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Metal Casting Technologies : June 2008
Back to the Alloying Principles. The aluminium-base alloys may, in general, be characterised as eutectic systems, containing intermetallic compounds or elements as the excess phases. Because of relatively low solubilities of most of the alloying elements in aluminium and because of the complexity of the alloys that are produced, any one aluminium-base alloy may contain several metallic phases which sometimes are quite complex in composition. These phases usually are appreciably more soluble near the eutectic temperatures than at room temperature, making it possible to heat-treat some of the alloys by solution and aging heat-treatments. All the properties of interest are, of course, influenced by the effects of the various elements with which aluminium is alloyed. The principal alloying elements in aluminium-base casting alloys are copper, silicon, magnesium, zinc, chromium, manganese, tin, and titanium. Iron is an element normally present and usually considered as an impurity. Complex Alloys. Improvements in strength and hardness and response to heat-tretment are obtained with proper percentages of copper, magnesium, zinc, or certain combinations of magnesium and silicon in aluminium alloys. Excessive percentages of these elements, however, result in complete loss of ductility and toughness. Other elements, namely, silicon, show improved properties through alloying effects, but demonstrate no significant benefit from heat- treatments. The latter alloys are especially susceptible to improvement by modification treatments or “chill casting.” Obviously, the foregoing details and principles have been greatly simplified by the consideration of only the simple binary-alloy systems. Commercial alloys are complex in composition. Also, only simple mechanical-property effects of alloying have been mentioned and considered. Changes in conductivity, corrosion resistance, machinability, thermal expansion, endurance limit, etc., have not been considered. However, the simple principles advanced are helpful in understanding the classes of aluminium casting alloys which have been developed. Classifications. Most of the aluminium casting alloys can be classified into one of the following groups: 1. Aluminium-copper alloys, heat-treatable and non-heat- treatable, identified by ASTM code letter “C” 2. Aluminium-silicon alloys, non-heat-treatable, identified by the code letter “S” 3. Aluminium-copper-silicon alloys, heat-treatable and non- heat-treatable grades, code letter “CS” or “SC” 4. Aluminium-silicon-magnesium alloys, heat-treatable, code letter “SG” 5. Aluminium-magnesium alloys, heat-treatable and non-heat- treatable, code letter “G” 6. Aluminium-zinc alloys, code letter “ZC” of “ZG” 7. Special alloys, copper-nickel-aluminium alloys, copper-tin alloys, high Si-Al alloys, etc. In the groupings above, alloys are classified as non-heat- treatable for one of two principal reasons. First, their properties 52 www.metals.rala.com.au are not significantly benefited by solution heat-treatment and aging, like the Al-Si alloys. Or second, if heat-treated, they become so brittle after aging as to be useless for casting-usage. Impurites have a great influence on the latter effect, so that in general, it is necessary to place much lower limits on impurities tolerable in the heat-treatable alloys. CASTING PROPERTIES The production of good aluminium castings requires that the casting alloys possess favorable foundry properties. Those considered of importance for aluminium casting alloys are the following: 1. Minimum solidification shrinkage (and maximum mould yield) 2. Adequate fluidity 3. Freedom from hot-tearing or cracking 4. Minimum difficulty in producing pressure-tight castings 5. Minimum problems with gas absorption and drossing The metallurgical principles relating to these properties follow the general precepts on the solidification of metals. However, some of the more notable effects in aluminium alloys will be considered briefly here. Shrinkage. Shrinkage problems are at a minimum in the Si- Al (Silicon-Aluminium) alloys containing 5-13% Si. Accordingly, foundry yield is at a maximum and difficulties with hot-tearing and microporosity are minimized. It may be noted that alloys having low shrinkage tendencies accompanied by a narrow freezing temperature have the better ratings of resistance to tearing, pressure-tightness, and fluidity. This follows the general principle related to solidification mechanisms. Rated second to the Si-Al alloys are the high-silicon-low-copper-aluminium alloys, the SC types. Because of their favorable casting properties, the silicon-rich aluminium alloys are extensively used for permanent-mould and die-casting processes. Percentages of 5, 7, 9, and 12% Si are used in combination with other elements for castings made by the metal-mould processes. Since most of the same alloys can be heat-treated, it is evident that this class of alloys is good for general-purpose use. Patternmakers’ shrinkage for the various alloys are 1.56 mm./cm. (5/32 in./in.) for average castings of the C, CG, and SC types and 1.875 mm./cm. (3/16 in./in.) for the ZG types. Shrinkage requirements may vary with the intricacy of the design and dimensions. However, a low value of patternmakers’ shrinkage does not mean that the shrinkage problems of microporosity, pressure-tightness, and cracking will be low. The mechanism of freezing rather than the total contraction is a dominant factor in the latter problem. Fluidity. The silicon-rich alloys favor fluidity and resistance to tearing. Most die-casting alloys are silicon-rich alloys. Thus it appears that silicon improves many of the casting properties of the aluminium alloys. This appears to be the reason that the general-purpose casting alloys contain substantial amounts of silicon along with smaller percentages of copper or other elements.