Manufacturing
Powder bed fusion +ve, -ve
+Ve 1) Range of materials, including semi-crystalline and amorphous polymers 2) Support materials, loose powder is sufficient support material for polymer. Saves significant time during building 3) Complex geometries, as part made it remains incased in loose (non-sintered) powder enabling generation of geometrically intricate components 4) mech properties, high mech resistance -ve 1) surface finish and accuracy, influenced by operating conditions and particle size. (FINER PARTICLE SIZE PRODUCE SMOOTHER MORE ACCURATE PARTS BUT ARE MORE DIFFICULT TO SPREAD AND HANDLE) 2) shrinkage, build materials exhibit shrinkage leading to part distortion 3) productivity, total part construction can take long due to preheat and cool down cycles 4) high energy, compared to other process it require high levels of energy to produce parts. Including power generated to sinter or melt powder and heating of building chamber/platform
Stereolithography(SLA)- +ve, -ve
+ve - Accuracy and flexabitily (can support many different configs etc) -build speed- mask projection have inherant speed advantage over laser. By utilising mask, an entire part cross section can be projected rather than having to sequentially scan the vector pattern for cross section -ve -materials- limited to acrylates and epoxies -strength and durability -post curing- most parts produced via stereolithography require an additional stage of post processing, increasing production time and cost
Binder jetting Drop on demand
+ve 1. smaller drop size 2. High placement accuracy
Binder Jetting advantages and disadvantages
+ve: 1) low cost (standard componenets) 2) high speed (print heads with thousands of nozzles) 3) Scalability 4) multiple materials + print in colour 5) induced micro porosity 6) water can be used as binder -ve 1) limited choice of materials (only waxes and photopolymers) 2) Limited part accuracy 3) Low mech prop of produced parts 4) trapped material 5) post processing required
Binder jetting Continous stream
+ve: high throughput rate -ve: two major contrains 1) material must be able to carry charge 2) fluid deflected into catcher needs to be disposed or reprocessed
Shielded Metal Arc Welding (SMAW)
-A manual process; -Hand-held electrode holder; -Consumable "stick" electrode; -Flux coating on electrodes acts to protect weld pool from atmosphere; -Flux can also assist with control of weld pool when out of position; -All welding positions possible; -Requires high level of skill; -Relatively low productivity.
Submerged Arc Welding (SAW)
-A mechanised process; -Filler wire fed continuously from spool; -Flux granules fill welding groove to protect weld pool from atmosphere; -Can use multiple welding heads concurrently to increase productivity; -Welding in 1G position only; -Requires modest level of skill; -High productivity.
Flux-Cored Arc Welding (FCAW)
-A semi-automatic or automatic process; -Hand-held torch with trigger; -Filler wire fed continuously from spool; -Flux granules embedded in core of filler wire to protect weld pool from atmosphere; -Flux can assist with control of weld pool when out of position; -All welding positions possible; -Requires intermediate level of skill; -Intermediate productivity.
Gas-Metal Arc Welding (GMAW)
-A semi-automatic or automatic process; -Hand-held torch with trigger; -Filler wire fed continuously from spool; -Molten weld pool is protected from atmosphere by inert gas shield; -Mode of metal transfer changes according to welding parameters; -All welding positions possible; -Requires intermediate level of skill; -Intermediate productivity.
What would an ideal welded joint be like?
-An ideal welded joint would be indistinguishable from the parent material -No point having welded joint that is stronger than parent materials. -If the weld zone is stronger than the parent material, it is likely to have a lower toughness and be susceptible to brittle fracture. -If the welded joint has a different microstructure or composition to the parent material, this raises the possibility of galvanic corrosion and anisotropy in mechanical properties.
The HAZ in Steel Welds
-At high temperatures (> ~ 850°C) the stable phase is austenite, which has a face- centred cubic crystal structure. Austenite can dissolve up to 2 wt. % carbon. -At lower temperatures (< ~ 700°C) the stable phase is ferrite, which has a body- centred cubic crystal structure. Ferrite cannot dissolve much carbon, so it often co- exists with a mixture of ferrite and iron carbide which is known as pearlite. -When a steel is cooled sufficiently slowly, it will transform from austenite to a mixture of ferrite and pearlite.
PWHT in a furnace Local PWHT (moveable)
-Can lead to the restoration of toughness and the relief of residual stresses. -Local PWHT can temper the microstructures to improve toughness but the relief of residual stresses in the structure as a whole is likely to be less effective.
CNC machining
-Computer based technology, file is made on cad, transferred to machine. Subtractive machine. Takes a block of material that is bigger than part to be made and cuts it away while rotating it. https://www.youtube.com/watch?v=8CSwOebmb0A -Works well for hard brittle material, like steel and aluminium. -Parts are homogenous (all same) + predictable in quality. -Removes material very fast, however if not an expensive machine it can be a multistage process, requiring repositions and can take weeks -When creating a part directly in as single piece there may be some geometries that cannot be fabricated. As machining tool is carried out in spindle there are certain accessibility constraints. Some parts have to be broken into components and reassembled at later stage. -Very accurate and high precision with right equipment. -Needs skilled workers as program sequence is very involved.
Continuous Cooling Transformation (CCT) Diagrams
-Continuous cooling transformation (CCT) diagrams are often used to predict the phases that will form when austenite transforms to a daughter phase upon cooling. -CCT diagrams usually plot the extent to which a transformation has taken place as a function of temperature and time. -CCT diagrams are used when it is likely that the cooling rate will be too fast for thermodynamic equilibrium to be maintained. -The transformation-start and transformation-finish lines on a CCT diagram are very sensitive to changes in the composition of the steel.
Destructive Testing of Welds
-Destructive testing involves the physical destruction of a completed weld in order to evaluate its characteristics. -An important evaluation technique is the preparation of a macrograph section. This involves extracting a cross-section from the weld, polishing it and etching it so it can be visually inspected. -A number of other tests may be required (e.g. tensile, bend, fatigue, creep) in order to assess fitness for intended application.
Electron Beam Welding
-Electron beam welding is possible when a focussed beam of electrons is incident on metal or alloy components; -Electron beam welding is usually carried out in a high vacuum so that the electron beam does not scatter; -Electron beam welding can be carried out with or without the addition of filler metal; -Weld penetration depends on the beam power, power density, welding speed, materials properties and the joint geometry.
Material extrusion Process- FDM (Fused deposition modelling, type of am)
-Extrusion machines forces material through nozzle as extrusion head or build plate moves in x-y plane -After layer is completed build platform moves down or head moves up -Raw material is filament of thermoplastic coiled onto spoon that is melted as extruded. +ve: easy operation, diversity of material and low cost equip -ve: high processing temp, pre-filament required, support structures
Non-Equilibrium Transformations in Steels
-For a steel to transform from austenite to a mixture of ferrite and pearlite upon cooling, there must be sufficient time for the diffusion of atoms to take place. -In many cases the HAZ cools rapidly during welding, so there is insufficient time for this diffusion to occur. -If there is no time for any diffusion to take place, the carbon in the austenite phase becomes trapped within the crystal lattice, and the austenite transforms to a hard brittle phase called martensite. -Martensite is a distorted form of ferrite that forms via a shear transformation of the crystal lattice, rather than by diffusion.
Friction Welding
-Heat is generated through mechanical friction between a moving workpiece and a stationary component; -An upset force is usually required to fuse the materials; -No melting occurs, so friction-based processes are solid-state welding processes; -Friction welding avoids degradation of material properties through phenomena such as grain growth; -It is possible to join dissimilar materials.
Arc Welding Processes
-In arc welding a welding power supply is used to establish an electric arc between the welding electrode and the workpiece. -This is established through the electrical breakdown of a gas that produces an ongoing plasma discharge at relatively low voltages. -The welding electrode may either be a non-consumable or a consumable electrode. -The power supply may be either direct current or alternating current. -The molten weld pool is shielded from the atmosphere either by flux forming a protective layer of slag, or by the employment of an inert gas.
Laser and Electron Beam Welding
-In laser and electron beam welding, the power densities can be high enough to vapourise the molten metal. -The power densities that can be achieved will depend on the laser optics or electron optics. -The vapour pressure can be sufficient to displace molten metal and allow the formation of a keyhole. -Surface tension helps to stabilise the keyhole. -Laser and electron beam welding can both be carried out in conduction mode or keyhole mode. -The depth of penetration increases with the beam power and the power density. -Both processes can involve the addition of filler metal, but often no filler metal is added. -The geometry of keyhole welds leads to low levels of "butterfly" distortion. -One of the primary challenges of keyhole welding is maintaining a stable keyhole. -Welding with the beam in a horizontal position can help to stabilise the weld pool. -Weld pool instability can lead to the formation of gas pores or blowholes. -The stability of the keyhole often improves when the keyhole penetrates through the full plate thickness -Electron beam welding requires a partial vacuum or a high vacuum. The use of a vacuum avoids the possibility of the plasma scattering the beam. -Electron beam welding can be prone to magnetic deflection of the beam. -Steel components will need to be de- magnetised prior to welding, especially if thickness is high.
Laser and Electron Beam Welding (advantages and disadvantages)
-Laser and electron beam welding can both offer significant productivity gains when compared to arc welding processes. -A significant advantage of both processes is their ability to form deep narrow welds. This reduces welding time and reduces distortion. -Both processes can be expensive when compared to arc welding, particularly with respect to the cost of equipment, and there are practical limitations e.g. size of vacuum chamber for EB, and the need for shielding from laser light. -Both processes should be considered if a high integrity low-distortion weld is required.
Hybrid Laser-Arc Welding
-Laser welding offers rapid welding of parts with low heat input and low distortion; -Laser welding is often implemented by mounting the welding head on a robot; -Laser welding requires good joint fit-up. It does not accommodate poor joint fit-up as well as an arc welding process; -Laser welding can be combined with an arc welding process in order to improve the tolerance to poor joint fit-up; -The most common form of hybrid-laser arc welding is Laser-GMAW;
Post Weld Heat Treatment
-Many steels are supplied in a heat treated condition. The purpose of the heat treatment is to achieve increases in strength while maintaining adequate toughness. -The quenching stage of the heat treatment will usually produce martensite, which is a hard and brittle phase, but which subsequently undergoes a tempering heat treatment to restore toughness. -When such steels are welded it is possible that, even when precautions are taken, martensite or a low-toughness microstructure will form in the HAZ. -Inadequate toughness in the HAZ of welds can render the joint inadequate for the intended surface environment. -PWHT is also carried out to relieve residual stresses in the vicinity of the welded joint. -Ideally PWHT would be carried out in a furnace, but this is not always feasible. -The ramp up rate, the soak time, and the cooling rate must be controlled in order to successfully relieve residual stresses.
AM (additive manufacturing)
-Parts are fabricated by adding material in layers -Works for polymeric materials but also, composites, metals and ceramics -Can produce part in single stage,may take few hours. -Multiple parts are often batched together inside single build -Not constrained with accessibility, undercuts and internal features can be easily built without specific process plaining. -Accurate, but is function of system and size dimension. As dimension increases so does inaccuracy.
Common welding defects
-Porosity -Lack of sidewall fusion -Slag inclusions -Lack of penetration
Martensite
-Properties: hard, brittle (esp as carbon increases and fine grain size) -High hardness and brittleness is due to carbon trapped in solution. -The small grain size is due to the high nucleation rate at the large undercooling of the transformation.
Ultrasonic Testing (UT)
-Pulse waves are transmitted into the material to detect internal flaws. -Waves are reflected from surfaces. If a crack is present, an extra reflection will be detected. -A high level of skill is required to interpret signals. -The technique is sensitive to microstructure gradients, and is better suited to planar defects (cracks) than volumetric defects (porosity).
Residual Stresses in Welds
-Residual stresses in welds tend to have magnitudes that are comparable to the yield stress of the materials being welded -Residual stresses in thick section welds can be significantly higher than the yield stress of the material, since material constraint can lead to the generation of significant hydrostatic components of stress. -In multipass welds, the thermo-mechanical cycles associated with the deposition of successive weld beads can lead to significant cyclic hardening of the material, and hence larger residual stresses. -Residual stresses in welds often need to be relieved by post-weld heat treatment in order to avoid problems with fatigue or stress-corrosion cracking.
Radiographic Testing
-Short wavelength electromagnetic radiation is used to penetrate material. -Radiation emerging from the reverse side of the material can be detected. -Defects such as porosity or inclusions are associated with a change in the attenuation. -Radiographic testing is better suited to detecting volumetric defects (pores) than to planar defects (cracks).
Metal casting- effect of cooling rate
-Slow cooling rate: result in coarse dendritic structures with large spacing between arms -High cooling rate: structure becomes finer with small dendrite arm spacing, for higher cooling rates structures are amorphous As grain size decreases, strength and ductility of cast alloy INCREASES, microporosity in casting DECREASES, tendency to crack (hot tearing) during solidification DECREASES
Fusion Zone
-The grain structure within the fusion zone is often influenced by the shape of the weld pool and the direction of heat flow. In some cases significant weld metal texture can develop, where the orientation of grains has a bias. This leads to significant anisotropy in the weld fusion zone -Heterogeneous solidification is common in casting and welding processes. -In fusion welding, epitaxial growth occurs first, whereby the existing grains at the interface grow into the weld pool. -Those grains with <100> crystal directions parallel to the direction of heat flow grow fastest, and the growth of grains with less favourable orientations is stifled. -A columnar grain structure usually results, and the preferred orientation in the crystal structure leads to significant anisotropy.
The Heat Affected Zone (HAZ)
-The heat-affected zone (HAZ) does not melt during welding, but it undergoes microstructural changes due to the welding thermal cycle. -The significance of the microstructural changes will depend on the alloy that is being welded, and whether or not the material has been heat treated. -In contrast to the fusion zone, there is less scope to mitigate the effects of the welding thermal cycle, since the composition of the HAZ will remain identical to that of the parent material. -The HAZ is often the most problematic region in the welded joint, and it is prone to various forms of cracking.
Weld Distortion
-The localised shrinkage and plastic deformation that takes place during fusion welding leads to distortion. -The material in the vicinity of the welded joint deflects or distorts in order to minimise the strain energy within the material. -The extent to which distortion takes place is a function of the degree of restraint that is applied. This restraint may be applied externally or (in practice) it will be inherent in the structure. -In cases where the restraint is significant the distortion will be lower but the levels of residual stress are likely to be higher. -Measures are usually taken to ensure that distortion is not excessive "Butterfly" distortion can be reduced by adopting a double sided welded strategy or using a keyhole welding process such as electron beam welding or laser welding.
Non Destructive Examination (NDE)
-The purpose of nondestructive examination is to establish whether a welded joint has any structural defects, while not causing damage to the material. -NDE is often carried out immediately after welding (i.e. before a joint goes in to service), and in many cases at specified intervals throughout the service life of the component. -A wide range of examination techniques are available. -These include visual inspection, die penetrant testing, magnetic particle testing, eddy current testing, ultrasonic testing and radiography. -All techniques have advantages and disadvantages. For example, visual inspection and die penetrant testing can only detect surface breaking defects. -Ultrasonic testing and radiography are widely used on critical components.
t8/5 Cooling Time
-The transformation of austenite to ferrite and pearlite first becomes possible when the temperature drops below approximately 800°C in many steels. -However, it is difficult for the diffusion of atoms to take place in steels once the temperature drops below approximately 500°C. -Therefore, the time that is taken to cool from 800°C to 500°C will have an important bearing on the likelihood of forming brittle phases such as martensite. -The t8/5 cooling time is the time that is taken to cool between 800°C to 500°C, and it is often estimated for a welding process in order to assess the risk of forming brittle phases. An increase in the t8/5 cooling time decreases this risk
Weld pool behavior
-Weld pool convection plays an important role in determining the shape of the fusion zone. -The hottest liquid is immediately under the arc. -Weld pool convection will determine how the arc's heat is distributed. -Surface tension effects are always present. Electromagnetic effects are greater at higher currents -The welding arc exerts a pressure on the weld pool that is proportional to the square of the welding current. -This depression becomes significant at welding currents exceeding 200 Amperes. -Weld pool depression can change the fusion boundary profile and the shape of the weld bead.
Why use welding process?
-Welding can provide a seamless joint with continuity of material. -A high quality weld will generally be stronger than a riveted or bolted joint. -A welded joint will be lighter than a bolted or riveted connection since there is no need to overlap panels, sheets or plates. -Welding avoids the need for sealants and gaskets. -A welded joint does not have any crevices, which is thought to reduce the likelihood of corrosion.
Injection moulding process parameters
1 - Melting temperature: generally below the melting point of the plastic depends on the polymer morphology 2 - Temperature of the mould: controlled with water that can be circulated through the mould. Some factors that affect the temperatures of the system(melt temperature and the mould temperature) are; 1) Shot size - larger shots take more heat 2) Injection rate - faster filling creates higher melt temperature because of shearing 3) Size of the runner system - long runners require higher temperatures 4) Part thickness - thick parts require more cooling time and are moulded at lower temperatures
Because metals contract during solidification, different defects can be originated. Which are the two major defects that can occur during casting? Explain the differences between them.
1 - Porosity caused by gases evolved during solidification can be a significant problem, particularly because of its adverse effect on the mechanical properties of the casting. 2 - Hot tears defects in castings occur because the casting cannot shrink freely during cooling, owing to constraints in various portions of the moulds and cores 3 - The main difference between Hot Tears and Porosity is related with the solidification stage where they occur. Porosity can develop when the material is solidified while Hot Tears occur during solid/liquid stage (mushy state).
Defects (metal casting)
1) Blow: is relatively large cavity produced by gases which displace molten metal form. Occurs on convex casting surface 2) Scar: is a shallow blow and occurs due to improper permeability or venting. It generally occurs on flat surfaces 3) Blister: is a shallow blow like a scar with thin layer of metal covering it 4) Scab: is a defect that occurs when a portion of the face of a mould lifts or breaks down and the recess thus made is filled by metal. The reasons can be: to fine sand, low permeability of sand, high moisture content of sand and uneven moulds ramming. 5) Cut or Wash: is a low projection on the drag face of a casting that extends along the surface, decreasing in height as it extends from one side of the casting to the other end. Usually occurs with bottom gating castings in which the moulding sand has insufficient hot strength, and when too much metal is made to flow through one gate into the mould cavity, 6) Incomplete filling of the casting: due to inadequate metal supply, too low mould or melt temperature, improperly designed gates, or length to thickness ratio of the casting is too large. 7) Cold shut (discontinuity): When two streams of liquid metal from different gates meet, an interface that lacks complete fusion, named a cold shut is formed
Major features in sand moulds
1) Flask: Supports the mould itself. Two pieces mould consists of a cope and a drag with a parting line in between 2) Pouring basin 3) Sprue: metal flows downward 4) Runner system: carries the metal to the mould cavity 5) Gates: inlets into the mould cavity 6) Risers: Supply additional metal 7) Cores: inserts made from sand to form hollow regions or define surfaces 8) Vents: Placed in the mould to carry off gases
Overview of Welding Processes
1) Heat-Based Processes: Gas flame welding, Electric arc welding, Laser welding ,Electron beam welding 2) Pressure-Based Processes: Friction welding, Friction stir welding, Ultrasonic Welding, Explosion welding 3) Combined Pressure and Heat Forge welding, Resistance welding, Induction Welding
More injection moulding process parameters
1) Injection pressure and holding pressure: This should be done at the lowest practical pressures possible without resulting in short shots (mould partially filled). The pressures should also be high enough and maintained long enough to minimise shrinkage by adding more plastic into the mould (packing process) during the time that the part is shrinking (cooling process). 2) Injection or fill rate and time: The injection time is the time to force the molten material into the cavity, controlled by the fill rate. -The mould temperature is below the freezing point of the material, so long injection times will increase the likelihood of premature freezing. -In parts with thick sections or with very small gates, the injection rate is sometimes lowered so that a continual passage of molten material through the gate will prevent freezing and allow the part to fill completely. Parts with only thin sections are filled at high injection rates to prevent freezing before filling. 3) Dwell time: Time that force or pressure is applied to the cavity after the cavity is filled. This force helps with packing and ensures that the dimensions of the part are set. 4) Freeze time or cooling time: Is the interval after the pressure is relieved and before the mould is opened. This time is governed by the ability of the part to harden sufficiently to be ejected. This time is often shortened by cooling the mould. 5) Ejection time: counts the time to open the mould, eject the part and close the mould.
Injection moulding units! IMPORTANT
1) Injection unit 2) Mould 3) Clamping system (clamping F=PA)
Injection moulding- Incomplete filling of cavity (short shots)
1) Melt temperature: should be as high as possible 2) Pressure: should be as high as possible 3) Injection speed: if too slow, the plastic may freeze before the mould is filled 4) Mould temperature: increase if possible
When compared to metals and ceramics, what are the attributes of plastics that make them of value from manufacturing and functional considerations?
1) Plastics can be formed by moulding into intricate part shapes, usually with no further processing required 2) On a volumetric basis, plastics are cost competitive with metals 3) Generally require less energy to produce than metals 4) Certain polymers are translucent and/or transparent 5) Plastics are also low density materials relative to metals and ceramics, presenting good strength-to-weight ratios, high corrosion resistance, low electrical and thermal conductivity
Because metals contract during solidification, different defects can be originated. Discuss the two major defects that can occur during solidification of casting? At what stages of the solidification do they occur? The main differences between Porosity and Hot Tears are?
1) Porosity caused by gases evolved during solidification can be a significant problem, particularly because of its adverse effect on the mechanical properties of the casting. 2) Hot tears defects in castings occur because the casting cannot shrink freely during cooling, owing to constraints in various portions of the molds and cores The main difference between Hot Tears and Porosity is related with the solidification stage where they occur. Porosity can develop when the material is solidified while Hot Tears occur during solid/liquid stage (mushy state).
Design of cast parts - General rules in casting
1) Sharp corners, angles, and fillets should be avoided as much as possible because they act as stress raisers and may cause tearing and cracking of the metal during solidification [Fillet radii should be selected to reduce stress concentrations and to ensure proper liquid-metal flow during pouring] 2) Section changes in castings should be blended smoothly into each other. The location of the largest circles that can inscribed in a particular region is critical so far as shrinkage cavities are concerned. Because the cooling rate in these regions is lower they are called hot spots which can develop shrinkage, cavities and porosity. They can be eliminated by using small cores, metal padding or chills in the mould. [It is important to maintain uniform cross section and wall thickness throughout the casting to avoid or minimize shrinkage cavities. 3) Large flat areas should be avoided since they may warp during cooling because of temperature gradients, or they develop poor surface finish because of uneven flow of metal during pouring [Breaking up flat surfaces with staggered ribs and serrations may avoid these problems by increasing the resistance of the casting] Draft: A small draft (tapper) typically is provided in sand mould patterns to enable removal of the patterns without damaging the mould.
Crystalline polymers
1) Sharp melting point 2) Usually opaque - the difference in refractive indices between the two phases (amorphous and crystalline) causes interference so the material appears translucent or opaque 3) High shrinkage - As the material solidifies from the amorphous state the polymers take up a closely packed, highly aligned structure. This produces a significant volume change manifested as high shrinkage 4) Good fatigue and wear resistance
Amorphous polymers
1) Usually transparent - the looser structure transmits light so the material appears transparent 2) Low shrinkage - On solidification, the random arrangement of molecules produces little volume change and hence low shrinkage 3) Low chemical resistance 4) Poor fatigue and wear
Characteristics of molten metal that influence fluidity (capability to fill mould cavity)
1) Viscosity: as viscosity and its sensitivity to temperature increase, fluidity decrease 2) Surface tension: high surface tension of the liquid metal reduces fluidity. Because of this oxide films on surface of molten metal having significant adverse effect on fluidity 3) Inclusions: as they are insoluble, inclusions have adverse effect on fluidity. This effect can be verified by observing the viscosity of a liquid (such as oil) with and without sand particles in it; the liquid with sand in it has higher viscosity and, hence, lower fluidity 4) Solidification pattern of alloy: Fluidity is inversely proportional to freezing range. Shorter the range (pure metals) the higher the fluidity. Alloys with long freezing ranges have lower fluidity 5) Degree of superheat: superheat (increment of temp of an alloy above melt temp) improves fluidity by delaying solidification. 6) Rate of pouring: slower the rate of pouring molten metal into mould the lower the fluidity because of higher rate of cooling when poured slowly 7) Heat transfer: directly affects viscosity of liquid metal
Metal forming: Various defects can arise in metal extrusion.
1)Piping: -Cause: Oxide skin at the surface of the billet can sometimes become entrained into the body of the extrude as an impurity defect. -Solution: Punch of smaller diameter than the billet. This collects the skin as a "skull". 2) Fir tree cracking -Cause: Combination of large reductions, fast extrusion rates and low melting point of materials -Solution: Reduce speed of extrusion 3) Rear cavity defects -Cause: Near to the die, in the plastically deforming region of the billet, the billet material is moving faster at the centre than nearer to the container walls. (ie. there is a non-uniform velocity distribution across the billet near to the die due to friction at the walls). Since the punch is rigid and cannot participate in the flow, the billet material near the centre moves ahead of the punch, creating a void (cavity). The cavity becomes wider and deeper as extrusion proceeds further and can extend through the die into the extruded product.
Polymer additives
1)Reinforcing fibres: Increase tensile strength, increase flexural modulus, increase heat-deflection temperature and resist shrinkage and warpage. 2)Conductive fillers: Improve electrical and thermal conductivity 3)Flame retardants: Reduces the occurrence and spread of combustion. 4) Extender fillers: Reduce material cost 5) Plasticisers: improves melt flow properties and enhances flexibility 6) Colorants: Providers colorfastness and protects from thermal and UV degradation
Describe the process of friction welding
1. In friction welding the required heat for welding is generated through friction at the interface of the two members being joined. 2. One of the members remains stationery while the other is placed in a chuck or collet and rotated at a high constant speed. One of the components to joined needs to have rotational symmetry. 3. The two members to be joined are then brought into contact under an axial force. 4. After sufficient contact is established the rotating member is brought to a quick stop, so that the weld is not destroyed by shearing, while the axial force is increased. 5. The pressure at the interface and the resulting friction produce sufficient heat for a strong joint to take place 6. Oxides and contaminants are flushed out. 7. The shape of the weld joint depends on the rotational speed and axial pressure applied. The flash produced by plastic deformation (upsetting) can be machined away.
FDM Process parameters
1. Melt temp: Increasing temp of material increases fluidity of molten material and extrusion flow, hence increasing filament diameter 2.Inlet pressure: in the case of pressure assisted system, increasing pressure, increases flow of extruded material and filament diameter 3. Screw rotation speed: in case of screw assisted systems, increasing screw rotation speed increases flow of extruded material and filament diameter 4. Scanning speed: keeping all other parameters constant, an increase in scanning speed will decrease filament diameter due to stretching effect
FDM- Pressure vs Screw assisted system
1. Pressure assisted system- low viscosity materials, lower accuracy, longer production time. Compatible with living cells for medial application 2. Screw assisted system- High viscosity materials, high accuracy, shorter production time, not compatible with living cells
Deformations during drawing of a cup
1. Pure radial drawing occurs between die and blankholder. The material is drawn towards the die under radial tensile stress. Since the radius is progressively decreasing, the material of the flange has to thicken due to induced compressive (hoop) stresses. 2. Stretching between die and punch. 3. Bending and sliding over the die profile. 4. Bending and sliding over the punch profile radius. 5. Stretching and sliding over the punch head.
AM Positives
1.Low vol production- replaces machine tooling 2.Low cost production 3.Responsive production- for low volume, has fast lead times 4. short supply chains 5. freedom of design 6. part consolidation- reduces assembly requirements by consolidating parts into 1 component 7. elimination of tooling
Metal forming, Friction in closed die forging
A flash can be seen around the forging at the punch/die interface. It is the high deformation resistance of the flash that results in material completely filling the cavity rather than being extruded sideways out of the die. Without the frictional resistance at the flash, closed die forging could not work. The flash has to be subsequently trimmed away from the forging.
The capability of molten metal to fill mould cavities is called fluidity. How does surface tension influence the fluidity of the molten metal?
A high surface tension of the liquid metal reduces fluidity
Describe the working principle of SLA and the two primary configurations for photopolymerization
A laser (normally ultraviolet but gamma rays, X-rays, electron beams and in some cases visible light can also be used ) is used to scan the surface of the resin promoting a chemical reaction and transforming it into a solid. This reaction is called photopolymerization, and is typically complex, involving many chemical participants. Once the first layer is built, the platform lowers, a new layer of resin is deposited and the process is repeated.
Metal forming: forward extrusion
A ram (or punch) pushes billet material through a die to obtain the desired uniform section of the extruded part. Extrude is moving in the same direction as the punch. This combination of motions defines forward extrusion.
Discuss drawbacks of having riser that is i) too large, ii) too small
A riser that is too large increases the material loss factor and also pushes up product cost. A riser that is too small will not be able to fill the crater created by shrinkage and hence this will affect the shape of the casting
Inkject printing
AM process in which liquid bonding agent is selectively deposited to join powder materials +ve: high processing speed, induced micro porosity, water as binder -ve: low mech prop, trapped material, powder material, post processing
Powder bed fusion (SLS)
AM process where thermal energy selectively fuses regions of powder bed. Most frequently used sintering/melting technique; powder dispensed, parts fused or melted via laser or electric beam +ve: high mech resistance, diversity of materials, support structures -ve: high energy, shrinkage, surface finish
Powder bed fusion materials
ANY MATERIAL THAT CAN BE MELTED AND RE-SOLIDIFIED
What are the main advantages of SLA compared to other Additive Manufacturing processes?
Accuracy; Flexibility; Built speed
Electron Beam Welding (positives and negatives)
Advantages -Can make very deep and narrow welds in a single weld pass. -Electron guns are available with powers of up to 100 kW. It is possible to penetrate up to 150 mm for steels. -Less distortion than for arc welding processes. Disadvantages -Components generally need to be placed (and fit) inside a vacuum chamber. -Often difficult to weld dissimilar materials.
Gas Tungsten Arc Welding (GTAW)
Advantages -The operator has independent control over the heat source and the addition of filler metal. -The process is readily automated and does not involve fluxes. GTAW produces very high quality welds. Disadvantages -The productivity of GTAW is generally low. -A high level of operator skill is required for manual welding.
Some Features of Gas-Metal Arc Welding (positives and negatives)
Advantages -With a constant-voltage power source characteristic, GMAW has a "self- correcting" arc length. -The process is readily automated and skill requirements less than for GTAW. -GMAW can produce very good weld quality, with good deposition rates. Disadvantages -The process can be prone to spatter. -Range of filler metal compositions can be limited for solid wire process.
Some Features of Flux Based Processes (advantages and disadvantages)
Advantages -With the exception of submerged arc welding, flux-based processes are generally well suited to out-of-position welding, and welding outdoors. -A wide range of electrode or filler wire compositions are available, or can be manufactured, since it is easy to change the composition of the flux or core constituents. Disadvantages -There is a significant risk of flux inclusions with these processes. -Fluxes tend to attract moisture, which can contribute to problems such as delayed (cold) cracking in steels.
In Fused Deposition Modelling (FDM) process an increase of the melting temperature of the material within the chamber will result in:
An Increase of the fluidity of the molten material and the extrusion flow hence increasing the filament diameter
Directional properties of sheet metal
Anisotropy in sheet metal is revealed if specimens are cut out at different angles to the rolling direction and are then subjected to tensile tests. Different stress strain properties are recorded. The anisotropy is also evidenced by the ears observed after deep drawing of a cup.
The phenomenon of earing in deep drawing is due to which one of the following?
Anisotropy in the blank material.
Metal casting- solidification of pure metals solidification of alloys
As pure metal has clear defined melting/freezing point. Solidifies at const temp. After temp of molten metal drops to freezing point, temp remains constant while latent heat of fusion is given off. In alloys solidification begins when temperature drops below the liquidus temp TL and is complete when it reaches solidus Ts. Within this temp range alloy is mushy state consisting of columnar dendrites (crystal mass with tree like branches)
Strength-to-weight ratio of die cast parts increase with decreasing wall thickness. Explain why
As section thickness increases the walls of the casting cool at a faster rate compared to the core. Thus a transition exists from equi-axial grains to elongated grains. A thinner casting has smaller (large relative surface area to volume ratio) and more uniform grains and hence higher strength.
Deep drawing of a cup
As the punch pushes the centre of the blank into the die, the rest of the blank is drawn radially inwards towards the die opening. Hence the term drawing. Because many of the parts made by this process are fairly deep, the process is termed deep drawing.
Deep drawing: Compromise in operating systems.
Assuming the drawability limit is not exceeded, wrinkling can be alleviated by increasing the blankholder pressure and reducing the amount of lubrication. On the other hand , tearing may be overcome by increasing the amount of lubrication and reducing the blankholder pressure. A compromise in operating conditions might be needed to overcome the occurrence of both defects. In the case of tearing, the improvement that may be obtained in this way cannot greatly exceed the basic range of drawability.
Stress and strain in forming
Assumtions: ●Elastic strains are often assumed to be zero since they are small compared with the strains encountered in plastic deformation. ● Volume is constant during metal forming. This applies providing there are no great temperature changes.
Identify and describe the importance of the two main analytical tools related with flow rate of castings
Bernoulli's theorem and the continuity law are the analytical tools used in designing castings with the goals of achieving an appropriate flow rate and eliminating defects associated with the flow.
Metal casting equations
Bernoulli's theorem and the continuity law are the analytical tools used in designing castings with the goals of achieving an appropriate flow rate and eliminating defects associated with the flow.
Injection moulding- Burn marks
Burn marks and/or discolorations are common causes of part rejection. These types of defects are generally caused by trapped gasses in the cavity or thermal degradation of the polymer 1) Cause: Trapped gasses in the cavity Fix: Clean vents, Provide additional vents, Decrease injection speed 2) Cause: Thermal degradation Fix: Decrease injection speed, Decrease melt temperature
Power bed fusion (SLS- selective laser sintering) Working principle
CO2 laser beam directed on powder bed. Moved so thermally fuses material to form slice cross section. Surrounding powder remains loose and serves as support. When new levels are layed it is leveled using counter-rotating roller.
Describe the casting process and its main features.
Casting is a solidification process in which molten metal is poured into a mould and allowed to cool. The metal may flow through a variety of passages (pouring basins, sprues, runners, risers and gating systems) before reaching the final mould cavity.
Metal casting process
Casting process involves a) pouring molten metal in a mould patterned after the part to be manufactured b) allowing for it to solidify c) removing part from mould
Formation of cracks in welds
Cracking can occur during welding, during PWHT, or in service, due to a variety of mechanisms. Examples are: • Solidification cracking • HAZ liquation cracking • Hydrogen cracking • Reheat cracking • Lamellar tearing
Injection cycle effect of high crystallinity
Crystallinity has a significant impact on the injection moulding process (cooling and holding pressure characteristics) and part quality. -High crystalline polymers are characterised by high shrinkage values, and high melting temperatures to be processed. -Additional changes in part dimensions can also occur after the part has been removed from the mould. This is caused by continued internal changes in the part due to crystallization. -Some plastics require considerable time to complete the crystallization time and for practical purposes cannot be kept in the mould until the crystallization time is complete. In parts with complex structures this can cause warping.
The main distinction in categorizing the most common Binder Jetting technologies refers to the possible modes of expulsion: Continuous Stream (CS) and Drop on Demand (DOD). What are the main advantages of DOD over CS?
DOD produces smaller drop size (often of diameter similar to the orifice); Higher placement accuracy in comparison to CS methods.
Crystallinity and Glass Transition Temperature Tg
Degree of structural order in a solid and is usually specified as a percentage of the volume of the material that is crystalline Glass Transition Temperature Tg: is the temperature at which an amorphous solid becomes soft upon heating or brittle upon cooling. The glass transition temperature is lower than the melting point of its crystalline form, if it has one. (above trans temp it acts like rubber, below like glass) Melting Temperature: is the temperature at which a crystalline polymer changes state from solid to liquid at atmospheric pressure
Describe three design considerations in metal casting
Design considerations in casting (i) the requirement to avoid turbulent flow in filling the mould cavity and to exclude entrapment of gases (ii) designing to eliminate hot spots so that near uniform cooling is provided (iii) design of risers to ensure adequate compensation for volumetric shrinkage (iii) design and selection of mould to optimise the solidification time for the casting and contain grain morphology.
Shrinkage (metal casting)
Due to thermal expansion characteristics, metals usually shrink (contract) during solidification and while cooling to room temperature. Shrinkage, causes dimensional changes and sometimes warping and cracking, is the result of the following three sequential events: 1 - Contraction of the molten metal as it cools prior to its solidification 2 - Contraction of the metal during phase change from liquid to solid (latentheat of fusion) 3 - Contraction of the solidified metal (casting) as its temperature drops to ambient temperature
Deep drawing: punch force and ironing
During deep drawing, The punch force versus punch stroke diagram is approximately like a sine wave. However, a secondary rise in load can occur towards the end of the process as material at the blank periphery thickens to an extent that it is greater than the radial gap between the punch and die. The consequent thinning of this thicker portion is known as "ironing". Sometimes, the tooling is deliberately designed to cause ironing and thus produce a more uniform wall thickness in the product.
What are the main advantages of die casting process?
Excellent dimensional accuracy and surface finish; high production rate
Metal Casting - Moulds classifications -Expendable mould and permanent pattern -Expendable mould and expendable pattern -Permanent moulds -Composite moulds
Expendable mould and permanent pattern: In sand and shell casting the mould is expendable but the pattern is reused to produce several moulds. Expendable mould and expendable pattern: Investment casting consumes a pattern for each mould produced. Permanent moulds: Typically made of metals that maintain their strength at high temperatures. Used repeatedly and designed in way that the casting can be removed easily and the mould used for the next casting. Metal moulds are better heat conductors than expendable non-metallic moulds. Composite moulds: are made of two or more different materials (such as sand, graphite, and metal) combining the advantages of each material. These moulds have a permanent and an expendable portion and are used in various casting processes improving: - mould strength - control the cooling rates - optimizing the overall economics of the casting process
With reference to casting process suggest how to reduce porosity on heavier sections of casting like corners
External or internal chills (dark areas at corners) can be used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metals.
Forward extrusion?
Extrude moves in same direction as punch.
The spatial distribution of the micro-constituents in a carbon steel that is cooled slowly from above the austenisation temperature depends on the carbon content. Which microstructure would you expect for a hypo-eutectoid steel (<0.8% carbon)?
Ferrite and Pearlite
Suggest a method of joining a metal shaft to a polymer plate
Fiction welding can be used for joining dissimilar materials like a shaft to a polymer plate. The shaft has rotational symmetry and can be spun against the polymer plate to generate the heat for welding.
FDM working principle
Filament spool, led to extruder, extruder uses torque and pinch system to feed and retract filament precise amounts. A heater block melts filament to usable temp, heated filament forced out of the heated nozzle at smaller diameter, extruded material laid down
Fluidity of molten metal
Fluidity is the capability of molten metal to fill mould cavities which consists of two basic factors: 1) characteristics of the molten metal 2) casting parameters
Direct energy deposition
Focused thermal energy is used to fuse materials by melting as the material is being deposited
If you need only a few units of a particular casting which process would you use and why?
For few cast parts use sand casting since the tooling cost is very low and hence the process breaks even at low production runs.
Friction in metal forming
Friction is present at the interfaces between tools and workpiece and results in extra force needed to perform the deformation. Thus lubrication is often essential. Friction can also be useful in sealing the die interface during closed die forging.
Powder bed Fusion- liquid phase sintering
Fusion of powder particles when portion of material becomes molten while other portions remain solid. Molten constituents act as glue which binds the solid particles together.
Present reasons why heat transfer and fluid flow are important considerations in metal casting
Heat transfer enables the estimation of the cooling time and optimisation of cast geometry to enable near uniform cooling. Fluid flow is used to control molten metal supply into the mould to ensure that the mould is completely filled and to ensure that gases are not trapped in the metal.
How to reduce turbulence in metal casting
Higher Re number higher turbulent flow. Techniques for minimizing turbulence generally involve avoiding sudden changes in flow direction and in the geometry of channel cross sections in gating system design!
Defects (metal casting) Hot tears and preventions
Hot tears, occur because the casting cannot shrink freely during cooling, owing to constraints in various portions of the moulds and cores. Exothermic (heat-producing) compounds may be used (as exothermic padding) to control cooling at critical sections to avoid hot tearing. Prevention 1) Alter casting design: to reduce stress concentrations and hot spots. 2) Use chills: Local hot spots can be reduced by local chilling which will strengthen the metal by taking it out of the susceptible temperature range 3) Mould strength: Reduce the mould strength so that it provides less constraint to the contracting casting. 4) Grain refinement: should help to reduce tear initiation since the strain will be spread over a greater number of grain boundaries. 5) Casting temperature: A reduction in the casting temperature can reduce the grain size.
Solution to drawability limit
If it is necessary to exceed the drawability of a material due to a particularly large diameter of blank needed to be drawn into a much smaller diameter of cup, then a different approach will be needed. The technique of re-drawing then becomes applicable. For a single re-drawing operation, a punch and die set of smaller diameter than those for the initial draw must be used.
Fluidity test
In a common test the molten metal is made to flow along a channel that is at room temperature. The distance the metal flows before it solidifies and stops flowing is a measure of its fluidity. The fluidity index is the length of the solidified metal in the spiral passage
Metal casting- fluid flow
In basic system, molten metal poured through pouring basin to cup, then flows through gating system (sprue, runners, gate) into mould cavity Sprue: vertical channel in which molten metal flows downward in mould Runners: channels that carry molten metal from sprue to mould cavity/or connect sprue to gate Gate: portion of runner through which molten metal enters mould cavity Risers: reservoirs of molten metal to supply any molten metal necessary to prevent porosity due to shrinkage (COMPENSATE FOR SHRINKAGE AS CASTING SOLIDIFIES)
Give an example of how the design of a component can affect the quality of a product if the component is made by a metal casting process
In casting design, corners, angles and section thickness are key design considerations for cast parts. Sharp corners, angles, and fillet should be avoided as much as possible, because the act as stress raisers and may cause cracking and tearing of the metal (as well as the dies) during solidification. Fillet radii should be selected to reduce stress concentrations and to ensure proper liquid-metal flow during pouring. So blend fillet radii to 3 to 15 mm.
Permanent mould - Die Casting
In die casting process, molten metal is forced into the die cavity at pressures ranging from 0.7 to 700 Mpa. There are two types of die casting machines: hot chamber and cold chamber machines. 1)The hot chamber process involves the use of a piston, which forces a certain volume of metal into the die cavity through a gooseneck and nozzle. -The metal is held under pressure until it solidifies in the die -To improve die life and to aid rapid metal cooling dies usually are cooled by circulating water or oil. -Low melting point alloys, such as zinc, magnesium, tin and lead are casted. 2) In the cold chamber process, molten metal is poured into the injection cylinder (shot chamber). The chamber is not heated and the metal is forced into the die cavity. Die casting has the capability for rapid production of strong, high quality parts with complex shapes. It also produces good dimensional accuracy and surface details so that parts require little or no subsequent machining or finishing operations. Because of the pressures involved, walls as thin as 0.38 mm are produced which are thinner than those obtained by other casting methods.
Permanent mould casting processes
In permanent mould casting two halves of a mould are made from materials with high resistance to erosion and thermal fatigue. 1) The mould cavity and gating system are machined into the mould and thus become integral part of it. 2) Typical core materials are oil-bonded or resin-bonded sand, plaster, graphite, gray iron, low carbon steel and hot work die steel. 3) In order to increase the life of permanent moulds the surfaces of the mould cavity usually are coated with a refractory slurry (such as sodium silicate and clay) or sprayed with graphite every few castings. 4) Mechanical ejectors (such as pins located in various parts of the mould) may be required for the removal of complex castings 1) The moulds are clamped together by mechanical means and heated to about 1500C to 2000C to facilitate metal flow and reduce thermal damage to the dies due to high temperature gradients. 2) The process is used for materials with low melting points. 3) Permanent mould castings generally present good surface finish, close dimensional tolerances, uniform and good mechanical properties and high production rates.
Metal forming: reverse extrusion
In reverse extrusion, the hollow punch acts as a die, and the extrude is caused to move into the inside of the punch. The length of the extrude is limited by the length of the punch which cannot be too long to avoid buckling. Punch and extrude have opposite movement.
Injection moulding
In the process a plastic is melted and then forced into the cavity of a closed mould which gives shape to the plastic after sufficient time for the plastic part to solidify (usually by cooling) the mould opens and the part is removed.
With reference to casting process suggest how to reduce the chance of having missing features on a casting can be achieved
Incomplete castings could be mis-runs (due to premature solidification), insufficient volume of metal poured, and runout (due to loss of metal from the mould after pouring). Slow mould fill rate or low temperature can cause the defect. These need to be investigated.
Provide three possible reasons why a casting may have incorrect dimensions or shape
Incorrect dimensions or shape of a casting may be attributed to (i) specifying incorrect shrinkage allowance (ii) incorrect location of the parting line (iii) locating gates and risers can non-optimum locations (iv) in correct control of liquid metal flow and filling etc.
Weld lines are a common injection moulding defect. How can it be prevented?
Increase injection speed
Which are the three main functional units of the injection moulding equipment?
Injection unit; Mould unit; Clamping unit
Expendable mould / Expendable pattern casting [investment casting]
Investment casting is sometimes referred to as expendable pattern casting process or expendable mould/expendable pattern process or even more commonly as lost wax process. It is unique in that a mould and a pattern must be produced for each casting. 1) The pattern is made of wax or plastic using moulding 2) The pattern is assembled in a tree and dipped in a slurry of refractory material such as silica; 3) the pattern is coated repeatedly to increase its thickness for better strength; 4) the one piece mould is dried in air and heated of 900 C to 1750C to melt out the wax. 5) After the metal has been poured and has solidified the mould is broken up and the casting removed
How can you define Glass Transition Temperature (Tg)?
Is the temperature at which an amorphous solid becomes soft upon heating or brittle upon cooling. The glass transition temperature is lower than the melting point of its crystalline form, if it has one.
In injection moulding, what is the Dwell Time?
Is the time that force or pressure is applied to the cavity after the cavity is filled
What is the T8/5 cooling time, and why is it important?
It is the time taken to cool between 800°C and 500°C. Phase transformation on cooling can start at around 800°C, and atomic diffusion becomes difficult below 500°C. A fast T8/5 time makes it more likely to form brittle phases such as martensite.
Metal forming: Compression test (open die forging)
Johnson carried out compression tests on cylindrical billets of pure lead to determine stress-strain characteristics of the material. However, the problem of barreling arises This phenomenon is due to radial friction at the tool/workpiece interface and affects the accuracy of experimental data To avoid barreling, concentric grooves were made in the flat surfaces of a cylindrical compression test billet to contain grease lubricant and thereby reduce radial friction forces.
Laser Welding
Laser welding has some similarities to electron beam welding: -Power densities are high enough to weld in "keyhole" mode; -Narrow welds can be made, with relatively low distortion; Laser welding is also different to electron beam welding in some ways: -Laser welding has the advantage that it does not require a vacuum; -Laser welding is less vulnerable to dissimilar material properties; -Penetration is generally less than for electron beam welding;
Binder Jetting- working principle
Liquid binder droplets are deposited over powder bed. Once layer printed powder bed is lowered. Printed part is left in powder bed after completion for binder to fully set and the green part to gain strength. Post processing involves removing part from powder bed, removing unbound powder via pressurized air and infiltrating the part with infiltrant to make it stronger.
VAT photopolymerization
Liquid photopolymer in a VAT is selectively cured by light-activated polyermerisation
In Fused Deposition Modelling (FDM), two extrusion principles can be employed: Pressure or Screw assisted. What characterizes the Pressure assisted systems?
Low viscosity materials; lower accuracy and longer production times compared with Screw assisted systems; compatible with living cells for medical applications
Additive Manufacturing processes fabricate parts by adding material in layers. What are the main advantages of AM?
Low volume production; Freedom of design; Complexity for free
Mask Projection (SLA, primary config for process)
Mask projection(layer wise)--> Irradiate entire layers one at a time enabling higher production rate but is less efficient in terms of energy compared to vector scan
How would you classify Fused Deposition Modelling (FDM) according to ASTM F42?
Material Extrusion Process
Metal forming
Metal forming processes achieve a pre- determined component shape by re- distributing raw material under the action of applied forces.
Reverse engineering
Models built from data generated using reverse engineering equipment from another object. Initial object, scanned, into cad file, object printed
Injection moulding- What is the importance of designing parts with uniform thickness?
Non-uniform section thickness results in distortions of the part because of differential cooling resulting in non-uniform internal stresses Uneven shrinkage causes sink marks, voids, stress and warpage
In cold forward extrusion of metals, a cavity defect can occur at the rear of the billet. What is responsible for this phenomenon?
Non-uniform velocity profile.
Deep drawing: formation of ears
Often, after drawing is completed, lobes or ears can be observed at the rim of the drawn part. Sheet steel is produced by rolling a billet many times until it reaches the correct sheet thickness. Thus the structure of the sheet and its properties are very directional (ie. anisotropic). The stresses imposed on the cup flange are axisymmetric. However because of anisotropy, the resulting strains are not equal about the major axis of the cup. Typically, four ears are formed at the periphery. -Solutions are to either anneal the sheet, or to accept the phenomenon and machine away the eared edge.
Consider a 1.5 kW hair dryer which is turned on and is very close to an AISI grade 304 stainless steel sheet, 1.6 mm in thickness. The hair dryer is heating the stainless steel plate by transferring 1.5 kJ.s-1 to the plate over an area approximately 50 mm in diameter. However, the stainless steel plate does not melt. If, however, we strike an arc between a tungsten electrode and the 1.6 mm thick stainless steel plate (10 Volts, and 150 Amperes), we will see that the plate melts. The total heat flux is the same, so why is this the case?
POWER DENSITY: The reason for the difference in the response of the material lies in the difference in power density. In welding applications, the power density refers to the welding power per unit area, and it is quoted in Watts per unit area. If the power density is too low for welding (e.g. for the hair dryer), then all of the heat that is transferred to the plate will be dissipated by thermal conduction. If, however, the power density increases (e.g. by using a welding arc) then the power density can be sufficient to cause melting of the material, or even vaporisation.
Deep drawing process
Parts produced by deep drawing are made from flat sheet metal which has been cut to a predetermined shape. These flat sheets are known as 'blanks'. Because the tooling is expensive to design and to develop, the process is suited to manufacture of large quantities.
Metal Casting - Patterns and cores (sand moulds)
Patterns: are used to mould the sand mixture into the shape of the casting and may be made of wood, plastic or metal. 1) One-piece patterns: used for simpler shapes and low quantity production. Normally made of wood and are inexpensive. 2) Split patterns: are two piece patterns, made such that each part forms a portion of the cavity for the casting. In this way castings with complicated shapes can be produced. 3) Match-plate patterns: are a common type of mounted pattern in which two-piece patterns are constructed by securing each half of one or more split patterns to the opposite sides of a single plate. Cores: For castings with internal cavities or passages. Cores are placed in the mould cavity to form the interior surfaces of the casting and are removed from the finished part during the shakeout and further processing. The cores are anchored by core prints which are recesses added to the pattern to locate and support the core and provide vents for the escape of gases.
Die casting process can be classified as: (mould)
Permanent mould
Why phase diagrams are so important in metal casting?
Phase diagrams are important tools for identifying the solidification point or points for technologically important metals.
Manufacturing functional areas
Planning, operation, research and control
When compared to metals and ceramics, what are the attributes of plastics that make them of value from a manufacturing and functional point of view?
Plastics can be formed by moulding into intricate part shapes, usually with no further processing required; On a volumetric basis, plastics are cost competitive with metals
Polymers
Polymers are large molecules made by joining together thousands of small molecular units known as monomers.
Reducing the porosity of sand castings (how to be achieved)
Porosity for sand cast castings can be reduced by (i) having a tapered sprue (ii) using more coarse sands (ii) perceiving the mould to aid gas escape.
Porosity caused by shrinkage (Metal Casting) and how to eliminate
Porous regions can develop in castings because of shrinkage of the solidified metal. Porosity is detrimental to the ductility of a casting and its surface finish, making the casting permeable and thus affecting the pressure tightness of a cast pressure vessel. Elimiate by; 1 - Supplying adequate liquid metal to avoid cavities caused by shrinkage 2 - Internal or external chills as those used in sand casting. 3 - With alloys, porosity can be reduced or eliminated by making the temperature gradient steep. For example mould materials that have higher thermal conductivity may be used. 4 - Subjecting the casting to hot isostatic pressing
Binder jetting- droplet formation (CS vs DOD)
Possible modes of expulsion; 1. Continuous stream (CS)- continuous column of liquid when exits nozzle. After stream leaves nozzle it becomes droplets due to Rayleigh instability, deposition controlled by charging field and attain electrostatic charge on droplets which pass through deflection field 2. Drop on Demand (DOD) discrete droplets exiting nozzle. Formed only when individual pressure pulses in nozzle causing fluid to be expelled. At present all commercial AM printing machines uses DOD print heads
How would you classify Laser Sintering (LS) process according to ASTM F42?
Powder Bed Fusion Process
Binder jetting
Process in which a liquid bonding agent is selectively deposited to join powder materials
Material jetting
Process in which droplets of build material are selectively deposited
Material extrusion
Process in which material is selectively dispensed through nozzle or orifice
Sheet lamination
Process in which sheets of materials are bonded to form an object
Epoxy based resin (SLA)
Produce more accurate, harder and stronger parts. Low shrinkage therefore reduced curling (excellent adhesion) Polymerization of epoxy based resins are not inhibited by atmospheric oxygen, enabling low concentrations of photoinitiator
The phenomenon of wrinkling in deep drawing can sometimes be reduced by ....
Reducing lubrication and increasing blankholder pressure.
Residual Stresses
Residual stresses are stresses that remain in a body when it is stationary and at equilibrium with its surroundings -Residual stresses result from the generation of misfit strains through localised plastic deformation. Such misfit strains may arise through mechanical deformation of a component, such as through rolling and forming. -Non-uniform heating and cooling during fusion welding can also lead to the creation of misfit strains through localised plastic deformation. -Since residual stresses exist in the absence of any external loading on a body, they must be self-equilibrating over any plane within the body.
Design for Expendable mould casting [riser]
Riser design: A major concern in the design of castings is the size and placement of risers. Risers are useful in affecting the solidification front progression across a casting and are an essential feature in the mould layout. Risers are designed according to six basic rules: 1. The riser must not solidify before the casting 2. The riser volume must be large enough to provide a sufficient amount of liquid metal to compensate for shrinkage 3. junctions between the casting and the riser should not develop a hot spot where shrinkage porosity can occur 4, risers must be placed so that the liquid metal can be delivered to locations where it is most needed 5. there must be sufficient pressure to drive the liquid metal into location in the mould where it is needed 6. the pressure head from the riser should suppress cavity formation and encourage complete cavity filling
Describe the process of sand casting. Cover all steps required to obtain a cast product.
Sand casting, consists of placing a pattern in sand to make an imprint, incorporating a gating system, filling the resulting cavity with molten metal, allowing the metal to cool until it solidifies, breaking away the sand mould thus leaving the casting and subsequently cleaning it.
Gas Tungsten arc welding (GTAW) is a common joining process. Describe the process characteristics?
Separate wire feed and non-consumable electrode, and air excluded via inert shielding gas.
Fdm +ve and -ve
Several drawbacks on top of other ones -Build speed: speed of fdm system is reliant on feed rate and plotting speed. Feed rate is dependant on ability to supply material.Improvements required to reduce friction of system to improve build speed. -Accuracy: layer thickness option 0.078mm only available with high cost machine. Use of this level slows build time and as nozzles are circular its IMPOSSIBLE to draw sharp corner -Part Anisotropy: Properties are isotropic in x-y plane. Strength in direction z is less than in x-y plane therefore it is important to build part such that major stress is aligned with x-y
What are the main characteristics of crystalline polymers?
Sharp melting point; Usually opaque; High shrinkage; Good fatigue and wear resistance
Metal forming, Friction in rolling and backing rolls
Since the metal elongates plastically as it passes through the rolls, it speeds up as it goes. Thus it goes slower than the rolls at entry and faster than the rolls at exit. At some point, in the roll gap, the material is moving at the same speed as the rolls. This implies that frictional forces vary through the roll gap. Without friction, rolling would not happen because the billet could not then be forced into the gap. Well-designed rolling mills use smaller diameter rolls to reduce rolling pressures. Such rolls are subject to elastic deflection and consequential inaccuracy in rolled work. Thus larger backing rolls are used to avoid deflection
Injection moulding- Sink Marks
Sink marks: a depression, resembling a dimple or groove, caused by excessive localized shrinking of the material after the part has cooled. Usually formed over the thicker sections of a part. This is due largely to the difference in cooling between the thick and thin sections. As thicker sections cool, the molten polymer shrinks, pulling the surface toward the centre 1) Cause: Insufficient polymer in the mould Fix: Increase packing pressure, Increase injection hold time, Increase injection speed, Increase gate size. 2) Cause: Polymer "flowing back" out of the mould into the runner system and barrel Fix: Increase injection hold time, Decrease mould temperature and/or increase cooling time 3) Cause: Polymer temperature too high Fix: Reduce back pressure, Reduce barrel temperature, Improve mould temperature controls, Ejected parts too hot, Increase cooling time
In deep drawing, there is often a secondary rise in force towards the end of the process. Which one of the following is responsible for this phenomenon?
Small radial clearance between punch and die.
Which are the main differences in terms of solidification process of pure metals and alloys?
Solidification of pure metal takes place at a constant temperature, whereas solidification of alloys occurs over a range of temperatures.
Volume and Area of sphere " " cube " " cylinder
Sphere: 𝑉= 4/3(𝜋𝑟^3) 𝐴 = 4𝜋𝑟^2 Cube: V=𝑎^3 𝐴 = 6𝑎^2 Cylinder: 𝑉= 𝜋𝑟^2h 𝐴 = 2𝜋𝑟^2 + 2π𝑟h
Sand Casting process
Steps of sand casting process; a) placing a pattern (having the shape of the desired casting) in sand to make an imprint; b) incorporating a gating system; c) removing the pattern and filling the mould cavity with molten metal; d) allowing the metal to cool until it solidifies; e) breaking away the sand mould; f) removing the casting Types of sand mould -Green sand: which is a mixture of sand, clay and water. The term green refers to the fact that the sand in the mould is moist or damp while the metal is being poured into it. Green sand mould is the least expensive and the sand is recycled easily for subsequent reuse. -Cold-box mould: various organic and inorganic binders are blended into the sand to bond the grains chemically for greater strength. These moulds are more dimensionally accurate than green-sand moulds but are more expensive. - No-bake moulds: a synthetic liquid resin is mixed with the sand and the mixture hardens at room temperature. Because the bonding of the mould in this and in the cold- box process takes place without heat, they are called cold- setting processes
Porosity in casting caused by shrinkage can be eliminated by:
Subjecting the casting to hot isostatic pressing
Support structures AM
Support material is deposited to allow fabrication of overhanging structures.
Deep drawing: Tearing
Tearing happens when the local tensile stress equals the ultimate tensile strength of the metal. The location of failure is associated with tensile loading and thinner regions where the cross sectional area is small. The failure occurs at a limiting draw ratio. The drawability normally lies in the approximate range of 1.6 to 2.2 Tears can also occur due to a small punch corner radius or a small die corner radius as well as material thinning. However the main limitation is the drawability. For a given sheet thickness, drawability increases with increases in both punch edge radius and die edge radius, but non-linearly. Solution: Correctly designed tooling requires that the edge radius of a punch or die should not be too sharp. Up to a point, tearing may be avoided by increasing the amount of lubrication present, or/and by reducing the blankholder pressure.
Testing in metal forming
Tensile test is only of value for relatively small strains, because the specimen goes unstable before the large strains associated with forming can be reached. USE Bulge test which applies a hydraulic pressure to one side of a circular sheet specimen that is thus deformed like a dome. Recordings of the pressure can be interpreted as stress values, whilst measurements of the deformed sheet give values of strain which are more representative of sheet metal forming.
Metallurgical Zones in a Weldment
The 'fusion zone' refers to the region that has undergone melting and solidification during welding. In general, it comprises a mixture of the parent material and the filler material. It is also referred to as the 'weld metal'. The 'heat-affected zone' (HAZ) is the zone that did not melt during welding but in which the microstructure and properties of the material were affected by the thermal cycle. The 'parent material' comprises material that has not been affected by the welding thermal cycle.
In a basic gravity-casting system the runners are:
The channels that carry the molten metal from the sprue into the mould cavity or connect the sprue to the gate.
Inspection of castings - Non-destructive methods
The control of all casting stages is essential to maintaining good quality. Castings can be inspected visually, or optically for surface defects. 1) Liquid penetrants: in this technique fluids are applied to the surfaces of the part and allowed to penetrate into cracks, seams and pores. 2) Radiography: X-ray inspection to detect internal flaws such as cracks and porosity. The technique detects differences in density within a part. The source of radiation is typically an X-ray tube and a visible, permanent image is made on a film or radiographic paper.
Heat transfer - Solidification time CHVORINOVS RULE
The cooling rate is slower towards the centre of the casting, thus equiaxial crystal are formed at the edge of the casting (thin skin) and columnar crystals towards the centre. Since thinner sections will cool faster. Heavier sections are provided with risers to minimise porosity, cracks and voids.
Main tooling for deep drawing
The die and punch. These are made of hardened steel. The punch applies pressure to the sheet blank and pushes it into the die to achieve the desired shape. This action requires that the sheet material is drawn inwards towards the die. The blankholder controls the sheet as it moves into the die, keeping it flat up to the point of entry. The flat part is called the flange.
Metal forming: indirect extrusion
The extrude takes the form of a tube, and the extrude moves in the opposite direction to the punch. Note also, that the punch defines the extrude profile, so is also acting as a die.
Porosity - Internal/external chills
The function of the chills is to increase the rate of solidification in critical regions. Internal chills usually are made of the same material as the casting and are left in the casting. It is common to avoid internal chills due to difficulties in fusing the chill with the casting. External chills may be made of the same materials as the casting or may be iron, copper or graphite.
Explain difference between runner and gate
The gating system is the network of channels used to deliver the molten metal to the mould cavity. The metal is introduced through a mouth called the pouring cup, it travels vertically down through the sprue and then along horizontal channels called runners through a gate into the mould cavity. Runners are the channels that carry the molten metal from the sprue into the mould cavity or connect the sprue with the gate. The gate is the portion of the runner through which the molten metal enters the mould cavity.
Heat transfer (metal casting)
The heat transfer during the complete cycle (from pouring, to solidification, and to cooling at room temperature) is another important consideration in metal casting. Heat from the liquid metal is given off through the mould wall and to the surrounding air. The temperature drop at the air-mould and mould-metal interfaces is caused by the presence of boundary layers and imperfect contact at these interfaces. The shape of the curve depends on the thermal properties of the molten metal and mould
Economics of casting - Sand vs Investment casting
The initial costs of mould and equipment are higher in the case of die-casting than for sand-casting. Labour costs are higher for sand casting. The break-even value, i.e. the number of castings which must be made before the use of die-casting is justified, may be of the order of several thousands. Once the break-even value has been reached, the cost per die-casting becomes less than the cost per sand-casting.
Injection moulding cycle
The injection cycle is composed of 4 stages: Plastification, Injection, Packing/Cooling and Ejection Conformal cooling makes use of cooling lines in an injection mould that curve and closely follow the geometry of the part to be produced. The objective of using "conformal cooling" channels is to cool the part uniformly, reducing the cooling time and improving part quality. Conformal cooling provides a tremendous advantage in mould tooling through significant reductions in injection cycle times.
Melting practice and furnaces in casting
The melting furnaces commonly used in foundries are electric furnaces, induction furnaces, crucible furnaces and cupolas: 1- Electric arc furnaces: used extensively in foundries and have such advantages as a high rate of melting, less pollution and the ability to hold the molten metal for alloying purposes 2 - Induction furnaces: especially useful in smaller foundries and to produce smaller composition-controlled melts. This type of furnace has excellent mixing characteristics for alloying and adding a new charge of metal 3 - Crucible: have been used extensively throughout history and are heated with various fuels such as commercial gases, fuel oil, fossil fuel and electricity.. May be stationary tilting or movable 4 - Cupolas: are basically vertical, refractory-lined steel vessels charged with alternated layers of metal, coke, and flux. Although they require major investments and increasingly are being replaced by induction furnaces, cupolas operate continuously, have high melting rates and produce large amounts of molten metal
Thermoplastics are one of the most important polymers used in injection moulding. How would you characterize them?
The molecules have weak, straight-chain bonds between them that can be broken by heating; They dissolve in organic solvents; On heating, they will soften and ultimately melt.
The major limitations of Fused Deposition Modelling (FDM) process are:
The need for a pre-filament, high processing temperatures and support structures
Design of cast parts - Locating the parting line
The parting line is the line or plane separating the upper (cope) and lower (drag) halves of the mould. Rules: 1) In general the parting line should be along a flat plane rather than be contoured. 2) The parting line should be at the corners or edges of casting rather than on flat surfaces in the middle of the casting (so that material squeezing is not visible) 3) The parting line should be placed as low as possible (relative to the casting) for less dense materials and located around mid-height for denser materials
Ironing in deep drawing is used to improve the quality of...
The product wall.
Two main events occur during the cure reaction of thermosetting resins namely Gelation and Vitrification. How can you describe the gelation process?
The resin transforms from a liquid to a rubbery state; At this point, the system will no longer flow and two phases coexist: a gel phase and a sol phase. The gel phase is the gelled part, insoluble in non-degrading solvents while the sol phase, which remains soluble, can be extracted with solvents. There is a drastic increase in viscosity.
Which of the following considerations are important for a riser to function properly?
The solidification time of the riser should always be higher compared to the casted part in order to guarantee the compensation of volumetric shrinkage that occurs during the solidification process.
The structures developed during the solidification process and the resulting grain size influence the properties of the casting. As grain size decreases:
The strength and ductility of the cast alloy increases
In Johnson's empirical method for estimating the punch load in cold forward extrusion of metals, a point on the punch load versus punch travel diagram is chosen as most representative of the extrusion load.
The transition between the steady state phase and the non-steady state phase.
With reference to casting process suggest how to enable the production of internal open features such as holes within castings
The use of cores. These are collapsible after the casting solidifies.
Metal forming classifications
There are essentially three classifications of metal forming processes. These are: • bulk deformation processes (eg. involving a billet or block) and sheet processes (involving a relatively thin sheet); • hot or cold processes: (hot processing requires material to be pre-heated to high temp, metal becomes easier to form but reacts with oxygen to form an oxide skin at surface) (cold processing is at room temp, metal is harder to form but has a 'bright' finish and improved strength) BLACK BARS FROM HOT FORMING, BRIGHT BARS FROM COLD WHICH HAVE CLOSE TOLERANCES • steady state (if process photographed at two separate moments and no change then it is steady state) or non-steady state processes.
Blankholder
There are several types of blankholder, but the main one is the controlled pressure type. This means that the pressure supplied by the blankholder can be controlled by means of springs or with a hydraulic supply. The latter is the type most often used. • Thus to avoid wrinkling, the blankholder pressure can be increased. Also, or alternatively, a reduction in lubrication of the blank will increase friction and reduce the tendency of the flange to wrinkle.
Powder bed fusion
Thermal energy selectively fuses regions of powder bed
Binder jetting- thermal and piezoelectric
Thermal: in thermal systems resistor causes heating of liquid within reservoir forcing bubble to expand and droplet to be ejected through nozzle. Piezoelectric: Actuator cause deformation of piezoelectric element to reduce volume of liquid reservoir hence forcing droplet to be ejected
Thermoplastic polymers
Thermoplastic polymers always possess linear or branched structures, or a mixture of the two. Branches increase entanglement among the molecules, which makes the polymers: - Stronger in solid state - More viscous at a given temperature 1)Solid materials at room temperature but viscous liquids when heated 2)They can be easily and economically shaped into products 3)They can be subjected to heating and cooling cycles repeatedly without significant degradation
Thermoplastic vs Thermoset
Thermoplastic: (recyclable) 1)The molecules have weak, straight-chain bonds between them that can be broken by heating. 2)They dissolve in organic solvents. 3)On heating, they will soften and ultimately melt. 4)The melting point of thermoplastics is lower than their degradation point. 5)In solid state, their structure consists of hard crystalline regions together with elastic amorphous regions. Thermoset: 1)They have strong chemical bonds between molecules, including cross-linking, due to which they do not separate on heating. 2)They do not dissolve in organic solvents. 3)On heating, thermoset polymers will char, not melt. 4)Their degradation point is lower than their melting point. 4)When solid, their structure consists of the thermosetting resin interspersed with reinforcing fiber.
Thermo set polymers
Thermoset or thermosetting plastic parts are made from polymeric resins that are capable of forming chemical cross-links. Thermosets are shaped (usually by placing them into moulds) and then are chemically cross-linked. The process is called curing. Curing reaction: Two main events occur during the cure reaction of thermosetting resins: i) gelation, i.e. liquid-to-rubber transition ii) vitrification, i.e. liquid or rubber-to-glass transition
VAT photo-polymerization (SLA- stereolithography technology)
Use light to solidify liquid photopolymer -Laser spans surface of resin promoting chemical reaction and transforming it into solid (polymerization) +ve: high dimensional accuracy, complex geometries -ve: photosensitive materials, post curing, use laser Materials: thermoplastic polymers (linear or branched structure allowing recycling) Photopolymers are cross linked and therefore do not melt (less cress creep and stress relaxation)
Stereolithography (SLA) is an AM process that can be classified according to ASTM F42 as:
Vat Photopolymerization Process
Vector scan (SLA, primary config for process)
Vector scan(point wise)--> monochromatic radiation produced by laser polymerise thick layers of resin in more uniform way
Injection moulding- Weld lines
Weld lines form whenever two molten polymer flow fronts meet. Problems in strength and appearance occur when there is insufficient knitting or joining of the flow fronts. 1) Cause: Slow injection speed Fix: Increase injection speed 2) Cause: Under-packing of the part Fix: Increase hold pressure 3) Cause: Polymer too cold Fix: Increase melt temperature, Increase mould temperature, Increase injection speed 4) Cause: Multiple gates, Decrease the number of gates, Increase gate size, Change gate locations
Weld quality
Weld quality is controlled through the development of a welding procedure specification (WPS), which is supported by a procedure qualification record (PQR). A welder qualification test record (WQTR) provides documented evidence that an individual welder has a demonstrated ability to adhere to a WPS.
How would the strength-to-weight ratio of cold isostatic component vary with section thickness?
When components are cold iso-static pressured, uniform pressure is applied on the outside components. The particles close to the outside have higher density and hence higher strength. If section thickness is increased then the differential density from the wall to the core is increased. Thus relatively thinner components have more uniform properties across the section thickness and also higher strength components to components with wider section thickness.
Three factors that need to be accounted for when estimating cost of manufactured product
When estimating unit cost of manufacture, (i) materials cost (ii) tooling cost and (iii) labour/overhead rate and production rate needs to be considered.
Deep drawing: wrinkling
Wrinkling: Wrinkling can occur in the flange of a partly drawn cup of thinner sheet due to compressive (hoop) stresses arising from material occupying a decreasing circumference as it is drawn radially inwards / As material is drawn radially inwards, compressive (hoop) stresses are set up circumferentially around the flange. If the flange is insufficiently supported normal to its surface, then the material will buckle due to the compression and this results in out-of-plane wrinkling. To avoid this, a blankholder has to be used to provide restraining pressure normal to the flange.
'Product'
an object that has value (forming, matching) added to it during the production process
Photo polymers (SLA)
composed of photo-initiators, reactive diluents, liquid monomers etc.... When uv radiation impinged on resign, photoinitiators undergo chem transformation and become reactive with liquid monomers. starting polymer chain
Acrylate based resin (sla)
highly reactive but typically produce weak parts due to inaccuracy caused by shrinkage and curling. Low curing can increased stresses in layer
Solidification time equation
𝑆𝑜𝑙𝑖𝑑𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 ∝ 1 /(𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎)^2
