimize thermal stresses from shrinkage during cooling • change welding process parameters, procedures and/or sequence • preheat components being welded • avoid rapid cooling after welding • induce residual compressive stress in weld metal by shot peening

Casting alloys vs wrought alloys

Wrought processes dominated by solid-state formability (i.e. large strains without failure), and microstructural control for properties (e.g. by heat treatment) Castings generally have poorer mechanical properties than their wrought counterparts, largely because of porosity, and a high content of second phases (often brittle). But casting (plus heat treatment) can be the route to high-quality property-critical components (e.g. internal Al alloy frame of Airbus doors, jet engine nickel alloy turbine blades)

Why is yield dependent on deviatoric stress and not hydrostatic stress?

Yield is controlled by dislocations, driven by shear stresses. Hydrostatic stress produces no shear stress in any orientation; deviatoric stress measures the deviation away from hydrostatic stress (and hence characterises shear). From Mohr's circle, the difference between any pair of principal stresses is twice the maximum shear stress on a plane at 45o to both of the principal directions. Tresca considers only the largest of these three shear stresses; von Mises takes a balance of all three.

Anodising

a way of protecting aluminium and titanium from corrosion, by deliberately creating a layer of aluminium oxide over it The hollow cells that are part of the structure of the thicker layers can be used to incorporate dyes or other surface modifiers such as PTFE (e.g. non-stick coating on saucepans). The tops of the cells are sealed after filling.

Concentration gradients in solidification: segregation

a) Initial Transient - Solidificaiton starts at T_0 - no diffusion in the solid, so depleted concentration gradient "locked in" - segregation: excess solid pushed into liquid ahead of solid-liquid interface With each incremental advance of the interface, the solute level in the solid stays below the average for the alloy. Hence the peak solute concentration in the liquid at the interface rises, with that in the corresponding solid maintaining the partition coefficient ratio, such that CS = kCL b) Steady State As the interface advances, CS rises asymptotically towards Co, at which point steady state is reached. The solidification temperature = T1, with CS = Co, and CL = Co/k. The "bow-wave" of elevated solute advances at the same speed as the interface. c) Final Transient In a real casting, solidification proceeds from opposing walls of the mould towards the centre. Hence the excess solute from both sides concentrates into the last part to solidify. This is called macrosegregation (i.e. segregation on the scale of the whole casting) and may lead to poor mechanical properties (e.g. high concentrations of weak second phases in the central grain boundaries). And if the segregating solute is a gas, this may lead to the formation of macroporosity in the centre of the casting

Solidification rate in castings and significance

affects - production rate, and hence process economics - the resulting microstructure, and hence properties CHVORINOV'S RULE: solidification time of a section is proportional to [Volume/Surface area]^2 Specific microstructural features affected - Grain structure (e.g. columnar vs. dendritic/equiaxed), affecting distribution of impurities and porosity at macro scale; - Grain size and dendrite arm spacing, affecting distribution of impurities and porosity at micro scale.

DaF: Wet Corrosion of metals 2

b) cont. Crevice corrosion Examples of good and bad joint design See IMAGE Pitting is a particularly dangerous form of crevice corrosion since it can lead to local perforation of sheet or plate (e.g. a tank, pipe, car body etc.) by the formation of deep pits when most of the rest of the object is relatively undamaged. The conditions in a pit are 'autocatalytic' - i.e. they tend to further enhance the local corrosion rate within the pit. Chloride ions (present in sea water) are particularly effective in assisting this mechanism, so that pitting corrosion is particularly prevalent in marine applications. The use of salt (sodium chloride) on roads in winter to avoid ice formation can also lead to severe corrosion of vehicle components. Local concentration corrosion can also result from broken or scratched paint coatings. c) Differential Energy Corrosion Features causing a local increase in energy in a metal (e.g. grain boundaries, dislocations, precipitate interfaces) act as anodic regions and can dissolve rapidly; other regions form cathodes. This can be useful in etching polished specimens to reveal microstructures. However, 12 C Y Barlow, November 2019 problems can be caused by (e.g.) cold-worked regions of a structure corroding preferentially. An example of localised corrosion caused by composition variation is weld decay in austenitic stainless steels (e.g. type 304: Fe-18%Cr8%Ni). If the steel also contains carbon, heating during welding can cause the carbon to react with chromium to form precipitates of chromium carbide on grain boundaries in the HAZ, depleting the neighbouring metal of chromium. The steel is then said to be 'sensitized'. The regions close to the grain boundaries do not contain enough chromium to be able to repair the protective surface film of chromium oxide when it is damaged in service. Localised corrosion occurs, with the grain boundary region being anodic. Sensitization can be avoided by using a low-carbon alloy (e.g. 304L), or a 'stabilized' stainless steel containing carbide-formers such as Ti or Nb which react with the carbon in preference to chromium. (See Steels handout 2, section 3.2)

Combining fibres and matrix: Pultrusion

e Can be used either to process fibre bundles in a form which can be used for subsequent lay-up processes (pre-pregs, avoiding the need to deal with resin separately at this stage), or can be used to produce composite material articles in final form. Fibres pulled through a resin bath and then through a die. If the composite is being produced in final form, the die is heated to cure the resin. Pultruded product may be small bundles or tapes of multiple fibres for subsequent processing, sheets or any extruded sections (e.g. rods, I-beams). unipulllc.com Advantages: Fast, excellent fibre alignment (unidirectional: fibres all parallel - NB low lateral strength), good structural control. Good range of compositions. Disadvantages: Costly (particularly with heated dies), limited to constant-section components. Applications: Beams and girders, bridges, ladders. Sheets of fibre with uncured resin (no heated die), pre-preg, can be used for layup operations, below.

Combining matrix and fibres: article manufacture starting from pre-preg sheets

f Sheets of the fibre pre-impregnated with resin, or pre-preg, are stacked to provide the desired distribution and orientation of fibres. The stack is heated and pressed to shape, and cured (polymerization) to produce the final article. The sheets, often called laminae, may contain the fibres as woven sheets or in uniaxial arrays. Advantages: Fibres are placed in the required positions and orientations to optimise mechanical properties. No specialist equipment required for layup; heating and pressing and the curing can be outsourced to other facilities. Disadvantages: Largely manual process. Applications: Aero and car bodies, wind turbine blades.

Types of stainless steel

ferritic (Fe-Cr, low C) martensitic (Fe-Cr, higher C) austenitic (Fe-Cr-Ni, low C)

Combining matrix and fibres: Resin Transfer Moulding (RTM)

g Fibre cloth stacked up as a preform in a closed cavity mould, resin injected (if under vacuum, process known as Vacuum Assisted RTM), component cured in mould. Advantages: High fibre contents, low porosity. Disadvantages: Costly moulds, very expensive for large parts. Applications: Small complex aircraft components, train seats.

Hetergeneous nucleation

involves the formation of a spherical cap of solid on a substrate in contact with the melt (i.e. the mould wall, or a solid particle within the liquid). The undercooling for heterogeneous nucleation is typically a few degrees - much smaller than for homogeneous - so nucleation from surfaces and particles dominates in casting. Heterogeneous nucleation is characterized by the contact angle, which depends on the balance between three surface energies: γSL (between liquid and solid), and those between the nucleating surface and the liquid and solid, γNL and γNS.

Homogeneous nucleation

involves the formation of isolated solid spheres within the melt.

What is the partition coefficient?

k = C_s / C_l

Design of Castings: Shrinkage defects and solutions (from solidification)

metals shrink during casting due thermal contraction (in liquid and solid state) and due to crystallisation on solidification. Moulds designed to hold reserve of molten metal ('risers') allow metal to be fed in during solidifcation. Hence metal in feeder head must solidify last, otherwise casting will be starved, leading to porosity. Shrinkage defects mostly associated with changes in section Local shrinkage stresses can lead to hot tearing, formation of a crack in the casting SOLUTIONS - using eutectic compositions means that feeding mould is easier to accomodate shrinkage as less change of "mushy freezers" - use a more uniform cross section to avoid formation of shrinkage cavities (when internal volumes liquid becomes cut-off from bulk feeders giving rise to cavitation as trapped volume solidifies) - use chills, to cause early solidifcation (thus strengthening) in vulnerable regions (e.g. embedding metal insersts in a sand mould) Also porosity solidifcation defects porosity from dissolved gases released on solidification but unable to escape (improve by vacuum degassing the melt - expensive for a cooking pot - so promote fine microstructure and trapped microporosity, or alloy with a "killing" element to take up dissolved oxygen in solid form;

Degredation of polymer-matrix composites

polymer-matrix composites preferred to metals in aggressive environments. No visible corrosion but degredation still happens. (focus only on long-fibre composites) Matrix degradation: As for bulk polymers (section 4.2). All can take place. Water/solvent swelling. Generally non-uniform (solvent entering and leaving via the surface) so internal stresses can result. Polymers can crack or craze (particularly at the surface). ESC at stress concentrations. Fibre degradation: Polymers: Kevlar may be affected by UV and oxidation, leading to loss of strength and toughness Glass suffers leaching (surface dissolution), so degrades in water. Carbon fibres do not degrade. Interface degradation: Swelling stresses can result in cracking of interfaces. Capillary flow ('wicking') of solvent along cracked interfaces can dramatically increase degradation rates, because it allows a 'short-cut' route for solvent to reach the interior of structures without having to diffuse through the matrix. ESC resulting in rapid and specific attack of fibre or matrix at the interface. Plasticisation of matrix at interface, leading to creep and distortion

Polymer processing overview

processes, characteristics and analogous metal forming process Process selection crucially depends on number of parts required

Reovery and recrystallisation

purposes: • to maintain ductility (enabling large strains without cracking); • to reduce forming loads (dynamic softening balances work hardening; annealing eliminates prior work hardening); • to control final grain structure Recrystallisation stems from the recovered subgrain structure, determined by: • prior deformation conditions (T, and ); • alloy composition. Sites for nucleation 1. Grain boundary nucleation: Larger subgrains at grain boundaries (GB) act as the nuclei for recrystallised grains. 2. Particle-stimulated nucleation: Wrought alloys contain fine-scale, hard, second phase particles and dispersoids (e.g. in Al alloys, intermetallic compounds of Al with Mn, Cr, Fe, Zr). The dislocation density is greater around the hard particles, locally increasing the driving force for recovery and recrystallisation. Grain size control Recrystallisation requires a minimum strain level - typically 5% for cold deformation (but higher for hot deformation). This reflects the need to store sufficient energy in the form of dislocations. Min temperature needed to trigger recrystallisation, and this falls with increasing strain. This reflects the thermal activation needed to nucleate new grains. Recrystallised grain size has a complex dependence on: • deformation strain-rate and temperature • strain during deformation • recrystallisation temperature In (hot) dynamic recrystallisation, deformation and recrystallisation temperatures are the same (as they are concurrent processes). Static recrystallisation occurs after deformation (e.g. in the coiled sheet). In hot rolling, this is immediately afterwards so the temperature is similar (but lower). In cold rolling, the metal must be reheated to trigger static recrystallization. It is most energy efficient to hot-roll and anneal directly after casting, e.g. continuously cast steel is hot-rolled immediately after solidification. Deformation processing is always inhomogeneous (due to geometric complexity, friction and heat transfer). Even in simple geometries such as flat strip rolling it is difficult to maintain uniform deformation across a rolled strip, and from one end of a coil to the other. In forging and extrusion deformation is very inhomogeneous. - different grain sizes in different parts of component, or sometimes no recrystallisation - variable properties: anisotropic deformation behaviour, poor surface finish, localised corrosion

Critical diameter D_0 for steel

the diameter of bar, quenched in a given medium, which forms 50% martensite at its centre.

Equivalent diameter

the equivalent diameter of a component is the diameter of an infinitely long circular cylinder which, if subjected to the same cooling conditions as the component, would have a cooling rate on its axis equal to that at the position of slowest cooling in the component

Powder manufacture: why is particle shape important?

• During pressing of metal powders, irregular particles experience more deformation than rounded particles. The high pressure at particle contact points leads to heavy plastic deformation, interlocking and local cold welding which increases their adhesion, leading to stronger 'green' components. • Rounded particles flow more easily than irregular particles and can pack together more uniformly and densely than irregular particles. For highest packing density we use mainly rounded particles with a range of particle sizes (small ones to fill up the gaps between big ones). Controlled low packing density leading to high porosity can be achieved by using a single particle size

3 elements of manufacturing triangle

• Technical limits: material compatibility, shape and size limits • Quality limits: tolerance and roughness • Economic limits: economic batch size, manufacturing cost per part

Benefits of additive manufacturing vs other processes

• avoid the need for moulds or dies - low tooling costs; potential to integrate AM deposition head with machining head on one machine • (mostly) net-shape processes - low material waste • unlimited form freedom and complexity - revolutionary component geometries, and easily customised • integration of parts into single component, reducing part count and assembly processes - savings in cycle times • both of the above may enable lightweighting and lower material usage • automatically integrates CAD with manufacturing instructions, readily distributed globally • reduction in need to maintain inventories of spare parts • vary composition across components Why are mechanical properties of AM components inferior to those produced in casting or moulding? Example metal AM process, powder bed fusion: scanning pulsed laser used to sequentially melt spots of material from a pre-deposited powder. Example polymer AM process: polymer extrusion or jetting, intermittent deposition of droplets of molten polymer. Both processes build up solid with large numbers of small droplets that solidify, often somewhat flattened in a plane normal to the build direction. This leaves a degree of porosity, and also a large number of interfaces between the layers where the bonding may be weaker (e.g. due to trapped oxide in metals). This is detrimental to mechanical properties, particularly in the build direction (normal to the layers).

Effect of carbon content on hypo-eutectoid plain carbon steel (<0.8% C)

• ferrite formation precedes the formation of pearlite. • hardenability increases as C content increases (curves for bainite/pearlite move to the right) • martensite start (Ms) and finish (Mf) temperatures fall with increasing C content (reflecting greater strain energy in higher C martensite) However, Eutectoid steel only has sufficient hardenability to form martensite through-thickness in sections of diameter 20-30 mm (hence need for ALLOYING)

Outcome of a manufacturing operation often depends on coupling between

• material: composition, properties during processing • process: design of the equipment, process operating conditions • design: product size and shape OFTEN, THESE FINAL DECISIONS MUST BE MADE SIMULTANEOUSLY