Finals Review EME50
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This 10 page Study Guide was uploaded by Mae Underwood on Sunday December 6, 2015. The Study Guide belongs to EME50 at University of California - Davis taught by Rida Farouki in Summer 2015. Since its upload, it has received 16 views. For similar materials see Manufacturing Processes in Mechanical Engineering at University of California - Davis.
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Date Created: 12/06/15
Metal Forming Changing shape of workpiece through plastic deformation (exert compressive stresses exceeding yield strength of metal) o Plastic deformation: non-recoverable deformation o Tools: press or hammer and dies o Lubricants used to minimize friction and tool wear Desired characteristics of metals: high malleability (low yield strength) and ductility (harder to fracture). o These properties can be temperature-dependent Stress-Strain curve (σ-ε) o Plastic deformation region: Ultimate tensile strengtultS ) > σ > Yield streygth (S ) Heating workpiece: can perform forming processes with lower forces and power consumption o Generally leads to poorer quality Cold Working (~room temperature) Good accuracy, surface finish, repeatability Strain hardening increases part strength Controllable directional properties of part × Requires high deformation forces, only limited geometry changes possible Hot working (~50-75% of melting temperature) Above recrystallization temperature (where internal structure of metal can reorganize) Requires lower forces than cold working & substantial shapes are possible More feasible for brittle metals @ room temp Isotropic strength properties × No strength improvement × Poorer accuracy, surface finish, repeatability Warm working (~30% of melting temperature) Strain rateε = =v deformation speed (v) h workpiece height (h) Forging press = gradual force applied Forging hammer = impact load Open dies: allow lateral flow of workpiece material Impression dies: significantly constrains shape Flashless dies: prevent flow of excess metal along the parting line between 2 die halves. “flash” which must be machined off o Volume of workpiece must carefully match flashless die cavity volume Bulk deformation processes Parts with low Surface Area:Volume ratio Forging, rolling, extrusion/drawing Sheet metal forming (presswork or stamping) High SA:V ratio Forming Compression of workpiece between 2 die halves defining desired part shape (mostly only a hot working process) Parts near net shape but may require more forging operations (machining, heat treatment, pressing etc.) Open die forging: “upsetting” or “upset forging” Reduces height & increases diameter of workpiece, μ0 Impression (closed) die forging: Dies match part shape Friction of flash forced between die halves holds workpiece in place & promotes full metal flow into entire die cavity May need a succession of dies for complicated final part shapes with several hammer blows per die Fine machining is required for high precision surfaces Flashless forging: Completely enclosed die halves Requires stringent process contours Used for simple symmetrical shapes & soft metals (Al, Mg, etc.) Incomplete forging (under-fill) or excessive forces (over-fill) – due to mismatch of workpiece & die volumes Rolling Reduction of part thickness or changing cross-sectional shape Feed metal between counter rotating tools “rolls” on a rolling mill Rolls draw workpiece into gap between the mills through friction, then compresses it with an applied force, F Rolling mills are massive and consume a lot of power Usually a hot-working process, but some can be cold-rolled Flat rolling – cylindrical rolls to reduce thickness of metal slab Shape rolling – contoured rolls to produce desired cross-sectional shape. A succession of progressively shaped rolls “roll pass” is required. Thread rolling – produce helical screw threads on cylindrical stock (bolts, screws). Pairs of flat dies with reciprocating slanted V-grooves Extrusion Shaping a metal bar to desired cross-sectional shape by forcing it through a shaped die opening Exit aperture is the cross-sectional shape Performed with hot or cold metal billet Limited to parts with uniform cross-sectional shape Close tolerances are possible Direct (forward) extrusion Forces metal billet through die using a ram Ram must overcome friction of billet against container wall and mechanical deformation forces Hollow or semi-hollow extrusions are possible using a shaped ram (mandrel) Indirect (backward) extrusion Die attached to a hollow ram, material is forced through die in a direction opposite to ram motion Avoids friction of billet motion against container walls, so ram force is lower Hollow ram is less rigid than solid ram Drawing Workpiece is shaped by pulling (rather than pushing) it through a die Makes wire of various diameters Final shape may be achieved through pulling through progressively smaller dies Maximum percent area reduction in a die depends on tensile strength of material Dies can be lubricated with oil or soap Final wire strength depends on total amount of cold-working Sheet Metal Working Performed cold on thin workpiece: compress it between positive (punch) and negative (die) tool pairs Bending Deep drawing: forming a concave shape in a workpiece held by blankholder o Punch and die must have suitable corner radii and clearance (to prevent cutting the metal) o Friction plays key role o Holding force of blankholder against workpiece must be adjusted Cutting by shear action Plastics Manufacturing Polymer – substance made of long molecules with repeating units, usually carbon-based Synthetic (plastics) or natural (rubber) Basic processes: injection molding, blow molding, extrusion, casting and thermoforming Thermoplastics (linear/reversible) Solid at room temp Melted at relatively low temp Reformed into diff. shapes repeatedly Thermosets (crosslinked/irreversible) Chemical reaction on initial heating and forming, prevents further melting and reshaping Elastomers (rubber): highly elastic – can exhibit elastic (recoverable) strains under relatively low stress Pros & Cons of Plastics Cheap Easily formed Light weight Easily painted/coated/plated Corrosion resistant Electrically and thermally insulating Can be used in composites × Relatively low strength × Stiffness × Brittle, not ductile × Low melting point (thermoplastic) × Creep under steady loads × Degrade under exposure to sunlight/atmosphere Injection Molding Heated plastic forced into mold cavity under high pressure by reciprocating plunger or screw o Pressure required to overcome viscosity of molten plastic Allows “net shape” production of complex parts with wide range of sizes & 10-30 second cycle times Used with thermoplastics, but also modified thermosets and elastomers Thermosets require “reaction injection molding” o Constituents are introduced into a mixing chamber before injection o Automobile bumpers, steering wheels, sports equipment Machining of molds for large parts can be expensive – so only suitable for large production runs Blow Molding Used for thin walled plastic containers A tube of heated, soft plastic “parison” is extruded into mold cavity Pressurized air forced into parison causes it to expand and assume shape of mold Control of parison thickness is important Typically a high-volume production process Extrusion Molten plastic is forced through a die opening to produce parts of uniform cross-sectional shape Pipes, door/window moldings, insulating coating on electrical wire Typically a continuous process Extruded product cut to desired length on exit Used with thermoplastics and elastomers but not with thermosets Extruders o Typically 1-6” in diameter o Length:diameter ~10-30 o Can be fitted with a variety of dies o Extruder screw operates typically at 60 rpm o Friction in screw mechanism may heat plastic to achieve melting Thermoforming Deformation of a heated plastic sheet into a desired shape against a mold Conformation of sheet against mold may be achieved by vacuum suction, positive pressure or mechanically using positive & negative mold halves Used extensively in packaging (plastic “blister packs”) Large parts can be made: bathtubs, shower stalls o Only thermoplastics (ABS, PVC, polystyrene, polyethylene, etc) Casting Gravity casting is a simpler and cheaper method than injection molding Molten plastic must be sufficiently hot & fluid to ensure complete mold filling Polystyrene, acrylics, vinyls (PVC), polyamides (nylon) Composite Materials Typically composed of 2 heterogeneous materials, in physically distinct phases o Combination provides superior engineering properties (stiffness, strength/weight ratio, etc.) than the individual materials. Typical combination involves embedding a reinforcing agent (secondary phase) in a continuous matrix (primary phase) o Reinforcing agent in the form of particles, flakes, fibers or sheets Depending on how it is embedded in the matrix, the composite material may have anisotropic (direction-dependent) properties The 2 phases remain physically distinct, but must have strong adhesion at their interfaces “Interphase” agent may be added to promote bonding of primary & secondary phases Reinforcing agent may have either a random or geometrically organized arrangements Matrix provides compressive strength and transfers tensile stress to the bonded reinforcing agent Composite Manufacturing vs Metals Labor intensive Costly Slow Used only for critical specialized applications, but now finding more widespread use Common “Simple” Composites Adobe bricks – made of dried compacted mud reinforced with straw Steel reinforced concrete Plywood – laminate of cross-grained veneer sheets, bonded with resin Modern Composites Polymer matrix composites (PMCs) o Thermoset resins (epoxy/polyester) reinforced with fibers o Thermoplastics with particle/flakes for reinforcement o Elastomers reinforced with “carbon block” – nanoparticles produced by combustion E.g. automobile tires – help conduct heat away from tread Metal matrix composites (MMCs) o Metal/ceramic mixtures (cemented carbides) o Fiber-reinforced aluminum/magnesium Ceramic matrix composites (CMCs) o “cermets” (ceramic + metal) o Aluminum oxide or silicon carbide w/ embedded fibers (highly temp-resistant) o Provide extreme hardness & wear resistance o Tungsten carbide is use for cutting tools, drill bits, indentation tools, dies, etc. Fiber reinforcement may be continuous (arranged as longitudinal wires, or a fabric weave) or randomly oriented short strands Fiber materials: glass, fiberglass (glass-reinforced plastic), carbon (graphite), kevlar, ceramics, metals Fiber-reinforced polymers are the most commonly used composites o Typically >50% fiber content in an epoxy matrix o Lamination of fiber sheets with prescribed orientations is used to fabricate thin cross- section parts (aircraft wings, boat hull & rudder/keel, vehicle body panels) Advantageous Properties of Composite Parts: Strength similar to steel, but with ~20% of weight Good fatigue life and corrosion resistance Low thermal expansion, dimensionally stable Parts with anisotropic (directional) strength are possible Good creep resistance under steady loads Manufacturing of composite parts is expensive and time-consuming. Stringent quality control is required for consistent results Reinforcing fibers must be impregnated with matrix polymer for good adhesion to matrix. “Prepreg” tapes or sheets (require refrigerated storage) may be cut, resin dipped and stacked to form laminate structures (manual or by CNC) Reinforced plastic (possibly heated) can be formed by various molding processes: Compression Vacuum or contact molding (e.g. hand “clay up”) Filament winding Pultrusion Pulforming The latter three combine continuous reinforcement fiber with resin where claying on a mold or pulling through a die Rapid Prototyping Family of technologies that produce physical prototypes from CAD models through curing, depositing, sintering or bonding material in parallel layers Also called “solid freeform fabrication” (SFF) and “layered manufacturing” (LM) Software slices CAD model into many horizontal layers for path planning Support structures (removed after process completion) required for parts with overhangs Part build orientation is important to avoid “stair-casing effect” on smooth part surfaces Accuracy depends on chain slice thickness Rapidly create physical 3D models to check shape, fit, function and avoid costly errors Commonly used to fabricate tooling for other manufacturing processes (e.g. casting patterns) Can fabricate “smart” structures with embedded sensors Many other applications Stereolithography (SLA): 3D Systems http://www.3dsystems.com Original RP process Uses liquid photopolymer that cures (hardens) on UV exposure A UV laser writes on surface of photopolymer reservoir to create the current part layer Platform supporting part is lowered to allow a new layer to cure Support structures are removed and completed part is flooded with UV light in oven (final hardening) Fabrication Times Hours to days System cost $100,000 - $500,000 Part sizes 20 x 20 x 24 in Material cost $300 / gallon Fuse Deposition Modeling (FDM): Stratasys http://www.stratasys.com Employs on extrusion nozzle mounted on an (x,y) stage, above a platform that moves in z- direction Thermoplastic supplied from a coil in filament form is heated to melting point on passing through the extruder Solidifies on exit After area of current slice is covered, platform lowers by slice thickness to build next layer Different materials can be used for supports to facilitate removal “Office-friendly” process involving no lasers or toxic chemicals Fabrication Times Hours System cost $30,000 - $150,000 Part sizes 9 – 30 in Material cost ~$100 / lb Selective Laser Sintering (SLS): DTM Corp – now merged with 3D systems http://www.3dsystems.com Sintering = fusing powder particles by raising temperature close to melting point Can use polymer, metal or ceramic powders Powder heating achieved by laser beam Once layer is sintered, platform lowers & new powder layer is rolled over it Only RP process that can make metal parts, but quality is not comparable Cast/forged/machined parts Sintered parts may be porous Fabrication Times Hours to days System cost $500,000 Part sizes 24 x 24 x 24 in Material cost $5-60 /lb Laminated Object Manufacturing (LOM): Helisys Inc (bankrupt in 2000), now cubic technologies Uses laser to cut coated paper or other sheet into shape of each part slice Bonds layers using a heated roller Can build large parts (engine blocks) with complex geometry Models have “sculpted wood” appearance Part is manually extracted by “de-cubing” process Parts not as accurate as SLA or SLS Cheap materials Fabrication Times Hours to days System cost N/A Part sizes Several feet (largest of all RP tech) Material cost ~$10-20 / lb Solid Object Printing (SOP) or “3D Printing”: ZCorp http://www.zcorp.com Inkjet nozzle sprays binding agent onto fine powder (plaster or starch) Very cheap and fast Limited part strength Can build multi-colored parts Competes directly with FDM processes Fabrication Times Hours System cost $20,000 - $70,000 Part sizes Up to 12 in Material cost Very low
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