OTHER NONFERROUS ALLOYS In addition to the four major nonferrous casting alloys, a
variety of niche materials are available for certain applications. Following is a brief look at nickel-base alloys, cobalt-base al- loys, titanium, low melting point alloys and precious metals. • Nickel-Based—These alloys contain virtually no iron. Their primary constituents are nickel (50-80%), chromium (20%) and a combination of molybdenum, aluminum, tungsten, cobalt and niobium. These alloys get their strength from solid solution strengthening and can be used in temperatures up to 2,200F (1,204C). Nickel-based alloys originally were developed for turbo-superchargers and aircraft turbine engines. The alloys now are used in aircraft and land-based gas turbines, rocket engines, and chemical and petroleum plants.
• Cobalt-Based—This group of alloys is more ductile than other superalloys. These are age-hardenable alloys whose primary constituents are nickel, iron, chromium, tungsten and cobalt. They can be used in applications where temperatures reach 1,900F (1,038C). Cobalt- based alloys were designed for gas turbine nozzle vanes. They are preferred for stator vanes and diaphragms in gas turbines because of their excellent thermal shock and corrosion resistance.
• Titanium—These alloys are corrosion-resistant metals
that were developed for use in jetliner airframes and landing gears and then for space applications because of their high strength-to-weight ratio, excellent corrosion resistance, fatigue strength and fracture toughness. Titanium’s melting point is 3,030F (1,666C). Its density is about one-half that of steel, but its mechanical properties are similar. Tita- nium has found signifi cant markets in the aerospace, energy and chemical industries. Typical applications include aircraft structural and engine components, steam turbine blades, offshore drilling equipment, marine components, and pumps and valves for the chemical processing industry. Titanium’s uses have been somewhat limited because of the relatively high cost associated with the metal.
• Low Melting Point—Low melting point metals include lead, bismuth and tin. Lead, the most common low melting point alloy, often is used in counterweights. Because of its inherent density and lower price tag than other dense materials like tungsten, gold and platinum, lead often is the material of choice when a heavy material is needed within tight confi nes. Lead is also suited to a large number of radioactiv- ity shielding applications due to its density. The health concerns regarding overexposure to lead are well-known, and lead has been removed from certain assemblies and alloys, such as those in potable water applications. • Precious—Precious metals, such as gold, silver and platinum, also can be poured in the casting process. Precious metals often are used in ornamental casting, particularly in the investment casting of jewelry. Precious metal casters are particu- larly focused on eliminating waste. Precious metals are generally alloyed to produce the different colors desired by jewelry purchas- ers, and the diffi culties in casting them are dealt with secondarily. For example, gold has hundreds of alloys in slight color variations, such as green-gold or yellow-gold. White gold becomes brittle when alloyed with nickel to achieve its appearance. Silver has long had a propensity to produce castings with heightened porosity. However, a de-ox family of the metal has been produced to combat that tendency and make the material more castable and resistant to de- fects. Other silver alloys that have emerged can prevent fi re-stain, the purplish discoloration on silver caused by copper-oxide.
38 | METAL CASTING DESIGN & PURCHASING | Jul/Aug 2012
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