Fabrication and Processing Methods PDF

Fabrication of Materials Chapter 11 - 1  Metal Fabrication How do we fabricate metals? – Blacksmith - hammer (forged) – Molding - cast Forming Operations – Rough stock formed to final shape Hot working vs. Cold working T high...

Fabrication of Materials Chapter 11 - 1  Metal Fabrication How do we fabricate metals? – Blacksmith - hammer (forged) – Molding - cast Forming Operations – Rough stock formed to final shape Hot working vs. Cold working T high enough for well below Tm recrystallization work hardening Larger deformations smaller deformations Chapter 11 - 2 Metal Fabrication Chapter 11 - 3  Metal Fabrication Methods - I FORMING CASTING JOINING Forging (Hammering; Stamping) Rolling (Hot or Cold Rolling) (wrenches, crankshafts) (I-beams, rails, sheet & plate) force die roll Ad A o blank A d often at Ao elev. T roll Adapted from force Drawing Extrusion Fig. 11.8, Callister 7e. (rods, wire, tubing) (rods, tubing) Ao die Ad container tensile die holder Ao force force ram billet extrusion Ad die container die die must be well lubricated & clean ductile metals, e.g. Cu, Al (hot) Chapter 11 - 4  Metal Fabrication Methods - II FORMING CASTING JOINING Casting- mold is filled with metal – metal melted in furnace, perhaps alloying elements added. Then cast in a mold – most common, cheapest method – gives good production of shapes – weaker products, internal defects – good option for brittle materials Chapter 11 - 5  Metal Fabrication Methods - II FORMING CASTING JOINING Sand Casting (large parts, e.g., trying to hold something that is hot auto engine blocks) what will withstand >1600ºC? Sand Sand cheap - easy to mold => sand!!! molten metal pack sand around form (pattern) of desired shape Chapter 11 - 6  Metal Fabrication Methods - II FORMING CASTING JOINING Sand Casting (large parts, e.g., auto engine blocks) Investment Casting pattern is made from paraffin. Sand Sand mold made by encasing in molten metal plaster of paris melt the wax & the hollow mold Investment Casting is left (low volume, complex shapes e.g., jewelry, turbine blades) pour in metal plaster die formed around wax wax prototype Chapter 11 - 7  Metal Fabrication Methods - II FORMING CASTING JOINING Sand Casting Die Casting (large parts, e.g., (high volume, low T alloys) auto engine blocks) Sand Sand molten metal Continuous Casting Investment Casting (simple slab shapes) (low volume, complex shapes molten e.g., jewelry, turbine blades) plaster solidified die formed around wax wax prototype Chapter 11 - 8  Metal Fabrication Methods - III FORMING CASTING JOINING Powder Metallurgy Welding (materials w/low ductility) (when one large part is impractical) pressure filler metal (melted) base metal (melted) fused base metal heat heat affected zone area unaffected unaffected contact piece 1 piece 2 Adapted from Fig. 11.9, Callister 7e. densify (Fig. 11.9 from Iron Castings Heat affected zone: Handbook, C.F. point contact densification Walton and T.J. by diffusion at (region in which the Opar (Ed.), 1981.) at low T higher T microstructure has been changed). Chapter 11 - 9  Thermal Processing of Metals Annealing: Heat to Tanneal, then cool slowly. Stress Relief: Reduce Spheroidize (steels): stress caused by: Make very soft steels for -plastic deformation good machining. Heat just -nonuniform cooling below TE & hold for -phase transform. 15-25 h. Full Anneal (steels): Types of Make soft steels for good forming by heating Annealing to get g, then cool in furnace to get coarse P. Process Anneal: Negate effect of Normalize (steels): cold working by Deform steel with large (recovery/ grains, then normalize recrystallization) to make grains small. Based on discussion in Section 11.7, Callister 7e. Chapter 11 - 10  Ceramic Fabrication Methods-I GLASS PARTICULATE CEMENTATION FORMING FORMING Pressing: Pressing Gob operation plates, dishes, cheap glasses --mold is steel with Parison graphite lining mold Fiber drawing: Compressed Blowing: air suspended Parison Finishing mold wind up Adapted from Fig. 13.8, Callister, 7e. (Fig. 13.8 is adapted from C.J. Phillips, Glass: The Miracle Maker, Pittman Publishing Ltd., London.) Chapter 11 - 11  Sheet Glass Forming Sheet forming – continuous draw – originally sheet glass was made by “floating” glass on a pool of mercury Adapted from Fig. 13.9, Callister 7e. Chapter 11 - 12  Heat Treating Glass Annealing: --removes internal stress caused by uneven cooling. Tempering: --puts surface of glass part into compression --suppresses growth of cracks from surface scratches. --sequence: before cooling surface cooling further cooled cooler compression hot hot tension cooler compression --Result: surface crack growth is suppressed. Chapter 11 - 13  Ceramic Fabrication Methods-IIA GLASS PARTICULATE CEMENTATION FORMING FORMING Milling and screening: desired particle size Mixing particles & water: produces a "slip" Form a "green" component Ao container die holder Adapted from --Hydroplastic forming: force ram bille extrusion Ad Fig. 11.8 (c), extrude the slip (e.g., into a pipe) t Callister 7e. container die --Slip casting: pour slip absorb water pour slip drain “green into mold into mold into mold mold ceramic” Adapted from Fig. “green 13.12, Callister 7e. ceramic” (Fig. 13.12 is from W.D. Kingery, Introduction to Ceramics, John Wiley and Sons, Inc., 1960.) solid component hollow component Dry and fire the component Chapter 11 - 14  Ceramic Fabrication Methods-IIB GLASS PARTICULATE CEMENTATION FORMING FORMING Sintering: useful for both clay and non-clay compositions. Procedure: -- produce ceramic and/or glass particles by grinding -- place particles in mold -- press at elevated T to reduce pore size. Aluminum oxide powder: -- sintered at 1700°C for 6 minutes. Adapted from Fig. 13.17, Callister 7e. (Fig. 13.17 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 m Chapter 11 - 15  Powder Pressing Sintering - powder touches - forms neck & gradually neck thickens – add processing aids to help form neck – little or no plastic deformation Uniaxial compression - compacted in single direction Isostatic (hydrostatic) compression - pressure applied by fluid - powder in rubber envelope Hot pressing - pressure + heat Adapted from Fig. 13.16, Callister 7e. Chapter 11 - 16  Tape Casting thin sheets of green ceramic cast as flexible tape used for integrated circuits and capacitors cast from liquid slip (ceramic + organic solvent) Adapted from Fig. 13.18, Callister 7e. Chapter 11 - 17  Ceramic Fabrication Methods-III GLASS PARTICULATE CEMENTATION FORMING FORMING Produced in extremely large quantities. Portland cement: -- mix clay and lime bearing materials -- calcinate (heat to 1400°C) -- primary constituents: tri-calcium silicate di-calcium silicate Adding water -- produces a paste which hardens -- hardening occurs due to hydration (chemical reactions with the water). Forming: done usually minutes after hydration begins. Chapter 11 - 18  Processing of Plastics Quite a variety of different techniques are employed in the forming of polymeric materials. e method used for a specific polymer depends on several factors: (1)Whether the material is thermoplastic or thermosetting; (2) if thermoplastic, the temperature at which it softens; (3)the atmospheric stability of the material being formed; and (4) the geometry and size of the finished product. There are numerous similarities between some of these techniques and those utilized for fabricating metals and ceramics. Chapter 11 - 19  Processing of Plastics Thermoplastic – – can be reversibly cooled & reheated, i.e. recycled – heat till soft, shape as desired, then cool – ex: polyethylene, polypropylene, polystyrene, etc. Thermoset – when heated forms a network – degrades (not melts) when heated – mold the prepolymer then allow further reaction – ex: urethane, epoxy Chapter 11 - 20  Processing Plastics - Molding Molding is the most common method for forming plastic polymers. The several molding techniques used include compression, transfer, blow, injection, and extrusion molding. For each, a finely pelletized or granulized plastic is forced, at an elevated temperature and by pressure, to flow into, fill, and assume the shape of a mold cavity. Chapter 11 - 21  Compression and transfer molding For compression molding, the appropriate amounts of thoroughly mixed polymer and necessary additives are placed between male and female mold members, as illustrated in Fig. Both mold pieces are heated; however, only one is movable. The mold is closed, and heat and pressure are applied, causing the plastic to become viscous and flow to conform to the mold shape. Before molding, raw materials may be mixed and cold pressed into a disc, which is called a preform. Preheating of the preform reduces molding time and pressure, extends the die lifetime, and produces a more uniform finished piece. This molding technique lends itself to the fabrication of both thermoplastic and thermosetting polymers; however, its use with thermoplastics is more time-consuming and expensive than the more commonly used extrusion or injection molding techniques discussed below. In transfer molding, a variation of compression molding, the solid ingredients are first melted in a heated transfer chamber. As the molten material is injected into the mold chamber, the pressure is distributed more uniformly over all surfaces. This process is used with thermosetting polymers and for pieces having complex geometries. Chapter 11 - 22  Processing Plastics - Molding Compression and transfer molding – thermoplastic or thermoset Adapted from Fig. 15.23, Callister 7e. (Fig. 15.23 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley & Sons, 1984. ) Chapter 11 - 23  Injection molding Injection molding, the polymer analogue of die casting for metals, is the most widely used technique for fabricating thermoplastic materials. A schematic cross section of the apparatus used is illustrated in Fig. The correct amount of pelletized material is fed from a feed hopper into a cylinder by the motion of a plunger or ram. This charge is pushed forward into a heating chamber where it is forced around a spreader so as to make better contact with the heated wall. As a result, the thermoplastic material melts to form a viscous liquid. Next, the molten plastic is impelled, again by ram motion, through a nozzle into the enclosed mold cavity; pressure is maintained until the molding has solidified. Finally, the mold is opened, the piece is ejected, the mold is closed, and the entire cycle is repeated. Chapter 11 - 24  Processing Plastics - Molding Injection molding – thermoplastic & some thermosets Adapted from Fig. 15.24, Callister 7e. (Fig. 15.24 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition, John Wiley & Sons, 1971. ) Chapter 11 - 25  Extrusion The extrusion process is the molding of a viscous thermoplastic under pressure through an open-ended die, similar to the extrusion of metals. A mechanical screw propels through a chamber the pelletized material, which is successively compacted, melted, and formed into a continuous charge of viscous fluid. Extrusion takes place as this molten mass is forced through a die orifice. Solidification of the extruded length is expedited by blowers, a water spray, or bath. The technique is especially adapted to producing continuous lengths having constant cross-sectional geometries—for example, rods, tubes, hose channels, sheets, and filaments. Chapter 11 - 26 Processing Plastics – Extrusion Adapted from Fig. 15.25, Callister 7e. (Fig. 15.25 is from Encyclopædia Britannica, 1997.) Chapter 11 - 27  Blow Molding The blow-molding process for the fabrication of plastic containers is similar to that used for blowing glass bottles, as represented in Fig. First, a parison, or length of polymer tubing, is extruded. While still in a semimolten state, the parison is placed in a two-piece mold having the desired container configuration. The hollow piece is formed by blowing air or steam under pressure into the parison, forcing the tube walls to conform to the contours of the mold. Of course the temperature and viscosity of the parison must be carefully regulated. Chapter 11 - 28 Processing Plastics – Blow molding Chapter 11 - 29  Casting Like metals, polymeric materials may be cast, as when a molten plastic material is poured into a mold and allowed to solidify. Both thermoplastic and thermosetting plastics may be cast. For thermoplastics, solidification occurs upon cooling from the molten state; however, for thermosets, hardening is a consequence of the actual polymerization or curing process, which is usually carried out at an elevated temperature. Chapter 11 - 30