Modern Manufacturing Processes

Electrochemical machining

"Reverse" Electroplating •MRR = C*I*η •Limited to electrically conductive materials •Does not create: •Burrs •Thermal stresses •Tool wear

Baushinger Effect

(Strain Softening) Note the decrease in yield stress in compression after the specimen has been subjected to tension. The same result is obtained if compression is applied first, followed by tension, whereby the yield stress in tension decreases.

Sprue

A channel through which metal or plastic is poured into a mold.

Shielded Metal Arc Welding

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 •Straight polarity (positive workpiece) gives shallow penetration, reverse polarity gives deeper penetrations, AC is best for thick metal plates

Extrusion

A compressive forming process in which the material is forced to go through a die opening in a shape of desirable crosssection Advantages: •Uniform cross-section •No waste of material

Friction hill

A friction hill refers to the coefficient of friction reaching its peak if you look at a graph of the role length of contact vs. role pressure

Parting line

A parting line is a mold making piece where two or more parts of the mold meet

Ingot preparation

A seed crystal with the desired properties is mounted to the end of a puller rod •The seed crystal is lowered into molten silicon, then slowly removed resulting in an ingot made of a single crystal •Larger ingots increase manufacturing efficiency •Industry standards are 200 mm and 300 mm diameter ingots, but 450 mm diameter ingots are scheduled Finishing Operations on Silicon Ingot •The ingots then go through a number of finishing steps (grinding, polishing) and are then sliced into wafers •These wafers are typically ~1 mm thick to allow for sufficient mechanical strength for the rest of the manufacturing process

Injection blow molding

A short tubular piece is first injection molded. The dies are then opened, and the parison is transferred to a blow-molding die.

Extrusion blow molding

A tube is extruded, usually vertically, clamped into a mold with a cavity much larger than the tubes diameter, and then blown outward to fill the mold cavity much larger than the tube's diameter, and then blown outward to fill the mold cavity. Blowing is usually done with an air blast, at a pressure of 350 to 700 kPa.

Abrasive jet machining

Abrasives (e.g. boron nitride) are added to a high pressure gas stream •The nozzles must be made of an abrasive resistant material (e.g. tungsten carbide) •Useful for material removal, deburring and finishing

Hardness vs. Friability

Abrasives are extremely hard; often 3-4 times harder than standard cutting tools •Friability is a measure of an abrasive's ability to break into smaller pieces •This is important, because a friable abrasive will break as it begins to wear, making it self-sharpening •When selecting an abrasive, you must balance a material that is hard enough to affect the workpiece but friable enough to self-sharpen •Example: mined diamonds vs. laboratory diamonds

Sand Casting pros and cons

Advantages • Almost any metal cast • No limit to the size, shape or weight • Low tooling cost • Disadvantages • Some finishing required • Coarse finish

Machining pro's and con's

Advantages Disadvantages •Good dimensional accuracy •Complicated geometric features possible •Economical for producing limited number of parts (i.e. prototyping) •Wastes material •Slow •Requires more energy •Can affect the fatigue life of the part

Die casting pro's and con's

Advantages: • High production rates (1000/hr) • Good strength • Intricate shapes • Dimensional precision • Excellent surface qualities • Small-medium sized castings • Disadvantages • Limited to metals with low melting temperatures • Part geometry must allow removal from the die

Lost foam (lost wax) casting

Advantages: • Pattern need not be removed from the mold • Simplifies and expedites mold-making, since two mold halves (cope and drag) are not required as in a conventional greensand mold • No parting lines, cores, risers -> design flexibility • Automated mass production of castings for automobile engines • Disadvantages: • A new pattern is needed for every casting • Economic justification of the process is highly dependent on cost of producing patterns

Riser

Also called feeders, serve as reservoirs of molten metal to supply the metal necessary to prevent shrinkage (which could lead to porosity).

Water Absorption in thermoplastics

An important limitation of some polymers such as nylons is their ability to absorb water. Water acts as a plasticizing agent, thus, in a essence, it lubricates the chains in the amorphous region. Typically, with increasing moisture absorption, the glass-transition temperature, yield stress, and elastic modulus of the polymer are lowered severely.

Chill zone

At the mold walls, the metal cools rapidly (chill zone) because the walls are at an ambient or slightly elevated temperature, and as a result, the casting develops a solidified skin (shell) or fine equiaxed grains

Diffusion

At this point we have a semiconductor wafer that needs to be doped to create the electrical conditions for a transistor Diffusion and Ion Implantation •The performance of the exposed substrate can be altered at this stage by introducing dopants •This is done using two main processes, diffusion and ion implantation •Diffusion: •produces a dopant gradient high at the surface but lower toward the middle •a more uniform dopant gradient can be achieved by elevating the temperature, called drive-in diffusion •Ion implantation uses high energy ions that are shot at the surface, allowing more control but expensive and requires more post-processing (annealing, etc.)

Grinding wheel wear

Attritious Wear •The cutting edge becomes dull by attrition •A complex process involving diffusion, so dependent on the affinity between the abrasive and the workpiece •Grain Fracture •Grains are hard but brittle, so will catastrophically fail •In moderation, this process allows grains to be selfsharpening •Bond Fracture •Bonds must be strong enough to hold chips in place during grinding •Dull grains should be able to be dislodged

Creep and stress relaxation in Thermoplastics.

Because of their viscoelastic behavior, thermoplastics are particularly susceptible to creep and stress relaxation

Broaching

Broaching is a process for creating specialized shapes (e.g. keyholes) in the workpiece •Requires a press with large forces (up to 0.9 MN)•The broach cutting tool is designed to remove the desired volume in a single pass •The total volume is equal to the amount removed per tooth •Can be done in a push or pull step; push requires less force but is more unstable

Machining burs

Burrs form during a variety of manufacturing processes: •Casting •Forging •Machining •Burrs can scratch surfaces, jam drilled holes or present safety hazards •Removing burrs (deburring) can be a significant portion of manufacturing expense

Front and back tension

By controlling the speed of the "uncoiler" relative to the rolling speed and "coiler", front and back tensions are created which are used to reduce rolling loads • A reduction in rolling loads results in • less wear of rollers • improved sheet flatness • Uniformity of thickness

Sand Casting vocab

Chaplet - metal supports (usually same as molten metal) Core - internal cavities and passages

Drilling

Chips cannot easily leave the machined area •High length-todiameter ratio Excessively deep holes (> 8:1 length:diameter) are challenging to machine and should be avoided

Cold chamber die casting

Cold Chamber (Pressure of 20 to 140 MPa) • Molten metal into shot chamber (not heated) • Aluminum, magnesium, copper alloys

Oxyfuel welding

Combination of a fuel gas (often acetylene) and oxygen Mechanics of Oxyfuel Welding •Flame is applied to the metal to produce a small pool of molten metal •This pool can be manipulated with the welding torch; it will flow to the hottest location on the metal •Metal from a filling rod (or welding rod) is added to the pool •The flame protects the surface of the base metal and the welding rod from oxidation

Electrochemical grinding

Combines grinding and electrochemical machining •The insulate the workpiece from the grinding wheel •Removes surface layers with very little pressure required •Low wear on the abrasives

Thermal and electrical properties of Thermoplastics

Compared with metals, plastics generally are characterized by low thermal and electrical conductivity, low specific gravity, and a relatively high coefficient of thermal expansion

Conventional vs. climb milling

Conventional Climb •Width of chip starts from zero •Upward forces are created that lift the workpiece away •Surface finish is worse because chips are dropped in front of cutter •Increases tool wear •Width of chip starts at maximum thickness •Downward forces are created •Surface finish is better because no chips in the way •Decreased tool wear •Potential for catastrophic failure

Three Dimensional (3D) Printing

Depositng a layer of powder by dispensing measured quantiy of powder from suply chamber. Roler compreses the powder at he top. Uses Inkjet print head (instead of ink, liquid adhesive or binder) to create patern onto the powder layer

Wafer identification

During the ingot finishing process, a notch or flat is added to indicate the semiconductor properties •Also possible to laser etch a marking into the wafer containing manufacturing information (manufacturer, date, etc.)

Bonded Abrasives

Each grain removes a small amount of material, so requires many grains working together •Typically held together by bonding material to form a grinding wheel •Some porosity is required to allow for chip removal

PM Design and Economic Considerations

Easier to manufacture uniform shapes •Parts should be designed for ejection from the mold •Large steps may require multi-action presses Economic Considerations for P/M •Costs may be 1.5 to 7 times more than cast or forged parts of equivalent size/weight •The major expenses in P/M are the powder, the press and the furnace •Powder cost is proportional to the type of metal, the particle size and the distribution •The press requires large forces, and complex parts may require multiple-actions •Furnaces may require moderately high temperatures for long periods of time •Typically reserved for parts that cannot be manufactured in other methods

Etching

Etching is used to remove films and/or substrate •Selectivity refers to the ability to remove certain material without removing other (e.g. removing silicon dioxide without removing the silicon) •Can be broadly separated into wet and dry processes •Wet: less expensive but results in isotropic etching •Dry: technically complicated but more control over etch direction

Filing

Filing is small scale removal of material from a surface, corner, edge, or hole. Files are usually made of hardened steels and are available in a variety of cross sections, including flat, round, half-round, square, and tri-angular.

Extrusion force

Force depends on • Strength of the billet material • Extrusion ratio • Friction • Process (i.e. temperature, speed)

Fused deposition modeling

Fused Depositon Modeling (FDM): A filament of thermo- plastic polymer is fed into a heated extruding head and selectively deposited for the direct building of layered parts. Each layer thicknes: 120 ~ 30 microns

Gating system

Gating system: network of channels that delivers the molten metal to the mold

Grinding design considerations

General Grinding •Minimize grinding by using near net shape manufacturing •Design sections that need to be ground so that they can be securely mounted and supported •Parts that need high dimensional accuracy should have primarily flat surfaces to minimize vibrations Cylindrical Grinding •Parts should be internally axisymmetric to minimize vibrations Centerless Grinding •Short pieces may be hard to grind, so cut after grinding when possible

Columnar

Grains that have a favorable orientation, grow preferentially

Submerged Arc Welding

Granular flux is fed to the weld zone to protect from oxidation and create thermal insulation allowing deeper heat penetration •Consumable electrode tip •Specialized process for high production rates

Grinding economic considerations

Grinding as a finishing operation adds considerable expense •Much of the expense comes from the need to be done by hand and the relatively low speeds that finishing is done at •Newer machines are capable of reducing human interaction but require large capital outlays

Mechanics of grinding

Grinding is a chip removal process •The grains are irregular and spaced randomly throughout the grinding surface •Very high cutting speeds •The average rake angle is highly negative (-60° or more) so low shear angles

Temperature rise in grinding

Grinding is often used as a finishing step, so it is particularly important to consider temperature effects on surface finishes •High grinding speeds mean high temperatures •As in cutting, the chips remove most of the heat so the workpiece can typically remain below critical temperatures •Friction from sliding and plowing can raise temperature regardless of chip formation •Sparks observed in metal machining are metal chips oxidizing which is an exothermic reaction

Water-jet machining

High pressure water (>400 MPa) is directed through a nozzle to the workpiece •Large surface forces but low temperature rise •Useful for a variety of materials (metals, polymers, ceramics)

Rapid prototyping

Host of related technologies that are used to fabricate physical objects directly from CAD data sources. Additive manufacturing: Ad and bond materials in layers to form objects (each layer: ~10 to 50 microns) Also caled as solid freform fabrication and layered manufacturing

Hot chamber die casting

Hot Chamber (Pressure of 7 to 35 MPa) • The injection system is submerged under the molten metals (low melting point metals such as lead, zinc, tin and magnesium)

Precision forging

In precision forging, special dies are made to a higher accuracy than in ordinary impression-die forging. This operation requires higher capacity forging equipment than do other forging processes because of the higher stresses required to precisely form the part. Aluminum and magnesium alloys are particularly suitable for precision forging because the forging loads and temperatures required are relatively low, die wear is low, and the forgings have good surface finish. The choice between conventional forging and precision forging requires an economic analysis. Although precision forging requires special dies, much less machining is involved because the part is closer to or at the desired final shape.

Centrifugal casting

Inertial forces due to spinning distribute the molten metal onto the walls of the mold cavity • True centrifugal casting • Dry-sand, graphite or metal mold can be rotated horizontally or vertically • Exterior profile of final product is normally round • Gun barrels (cannon), pipes, tubes • Interior of the casting is round or cylindrical • If the mold is rotated vertically, the inner surfaces will be parabolic

Injection molds

Injection molds have several components, depending on part design, such as runners, cores, cavities, cooling channels, inserts, knockout pins, and ejectors. There are three basic types of molds: a: Cold-runner two-plate mold, which is the basic and simplest mold design. b: Cold-runner three-plate mold, in which the runner system is separated from the part after the mold is opened. c: Hot-runner mold, also called runnerless mold, in which the molten plastic is kept hot in a heated runner plate Injection molding is a high-rate production process, with good dimensional control. Typical cycle times range from 5 to 60 s but can be several minutes for thermosetting materials

Laminated Object Manufacturing

Laminated Object Manufacturing (LOM): A new layer (laminate) is roled and presed over the previous layer for god adhesion and, subsequently, selectively cut via a CO2 laser.

Streolithography

Laser lithography: A UV laser-beam scans the surface of a vat of photosensitve resin (typicaly, acrylic or epoxy based) to selectively cure necesary regions. Light- beam delivery is achieved via fast and acurate, servo- controled galvanometer mirors

Selective Laser Sintering

Laser sintering: A CO2 laser is used to selectively scan the necesary regions of the deposited powder (plastic, metal or ceramic) layer in order to bind the particles into a desired cros-sectional geometry:Instead of a liquid polymer, powders of diferent materials are spread over a platform by a roler. A laser sinters selected areas causing the particles to melt and then solidify. Materials being used or investigated include plastics, wax, metals, and coated ceramics.

Drag

Lower half of casting mold

Rolling

Metal is deformed to reduce the thickness (or cross section) of a long workpiece by the action of compressive forces.

Types of Abrasives

Naturally occurring abrasives exist (e.g. quartz, garnet, diamond but may contain impurities that make their performance unpredictable •Synthetic abrasives (e.g. aluminum oxide, diamond) tend to be harder than naturally occurring abrasives, but are also typically more friable

Forging

Oldest known metalworking process • A process of shaping materials into components using solid-state methods • The stresses needed to make solids flow are very high and require a lot of energy and strong tooling • Due to costs involved, forging is only viable for large production volumes

Upsetting

Open-die forging typically involves placing a solid cylindrical workpiece between two flat dies (platens) and reducing its height by compressing it, an operation know as upsetting.

Oxidation

Oxides perform a critical role in semiconductor performance •When grown from the substrate, (e.g. silicon dioxide) this process is called oxidation •Dry oxidation: high temperature •Wet oxidation: elevated temperature and water vapor

Injection molding

Pellets, or granules, are fed into a heated cylinder, where they are melted. The melt is then forced into a split-die chamber, either by a hydraulic plunger or by a rotating-screw of an extruder. As the pressure builds up at the mold entrance, the rotating screw starts to move backward, under pressure, to a predetermined distance, thus controlling the volume of material to be injected. The screw then stops rotating and is pushed forward hydraulically, forcing the molten plastic into the mold cavity.

Steps of PM

Powder Metallurgy consists of the following steps: •Powder production: methods for converting bulk metals into powder •Blending: various metal powders can be mixed together to change the physical and mechanical properties •Compaction: powders are pressed into shapes using dies and presses •Sintering: moderate temperatures (<Tmelt) fuse the powders together •Finishing: various methods for improving powdermetallurgy products

Variables in rolling

Roll Diameter • Deformation resistance of the metal • Friction between rolls and work piece • Presence of front and back tensions in sheet

Sawing

Sawing is a cutting operation in which the tool consists of a series of small teeth, with each tooth removing a small amount of material. Saws are used for all metallic and nonmetallic machinable materials, and they are also capable of producing various shapes. The width of cut (kerf) in sawing is usually narrow, and hence sawing wastes little material.

Centrifugal casting variations

Semi-centrifugal casting • Slower than true centrifugal casting • Several molds may be stacked on top of one another • Used for gear blanks, pulley sheaves, wheels, impellers, etc. • Centrifuging • Uses centrifugal acceleration to force metal into cavities that are offset from the axis of rotation

Shrinkage

Shrinkage in a casting causes dimensional changes and sometimes cracking and is a result of the following phenomenon: 1. Contraction of the molten metal as it cools before it begins to solidify. 2. Contraction of the metal during phase change from liquid to solid (latent heat of fusion). 3. Contraction of the solidified metal (the casting) as its temperature drops to ambient temperature The largest amount of shrinkage occurs during cooling of the casting.

Polyjet

Similar to inkjet printing: printing photopolymer and UV light immediately cure and harden each layer. Similar to SLA (similar resins), but no lengthy post proces curing operation, smaler layer thicknes (as smal as 16 microns) -> beter resolution

Solidification

Solidification begins when the temperature of the molten metal drops below the liquidus and is completed when the temperature reaches the solidus

Cutting fluid

The cutting fluid provides several major functions: •Cools the tool and surface, reducing thermal issues (expansion, fatigue, diffusion) •Lubricates the surface, reducing friction •Reduces the forces/power required •Washes away chips •Typically done either by flood cooling or mist cooling •Has major environmental and health & safety impacts, so increasingly the trend is to minimize the use of cutting fluid (Near Dry Machining (NDM) or Dry Machining) •Cutting fluid is less necessary at high manufacturing speeds. Why?

Impression die forging

The flash helps to ensure the part completely fills the mold, but requires machining to finish the part

Laser machining

The laser beam locally heats the material sufficient to melt and/or evaporate the material •Effective on a variety of materials, including nonconductive materials •A gas stream can be added to prevent oxidation and remove vaporized materials from the surface

Slab analysis

The slab method is one of the simpler methods of analyzing the stresses and loads in forging as well as other bulk deformation processes. This method requires the selection of an element in the workpiece and identification of all the normal and frictional stresses acting on the element.

Barreling

The specimen develops a barrel shape during upsetting. Barreling is cause primarily by frictional forces at the die-workpiece interfaces that oppose the outward flow of the material at these interfaces. Barreling can also occur in upsetting of hot workpieces between cool dies. The reason is that the material at and near the die-specimen cools rapidly, whereas the rest of the specimen remains relatively hot. Since the strength of the material decreases with increasing temperature, the upper and lower portions of the specimen show a greater resistance to deformation than does the center. Due to barreling, deformation throughout the specimen becomes nonuniform or inhomogeneous.

Heat transfer in Arc Welding

The weld speed is dependent on the material (through u), the equipment (through V*I) and the process (through e) •Other considerations (such as thermal conductivity and thermal expansion) affect weldability (discussed in Wednesday's lecture)

Advanced machining design considerations

Ultrasonic Machining •Sharp edges will be eroded by the abrasive slurry •Holes produced will have tapered edges Chemical Machining •Difficult to produce constant cross-sectional areas because of the isotropic etch Electrochemical Machining and Grinding •The electrolyte wears away sharp profiles (sharp corners) Electrical Discharge Machining •Minimize the amount of material to be removed by EDM Laser-Beam Machining •Dull/unpolished surfaces are preferrable

Cope

Upper half of casting mold

Shell Mold Casting

Uses a minimum quantity of sand • Heat the metal pattern • Dry sand particles are held by a thermosetting resin binder (2.5-4% • Rapid cooling rate produces a fine grain structure than normal sand casting

Surface micromachining

Uses spacer layers (applied using techniques like CVD) to control etch rates •Allows for more complex shapes and uses less of the substrate

Thermosets

When a long-chain molecule in a polymer are cross-linked in a three-dimensional arrangement, the structure in effect becomes one giant molecule with strong covalent bonds. Such polymers are called thermosetting polymers, or thermosets, because during polymerization, the network is completed, and the shape of the part is permanently set. An important behavior is that the curing reaction, unlike thermoplastics, is irreversible.

Electrical discharge machining

When current carrying wires are brought near each other in a dielectric fluid, a spark forms •This spark removes material from both surfaces •Works with any conducting material

Crazing in Thermoplastics

When subjected to tensile stresses or to bending, some thermoplastics such as polystyrene and polymethlmethacrylate develop localize wedge shape narrow regions of highly deformed material, a phenomenon called crazing.

Orientation in Thermoplastics

When thermoplastics are permanently deformed or shaped, say by stretching, the long-chain molecules align in the general direction of elongation. This process is called orientation, and just as with metals, the polymer becomes anisotropic. The specimen becomes stronger and stiffer in the elongated direction than in the transverse direction. This process is an important technique to enhance the strength and toughness of polymers. Orientation weakens the polymer in the transverse direction

Effects of temperature and deformation rate on thermoplastics

With increasing temperature, the strength and modulus of elasticity decrease, and the toughness increases. If the temperature of a thermoplastic polymer is raised above its Tg, it first becomes leathery and then rubbery with increasing temperature. Finally, at higher temperatures, above Tm for crystalline thermoplastics, it becomes a viscous fluid, with viscosity decreasing as temperature and strain rate are increased. Thermoplastics display viscoelastic behavior

Sintering

a)In solid-phase sintering, temperatures ~70-90% of Tmelt cause diffusion between the particles; the part shrinks b)In liquid-phase sintering the secondary material melts and bonds the primary material together; no shrinkage intering Methods •Continuous-sintering furnaces •Step 1: Burn-off •Step 2: Sintering •Step 3: Cooling •Spark Sintering or Spark Plasma Sintering •Sintering occurs during the compaction step and uses electrical discharge to generate high temperatures •Direct Laser Metal Sintering •Allows for localized, layered sintering •Used in 3D printing (additive manufacturing)

Vulcanization

is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks (bridges) between individual polymer chains Uncured natural rubber is sticky, deforms easily when warm, and is brittle when cold. In this state it cannot be used to make articles with a good level of elasticity. The reason for inelastic deformation of un-vulcanized rubber can be found in its chemical structure: rubber is composed of long polymer chains. These chains can move independently relative to each other, which lets the material change shape. Crosslinking introduced by vulcanization prevents the polymer chains from moving independently. As a result, when stress is applied the vulcanized rubber deforms, but upon release of the stress, the article reverts to its original shape.

Direct extrusion (forward extrusion)

similar to forcing toothpaste through the opening of a tube. Note that the billet in this process slides relative to the container wall

Thermoplastic

substances that become plastic on heating and harden on cooling and are able to repeat these processes

Indirect extrusion (reverse extrusion)

the die moves toward the billet and there is no relative motion at the billet-container interface except at the die.

Impression die forging pros and cons

• Advantages compared to machining: • High Production rates • Less waste of metal • Greater strength • Favorable grain orientation in the metal • Limitations: • Not capable of close tolerances • Machining is often required to achieve accuracies and features needed

Investment casting

• Advantages: • Parts of great complexity and intricacy can be cast • Close dimensional control and good surface finish • Wax can usually be recovered for re-use • Additional machining is not normally required, this is a net shape process • Disadvantages • Many processing steps are required • Relatively expensive process

Shell Mold Casting pro's and con's

• Advantages: • Smoother cavity surface permits easier flow of molten metal and better surface finish on casting • Good dimensional accuracy • Machining often not required • Can be mechanized for mass production • Disadvantages • More expensive metal pattern • Difficult to justify for small quantities

Typical die castings

• Cell phone cases • Binoculars • Cameras • Transmission housings, valve bodies, motors, business machine and appliance components, hand tools, toys

Metal flow in extrusion

• Homogeneous flow pattern • When friction along interfaces is high, a dead metal zone develops. This is a high shear area as the material flows into the die exit • The high shear zone extends farther back due to frictions

Squeeze casting

• In squeeze casting, metal is injected at low pressure through larger gates • Once filled, the pressure is increased • Intricate shapes are possible • Shrinkage and porosity defects minimized • Aluminum and magnesium common • Metal matrix composites are emerging

Mold die casting

• Molds are made of tool steel, mold steel, tungsten and molybdenum • Single or multiple cavity • Lubricants and ejector pins to free the parts • Venting holes and passageways in die • Formation of flash that needs to be trimmed

Die casting

• Molten metal is forced into the mold under high pressure (0.7-700 MPa) • Held under high pressure during solidification • Castings can have fine sections and complex details • Long mold life (120,000 parts typical) • Typical metals cast • Zinc, copper magnesium, aluminum and their alloys

Sand Casting

• Most widely used casting process • Parts ranging in size from small to very large • Production quantities from one to millions • Sand mold is used • Patterns and Cores • One piece, Split, Match-plate Patterns • Cores — achieve the internal surface of the part • Molds • Green-sand, cold-box, no-bake molds • Green-sand mold is most common: sand with a mixture of water and bonding clay • Typical mix: 90% sand, 3% water and 7% clay • Gravity flow is the most common method of inserting the liquid metal into the mold • Metal is allowed to solidify and then the mold is removed

Investing casting today

• Rapid prototype process • Mold created directly from CAD file using stereolithography machines • Need to add shrinkage factor • Mold then coated with ceramic slurry • Usually assemble a complex (multi-piece) die using CNC driven machine tools • Can be modified as needed by metal removal or addition as needed on only some details

Defects in extrusion

• Surface cracking (tearing) • Occurs when extrusion temperature, friction and/or speed is too high • Surface temperature rises significantly • Extrusion defect • Draws surface oxides and impurities toward the center • Reduced by modifying the pattern

Abrasives and advanced machining summary

•Abrasives and advanced machining are necessary for hard or brittle workpieces, for complex shapes or when dimensional tolerances are very important •Conventional abrasives (e.g. aluminum oxide) and superabrasives (e.g. diamond) are used for material removal and finishing •The major considerations for abrasives are the size and shape, hardness, friability and chemical affinity with the workpiece material •Grinding is done at high speeds, and typically requires a grinding fluid to remove microchips and minimize temperature rise•Finishing operations often add a large expense to production •Manufacturing burrs may occur from a variety of manufacturing processes, and deburring may be required to minimize scratching and due to safety concerns •Advanced machining includes a variety of electrical and chemical methods •All advanced machining techniques are more expensive than their standard counterparts, but they allow for production that would be difficult or impossible otherwise

Wire EDM

•Also known as electrical-discharge wire cutting •Operates similar to a band saw and allows for precise cuts in hard materials •Modern CNC machines can cut while monitoring wire health

Production of PM

•Atomization •Reduction •Electrolytic deposition •Carbonyls •Comminution •Mechanical alloying

Boring mill

•Boring is a cutting process that can enlarge holes in a workpiece •Used to improve dimensional accuracy or surface finish •Small scale boring can be done on a lathe, but larger parts may need to be done on a boring mill

Cylindrical and internal grinding

•Cylindrical grinding is used on axisymmetric parts, including to produce threads •Internal grinding is used for interior surfaces

Soft lithography

•Describes a number of processes for pattern transfer •Uses standard photolithography to create a mold with the desired pattern •A stamp is made using an elastomeric material (typically PDMS) Soft Lithography Process •The stamp can then be filled with a liquid polymer •The polymer is transferred to the desired surface and allowed to cure •The stamp is removed, leaving the pattern behind

Blending

•Different metals can be mixed together for specific properties (analogous to alloying) •Powders with different size distributions can be mixed for an alternate distribution •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 •Care must be taken during the blending process, because of the high surface-area-to-volume ratio of the powder

Cutting mechanisms

•Drilling •Boring •Tapping •Sawing •Milling

Fatigue

•Due to cyclic loads •Fluctuating mechanical loads (gear teeth, dies, cutters) •Thermal stresses (a cool die coming into repeated contact with a work piece) •Fatigue tests: various stress amplitude, S and number of cycles, N Key concern: fatigue failure stress < static

Electron beam machining

•EBM is similar in concept to LBM, but requires a vacuum to operate •Best suited for small parts, with fine features •Used in aerospace, automotive and semiconductor industries

High energy beam welding

•Electron Beam Welding is performed in various levels of vacuum, which affects the weld depth •Laser Beam Welding produces better welds, is easier to control and automate and does not require a vacuum to work

Face milling

•Face milling is used to produce smooth surfaces and/or cutouts •The axis of rotation is perpendicular to the movement of the workpiece •High speeds with low depth of cut is used to create a smooth finish

Films and film disposition

•Films are used extensively in microelectronic-device manufacturing •Main processes are: •Evaporation •Sputtering •Chemical Vapor Deposition (CVD)

Electrical-discharge grinding

•Grinding wheel is made of graphite or brass and contains no abrasives •As the wheel rotates, a series of sparks remove a thin layer of material from the workpiece •The material removal rate is proportional to a material constant and the current •The wear rate of the tool is much less than during standard grinding and works on extremely hard surfaces with low pressures •Can be combined with electrochemical grinding: •Electrolytes break down oxide layers and remove excess material, while discharge removes surface layers

Plasma arc cutting

•High pressure gas is fed through a nozzle •An electrical current is used to convert the gas into a plasma •High temperatures (>9400℃) and high flow rates melt the workpiece material and blow it away from the surface •Typically used for stainless steels and refractory metals

Compaction

•High pressures are used to compact the blended powder into the desired shape •Spherical powders with similar distributions do not compact well; require a binder •Uniformity can be enhanced by adding lubricants

High speed machining

•Improvements in tool materials has enabled high-speed machining (HSM), as tools an now survive the temperatures and stresses •Typically refers to automated or semi-automated systems (e.g. CNC machines) •Approximate cutting speeds: •High: 600-1800 m/min •Very High: 1800-18,000 m/min •Ultra High: >18,000 m/min •Improves dimensional tolerances because more heat is lost with the chips

Conventional and climb milling

•In conventional milling, the workpiece moves opposite the rotation •Climb milling moves the workpiece in the direction of the rotation

Lithography

•Lithography is the process for transferring the geometric patterns required to the substrate •The pattern geometry is created using software •A mask (or reticle) of this geometry is created on glass or quartz •A beam (typically light) is passed through the mask on to the substrate that is coated in photoresistThe process is defined by the type of beam •Photolithography is the most common, but has significant limitations for smaller chips •Newer techniques are available, but are not typically used for mass production

Bulk micromachining

•Original MEMS manufacturing technique •Uses etchant to remove selected layers from a single crystal substrate •Works well for developing simple shapes

Diffusion welding

•Parts are heated to ~0.5 Tmelt and held under pressure •Bond formation time is dependent on diffusion rate •This process can be combined with superplastic forming to form lightweight reinforced structures

Creep

•Permanent elongation of a material under a static load maintained for a period of time •Creep rate can be estimated from secondary stage (linear slope) •Creep rate increases with temperature and applied load

Powder metallurgy

•Powder Metallurgy (P/M) uses high pressures (compaction) and moderate temperatures (sintering) to produce parts that are not possible with other methods •P/M is an old manufacturing technique (~300 A.D.) but modern high-technology industries (e.g. automotive) make use of the process •P/M is an important part of modern "additive manufacturing", including 3D printing of metal parts (e.g. Direct Laser Metal Sintering of SpaceX engine parts)

Geometry of orthogonal cutting

•Rake angle, α, is the angle between the tool and a line normal to the workpiece surface •Clearance (or relief) angle is the angle between the tool flank and the workpiece surface •Shearing occurs along a shear plane, which makes an angle ɸ to the workpiece surface •Depth of cut is denoted by to

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

•Shielding gas is fed to the welding region to protect against oxidation •Also known as MIG (Metal Inert Gas) welding •Allows for higher production rates than SMAW •Metal is transferred from the tip via spray transfer, globular transfer or short circuiting

Gas Tungsten Arc Welding

•Similar to GMAW, but with a nonconsumable tungsten tip •Filler metal must be supplied •Also known as TIG (Tungsten Inert Gas) welding •High quality surface finish

Residual Stresses

•Stresses that remain with a part after it has been deformed and all external forces have been removedWarping •Tensile residual stress -> undesirable, lower the fatigue life and fracture strength •Compressive residual stress -> desirable, increase fatigue life (shot peening, surface rolling)•Stress-relief annealing (thermal treatment) •Generally accompanied by warpage of the part •Further plastic deformation: stretching

Types of grinding

•Surface Grinding •The most common grinding operation •Used for gross material removal of hard/brittle materials and finishing of all types of materials •Cylindrical Grinding •Used for finishing axisymmetric parts •Internal Grinding •Used for finishing internal surfaces •Centerless Grinding •A high volume production method for grinding axisymmetric parts

Temperature effects on grinding

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

Ultrasonic Machining

•The surface is coated in a slurry containing an abrasive (typically boron carbide) •The tip of the tool is excited to a frequency of 20 kHz •This excitation is transmitted to the particles as high velocities •The particles microchip the surface •This process is suitable for hard and brittle surfaces

Abrasives

•There are many materials that are too hard or too brittle to machine using a cutting tool •In these situations, abrasives are are a good option •Other "new" machining processes have been developed for: •working with hard materials •parts that are too slender to be clamped for machining or grinding •parts that are too complex for traditional methods •obtaining improved dimensional accuracyAn abrasive is a small, hard particle that has sharp edges and an irregular shape Examples: Conventional •Aluminum oxide •Silicon carbide Superabrasives •Cubic boron nitride •Diamond

Turning

•Turning •Blanks to be machined should be as close to the final dimension as possible to minimize machining •Long, slender parts may deflect excessively, so keep parts as short and stocky as possible

Surface grinding

•Used to grind flat surfaces •Can be used to remove material (e.g. semiconductor back grinding) •Or to provide a surface finish within tight dimensional tolerances (e.g. glass wear)

Arcwelding

•Uses electrical discharge to produce the high temperatures necessary for welding •Related to electrical machining techniques discussed in Chapter 9 •An important process for automated manufacturing•An electrode is touched to the workpiece, which has been electrically charged •The electrode is then removed from the surface to generate an arc Consumable Electrode Shielded Metal Arc Welding Submerged Arc Welding Gas Metal Arc Welding Flux-Core Arc Welding Electrogas Welding Non-consumable Electrode Gas Tungsten Arc Welding Atomic Hydrogen Welding Plasma Arc Welding

EDM examples

•Varying the shape of the EDM cutting tools allows for complex external and internal geometries •The wear rate of the cutting tool is much less than the workpiece, which allows for precise shapes •High MRR results in rough surface finishes, but pulsed EDM results in a better overall finish

Deburring

•Vibratory and Barrel-finishing: part is placed in a container with abrasives •Shot blasting: abrasive particles are propelled at the surface •Abrasive-flow machining: abrasives in a putty are forced at high pressure through internal cavities •Thermal-energy Method: high temperature flashes (>3300℉) melt the burrs but not the workpiece •Robotic deburring: automated systems that may use various techniques