Manufacturing Processes Final Exam Review

Direct Laser Metal Sintering

1. Allows for localized, layered sintering 2. Used in 3D printing (additive manufacturing)

Methods for Powder Production

1. Atomization: molten stream broken up by jets of air, inert gas, or water 2. Reduction: Hydrogen or carbon monoxide used to reduce metal oxides to a metallic state by removing oxygen 3. Electrolytic Deposition 4. Carbonyls: Metal carbonyls are formed by allowing metal to react with carbon monoxide. The metal carbonyls are decomposed into small, dense, spherical particles of high purity metal. 5. Comminution: mechanical crushing, milling or grinding of metals into smaller particles 6. Mechanical Alloying: powders of two or more pure metals mixed in a ball mill

Continuous-Sintering Furnaces

1. Burn-off 2. Sintering 3. Cooling

High Energy Beam Welding

1. Electron-Beam Welding: heat generated by high-velocity, narrow-beam electrons. As the electron strikes the workpiece its kinetic energy is converted into heat. The level of vacuum used is responsible for how much the electron-beam penetrates into the part. No shielding gas, flux, or filler material is needed. Distortion and shrinkage in the weld area are minimal, and the weld quality is good with high purity. 2. Laser-Beam Welding: A high-powered laser beam is the source of heat that produces a fusion weld. A narrow diameter allows for deep material penetration. The laser beam may be pulsed for spot welding thin materials. An increased reflectivity of a material hinders the laser beam's penetration ability. Used to weld otherwise inaccessible locations, and can easily be automated. Higher quality than electron-beam welding with less tendency for incomplete fusion, spatter, porosity, and distortion. 3. High Energy Beam Welding produce much smaller weld beads than standard welding techniques, which is important for precision welding

Screw

1. Feed section: smallest section, brings material from hopper to melt section 2. Melt Section: heat generated begins melting the material 3. additional shearing and melting occur with the pressure buildup at the die.

Grinding

1. Grinding is a chip removal process 2. The grains are irregular and spaced randomly throughout the grinding surface 3. Very high cutting speeds 4. The average rake angle is highly negative (-60° or more) so low shear angles 5. porosity is required to allow for chip removal

Importance of Temperature Rise in Grinding

1. Grinding is a finishing step, so temperature effects on the surface finish must be considered 2. High grinding speed = high temp. 3. As in cutting, the chips remove most of the heat so the workpiece can typically remain below critical temperatures 4. Friction from sliding and plowing can raise temperature regardless of chip formation 5. Sparks observed in metal machining are metal chips oxidizing which is an exothermic reaction

Fusion Welding

1. Involves melting and re-coalescing materials by means of heat either electrically or high-energy means. 2. Typically involves a filler material to strengthen the bond 3. Preventing oxidation of the workpiece is a major consideration 4. Divided into three major categories •Oxyfuel welding •Arc welding •High-energy beam welding 5. Allows for fast, inexpensive welding (oxyfuel, GMAW), automated, high production rate welding (SAW, GTAW) or precision welding for small features (LBW)

Powder Metallurgy Steps

1. Powder Production: Converting bulk metals into powders 2. Blending: various metal powders may be blended to alter physical and/or mechanical properties 3. Compaction: Dies and presses compact powder into desired shape 4. Sintering: Heated enough for powders to fuse together, but not enough to melt the metal 5. Finishing Operations: methods to improve P/M

Ultrasonic Machining Design Considerations

1. Sharp edges will be eroded by the abrasive slurry 2. Holes produced will have tapered edges 3. Bottom may need a backup plate due to chipping at the exit of end holes

Compaction

1. Spherical powders with similar distributions do not compact well; require a binder 2. Uniformity can be enhanced by adding lubricants 3. The higher the density after compaction (green density), the greater the part will be able to resist external forces (higher strength and elastic modulus). Density may vary widely within the part due to friction present between metal particles in the powder and also between the punches and die walls.

Temperature Effects

1. Tempering: the large temperature rises can temper and soften the surface 2. Burning: possible to oxidize the surface or cause a chemical transformation 3. Heat checking: thermal stresses can cause cracks in the surface known as heat checking 4. Residual stresses: temperature gradients during grinding can cause residual stresses to develop on the surface

Electrochemical Machining Design Considerations

1. The electrolyte wears away sharp profiles (sharp corners)

Brazing Methods

1. Torch Brazing: a reducing oxyfuel flame is used to heat the joint, and then the braze metal is deposited 2. Resistance Brazing: the source of heat is resistance through the part; good for localized joint formation 3. Diffusion Brazing: the filler material is placed between the joints and held in a high temperature furnace. high production rate. 4. Furnace Brazing: the part is prepared and placed in a high temperature furnace. High production rate.

P/M Design Considerations

1. Uniform shapes are easier to manufacture 2. Parts should be easily ejected from the die without damaging the green compact. 3. Production costs may be reduced by using the widest allowable dimensional tolerances. 3. Large steps may require complex multiple-action tooling

Arc Welding

1. Uses electrical discharge to produce the high temperatures necessary for welding 2. Two Forms: Consumable or Non-Consumable electrode 3. An electrode is touched to the workpiece, which has been electrically charged. The electrode is then removed from the surface to generate an arc 4. Important piece of automated manufacturing; used for assembly lines

Duburring

1. Vibratory and Barrel-finishing 2.Shot blasting 3. Abrasive-flow machining 4. Thermal-energy Method 5. Robotic deburring

dielectric fluid

1. electrically nonconducting 2. acts as an insulator until the potential is sufficiently high 3. acts as a flushing and cooling medium for the process

Oxyfuel Gas Welding

1. fuel gas (typically use acetylene gas) is combined with oxygen to form a flame. Temperatures can reach 3300 C. 2. a small pool of molten metal forms at the surface of the workpiece. 3. This pool can be manipulated with the welding torch; it will flow to the hottest location on the metal 4. Metal from a filling rod (or welding rod) is added to the pool for extra strength 5. The outer flame protects the surface of the base metal and the welding rod from oxidation

Friability

1. measure of an abrasive's ability to break into smaller pieces 2. a friable abrasive will break as it begins to wear, making it self-sharpening

Machining Burrs

1. thin ridges, usually triangular in shape that develop on the edges of a workpiece 2. They may scratch surfaces, jam drilled holes, or present safety hazards 3. Burr Removal can be very expensive

Electrical-Discharge Machining (extra facts)

1. use with electrical conductors only 2. melting point and latent heat of melting of workpiece affect volume of metal removed per discharge 3. mechanical energy, hardness, strength, and toughness of the material do not necessarily affect the material removal rate 4. The electrode is normally made of graphite, but brass, and copper are sometimes used 5. Tool wear may be minimized by using copper tools and reversing the polarity 6. The wear rate of the cutting tool is much less than that of the workpiece 7. Shape of the tool may vary to cater to complex internal and external geometries 8. The high MRR causes a rough surface finish, but pulsed EDM may be used to improve overall quality of surface finish

Selective Laser Sintering

A CO2 laser is used to selectively scan the necessary regions of the deposited powder (plastic, metal or ceramic) layer in order to bind the particles into a desired cross-sectional geometry. Instead of a liquid polymer, powders of different materials are spread over a platform by a roller. A laser sinters selected areas causing the particles to melt and then solidify.

Shielded Metal Arc Welding (SMAW)

A coated, consumable electrode is used to generate the arc. The electrode and the base metal form the weld, which is protected from oxidation by the vaporized coating. DC power in straight polarity (positive workpiece) gives shallow penetration, reverse polarity gives deeper penetrations. AC pulsates the arc rapidly, and is best for thick metal plates

Bauschinger Effect

A material may be strain softened by applying a tensile load large enough to reach a material's plastic range and then applying the same compressive load. The yield stress in compression is lower than that in tension under the same loading conditions. This yield stress will continue to decrease with the number of cycles until failure. This is also true if the loading path is reversed.

Laminated Object Manufacturing

A new layer (laminate) is rolled and pressed over the previous layer for good adhesion and, subsequently, selectively cut via a CO2 laser.

Joint Clearance

A smaller gap creates higher shear strength in the joint. There is an optimal gap to achieve the maximum tensile strength. Surface roughness of the mating surfaces is also important due to small clearances.

Squeeze Casting

Combination of forging and casting. molten metal solidified under high pressure. Near-net-shape process using less pressuring than hot or cold forging.

Sintering

Compacted metals are heated in a controlled environment below the melting temperature (70% - 90% of Tmelt) to bond particles together. This improves strength and density within the part. Dimensional accuracy is the main concern because in solid-phase sintering particles bond by diffusing into each other, and shrink the part. Too much sintering causes large dimensional changes. Liquid or Vapor-phase sintering causes the particles to physically melt (if one particle has a lower melting temp than the other) or start vaporizing into each other forming a larger interface between the two particles. Shrinkage does not occur in these sintering methods.

Cooling Rate Effects

Cooling rate affects grain size. Fast cooling rate = fine crystal structure with small dentritic arms. Slow cooling = coarse dendritic structures. Small/fine grain structures promote strength and ductility along with preventing microporosity and hot tearing in casting.

Thermoset

Cross-linked materials that have strong covalent bonds and are very brittle. They are non-reversible.

Three Dimensional Printing

Depositng a layer of powder by dispensing measured quantity of powder from supply chamber. Roller compresses the powder at he top. Uses Inkjet print head (instead of ink, liquid adhesive or binder) to create patern onto the powder layer. Full color prototypes may be made, but parts may be somewhat porous affecting the strength. Metal infiltration and sintering can help develop fully dense parts. This combination forms an efficient approach to rapid tooling.

Flame Types

Different proportions of oxygen to acetylene cause different flame types. 1. neutral: Ratio is 1:1, perfect balance between gas and oxygen. 2. Oxidizing Flame: More oxygen than gas, so the flame becomes harmful to steels as it can oxidize the surface. Only used with copper based alloys because the flame forms a thin protective layer of slag (oxide compounds) over the metal. 3. Reducing (Carburizing) Flame: Insufficient oxygen causes the flame to be cooled and unable to fully combust. Used for applications requiring less heat such as brazing, soldering, and flame hardening operations.

Vulcanization

Elastomers (rubber) may be cross-linked to improve hardness...Natural rubber deteriorate after a few days due to sunlight and UV rays. Polymer molecules are cross-linked to be more durable. Actual chemical cross-linking is done with sulfur.

Electrochemical Machining (ECM)

Electrolyte current passes through canals in the tool and carries current that washes metal ions away from the workpiece. It may only be used on conductive materials. Some benefits include the lack of burrs, thermal stresses, and tool wear. Used in the production of complex cavities in high strength materials. Often applied in the aerospace industries for turbine blades, jet engine parts, and nozzles. May even be used as a deburring process.

Hot Isostatic Pressing

High pressure and temperature are used to produce essentially 100% density, strong bonding among particles, and good mechanical properties. Powder housed in high melting temp sheet metal, while inert gas or vitreous (glasslike) fluid is used as the pressurizing medium. Advantageous because the lack of friction produces fairly uniform grain structure and density, irrespective of particle shape. Some deficiencies include wider dimensional tolerances, more time is required, and expensive.

Particle Size Distribution

Important bc it affects the processing characteristics of the powder. Typically the powder is passed through a series of filters to achieve the desired maximum size.

Soldering

Lower temperatures (<450°C) than in welding/brazing. Produces low strength connections, unsuitable for load bearing joints. Solders are generally tin or lead based. Soldered joints are used extensively in electronics. Solderability is dependent on wettability of the surface and the formation of oxide layers.

Shape Index (Shape Factor)

Measurement of Surface Area to Volume of particle with reference to a spherical particle (a sphere has a smaller shape index than a flake)

Definition of Powder Metallurgy (P/M)

Metal powders are compacted in dies and sintered to a near-net-shape product

Electrical Discharge Machining Design Considerations

Minimize amount of material removal, because EDM is slow and costly

Blow Molding

Molding process in which air pressure is used to inflate soft plastic into a mold cavity. Important for making one piece hollow plastic parts with thin walls. Very high quantities. Limited to thermoplastics (HDPE, PP, PVC, PET etc.). Accomplished in two steps: 1. Fabrication of a starting tube, caled a parison 2. Inflation of the tube to desired final shape Forming the parison is accomplished by either Extrusion (used to produce corrugated pipes) or Injection molding (used to produce plastic beverage bottles, and hollow containers).

Spray Casting

Molten metal is atomized and deposited on a cooled preform mold where it solidifies. The metal particles weld together developing a density higher than 99% of the solid metal density. The grain size of the part is fine, and the mechanical properties are comparable with those of wrought products with the same alloys.

Soldering Methods

Most are very similar to brazing. 1. Torch soldering 2. Furnace soldering 3. Iron soldering 4. Resistance Soldering 5. Reflow Soldering: used for mounting components onto printed circuit boards. filler material/flux paste pushed over screen onto base material. 6. Wave Soldering: used for automated soldering of printed circuit boards. A laminar wave of molten solder is generated by a pump and introduced to the exposed surface. The solder only sticks to preheated metal, leaving the rest of the circuit board bare.

Cold Isostatic Pressing

Part is sealed in a mold, which is placed in an isostatic chamber. Liquid (typically water) acts as the pressure source. Wet-Bag method is easier to design, but dry-bag method is used for higher production rates.

Brazing

Performed at lower temperatures than welding, a filler material is placed at or between the faying surfaces to be joined. The filler material is melted without the base material melting, and closely fits the space by capillary action. Once the filler material cools, a strong joint is developed. May be done using a reducing flame. filler material may be coated with flux to reduce oxidation as it does not stick well to an oxidized layer. The flux may be applied via paste, slurry, or powder, but must be removed after brazing because it may be corrosive.

Blending

Powders with different size distributions can be mixed for an alternate distribution to try to improve part uniformity. Lubricants can be mixed in to the powder to ease flow and reduce die pressures and die wear. Binders can be added to increase the green (pre-sintered) strength of the part.

Die Casting

Process can be highly automated. High production rates are possible with good strength and high quality parts and complex shapes. Little to no finishing or machining is required due to good surface finish and close dimensional tolerance. Cooling rates are much faster than expandable mold casting causing finer grain structures and higher quality parts.

Solidification

Pure metals have clearly defined melting or freezing points, and solidification takes place at a constant temperature. The latent heat of fusion is given off as molten metal has cooled to its freezing point without temperature change. Alloys solidify over a range of temperatures. Solidification begins when the temperature of the molten metal drops below the liquidus and is completed when the molten metal reaches the solidus. Between these two temps the alloy is pasty or mushy.

Finishing

Re-pressing: Improve surface finish or dimensional accuracy Forging: Unsintered or sintered workpieces (preforms) are forged under high pressure Impregnating: Fluids (e.g. oil) enter the porous P/M part Infiltration: A metal slug is melted into the P/M part

Gas Metal Arc Welding (GMAW)(MIG)

Shielding gas is fed to the welding region to protect against oxidation. Consumable wire is fed through a nozzle into the weld arc. Allows for higher production rates than SMAW. Metal is transferred from the tip via spray transfer, globular transfer or short circuiting. (susceptible to air current)

Gas Tungsten Arc Welding (GTAW)(TIG)

Similar to GMAW but non-consumable tungsten electrode is used meaning a filler rod is necessary. The filler material must be similar to that of the welded metal. AC power may be used for aluminum or magnesium because it removes oxides and improves weld quality. Thorium or zirconium may be used in tungsten electrodes to improve their electron-emission characteristics. Contact of the electrode to the molten metal must be avoided because it may contaminate the tungsten electrode and cause discontinuities in the weld. The need for an inert gas shield makes GTAW expensive, but it provides a high quality weld with good surface finish.

Diffusion Bonding (Welding)

Solid-state joining process in which the strength of the joint depends upon the diffusion of the atoms across the interface, and the plastic deformation of the faying surfaces. Executed at about 0.5Tmelt to ensure high diffusion rate. The bond strength depends on the pressure, temperature, time of contact, and cleanliness of the faying surfaces (filler material may be used to lower requirements). The parts are heated in a furnace and the diffusion rate dictates the bonding time. This process may be combined with superplastic forming lightweight reinforced structures.

Extrusion Defects

Surface Cracking: due to high temperature, speed, or friction during the process. May also be found in low temp extrusion if the billet sticks to the die. A very large increase in extrusion force moves the billet forward, but it then sticks to the die again. Increasing extrusion speed may help. Extrusion Defect: Dead zones formed by too much friction propagate through the billet rendering most of the material useless as impurities are drawn towards the center of the part. Internal Cracking: internal cracks may be formed by hydrostatic tensile stress in the center of the billet. The die angle, extrusion ratio (reduction in cross section), and friction are the main variable causing or stopping internal cracks from forming.

Barreling

Tendency for workpiece to bow at the sides during upsetting. Caused by frictional forces at the die-workpiece interface.

Powder Rolling

The powder is fed into the roll gap in a two-high rolling mill. It is compacted into a continuous strip. Used for producing electrical and electronic components and coins.

Blending Precautions

The powder may suffer from deterioration if it is mixed too much. The powder particles may change shape and become work hardened causing them to be more difficult to compact. Also the high Surface area to Volume ratio causes metal powders to be highly explosive. Grounding equipment, preventing the formation of sparks, and avoiding dust clouds, open flames, and chemical reactions are imperative to staying safe.

Front and Back Tension (Rolling)

The roll force may be reduced by a longitudinal tension. Tensile forces can be applied at the entry or exit of the part. Front tension is controlled by the torque on the delivery reel around which the rolled sheet is coiled. Back tension is controlled by a brake system in the uncoiler (payoff reel)

Reaction-Injection Molding

Two or more liquid reactants are forced into the mold cavity. chemical reaction causes the thermosets to solidify.

sterolithography

a laser cures a liquid photopolymer by providing the necessary energy for polymerization. A vat is filled with photocurable liquid acrylate photopolymer and a platform that can be raised or lowered. The liquid polymerizes when exposed to laser light. The platform is slowly lowered from the surface of the liquid as the laser cures the photopolymer one layer at a time. This process takes a long time but can form many objects. The part must be ultrasonically cleaned in an alcohol bath, and exposed to UV radiation for up to a few hours to ensure hardness of the polymer part.

Fused Deposition Modeling

a thermoplastic filament is extruded from an extruder head capable of moving in two directions. The table can be raised or lowered as needed. The filament superimposes onto the first extruded layer until the part is finished. Support material is often needed for complex parts. Surface roughness can also be an issue.

Shot blasting

abrasive particles are propelled at the surface. Typically results in a matte surface finish.

Abrasive-flow machining

abrasives in a putty are forced at high pressure through internal cavities, and may also act on the outer edges of the workpiece if it is held within a fixture.

chill zone

at the mold walls metal cools rapidly, and the casting develops a solidified skin of equiaxed grains.

Robotic deburring

automated systems that may use various techniques. Initial cost is very high, but allows for reduced cost in manual labor over time.

Electrical-Discharge Machining (EDM)

based upon the erosion of metals by spark discharges. It consists of an electrode (the tool) and the workpiece. They are placed in a dielectric fluid and given a DC power source. A bridge forms between the workpiece and electrode allowing the flow of current. An intense electrical arc is formed melting some of both the workpiece and tool. The arc becomes interrupted as the evaporation of the dielectric fluid causes the resistance between the tool and workpiece to increase significantly. The interruption causes a shock wave that flushes debris away from the workpiece and entrains the molten metal in the dielectric fluid.

Shell Molding

close dimensional tolerance, good surface finish, and cheap along with flexible. Sand bonds to parting agent on metal pattern to create shell mold. Thin shells allow gases to escape during solidification. The process can be automated fairly easily.

Polyjet

eight print heads deposit a photopolymer on a build tray. Ultraviolet bulbs instantly cure and harden each layer eliminating any postmodeling curing. Better resolution and easier part cleanup are associated with the polyjet method.

Forging Defects

excess material in the web may buckle and develop laps. If the web is too thick excess material flows past the forged portions and may develop internal cracks. Small die radii may cause incomplete forging or overlapping of material.

Die Swell

extruded product is larger than the die opening due to pressure relief.

Braze Welding

filler material deposited at the joint. Much more filler material is used than in brazing. Part distortion is minimal and flux is imperative for proper joint strength. It is usually only used to repair or maintain a part. Useful for joining dissimilar materials, as there is less thermal distortion and stress.

Sand Casting

fine grains so sand can be closely packed. Fairly permeable so entrapped gasses may escape. The mold must have good collapsibility so the cast can shrink without any defects.

Filler Metal

generally melt above 450C, but below the melting point of the base material. Localized grain structure may be somewhat affected. Typically have significantly different composition than the base metal. Embrittlement of the joint, brittle inter-metallic compounds at the joint, and galvanic corrosion in the joint may be caused by poor Braze metal selection. Also, due to diffusion of the filler material and base material, the metallurgic properties of the joint may change over time.

Columnar Zone

grains grow away from the mold surface. They all grow in one direction unlike equiaxed grains, giving them direction mechanical properties.

Upsetting

height of workpiece being reduced between two dies

Thermal-energy Method

high temperature flashes (>3300°F) melt the burrs but not the workpiece. This method does not affect the surface finish at all.

Investment Casting (lost-wax)

labor and material use are intensive, but little to no finishing is required. This process can create intricate shapes, and is suitable for casting high melting point alloys with a good surface finish and close tolerances.

Spark Sintering (Spark Plasma Sintering)

loose metal powders are placed in a graphite mold, heated by electric current, subjected to high-energy discharge, and compacted, all in one step.

endurance limit

max stress to which material can be subjected regardless of cycles without any failure

Stress Amplitude

max subjected stress

Equiaxed zone

middle of casting consisting of random crystal growth. Expanding the equiaxed zone is beneficial and may be done by lowering pouring temps, adding inoculants, or adding alloying elements.

Wire EDM

operates similar to a bandsaw except without teeth. A slow moving wire travels along a prescribed path cutting the workpiece via discharge sparks acting as the cutting teeth. The wire should have sufficient tensile strength and fracture toughness. It also should have high electrical conductivity (brass, copper, tungsten). Modern CNC machines monitor wire health while cutting, to improve efficiency and reliability.

Creep

permanent elongation of a material under a static load maintained for a period of time. There is instantaneous deformation, primary, secondary, and tertiary stages before the part ruptures due to creep. The secondary region is longer and has a constant slope that is useful in designing components for a specific creep life.

Injection Molding

plunger or screw pushes molten material into a split die chamber. Complex shapes and good dimensional accuracy may be achieved. A high production rate may also be achieved. Oversized molds must be used because plastics shrink a lot when cooled.

Extrusion

raw thermoplastic materials in the form of pellets, granules, or powder are place into a hopper and fed into the extruder barrel. A screw inside the barrel blends and conveys the pellets towards the exit of the barrel. internal friction along with shear stresses from mechanical action of the screw liquifies the pellets. The screw also helps build pressure inside the barrel. The material is then pushed out of the barrel through a filter and die.

Residual Stress

remnants of loading. internal stress that keeps the part in equilibrium. Tensile is bad (lower fatigue life and fracture strength, and may lead to stress cracking), compressive is good. May be relieved by further deformation (plastic) or part may relax over time dependent upon the temperature

Fatigue

repeatedly subject specimens to various states of stress. Provides max tension, then compression, then compression until failure.

Hardness

resistance to permanent indentation, scratching, or wear

Particle Shapes

size and shape of particles is dependent on the metal and the production method, and has an impact on flow under pressure and ability to be sintered

Ultrasonic Machining

small abrasives housed by a water slurry are located at the tip of the tool. The tool is vibrated at a frequency of 20 KHz transmitting high velocity to the abrasive grains. very good dimensional tolerances may be held in this process. Microchipping also occurs during this process.

Precision Forging

suitable for softer metals like aluminum and magnesium alloys because the required forging loads and temperatures are relatively low and do not cause much wear on the dies.

Solderability

the ability for a metal to be soldered. Copper and precious metals such as silver and gold are easy to solder. Most other metals are difficult to solder for different reasons. Some have an oxide film that makes soldering difficult, but coating such metals can improve solderability drastically.

Aspect Ratio

the ratio of the largest dimension to the smallest dimension of the particle

Injection Foam Molding

thermoplastics are mixed with blowing agent (inert gas) to expand the polymer. The core of the part is cellular and the skin is rigid. Part densities are as low as 40% of solid plastic.

Vibratory and Barrel-finishing

used to improve surface finish and remove burrs from large numbers of relatively small parts. Many small parts are placed in an abrasive filled container that is typically vibrated. It can be performed both dry and wet.

Thermoplastic

weak secondary bonds hold them together (linear, branched). May be heated to liquid consistency, and then shaped. This process may occur an infinite amount of times without losing hardness or strength (reversible)