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Aluminum casting alloys must contain, in addition to strengthening elements, sufficient amounts of eutectic-forming elements (usually silicon) in order to have adequate fluidity to feed the shrinkage that occurs in all but the simplest castings. Required amounts of eutectic formers depend in part on casting process. Alloys for sand casting generally are lower in eutectics than those for casting in metal molds, because sand molds can tolerate a degree of hot shortness that would lead to extensive cracking in non-yielding metal molds.
Aluminum casting alloys must contain, in addition to strengthening elements, sufficient amounts of eutectic-forming elements (usually silicon) in order to have adequate fluidity to feed the shrinkage that occurs in all but the simplest castings. Required amounts of eutectic formers depend in part on casting process. Alloys for sand casting generally are lower in eutectics than those for casting in metal molds, because sand molds can tolerate a degree of hot shortness that would lead to extensive cracking in non-yielding metal molds.
The range of cooling rates characteristic of the casting process being used controls to some extent the distribution of alloying and impurity elements. For example, the extremely high cooling rates inherent in die casting result in fine dispersion of strengthening and eutectic-forming constituents, and reasonably good castings can be obtained in spite of impurity contents that would render sand or plaster-mold castings unacceptable. However, with these minor exceptions, most aluminum foundry alloys can be cast by all processes, and choice of casting technique usually is controlled by factors other than alloy composition.
A large number of aluminum alloys has been developed for casting, but most of them are varieties of six basic types: aluminum-copper, aluminum-copper-silicon, aluminum-silicon, aluminum-magnesium, aluminum-zinc-magnesium and aluminum-tin alloys.
Aluminum-copper alloys that contain 4 to 5% Cu, with the usual impurities iron and silicon and sometimes with small amounts of magnesium, are heat-treatable and can reach quite high strength and ductility, especially if prepared from ingot containing less than 0.15% iron.
Manganese in small amounts also may be added, mainly to combine with the iron and silicon and reduce their embrittling effect. However, these alloys have poor castability and require very careful gating if sound castings are to be obtained. Such alloys are used mainly in sand casting; when they are cast in metal molds, silicon must be added to increase fluidity and curtail hot shortness, and this addition of silicon substantially reduces ductility.
AI-Cu alloys with somewhat higher copper contents (7 to 8%), formerly the most commonly used aluminum casting alloys, have steadily been replaced by AI-Cu-Si alloys and today are used to a very limited extent. The best attribute of these higher-copper Al-Cu alloys is their insensitivity to impurities, but they have very low strength and only fair castability. Also in limited use are AI-Cu alloys that contain 9 to 11 % Cu, whose high-temperature strength and wear resistance make them suitable for automotive pistons and cylinder blocks. These alloys usually contain manganese as an impurity because wrought metal scrap is used in preparing them. The manganese has little effect.
Very good high-temperature strength is an attribute of alloys containing copper, nickel and magnesium, sometimes with iron in place of part of the nickel.
Aluminum-copper-silicon alloys. The most widely used aluminum casting alloys are those that contain silicon together with copper. The amounts of both additions vary widely, so that the copper predominates in some alloys and the silicon in others. In these alloys, the copper contributes to strength, and the silicon improves castability and reduces hot shortness. Thus, the higher silicon alloys normally are used for more complex castings and for permanent mold and die casting processes, which cannot tolerate hot-short alloys.
Al-Cu-Si alloys with more than 3 to 4% Cu are heat treatable, but usually heat treatment is used only with those alloys that also contain magnesium, which enhances their response to heat treatment. Without magnesium, response is too slow for heat treatment to be economical.
High-silicon alloys (> 10% Si) have low thermal expansion, which makes them suitable for high-temperature operations. When silicon content exceeds 12 to 13% (silicon contents as high as 22% are typical), primary silicon crystals are present and, if properly distributed, cause excellent wear resistance. Automotive engine blocks and pistons are major uses of these alloys.
Aluminum-silicon alloys that do not contain copper additions are used when good castability and good corrosion resistance are needed. If high strength is also needed, magnesium additions make these alloys heat treatable.
Alloys with silicon contents as low as 2% have been used for casting, but silicon content usually is between 5 and 13%. Strength and ductility of these alloys, especially the ones with higher silicon, can be substantially improved by "modification".
Modification of the hypoeutectic alloys is particularly advantageous in sand castings, and can be effectively achieved through the addition of a controlled amount of sodium or strontium, which refines the silicon eutectic. Calcium and antimony additions are also used. Pseudomodification of sand castings, in which the size of the eutectic but not the structure is affected, may be achieved by solidification at high rates, such as occurs when chills are used. With permanent mold castings, modification of the eutectic also is advantageous, but the effect on properties is not as dramatic as with sand castings.
Aluminum-magnesium alloys. High corrosion resistance, especially to seawater and marine atmospheres, is the primary advantage of castings made of Al-Mg alloys. Best corrosion resistance requires low impurity content (both solid and gaseous), and thus 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 have the ability to naturally age, achieving full strength at room temperature 20 to 30 days after casting. This strengthening process can be accelerated by furnace aging.
The high-temperature solution heat treatment and drastic quenching required by other alloys (Al-Cu and AI-Si-Mg alloys, for example) is not necessary for optimum properties in most Al-Zn-Mg alloy castings.
However, microsegregation of Mg-Zn phases can occur in these alloys, which reverses the accepted rule that faster solidification results in higher properties. When it is found in an Al-Zn-Mg alloy casting that the strength of the thin or highly chilled sections are lower than the thick or slowly cooled sections, the weaker sections can be strengthened to the required level by solution heat treatment and quenching, followed by natural or artificial (furnace) aging. Castability of Al-Zn-Mg alloys is poor, but they have good general corrosion resistance despite some susceptibility to stress corrosion.
Aluminum-tin alloys that contain about 6% Sn (and small amounts of copper and nickel for strengthening) are used for cast bearings because of the excellent lubricity imparted by tin. Bearing performance of Al-Sn alloys is strongly affected by casting method. Fine interdendritic distribution of tin, which is necessary for best bearing properties, requires small interdendritic spacing, and small spacing is obtained only with casting methods in which cooling is rapid.
Each casting process requires specific metal characteristics. For example, die and permanent mold casting generally require alloys with good fluidity and resistance to hot tearing, whereas these properties are less critical in sand, plaster and investment casting, where molds and cores offer less resistance to shrinkage. Discussions of required alloy characteristics, and lists of alloys commonly used, are presented for the various casting processes in the section that follows.
The application for which a casting is to be made affects alloy selection by establishing requirements for strength and ductility, as well as special service requirements such as pressure characteristics, corrosion resistance and surface treatments.
Economic considerations also may be important in alloy selection. Total cost of making a casting is affected by required heat treatment and by weldability and machinability, in addition to ingot and melting costs.
Full development of the potential of any casting alloy depends in large part on foundry technique. Foundry personnel should be consulted on alloy selection; use of alloys with which such personnel are familiar often results in better and more economical castings.
Selection of the proper alloy requires careful consideration of all the factors discussed above, which are presented in the brief outline that follows.
Alloy characteristics necessary for casting process selected:
Casting design considerations:
Mechanical-property requirements:
Service requirements: