Powder-Metal Processing and Equipment Text Reference: Manufacturing Engineering and Technology, Kalpakjian & Schmid, 6/e, 2010 Chapter 17
Powder Metallurgy Metal powders are compacted into desired and often complex shapes and sintered* to form a solid piece * Sinter: To heat without melting
FIGURE 17.1 (a) Examples of typical parts made by powder- metallurgy processes. (b) Upper trip lever for a commercial irrigation sprinkler made by PM. This part is made of an unleaded brass alloy; it replaces a die-cast part with a 60% cost savings. (c) Main-bearing metal-powder caps for 3.8- and 3.1-liter General Motors automotive engines. Source: (a) and (b) Reproduced with permission from Success Stories on PM Parts, 1998. Metal Powder Industries Federation, Princeton, New Jersey, 1998. (c) Courtesy of Zenith Sintered Products, Inc., Milwaukee, Wisconsin.
FIGURE 17.2 Outline of processes and operations involved in producing powder-metallurgy parts.
FIGURE 17.3 Particle shapes in metal powders, and the processes by which they are produced. Iron powders are produced by many of these processes (see also Fig. 17.4).
FIGURE 17.4 (a) Scanning-electron microscope image of iron-powder particles made by atomization. (b) Nickel-based superalloy (Udimet 700) powder particles made by the rotating electrode process; see Fig. 17.5d. Source: Courtesy of P.G. Nash, Illinois Institute of Technology, Chicago.
FIGURE 17.5 Methods of metal-powder production by atomization: (a) gas atomization; (b) water atomization; (c) centrifugal atomization with a spinning disk or cup; and (d) atomization with a rotating consumable electrode.
FIGURE 17.6 Methods of mechanical comminution to obtain fine particles: (a) roll crushing, (b) ball mill, and (c) hammer milling.
FIGURE 17.7 Mechanical alloying of nickel particles with dispersed smaller particles. As nickel particles are flattened between two balls, the second, smaller phase is impressed into the nickel surface and eventually is dispersed throughout the particle due to successive flattening, fracture, and welding events.
FIGURE 17.8 (a) through (d) Some common bowl geometries for mixing or blending powders. (e) A mixer suitable for blending metal powders. Since metal powders are abrasive, mixers rely on the rotation or tumbling of enclosed geometries, as opposed to using aggressive agitators. Source: Courtesy of Kemutec Group, Inc.
FIGURE 17.9 (a) Compaction of metal powder to form a bushing. The pressed-powder part is called green compact. (b) Typical tool and die set for compacting a spur gear. Source: Courtesy of the Metal Powder Industries Federation.
FIGURE 17.10 (a) Density of copper- and iron-powder compacts as a function of compacting pressure. Density greatly influences the mechanical and physical properties of PM parts. (b) Effect of density on tensile strength, elongation, and electrical conductivity of copper powder. Source: (a) After F.V. Lenel, (b) After the International Annealed Copper Standard (IACS) for electrical conductivity.
FIGURE 17.11 Density variation in compacting metal powders in various dies: (a) and (c) single-action press; (b) and (d) double-action press. Note in (d) the greater uniformity of density from pressing with two punches with separate movements compared with (c). (e) Pressure contours in compacted copper powder in a single-action press. Source: After P. Duwez and L. Zwell.
TABLE 17.11 Compacting Pressures for Various Powders
FIGURE 17.12 A 7.3-MN (825-ton) mechanical press for compacting metal powder. Source: Courtesy of Cincinnati i Incorporated.
FIGURE 17.13 Schematic diagram of cold isostatic pressing. Pressure is applied isostatically inside a high-pressure chamber. (a) The wet bag process to form a cup-shaped part. The powder is enclosed in a flexible container around a solid-core rod. (b) The dry bag process used to form a PM cylinder.
FIGURE 17.14 Capabilities, with respect to part size and shape complexity, available from various PM operations. PF = powder forging. Source: Courtesy of the Metal Powder Industries Federation.
FIGURE 17.15 Schematic illustration of hot isostatic pressing. The pressure and temperature variation versus time are shown in the diagram.
FIGURE 17.16 A valve lifter for heavy-duty diesel engines produced from a hot-isostatic-pressed carbide cap on a steel shaft. Source: Courtesy of the Metal Powder Industries Federation.
Powder-Injection Molding (An alternative to conventional compaction) Very fine metal powders are blended with a 25 45% polymer or a wax-based binder Mixture is injected into a mold at 135 o 200 o C Debinding: Molded green parts placed in oven to burn off the plastic Sinter (up to 1375 o C) Useful to make complex shapes with small wall thickness
FIGURE 17.17 Powder-metal components for mobile phones to achieve a flip-open feature.
FIGURE 17.18 18 An illustration of powder rolling.
FIGURE 17.19 Spray deposition (Osprey process) in which molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipe.
TABLE 17.2 Sintering Temperature and Time for Various Metals
FIGURE 17.20 Schematic illustration of two mechanisms for sintering metal powders: (a) solid-state material transport; and (b) vapor-phase material transport. R = particle radius, r = neck radius, and ρ = neckprofile radius.
TABLE 17.3 Mechanical Properties of Selected PM Materials
TABLE 17.4 Comparison of Mechanical Properties of Some Wrought and Equivalent PM Metals (as Sintered)
TABLE 17.5 Mechanical Property Comparisons for Ti-6AL-4V Titanium Alloy
Secondary & Finishing Operations Coining & Sizing improve dimensional accuracy, strength, th surface finish i Forging of sintered preform to final desired shape produces part with good surface finish i and dimensional tolerances Machining, i Grinding, Plating, Heat-treating t ti Impregnate porous PM components with oil to produce permanently lubricated parts Infiltrate a lower melting point metal to fill pores, improve corrosion resistance, tensile strength th Electroplate (in some conditions) a PM part to seal surface & eliminate i its permeability
Design Considerations Keep shape of compact as simple and uniform as possible Provide for ejection of green compact from die without damaging the compact Make PM parts with widest acceptable tolerances Avoid very thin wall thickness Produce steps in parts if they are simple and their size doesn t exceed 15% of overall part length Press embossed or recessed letters perpendicular to the direction of pressing
(More) Design Considerations Make sure large flanges have generous fillets and tapers Avoid sharp corners; use chamfers & flats Form keys, keyways, etc. during powder compaction Orient notches & grooves perpendicular to pressing direction Maintain uniform wall thickness Dimensional tolerance range ±0.05 to 0.1 mm
FIGURE 17.21 Die geometry and design features for powder-metal compaction. Source: Courtesy of the Metal Powder Industries Federation.
FIGURE 17.22 Examples of PM parts showing poor and good designs. Note that sharp radii and reentry corners should be avoided and that threads and transverse holes have to be produced separately by additional machining operations. Source: Courtesy of the Metal Powder Industries Federation.
FIGURE 17.23 (a) Design features to use with unsupported flanges. (b) Design features for use with grooves. Source: Courtesy of Metal Powder Industries Federation.
Process Capabilities Can make parts from high melting point refractory metals, difficult otherwise High production rates possible on relatively complex parts using automated equipment Produce parts with good dimensional control, reducing need for machining Wide range of compositions for specialized applications Capability for impregnation & infiltration Limitations - High cost of metal powder - High cost of tooling for small production runs - Limitations on part size & shape complexity - Strength th & ductility < forging
TABLE 17.6 Forged and PM Titanium Parts and Cost Savings
FIGURE 17.24 Collection of PM parts in a commercial snowblower.
Summary Net shape forming process; low size & weight; good dimensional accuracy May perform secondary & finishing operations if desired Control product quality by powder quality, process variables, sintering atmospheres Powder injection molding by mixing very fine powders with polymers, later evaporated Design considerations