DOBSON+CH14 - SYNTHESIS, FABRICATION & ...

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Unformatted text preview: SYNTHESIS, FABRICATION & PROCESSING OF MATERIALS METALS CERAMICS POLYMERS Metal FabricaQon HOW DO WE FABRICATE METALS? –  Blacksmith - hammer (forged) –  Cast molten metal into mold FORMING OPERATIONS –  Rough stock formed to final shape Hot working DeformaQon temperature high enough for recrystallizaQon Large deformaQons vs. Cold working DeformaQon below recrystallizaQon temperature Strain hardening occurs Small deformaQons Metal FabricaQon Methods FORMING CASTING • Forging (Hammering; Stamping) (wrenches, crankshaWs) force die A o blank A d oWen at elev. T • Drawing force Ao die Ad • Rolling (Hot or Cold Rolling) (I- beams, rails, sheet & plate) roll A o roll • Extrusion A d Adapted from Fig. 14.2, Callister & Rethwisch 3e. (rods, tubing) (rods, wire, tubing) die MISCELLANEOUS Ao tensile force Die must be well lubricated & clean force container ram billet container die holder extrusion die DucQle metals, e.g. Cu, Al (hot) Ad Metal FabricaQon Methods FORMING CASTING MISCELLANEOUS CASTING - mold is filled with molten metal –  metal melted in furnace, perhaps alloying elements added, then cast in a mold –  common and inexpensive –  gives good producQon of shapes –  weaker products, internal defects –  good opQon for bri]le materials Metal FabricaQon Methods FORMING CASTING MISCELLANEOUS Example: Sand Casting (large parts, e.g., auto engine blocks) Sand Sand molten metal •  What material will withstand T >1600ºC and is inexpensive and easy to mold? •  Answer: sand!!! •  To create mold, pack sand around form (pattern) of desired shape Metal FabricaQon Methods FORMING CASTING Example: Die CasQng - - high volume - - for alloys having low melQng temperatures MISCELLANEOUS Example: ConQnuous CasQng - - simple shapes (e.g., rectangular slabs, cylinders) molten solidified Metal FabricaQon Methods FORMING CASTING POWDER METALLURGY (metals w/low ducQliQes) pressure WELDING (when fabricaQon of one large part is impracQcal) filler metal (melted) base metal (melted) fused base metal heat area contact densify point contact at low T densificaQon by diffusion at higher T MISCELLANEOUS unaffected piece 1 heat- affected zone unaffected piece 2 • Heat- affected zone: (region in which the microstructure has been changed). Adapted from Fig. 14.3, Callister & Rethwisch 3e. (Fig. 14.3 from Iron Cas7ngs Handbook, C.F. Walton and T.J. Opar (Ed.), 1981.) Thermal Processing of Metals ANNEALING: Heat to Tanneal, then cool slowly. • Stress Relief: Reduce stresses resulQng from: - plasQc deformaQon - nonuniform cooling - phase transform. • Spheroidize (steels): Make very soW steels for good machining. Heat just below Teutectoid & hold for 15- 25 h. Types of Annealing • Process Anneal: Negate effects of cold working by (recovery/ recrystallizaQon) • Full Anneal (steels): Make soW steels for good forming. Heat then furnace- cool to obtain coarse pearlite. • Normalize (steels): Deform steel with large grains. Then heat treat to allow recrystallizaQon and formaQon of smaller grains. Based on discussion in Section 14.5, Callister & Rethwisch 3e. Heat Treatment Temperature-Time Paths A a)  Full Annealing P b)  Quenching c)  Tempering (Tempered Martensite) A b) B a) c) Fig. 11.26, Callister & Rethwisch 3e. 9 Ceramic FabricaQon Methods PARTICULATE FORMING GLASS FORMING PRESSING: BLOWING OF GLASS BOTTLES: Gob CEMENTATION Pressing operation Parison mold plates, cheap glasses -- glass formed by application of pressure -- mold is steel with graphite lining FIBER DRAWING: Compressed air Suspended parison Finishing mold wind up www.youtube.com/watch?v=NVKcISj2LfA Adapted from Fig. 14.18, Callister & Rethwisch 3e. (Fig. 14.18 is adapted from C.J. Phillips Glass: The Miracle Maker, Pi]man Publishing Ltd., London.) Ceramic FabricaQon Methods GLASS FORMING PARTICULATE FORMING CEMENTATION Sheet forming – conQnuous casQng –  sheets are formed by floaQng the molten glass on a pool of molten Qn Adapted from Fig. 14.19, Callister & Rethwisch 3e. Glass Structure • Basic Unit: 4Si0 4 tetrahedron Si 4+ O2 - • Quartz is crystalline SiO2: Glass is noncrystalline (amorphous) • Fused silica is SiO2 to which no impuriQes have been added • Other common glasses contain impurity ions such as Na+, Ca2+, Al3+, and B3+ Na + Si 4+ O2 - (soda glass) Adapted from Fig. 3.41, Callister & Rethwisch 3e. Glass ProperQes Specific volume (1/ρ) vs Temperature (T): • Crystalline materials: Specific volume Liquid (disordered) Supercooled Liquid • Glasses: Glass (amorphous solid) - - do not crystallize Crystalline (i.e., ordered) Tg - - crystallize at melQng temp, Tm - - have abrupt change in spec. vol. at Tm Tm solid T - - change in slope in spec. vol. curve at glass transiQon temperature, Tg - - transparent - no grain boundaries to sca]er light Adapted from Fig. 14.16, Callister & Rethwisch 3e. Heat TreaQng Glass • Annealing: - - removes internal stresses caused by uneven cooling. • Tempering: - - puts surface of glass part into compression - - suppresses growth of cracks from surface scratches. - - sequence: before cooling hot initial cooling at room temp. cooler hot cooler -- Result: surface crack growth is suppressed. compression tension compression Ceramic FabricaQon Methods PARTICULATE FORMING GLASS FORMING CEMENTATION HydroplasQc forming: • Mill (grind) and screen consQtuents: desired parQcle size • Extrude this mass (e.g., into a brick) Ao force container ram billet container die holder extrusion Ad die • Dry and fire the formed piece Adapted from Fig. 14.2 (c), Callister & Rethwisch 3e. Ceramic FabricaQon Methods GLASS FORMING PARTICULATE FORMING CEMENTATION Slip casQng: solid component hollow component Adapted from Fig. 14.22, Callister & Rethwisch 3e. (Fig. 14.22 is from W.D. Kingery, Introduc7on to Ceramics, John Wiley and Sons, Inc., 1960.) Sintering: Powder pressing || Tape casQng Ceramic FabricaQon Methods GLASS FORMING PARTICULATE FORMING CEMENTATION • Hardening of a paste – paste formed by mixing cement material with water • FormaQon of rigid structures having varied and complex shapes • Hardening process – hydraQon (complex chemical reacQons involving water and cement parQcles) • Portland cement – producQon of: - - mix clay and lime- bearing minerals - - calcine (heat to 1400°C) - - grind into fine powder Polymer FormaQon Two types of polymerizaQon: –  AddiQon (or chain) polymerizaQon –  CondensaQon (step) polymerizaQon AddiQon (Chain) PolymerizaQon –  Ini4a4on –  Propaga4on –  Termina4on CondensaQon (Step) polymerizaQon Polymer addiQves Improve mechanical properQes, processability, durability, etc. •  Fillers •  Stabilizers •  PlasQcizers •  Lubricants •  Colorants •  Flame Retardants Processing PlasQcs www.youtube.com/watch?v=5dXMGDRWpqg •  ThermoplasQc –  can be reversibly cooled & reheated, i.e. recycled –  heat unQl soW, shape as desired, then cool –  ex: polyethylene, polypropylene, polystyrene, PMMA •  Thermoset –  when heated forms a molecular network (chemical reacQon) –  degrades (doesnt melt) when heated –  a pre- polymer molded into desired shape, then chemical reacQon occurs –  ex: urethane, epoxy Processing PlasQcs: Compression Moulding ThermoplasQcs and thermosets •  polymer and addiQves placed in mold cavity •  mold heated and pressure applied •  fluid polymer assumes shape of mold Fig. 14.29, Callister & Rethwisch 3e. (Fig. 14.29 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley & Sons, 1984.) Processing PlasQcs: InjecQon Moulding ThermoplasQcs and some thermosets: •  when ram retracts, plasQc pellets drop from hopper into barrel •  ram forces plasQc into the heaQng chamber (around the spreader) where the plasQc melts as it moves forward •  molten plasQc is forced under pressure (injected) into the mold cavity where it assumes the shape of the mold Fig. 14.30, Callister & Rethwisch 3e. (Fig. 14.30 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd ediQon, John Wiley & Sons, 1971.) Barrel Processing PlasQcs: Extrusion ThermoplasQcs •  plasQc pellets drop from hopper onto the turning screw •  plasQc pellets melt as the turning screw pushes them forward by the heaters •  molten polymer is forced under pressure through the shaping die to form the final product (extrudate) Fig. 14.31, Callister & Rethwisch 3e. (Fig. 14.31 is from Encyclopædia Britannica, 1997.) 25 25 Processing PlasQcs: Blown Film Extrusion www.youtube.com/watch?v=sFsiIsefgrk Fig. 14.32, Callister & Rethwisch 3e. (Fig. 14.32 is from Encyclopædia Britannica, 1997.) SUMMARY • METAL FABRICATION TECHNIQUES: - - forming, casQng, miscellaneous. • ANNEALING • CERAMIC FABRICATION TECHNIQUES: - - glass forming (pressing, blowing, fiber drawing). - - parQculate forming (hydroplasQc forming, slip casQng, powder pressing, tape casQng) - - cementaQon • HEAT TREATING PROCEDURES - - glasses—annealing, tempering - - parQculate formed pieces—drying, firing (sintering) • POLYMERS AND POLYMER PROCESSING - - compression and injecQon molding, extrusion, blown film extrusion ...
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