By definition, a metal powder is an aggregate of discrete metal particles that are usually in the size range of 1 to 1000 |im.
The metal powders in common use are lead-tin alloys for solders; iron-, nickel-, and cobalt-base alloys for hard facing; copper-, silver-, and nickel-base alloys for brazing; aluminum, bronze, and stainless steel for paint pigments; magnesium for pyrotechnics; iron for welding rods, torch cutting and scarfing, and metal powder parts; copper for metal-powder parts; and carbide for tools. Used in smaller quantities are iron powders for radio and television tuning cores; nickel-iron and silicon-iron alloy powders for other soft magnetic parts; aluminum, iron, nickel, and cobalt powders for small permanent magnets in the "Alnico" series; nickel and cobalt powders as binders in the production of carbide tools and alloy and stainless steel powders for high-strength and special property parts.
Of these uses, the fabrication of structural parts or machinery components accounts for the largest single use of metal powders and competes with such other metal-forming methods as machining, casting, stamping, and forging. The powder-metallurgy technique may be selected by the designer as the best way to make a particular part for one of several reasons:
1. The process is ideal for mass-producing machine components at low unit cost.
2. Residual porosity can be controlled to provide long-wearing qualities through the self-lubricating feature of the "oil-less" sleeve bearing.
3. Metal combinations are possible through powder metallurgy that cannot be melted.
4. Powder-metallurgy techniques provide the only practical method of forming high-melting-point metals.
The good surface finish and close dimensional tolerances possible are additional reasons why the designer may specify metal powder parts.
Although most metal powder parts are made from iron, copper, or mixtures of these primary powders with or without graphite additions, many other powders are used to develop special properties like high strength, magnetic or electrical properties, corrosion resistance, or oxidation resistance. These special powders include brass, bronze, alloy steels, stainless steel, and various nickel-base alloys. Many of these special powders are also employed in the production of metal filters.
Production Methods Electrolytic
These methods are used to make most of the copper powder used for metal powder parts and a special, high-purity iron powder often used for making high-density iron powder parts.
Atomization
This is a molten metal production method applied to a wide range of powders including aluminum, magnesium, brass, bronze, lead, tin, nickel-silver, and stainless and low-alloy steels. Other more complex iron-, nickel-, and cobalt-base alloys are also made by atomization.
