Nonferrous PM Materials

As discussed in the Section "Metal Powder Production and Characterization" in this Volume, a great many nonferrous metals are also produced in powder form, including:

• Copper: by reduction of oxides, atomization, electrolysis, and hydrometallurgical processing

• Aluminum: by atomization

• Magnesium: by mechanical comminution and atomization

• Nickel: by carbonyl vapormetallurgy, hydrometallurgy, and atomization

• Cobalt: by carbonyl vapor metallurgy, hydrometallurgy, reduction of oxides, and atomization

• Silver: by chemical precipitation, electrolysis, and reduction of oxides

• Gold, platinum, and palladium: by chemical precipitation

• Tungsten and molybdenum: by reduction of oxides

• Metal carbides: by carburization, Menstruum process, and exothermic thermite reactions

• Tantalum: by reduction of potassium tantalum fluoride and a sequence of electron beam melting, hydriding, comminution, and degassing (dehydriding)

• Niobium: by aluminothermic reduction of oxides

• Titanium: by reduction of oxides and atomization

• Beryllium: by reduction of vacuum-melted ingots by comminution

• Composite powders: by diffusion (alloy coating)

This section reviews copper-, titanium-, and aluminum-base P/M materials.

Copper-base alloys include pure copper for high-density electrical applications: 90Cu-10Sn bronzes for bearings and structural parts; brasses with 10, 20, and 30% Zn; and nickel silver (Cu-18Zn-18Ni). The brasses and nickel silvers are used for structural parts that require ductility, moderate strength, corrosion resistance, and decorative value. Copper exhibits a single-phase structure with some annealing twins. The most significant feature is the particle boundaries or their absence. There should be virtual freedom from particle boundaries from the surface to the center of the part. Bronzes should display all G -bronze with no gray copper-tin intermetallic compounds. Optimal mechanical properties and machinability dictate a minimum of reddish copper-rich areas and small grain clusters of bronze. Mixes containing admixed graphite show the mottled gray flakes in the pores of the part. Bearings exhibit varying degrees of sintering, depending on the final application. In general, however, a well-sintered bearing results in greater ease of oil impregnation. Bronze P/M structures are shown in Fig. 117, 118, 119, and 120.

Brasses and nickel silvers are generally single-phase structures. They should display good pore rounding and almost no original particle boundaries. Some of the materials may contain up to 2% Pb within the particles as an aid to machinability; this will appear as a fine, rounded gray phase (Fig. 121 and 122).

Titanium and titanium alloys such as Ti-6Al-4V are produced from metal powders in several ways. The powders may be prealloyed or may be an elemental mix of titanium and a master alloy of vanadium and aluminum. The latter can be pressed and vacuum sintered to an impermeable state, which may then be hot isostatically pressed to full density without a can. The prealloyed materials may be vacuum hot pressed or preformed, canned, and hot isostatically pressed to full density. Titanium alloys can also be consolidated by metal injection molding. Titanium alloy P/M structures are shown in Fig. 123, 124, and 125.

Aluminum P/M alloys are pressed and sintered to 90 to 95% density. The common alloys are 201AB and 601AB. The alloys are prepared using low-alloy aluminum powder with additions of elemental or master alloy copper, magnesium, and silicon. During sintering, the additions cause a liquid phase to form that fluxes away the surface oxides and allows bonding between the aluminum particles. Sintering in nitrogen is performed at approximately 595 or 620 °C (1100 or 1150 °F) at a dew point of -50 °C (-60 °F) to prevent further oxidation of the aluminum. After sintering, the alloys are often solutionized and quenched, then repressed or coined before aging. The repressing densifies the material and establishes close dimensional tolerances. The materials may also be cold forged or rolled to varying reductions in thickness because of their favorable as-sintered ductility. Aluminum P/M structures are shown in Fig. 126, 127, 128, 129, and 130.

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