These alloys, which contain 2 to 5% Mg and 0.1 to 0.4% Mn, have good weldability and high corrosion resistance, especially to seawater and marine atmospheres; this is the primary advantage of castings made of Al-Mg alloys. For best corrosion resistance, a low impurity content (both solid and gaseous) is required, and thus these alloys must be prepared from high quality metals and handled with great care in the foundry. The relatively poor castability of Al-Mg alloys and the tendency of the magnesium to oxidize increase handling difficulties and, therefore, cost.
Aluminum-zinc-magnesium alloys. These aluminum alloys contain 2 to 8 % zinc, 0.5 to 4% magnesium, and 0 to 3% copper. The total amount of zinc, magnesium, and copper controls the properties and consequently the uses. When the total amount is above 9%, high strength is greatest, and corrosion resistance, formability, and weldability are subordinate to it. Below a total of 5–6%, fabricability becomes greatest and stress corrosion susceptibility tends to disappear. Al-Zn-Mg alloys have the ability to naturally age, achieving full strength at room temperature 2 to 3 weeks after casting. This process can be accelerated by furnace aging.
Aluminum-tin alloys. Al-Sn alloys contain about 6% tin and a small amount of copper and nickel for improving strength; castability is good and they are used for cast bearings because of tin’s excellent lubrication characteristics.
b) Magnesium and Magnesium Alloys
Elemental magnesium is a fairly strong, silvery-white, lightweight metal; it is the world’s lightest metal. Its crystal structure is hexagonal close pocket (hcp), its melting point is 650°C (1202°F), and it has a density of 1740 kg/m3 (108.3 lb/ft3).
This lightness, combined with the good strength-to-weight ratio of magnesium, has made magnesium and its alloys very useful use in the airplane, missile, and automotive industries. Its compounds are used as refractory material in furnace linings for producing metals (iron and steel, nonferrous metals), glass, and cement.
Magnesium and its alloys are available in both wrought and cast forms. Magnesium is also the most electrochemically active metal. Therefore, in all processing of magnesium, small particles of the metal such as metal cutting chips oxidize rapidly, and care must be taken to avoid fire hazard.
Magnesium alloys. The main alloying elements are aluminum (Al), zinc (Zn), and manganese (Mn). Commercial cast alloy AZ81 (Mg-8, Al-0.5, Zn-03, Mn), contains 8% Al, 0.5% Zn, and 0.3% Mn. Magnesium alloy castings can be produced by nearly all of the conventional casting methods, namely, sand, permanent, and semipermanent mold and shell, investment, and diecasting. The choice of a casting method for a particular part depends upon factors such as the configuration of the proposed design, the application, the properties required, the total number of castings required, and the properties of the alloy. Typical applications include automotive wheels and air-cooled engine blocks.
c) Copper and Copper Alloys
The element copper is a reddish-yellow material and is extremely ductile. Copper has a face-centered cubic (fcc) crystal structure and a melting point of 1084.6°C (1984.6°F), with a density of 8920 kg/m3 (556.85 lb/ft3); it also has the second best electrical conductivity of the metals, second only to silver (redundant), with respect to which its conductivity is 97%.
Pure copper is extremely difficult to cast, and it is prone to surface cracking, porosity problems, and to the formation of internal cavities. The casting characteristics of copper can be improved by the addition of small amounts of elements, including beryllium, silicon, nickel, tin, zinc, chromium, and silver.
Copper alloys. Unlike pure metals, alloys solidify over a range of temperatures. Solidification begins when the temperature drops below the liquidus; it is completed when the temperature reaches the solidus. A wide variety of copper alloys are available including brasses, aluminum bronzes, phosphor bronzes, and tin bronzes.
Cast copper alloys are used for applications such as bearings, bushings, gears, fittings, valve bodies, and miscellaneous components for the chemical processing industry. These alloys are poured into many types of castings, such as sand, shell, investment, permanent mold, chemical sand, centrifugal, and die casting. The high cost of copper limits the use of its alloys.
REVIEW QUESTIONS
3.1Which rule covers every stage of good cast design?
3.2Describe general design considerations in metal casting.
3.3What are hot-spot effects?
3.4What is draft and is it necessary in all molds?
3.5What is shrinkage allowance?
3.6How many dimensional tolerances are defined in the ISO?
3.7What is the difference between shrinkage and machining allowance?
3.8What is a parting line and how it is determined?
3.9Name the factors that influence the design of the gating system.
3.10Describe a typical gating system for a sand-gravity casting process.
3.11Name the types of cast irons and describe their characteristics.
3.12What is casting steel? Describe its characteristics.
3.13List the most often used nonferrous alloys and describe their properties.
PARTICULATE PROCESSING FOR METALS
In this part, basic particulate processing, in which starting materials are metal powders, is discussed. The unit deals with characterization of metal powders, powder metallurgy (Fig. II.1) processes used in shaping these materials, pressing and sintering techniques, and the economics of the powder metallurgy process.
Fig. II.1 An outline of the particle processing described in Part II.
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POWDER METALLURGY
4.2 Characteristics of Metal Powders
4.3 Production of Metallic Powder
4.4 Powder Manufacturing Processes of Metal Parts
4.5 Powder Metallurgy Materials
4.6 Design Considerations in Powder Metallurgy
4.7 Economics of Powder Metallurgy
Powder metallurgy, or P/M, is a process for forming metal parts by heating compacted metal powders to just below their melting points. The heating treatment is called sintering. 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 P/M process, referred to as press-and-sinter, has been used to produce many complex parts such as the planetary carrier, helical gears and blades, piston rings, connecting rods, cams, brake pads, surgical implants, and many other parts for aerospace, nuclear, and industrial applications.
P/M’s popularity