Machinery's Handbook, 31st Edition
1488 NONFERROUS CASTING MATERIALS polish to an attractive silvery color, rather than the typical brassy color, which accounts for common use over several centuries. Beryllium-Copper (Be-Cu) Alloys: In the fully heat treated and cold-worked condition, this is the hardest and strongest copper alloy. It is similar in mechanical properties to many high-strength alloy steels but, compared to steels, has better corrosion resistance. Magnesium (Mg).— A shiny gray, solid, lightweight metal, magnesium has a melting point of 1202°F (650°C) and a density of 108.3 lb/ft 3 (1740 kg/m 3 ). As the world’s lightest metal, with good strength-to-weight ratio, magnesium and its alloys are prevalent in the au- tomotive, airplane, and missile industries. Its compounds also are used as refractory mate- rial in furnace linings for producing iron, steel, other nonferrous metals, glass, and cement. Magnesium is the most electrochemically active metal. Care must be taken in process- ing to avoid fire hazards, and small particles of the metal, such as metal cutting chips, oxidize rapidly. Magnesium and its alloys are available in both wrought and cast forms. Magnesium Alloys: These light alloys have received renewed interest as substitutes for some conventional structural materials to reduce weight in vehicles. Cast alloys, widely used in interior and power-train components, account for more than 99 percent of magne- sium alloys used today, while only a small number of wrought products are used. This is because magnesium alloys lack formability for wrought applications; their high cost also discourages use for some automotive applications. Rare earth (RE) elements, such as cerium (Ce), gadolinium (Gd), neodymium (Nd), and yttrium (Y), often are used as major alloying elements because of their relatively high solubility in magnesium and effectiveness in precipitation hardening and creep re- sistance. Commercial wrought magnesium alloys, such as ZK60 and AZ61, are based on magnesium-zinc (Mg-Zn) and magnesium-zinc-aluminum (Mg-Zn-Al) compounds, both of which are age-hardenable. ZK and AZ alloys attain good strength with hot extru- sion or rolling; however, age-hardening processes, such as T6 (solution heat treatment and aging) and T8 (cold-work and subsequent aging), do not add strengthening due to soften- ing by recrystallization at the temperature for artificial aging. CASTING OF METALS Casting is an age-old manufacturing process in which a liquid material usually is poured into a mold that contains a hollow cavity of the desired shape. As a result of new technological advances, this traditional form of manufacturing has become more essen- tial than ever. In metal casting, the mold generally includes runners and risers that facilitate metal filling the cavity. The mold and metal are cooled until the metal solidifies, then the solidi - fied part ( casting ) is recovered from the mold. Subsequent operations are used to remove excess material caused by the casting process (such as the runners and risers), after which the part may solidify further. Metal casting processes are divided into two broad types: expendable-mold casting processes and nonexpendable-mold casting processes. Expendable-mold casting includes sand, plastic, shell, plaster, and investment (lost-wax technique) molding processes, all of which rely on gravity to move the liquid material into casting cavities. The molds in which the molten material solidifies usually are made of non- metallic materials and are temporary—that is, they must be destroyed to remove the casting. Nonexpendable-mold casting refers to casting processes involving a reusable mold. Such molds are designed with two or more sections for easy, precise closing (for molding) and opening (to remove the casting) and can be used for multiple castings. For metal cast- ing, the mold, also called a die , is commonly made of metal or ceramic. Permanent mold casting refers to all casting technologies in which the mold cavity is reused many times and is made of a metallic material or graphite. This is the predomi- nant casting method for manufacturing metal shapes. Specifically, about 90 percent of all aluminum castings produced use metal molds, including gravity-fed, low-pressure, and high-pressure die castings.
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