Chapter 14: Synthesis, Fabrication, and Processing of Materials

Transcription

1 Chapter 14: Synthesis, Fabrication, and Processing of Materials ISSUES TO ADDRESS... What are some of the common fabrication techniques for metals? What heat treatment procedures are used to improve the mechanical properties of both ferrous and nonferrous alloys? How is processing of ceramics different than for metals? What are the polymerization and basic fabrication and processing techniques for polymers? Chapter 14-1

2 Metal Fabrication How do we fabricate metals? - hammer (forged) Cast molten metal into mold Rough stock formed to final shape Hot working vs. Cold working Deformation high enough for Large Deformation below temperature Strain hardening occurs Small Chapter 14-2

3 Metal Fabrication Methods (i) FORMING Forging ( ) Ao (wrenches, crankshafts) force die blank force Drawing (rods, wire, tubing) Ao die die Ad Ad tensile force often at elev. T die must be well lubricated & clean CASTING Rolling ( ) (I-beams, rails, sheet & plate) Ao force Ao MISCELLANEOUS ram roll roll (rods, tubing) container billet Ad Adapted from Fig. 14.2, Callister & Rethwisch 4e. die holder extrusion container die ductile metals, e.g. Cu, Al (hot) Chapter 14-3 Ad

4 Metal Fabrication Methods (ii) FORMING CASTING MISCELLANEOUS Casting- mold is filled with molten metal, perhaps alloying elements added, then in a mold gives good production of shapes weaker products, internal defects Chapter 14-4

5 Metal Fabrication Methods (iii) FORMING CASTING MISCELLANEOUS Casting (large parts, e.g., auto engine blocks) Sand Sand molten metal What material will withstand T >1600ºC and is and easy to mold? Answer:!!! To create mold, pack around form (pattern) of desired shape Chapter 14-5

6 Metal Fabrication Methods (iv) FORMING CASTING MISCELLANEOUS (low volume, complex shapes e.g., jewelry, turbine blades) Stage I formed by pouring around wax pattern. Plaster allowed to harden. wax I Stage II Wax is melted and then poured from mold hollow mold cavity remains. Stage III into mold and allowed to solidify. II III Chapter 14-6

7 Metal Fabrication Methods (v) FORMING CASTING MISCELLANEOUS Die Casting -- high -- for alloys having melting temperatures Continuous Casting -- shapes (e.g., rectangular slabs, cylinders) molten solidified Chapter 14-7

8 Metal Fabrication Methods (vi) FORMING CASTING MISCELLANEOUS (metals w/low ductilities) (when fabrication of one large part is impractical) point contact at low T densify pressure heat area contact densification by diffusion at higher T filler metal (melted) base metal (melted) fused base metal unaffected heat-affected zone unaffected piece 1 piece 2 Heat-affected : (region in which the microstructure has been changed). Adapted from Fig Callister 4e. (Fig from Iron Castings Handbook, C.F. Walton and T.J. Opar (Ed.), 1981.) Chapter 14-8

9 Thermal Processing of Metals Annealing: Heat to Tanneal, then cool slowly. Stress Relief: Reduce stresses resulting from: - plastic deformation - cooling - phase transform. (steels): Make very soft steels for good machining. Heat just below T eutectoid & hold for h. : Negate effects of cold working by (recovery/ recrystallization) Types of Annealing Full Anneal (steels): Make soft steels for good forming. Heat to get γ, then furnace-cool to obtain coarse pearlite. (steels): Deform steel with large grains. Then heat treat to allow and formation of smaller grains. Based on discussion in Section 14.5, Callister & Rethwisch 4e. Chapter 14-9

10 Heat Treatment Temperature-Time Paths a) b) A P c) Tempering (Tempered Martensite) A B Fig , Callister & Rethwisch 4e. b) a) c) Chapter 14-10

11 Hardenability -- Steels measure of the ability to form martensite Jominy end quench test used to measure. specimen (heated to γ phase field) 24ºC water flat ground Rockwell C hardness tests Adapted from Fig. 14.5, Callister & Rethwisch 4e. (Fig adapted from A.G. Guy, Essentials of Materials Science, McGraw-Hill Book Company, New York, 1978.) Plot versus distance from the quenched end. Hardness, HRC Distance from quenched end Adapted from Fig. 14.6, Callister & Rethwisch 4e. Chapter 14-11

12 Reason Why Hardness Changes with Distance The cooling rate with distance from quenched end. T(ºC) Hardness, HRC M(start) 200 A M 0 M(finish) distance from quenched end (in) 0% 100% Adapted from Fig. 14.7, Callister & Rethwisch 4e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 376.) Time (s) Chapter 14-12

13 Hardenability vs Alloy Composition curves for five alloys each with, C = Adapted from Fig. 14.8, Callister & Rethwisch 4e. (Fig adapted from figure furnished courtesy Republic Steel Corporation.) "Alloy Steels" (4140, 4340, 5140, 8640) -- contain Ni, Cr, Mo (0.2 to 2 wt%) -- these shift the "nose" to longer times (from A to B) -- martensite is easier to form Hardness, HRC Distance from quenched end (mm) 800 T(ºC) A B Cooling rate (ºC/s) T E %M M(start) M(90%) Time (s) Chapter 14-13

14 Influences of Quenching Medium & Specimen Geometry Effect of quenching medium: Medium air oil water Severity of Quench Effect of specimen geometry: When surface area-to-volume ratio increases: -- cooling rate throughout interior increases -- hardness throughout interior increases Position center surface Cooling rate low high Hardness Hardness low high Chapter 14-14

15 Ceramic Fabrication Methods (i) GLASS FORMING Blowing : Parison mold Suspended parison Gob PARTICULATE FORMING Pressing operation Compressed air CEMENTATION : plates, cheap glasses -- glass formed by of pressure -- mold is steel with lining drawing: Finishing mold Adapted from Fig , Callister & Rethwisch 4e. (Fig is adapted from C.J. Phillips, Glass: The Miracle Maker, Pittman Publishing Ltd., London.) wind up Chapter 14-15

16 Sheet Glass Forming Sheet forming casting sheets are formed by floating the glass on a pool of Adapted from Fig , Callister & Rethwisch 4e. Chapter 14-16

17 Basic Unit: 4- Si04 Quartz is SiO2: Glass Structure Si 4+ O 2- Glass is (amorphous) Fused is SiO 2 to which no impurities have been added Other common contain impurity ions such as Na +, Ca 2+, Al 3+, and B 3+ Na + Si 4+ O 2- (soda glass) Adapted from Fig. 3.42, Callister & Rethwisch 4e. Chapter 14-17

18 Glass Properties (1/ρ) vs Temperature (T): Specific volume Supercooled Liquid Liquid (disordered) materials: -- crystallize at melting temp, T m -- have in spec. vol. at T m Glass (amorphous solid) Crystalline (i.e., ordered) T g T m Adapted from Fig , Callister & Rethwisch 4e. solid T : -- do not crystallize -- change in in spec. vol. curve at glass transition temperature, T g -- transparent - no grain boundaries to scatter light Chapter 14-18

19 Glass Properties: Viscosity Viscosity, η: -- relates (τ) and (dv/dy): τ τ glass dy dv dv dy velocity gradient η = τ dv / dy η has units of (Pa-s) Chapter 14-19

20 Log Glass Viscosity vs. Temperature decreases with T soda-lime glass: 70% SiO 2 balance Na 2 O (soda) & CaO (lime) borosilicate (Pyrex): 13% B 2 O 3, 3.5% Na 2 O, 2.5% Al 2 O 3 Vycor: 96% SiO 2, 4% B 2 O 3 fused silica: > 99.5 wt% SiO 2 Viscosity [Pa-s] T melt T(ºC) point point : glass-forming carried out Adapted from Fig , Callister & Rethwisch 4e. (Fig is from E.B. Shand, Engineering Glass, Modern Materials, Vol. 6, Academic Press, New York, 1968, p. 262.) Chapter 14-20

21 Heat Treating Glass : -- removes stresses caused by uneven cooling. : -- puts surface of glass part into -- suppresses from surface scratches. -- sequence: before cooling hot initial cooling cooler hot cooler -- Result: surface crack growth is. at room temp. compression tension compression Chapter 14-21

22 Ceramic Fabrication Methods (iia) GLASS FORMING PARTICULATE FORMING CEMENTATION forming: Mill (grind) and screen constituents: Extrude this mass (e.g., into a brick) force Ao container ram billet container die holder extrusion die Ad Adapted from Fig. 14.2(c), Callister & Rethwisch 4e. the formed piece Chapter 14-22

23 Ceramic Fabrication Methods (iia) GLASS FORMING : Mill (grind) and screen : desired particle size Mix with and other constituents to form slip operation pour slip into mold absorb water into mold green ceramic solid component PARTICULATE FORMING pour slip into mold drain mold hollow component CEMENTATION green ceramic Adapted from Fig , Callister & Rethwisch 4e. (Fig is from W.D. Kingery, Introduction to Ceramics, John Wiley and Sons, Inc., 1960.) Dry and fire the Chapter 14-23

24 Typical Porcelain Composition (50%) 1. Clay (25%) 2. e.g. quartz (finely ground) (25%) 3. (Feldspar) -- plus K +, Na +, Ca + -- upon firing - forms low-melting-temp. glass Chapter 14-24

25 Hydroplasticity of Clay Clay is inexpensive When is added to clay -- water molecules fit in between layered sheets -- reduces degree of bonding -- when external forces applied clay particles free to move past one another becomes Structure of Kaolinite Clay: Adapted from Fig. 3.14, Callister & Rethwisch 4e. (Fig is adapted from W.E. Hauth, "Crystal Chemistry of Ceramics", American Ceramic Society Bulletin, Vol. 30 (4), 1951, p. 140.) charge neutral Shear Shear charge neutral weak van der Waals bonding Si 4+ Al 3+ - OH O 2- Chapter 14-25

26 Drying and Firing : as water is removed - interparticle spacings decrease. Adapted from Fig , Callister & Rethwisch 4e. (Fig is from W.D. Kingery, Introduction to Ceramics, John Wiley and Sons, Inc., 1960.) wet body partially dry dry Drying too fast causes sample to warp or crack due to non-uniform Firing: -- heat treatment between ºC -- : liquid glass forms from clay and flux flows between SiO2 particles. (Flux lowers melting temperature). Adapted from Fig , Callister & Rethwisch 4e. (Fig is courtesy H.G. Brinkies, Swinburne University of Technology, Hawthorn Campus, micrograph of porcelain 70μm Hawthorn, Victoria, Australia.) Si0 2 particle (quartz) Chapter glass formed around the particle

27 Ceramic Fabrication Methods (iib) GLASS FORMING PARTICULATE FORMING CEMENTATION : used for both clay and non-clay compositions. (plus binder) compacted by pressure in a mold -- compression - compacted in single direction -- (hydrostatic) compression - fluid - powder in rubber envelope -- Hot pressing - pressure + heat Chapter 14-27

28 Sintering Sintering occurs during of a piece that has been -- powder particles coalesce and reduction of pore size Aluminum oxide powder: -- at 1700ºC for 6 minutes. Adapted from Fig , Callister & Rethwisch 4e. Adapted from Fig , Callister & Rethwisch 4e. (Fig is from W.D. Kingery, H.K. Bowen, and D.R. Uhlmann, Introduction to Ceramics, 2nd ed., John Wiley and Sons, Inc., 1976, p. 483.) 15 μm Chapter 14-28

29 Tape Casting Thin sheets of cast as flexible tape Used for integrated circuits and Slip = suspended ceramic particles + organic liquid (contains binders, ) Fig , Callister & Rethwisch 4e. Chapter 14-29

30 Ceramic Fabrication Methods (iii) GLASS FORMING PARTICULATE FORMING CEMENTATION Hardening of a paste formed by mixing cement material with water Formation of structures having varied and complex shapes Hardening process (complex chemical reactions involving ) Portland cement production of: -- mix clay and -- calcine (heat to 1400ºC) -- grind into fine powder Chapter 14-30

31 Polymer Formation There are two types of polymerization polymerization Chapter 14-31

32 Addition (Chain) Polymerization Initiation Chapter 14-32

33 Condensation (Step) Polymerization Chapter 14-33

34 Polymer Additives Improve mechanical properties, processability, durability, etc. Fillers Added to improve resistance, toughness & decrease cost ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc. Added to the glass transition temperature T g below room temperature Presence of transforms brittle polymer to a one Commonly added to PVC - otherwise it is brittle Chapter 14-34

35 Stabilizers Polymer Additives (cont.) UV protectants Added to allow polymer slides through easier ex: sodium stearate Colorants Substances containing chlorine, fluorine, and boron Chapter 14-35

36 Processing of Plastics can be reversibly, i.e. recycled heat until soft,, then cool ex: polyethylene, polypropylene, polystyrene. Thermoset when heated (chemical reaction) (doesn t melt) when heated a prepolymer molded into desired shape, then chemical reaction occurs ex: urethane, epoxy Chapter 14-36

37 Processing Plastics Compression Molding thermoplastics and thermosets placed in mold cavity mold assumes shape of mold Fig , Callister & Rethwisch 4e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley & Sons, 1984.) Chapter 14-37

38 Processing Plastics Injection Molding and some thermosets when ram retracts, plastic pellets drop from into barrel ram forces plastic into the heating chamber (around the spreader) where the molten plastic is forced under pressure ( ) into the mold where it assumes the shape of the mold Fig , Callister & Rethwisch 4e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition, John Wiley & Sons, 1971.) Barrel Chapter 14-38

39 Processing Plastics Extrusion plastic pellets drop from hopper onto the plastic melts passed the heaters molten polymer is forced under pressure through the shaping die to form the shaped product ( ) Fig , Callister & Rethwisch 4e. (Fig is from Encyclopædia Britannica, 1997.) Chapter 14-39

40 Processing Plastics Blown-Film Extrusion Fig , Callister & Rethwisch 4e. (Fig is from Encyclopædia Britannica, 1997.) Chapter 14-40

41 Summary Metal fabrication techniques: -- forming, casting, miscellaneous. Hardenability of metals -- measure of ability of a steel to be heat treated. -- increases with alloy content. Ceramic Fabrication techniques: -- glass forming (pressing, blowing, fiber drawing). -- particulate forming (hydroplastic forming, slip casting, powder pressing, tape casting) -- cementation Heat treating procedures -- glasses annealing, tempering -- particulate formed pieces drying, firing (sintering) Polymer Processing -- compression and injection molding, extrusion, blown film extrusion Chapter 14-41