Powder-Metal P i d E i tProcessing and Equipment

Text Reference: “Manufacturing Engineering and Technology”, Kalpakjian & Schmid 6/e 2010Kalpakjian & Schmid, 6/e, 2010

Chapter 17

Powder MetallurgyPowder Metallurgy

Metal powders are compacted into desired and often complex shapes and sintered* to p pform 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 irrigationmetallurgy 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 automotivemetal-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, P i t N J 1998 ( ) C t f Z ith Si t dPrinceton, 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 thesewhich 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 (Udimetparticles 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,

ChicagoChicago.

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 ballssmaller 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 eventsflattening, 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 metalmixing 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 Incagitators. 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 p g p y g y

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 Internationalpowder. 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:contours in compacted copper powder in a single action press. Source:

After P. Duwez and L. Zwell.

TABLE 17 1 Compacting Pressures for Various PowdersTABLE 17.1 Compacting Pressures for Various Powders

FIGURE 17.12 A 7.3-MN (825-ton) mechanical press for compacting t l d S C t f Ci i ti I t dmetal powder. Source: Courtesy of Cincinnati 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 flexibleprocess 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: Courtesyavailable 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 thepressure 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:from a hot-isostatic-pressed carbide cap on a steel shaft. Source:

Courtesy of the Metal Powder Industries Federation.

Powder-Injection MoldingPowder 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 135o – 200oC• Debinding: Molded green parts placed in oven toDebinding: Molded green parts placed in oven to

burn off the plastic• Sinter (up to 1375oC)• Sinter (up to 1375 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 An illustration of powder rollingFIGURE 17.18 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 tubingmetal is sprayed over a rotating mandrel to produce seamless tubing

and pipe.

TABLE 17 2 Sintering Temperature and Time for Various MetalsTABLE 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 = neck-

profile 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)and Equivalent PM Metals (as Sintered)

TABLE 17.5 Mechanical Property Comparisons for Ti-6AL-4V Titanium Alloy

Secondary & Finishing OperationsSecondary & Finishing Operations• Coining & Sizing improve dimensional

t th f fi i haccuracy, strength, surface finish• Forging of sintered preform to final desired

h d t ith d f fi i hshape produces part with good surface finish and dimensional tolerancesM hi i G i di Pl ti H t t ti• Machining, Grinding, Plating, Heat-treating

• Impregnate porous PM components with oil to d ‘ tl l b i t d’ tproduce ‘permanently lubricated’ parts

• Infiltrate a lower melting point metal to fill pores, i i i t t il t thimprove corrosion resistance, tensile strength

• Electroplate (in some conditions) a PM part to seal f & li i t it bilitsurface & eliminate its permeability

Design ConsiderationsDesign Considerations• Keep shape of compact as simple and uniform

as possibleas possible• Provide for ejection of green compact from die

without damaging the compactwithout damaging the compact• Make PM parts with widest acceptable

tolerancestolerances• Avoid very thin wall thickness

P d t i t if th i l d• Produce steps in parts if they are simple and their size doesn’t exceed 15% of overall part lengthlength

• Press embossed or recessed letters perpendicular to the direction of pressingperpendicular to the direction of pressing

(More) Design Considerations(More) Design Considerations

• Make sure large flanges have generous fillets and tapers

• Avoid sharp corners; use chamfers & flats• Form keys, keyways, etc. during powderForm keys, keyways, etc. during powder

compaction• Orient notches & grooves perpendicular to• Orient notches & grooves perpendicular to

pressing directionMaintain uniform wall thickness• 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 Industriescompaction. 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 separatelyand 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(b) Design features for use with grooves. Source: Courtesy of Metal

Powder Industries Federation.

Process CapabilitiesProcess Capabilities• Can make parts from high melting point refractory

l diffi l h imetals, difficult otherwise• High production rates possible on relatively complex

parts using automated equipmentparts using automated equipment• Produce parts with good dimensional control, reducing

need for machiningg• Wide range of compositions for specialized applications• Capability for impregnation & infiltrationLimitations- High cost of metal powder- High cost of tooling for small production runs- Limitations on part size & shape complexity

St th & d tilit f i- Strength & ductility < forging

TABLE 17 6 Forged and PM Titanium Parts and Cost SavingsTABLE 17.6 Forged and PM Titanium Parts and Cost Savings

FIGURE 17 24 Collection of PM parts in a commercial snowblowerFIGURE 17.24 Collection of PM parts in a commercial snowblower.

SummarySummary

• 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 atmospheresprocess variables, sintering atmospheres

• Powder injection molding by mixing very fine powders with polymers later evaporatedpowders with polymers, later evaporated

• Design considerations