CHAPTER. Fabrication and Processing of Engineering Materials. Chapter 17 -

Transcription

1 CHAPTER 17 Fabrication and Processing of Engineering Materials Chapter 17-1

2 Chapter 17: Fabrication and Processing of Engineering 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 primary polymer processing methods? Chapter 17-2

3 1. Introduction 2. (Metal) Forming Operations 3. Casting 4. Miscellaneous Techniques 5. Annealing Processes 6. Heat Treatment of Steels 7. Precipitation Hardening 8. Fabrication and Processing of Glasses and Glass-Ceramics 9. Fabrication and Processing of Clay Products 10. Powder Pressing 11. Tape Casting 12. Polymerization 13. Polymer Additives 14. Forming Techniques for Plastics 15. Fabrication of Elastomers 16. Fabrication of Fibers and Films Chapter 17-3

4 Fabrication and processing of engineering materials Materials are formed or manufactured into components or products Applying some type of processing treatment to achieve the required properties Suitability of a materials for an application Is dictated by economic considerations with respect to processing methods Various techniques Chapter 17-4

5 The aluminum can in various stages of production From a single sheet of an Al alloy. Drawing, dome forming, trimming, cleaning, decorating, neck and flange forming Chapter 17-5

6 Metal Fabrication How do we fabricate metals? Blacksmith - hammer (forged) Cast molten metal into mold Forming Operations Rough stock formed to final shape Hot working vs. Cold working Deformation temperature high enough for recrystallization Large deformations Deformation below recrystallization temperature Strain hardening occurs Small deformations Chapter 17-6

7 Fig_17_01 Chapter 17 -

8 2. Metal Fabrication Methods (i) FORMING Forging (Hammering; Stamping) (wrenches, crankshafts) A o die blank force force Drawing (rods, wire, tubing) A o die die A d A d tensile force often at elev. T die must be well lubricated & clean CASTING Rolling (Hot or Cold Rolling) (I-beams, rails, sheet & plate) A o force A o MISCELLANEOUS ram roll roll Extrusion (rods, tubing) container billet container A d Adapted from Fig. 17.2, Callister & Rethwisch 9e. die holder extrusion die ductile metals, e.g. Cu, Al (hot) A d Chapter 17-8

9 3. Metal Fabrication Methods (ii) 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 production of shapes weaker products, internal defects good option for brittle materials Chapter 17-9

10 Metal Fabrication Methods (iii) FORMING CASTING MISCELLANEOUS Sand Casting (large parts, e.g., auto engine blocks) Sand Sand molten metal What material will withstand T >1600 C and is inexpensive and easy to mold? Answer: sand!!! To create mold, pack sand around form (pattern) of desired shape Chapter 17-10

11 Metal Fabrication Methods (iv) FORMING CASTING MISCELLANEOUS Investment Casting (Lost-wax) (Lost-foam) (low volume, complex shapes e.g., jewelry, turbine blades) Stage I Mold formed by pouring plaster of paris 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 Molten metal is poured into mold and allowed to solidify. II III Chapter 17-11

12 Metal Fabrication Methods (v) FORMING CASTING MISCELLANEOUS Die Casting -- high volume -- for alloys having low melting temperatures Continuous Casting -- simple shapes (e.g., rectangular slabs, cylinders) molten solidified Chapter 17-12

13 4. Metal Fabrication Methods (vi) FORMING Powder Metallurgy (metals w/low ductilities) point contact at low T pressure densify heat area contact densification by diffusion at higher T CASTING MISCELLANEOUS Welding (when fabrication of one large part is impractical) filler metal (melted) base metal (melted) fused base metal unaffected heat-affected zone unaffected piece 1 piece 2 Heat-affected zone: (region in which the microstructure has been changed). Chapter 17-13

14 Thermal Processing of Metals 5. Annealing: Heat to Tanneal, then cool slowly. Stress Relief: Reduce stresses resulting from: - plastic deformation - nonuniform cooling - phase transform. Spheroidize (steels): Make very soft steels for good machining. Heat just below T eutectoid & hold for h. Process Anneal: 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. Normalize (steels): Deform steel with large grains. Then heat treat to allow recrystallization and formation of smaller grains. Based on discussion in Section 17.5, Callister & Rethwisch 9e. Chapter 17-14

15 6. Heat Treatment Temperature-Time Paths a) Full Annealing b) Quenching c) Tempering (Tempered Martensite) A A B P b) a) c) Chapter 17-15

16 Hardness, HRC Hardenability -- Steels Hardenability measure of the ability to form martensite Jominy end quench test used to measure hardenability. specimen (heated to γ phase field) 24 C water flat ground Rockwell C hardness tests Plot hardness versus distance from the quenched end. Fig Distance from quenched end Chapter 17-16

17 Hardness, HRC Reason Why Hardness Changes with Distance The cooling rate decreases with distance from quenched end T( C) distance from quenched end (in) 0% 100% M(start) M(finish) A M Time (s) Chapter 17-17

18 Hardness, HRC Hardenability vs Alloy Composition Hardenability curves for five alloys each with, C = 0.4 wt% C Fig. 17.8, "Alloy Steels" (4140, 4340, 5140, 8640) -- contain Ni, Cr, Mo (0.2 to 2 wt%) -- these elements shift the "nose" to longer times (from A to B) -- martensite is easier to form Distance from quenched end (mm) 800 T( C) A B 4340 T E Cooling rate ( C/s) M(start) M(90%) %M Time (s) Chapter 17-18

19 Influences of Quenching Medium & Specimen Geometry Effect of quenching medium: Medium air oil water Severity of Quench low moderate high 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 low moderate high Hardness low high Chapter 17-19

20 Table_17_01 Chapter 17 -

21 Table_17_02 Chapter 17 -

22 7. Precipitation Hardening Particles impede dislocation motion. Ex: Al-Cu system 700 T(ºC) Procedure: 600 α -- Pt A: solution heat treat A (get α solid solution) Pt B: quench to room temp. 400 C (retain α solid solution) -- Pt C: reheat to nucleate small θ particles within (Al) α phase. Other alloys that precipitation harden: Cu-Be Cu-Sn Mg-Al Temp. Pt A (sol n heat treat) + L B Pt C (precipitate ) L α + θ θ + L CuAl 2 θ wt% Cu composition range available for precipitation hardening Fig Pt B Time Chapter 17-22

23 tensile strength (MPa) %EL (2 in sample) 2014 Al Alloy: Influence of Precipitation Heat Treatment on TS, %EL Maxima on TS curves. Increasing T accelerates process. Minima on %EL curves ºC 204ºC 1min 1h 1day 1mo 1yr precipitation heat treat time ºC 149ºC 1min 1h 1day 1mo 1yr precipitation heat treat time Fig , Chapter 17-23

24 A supersaturated solid solution A transition precipitate phase The equilibrium phase within the matrix phase Fig_17_19 Chapter 17 -

25 A TEM micrograph showing the microstructure of a precipitation hardened.7150-t651 aluminum alloy (6.2 wt% Zn, 2.3 wt% Cu, 2.3 wt% Mg, 0.12 wt% Zr, the balance Al) The light matrix phase in the micrograph is an aluminum solid solution. The majority of the small plate-shaped dark precipitate particles are a transition phase, the remainder being the equilibrium (MgZn 2 ) phase. Chapter 17 -

26 Precipitation Hardening Some alloys strengthened by small particles of a second, or precipitate, phase. Control of particle size and, subsequently, strength is accomplished by two heat treatments: 1. solution, heat treatment, all solute atoms are dissolved to form a single-phase solid solution; quenching to a relatively low temperature preserves this state. 2. precipitation, treatment (at constant T), precipitate particles form and grow; strength, hardness, and ductility depend on heat-treating time (and particle size). Strength and hardness increase with aging time to a maximum and then decrease during overaging. This process is accelerated with rising temperature The strengthening phenomenon is explained in terms of an increased resistance to dislocation motion by lattice strains that are established in the vicinity of these microscopically small precipitate particles. Chapter 17-26

27 Processing/Structure/Properties/Performance Summary At this time, we have completed our processing/structure/properties/performance commentary for steels. By way of summary, Figure shows relationships of these processing, structure, and property elements for this group of alloys. It was compiled from summaries provided in previous chapters and, in addition, includes topics discussed in this chapter. Chapter 17-27

28 Steel alloys: from a materials science perspective Fig_11-29 Chapter 17 -

29 Steel alloys: from a materials engineering perspective Chapter 17 -

30 1. Introduction 2. (Metal) Forming Operations 3. Casting 4. Miscellaneous Techniques 5. Annealing Processes 6. Heat Treatment of Steels 7. Precipitation Hardening 8. Fabrication and Processing of Glasses and Glass-Ceramics 9. Fabrication and Processing of Clay Products 10. Powder Pressing 11. Tape Casting 12. Polymerization 13. Polymer Additives 14. Forming Techniques for Plastics 15. Fabrication of Elastomers 16. Fabrication of Fibers and Films Chapter 17-30

31 8. Ceramic Fabrication Methods (i) GLASS FORMING Blowing of Glass Bottles: Parison mold Gob PARTICULATE FORMING Pressing operation Compressed air CEMENTATION Pressing: plates, cheap glasses -- glass formed by application of pressure -- mold is steel with graphite lining Fiber drawing: Suspended parison Fig , Finishing mold wind up Chapter 17-31

32 Sheet Glass Forming Sheet forming continuous casting sheets are formed by floating the molten glass on a pool of molten tin Fig Chapter 17-32

33 Basic Unit: 4- Si0 4 tetrahedron Si 4+ Glass Structure O 2 - Glass is noncrystalline (amorphous) Fused silica is SiO 2 to which no impurities have been added Other common glasses contain impurity ions such as Na +, Ca 2+, Al 3+, and B 3+ Quartz is crystalline SiO2: Na + Si 4+ O 2 - (soda glass) Chapter 17-33

34 Glass Properties Specific volume (1/ρ) vs Temperature (T ): Specific volume Supercooled Liquid Liquid (disordered) Crystalline materials: -- crystallize at melting temp, T m -- have abrupt change in spec. vol. at T m Glass (amorphous solid) Crystalline (i.e., ordered) T g T m Adapted from Fig , solid T Glasses: -- do not crystallize -- change in slope in spec. vol. curve at glass transition temperature, T g -- transparent - no grain boundaries to scatter light Chapter 17-34

35 Glass Properties: Viscosity Viscosity, η: -- relates shear stress (τ) and velocity gradient (dv/dy): τ τ glass dy dv dv dy velocity gradient η has units of (Pa-s) Chapter 17-35

36 Viscosity [Pa-s] Log Glass Viscosity vs. Temperature Viscosity 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 strain point annealing point 10 6 Working range: glass-forming carried out 10 2 T 1 melt Fig T(ºC) Chapter 17-36

37 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 Heat Treating Glass initial cooling cooler hot cooler -- Result: surface crack growth is suppressed. at room temp. compression tension compression Chapter 17-37

38 Fabrication and Heat-Treating of Glass-Ceramics Glass-ceramics are initially fabricated as glasses and then, by heat treatment, crystallized to form fine-grained polycrystalline materials. Two properties of glass-ceramics that make them superior to glass are improved mechanical strengths and lower coeff. of thermal expansion (improves thermal shock resistance). Typical time-versus-temperature processing cycle for a Li 2 O-Al 2 O 3 - SiO 2 glass-ceramic. Fig_17_28 Chapter 17-38

39 9. Ceramic Fabrication Methods (iia) GLASS FORMING Hydroplastic forming: PARTICULATE FORMING CEMENTATION Mill (grind) and screen constituents: desired particle size Extrude this mass (e.g., into a brick) force A o ram container billet container die holder extrusion die A d Fig (c), Callister & Rethwisch 9e. Dry and fire the formed piece Chapter 17-39

40 Ceramic Fabrication Methods (iia) GLASS FORMING Slip casting: Mill (grind) and screen constituents: desired particle size Mix with water and other constituents to form slip Slip casting operation pour slip into mold absorb water into mold green ceramic PARTICULATE FORMING pour slip into mold drain mold CEMENTATION green ceramic Fig solid component hollow component Dry and fire the cast piece Chapter 17-40

41 Typical Porcelain Composition (50%) 1. Clay (25%) 2. Filler e.g. quartz (finely ground) (25%) 3. Fluxing agent (Feldspar) -- aluminosilicates plus K +, Na +, Ca + -- upon firing - forms low-melting-temp. glass Chapter 17-41

42 Hydroplasticity of Clay Clay is inexpensive When water is added to clay -- water molecules fit in between layered sheets -- reduces degree of van der Waals bonding -- when external forces applied clay particles free to move past one another becomes hydroplastic Structure of Kaolinite Clay: Fig. 4.15, charge neutral Shear charge neutral weak van der Waals bonding Si 4+ Al 3+ - OH O 2- Shear Chapter 17-42

43 micrograph of porcelain Drying and Firing Drying: as water is removed - interparticle spacings decrease shrinkage. Fig wet body partially dry completely dry Drying too fast causes sample to warp or crack due to non-uniform shrinkage Firing: -- heat treatment between C -- vitrification: liquid glass forms from clay and flux flows between SiO2 particles. (Flux 70 μm Si0 2 particle (quartz) glass formed around the particle lowers melting temperature). Fig , Chapter 17-43

44 10. Ceramic Fabrication Methods (iib) GLASS FORMING PARTICULATE FORMING CEMENTATION Powder Pressing: used for both clay and nonclay compositions. Powder (plus binder) compacted by pressure in a mold -- Uniaxial compression - compacted in single direction -- Isostatic (hydrostatic) compression - pressure applied by fluid - powder in rubber envelope -- Hot pressing - pressure + heat Fig_17_32 Chapter 17-44

45 Sintering Sintering occurs during firing of a piece that has been powder pressed -- powder particles coalesce and reduction of pore size Aluminum oxide powder: -- sintered at 1700 C for 6 minutes. Fig ,. Fig , Callister & Rethwisch 9e. 15 μm Chapter 17-45

46 11. Tape Casting Thin sheets of green ceramic cast as flexible tape Used for integrated circuits and capacitors Slip = suspended ceramic particles + organic liquid (contains binders, plasticizers) Fig Chapter 17-46

47 Ceramic Fabrication Methods (iii) GLASS FORMING PARTICULATE FORMING CEMENTATION Hardening of a paste paste formed by mixing cement material with water Formation of rigid structures having varied and complex shapes Hardening process hydration (complex chemical reactions involving water and cement particles) Portland cement production of: -- mix clay and lime-bearing minerals -- calcine (heat to 1400 C) -- grind into fine powder Chapter 17-47

48 1. Introduction 2. (Metal) Forming Operations 3. Casting 4. Miscellaneous Techniques 5. Annealing Processes 6. Heat Treatment of Steels 7. Precipitation Hardening 8. Fabrication and Processing of Glasses and Glass-Ceramics 9. Fabrication and Processing of Clay Products 10. Powder Pressing 11. Tape Casting 12. Polymerization 13. Polymer Additives 14. Forming Techniques for Plastics 15. Fabrication of Elastomers 16. Fabrication of Fibers and Films Chapter 17-48

49 12. Polymer Formation There are two types of polymerization Addition (or chain) polymerization Condensation (step) polymerization For addition polymerization, monomer units are attached one at a time in chainlike fashion to form a linear molecule. Condensation polymerization involves stepwise intermolecular chemical reactions that may include more than a single molecular species. Chapter 17-49

50 Addition (Chain) Polymerization Initiation Propagation Termination Chapter 17-50

51 Condensation (Step) Polymerization Chapter 17-51

52 13. Polymer Additives Improve mechanical properties, processability, durability, etc. Fillers Added to improve tensile strength & abrasion resistance, toughness & decrease cost ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc. Plasticizers Added to reduce the glass transition temperature T g below room temperature Presence of plasticizer transforms brittle polymer to a ductile one Commonly added to PVC - otherwise it is brittle Chapter 17-52

53 Stabilizers Polymer Additives (cont.) Antioxidants UV protectants Lubricants Added to allow easier processing polymer slides through dies easier ex: sodium stearate Colorants Dyes and pigments Flame Retardants Substances containing chlorine, fluorine, and boron Chapter 17-53

54 14. Processing of Plastics Thermoplastic can be reversibly cooled & reheated, i.e. recycled heat until soft, shape as desired, then cool ex: polyethylene, polypropylene, polystyrene. Thermoset when heated forms a molecular network (chemical reaction) degrades (doesn t melt) when heated a prepolymer molded into desired shape, then chemical reaction occurs ex: urethane, epoxy Chapter 17-54

55 Processing Plastics Compression Molding Thermoplastics and thermosets polymer and additives placed in mold cavity mold heated and pressure applied fluid polymer assumes shape of mold Fig , Callister & Rethwisch 9e. (From F. W. Billmeyer, Jr., Textbook of Polymer Science, 3rd edition. Copyright 1984 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) Chapter 17-55

56 Processing Plastics Injection Molding Thermoplastics and some thermosets when ram retracts, plastic pellets drop from hopper into barrel ram forces plastic into the heating chamber (around the spreader) where the plastic melts as it moves forward molten plastic is forced under pressure (injected) into the mold cavity where it assumes the shape of the mold Fig , Callister & Rethwisch 9e. (From F. W. Billmeyer, Jr., Textbook of Polymer Science, 3rd edition. Copyright 1984 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) Barrel Chapter 17-56

57 Processing Plastics Extrusion thermoplastics plastic pellets drop from hopper onto the turning screw plastic 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 , Callister & Rethwisch 9e. (Reprinted with permission from Encyclopæ dia Britannica, 1997 by Encyclopæ dia Britannica, Inc.) Chapter 17-57

58 15. Fabrication of elastomers The same as for plastics -- compression and injection molding, extrusion, blown film extrusion Ruber materials are vulcanized Some are reinforced with carbon black Chapter 17-58

59 16. Processing Plastics Blown-Film Extrusion Fig , Callister & Rethwisch 9e. (Reprinted with permission from Encyclopæ dia Britannica, 1997 by Encyclopæ dia Britannica, Inc.) Chapter 17-59

60 Summary Metal fabrication techniques: -- forming, casting, miscellaneous. Hardenability of metals -- measure of ability of a steel to be heat treated. -- increases with alloy content. Precipitation hardening --hardening, strengthening due to formation of precipitate particles. --Al, Mg alloys precipitation hardenable. Chapter 17-60

61 Summary (Cont.) Ceramic Fabrication techniques: -- glass forming (pressing, blowing, fiber drawing). -- particulate forming (hydroplastic forming, slip casting, powder pressing, tape casting) -- cementation Heat treating procedures for ceramics -- glasses annealing, tempering -- particulate formed pieces drying, firing (sintering) Polymer Processing -- compression and injection molding, extrusion, blown film extrusion Polymer melting and glass transition temperatures Chapter 17-61