POWDER METALLURGY POWDER METALLURGY Introduction Machinery's Handbook, 31st Edition
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Powder metallurgy (PM) is a process for forming metal parts by heating compacted metal powders to just below their melting points. The heating treatment is called sinter- ing. Although the modern field of powder metallurgy dates to the early 19th century, over the past quarter century, it has become widely recognized as a superior way of producing high quality parts for a variety of important applications. Powder metallurgy actually comprises several different technologies for fabricating semidense and fully dense components. The conventional PM process, referred to as press-and-sinter, has been used to produce many complex parts, such as the planetary car rier, helical gears and blades, piston rings, connecting rods, cams, brake pads, bushings, tool steels, tungsten carbides, cermets tool and die materials, magnetic materials, graph- ite brushes impregnated with copper for electric motors, surgical implants, and many other parts for aerospace, nuclear, and industrial applications. PM’s popularity is due to a number of attributes: a) the advantage that the process offers over other metal-forming technologies such as forging and metal casting, b) its advan- tages in material utilization, c) shape complexity, d) near-net-shape dimensional control, and others. PM’s benefits add up to cost-effectiveness, shape and material flexibility, application versatility, and part-to-part uniformity for improved product quality. Advantages that make powder metallurgy an important commercial technology include the following: • eliminates or minimizes machining by producing parts at or close to final dimensions • eliminates or minimizes scrap losses by typically using more than 97 percent of the starting raw material in the finished part • permits a wide variety of alloy systems • produces good surface finishes and tolerances of ± 0.005 in. ( ± 0.13 mm) • provides materials that may be heat treated for increased strength or increased wear resistance • relatively low processing temperatures • provides controlled porosity for self-lubrication or filtration • facilitates manufacture of complex or unique shapes that would be impractical or impossible with other metalworking processes • is suited to moderate- to high-volume component production requirements • offers long-term performance reliability in critical applications • is cost-effective and environmentally friendly • can be automated. There are some disadvantages associated with PM processing. These include: • high tooling and equipment cost • expense of metal powder • difficulties with storing and handing metal powders (degradation of the metal over time and fire hazards with particular metals) • variations in material density can cause problems, especially for complex geometries • limitations on part geometry because metal powders do not readily flow laterally in the die during pressing. Most parts weigh less than 5.5 lb (2.5 kg), although parts weighing as much as 110 lb (50 kg) can be produced using conventional powder metallurgy equipment. While many of the early powder metallurgy parts, such as bushings and bearings, were very simple shapes, today’s sophisticated powder metallurgy process produces components with complex contours and multiple levels and does so quite economically.
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