Aluminum Foundry Products

ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials Copyright © 1990 ASM International® ASM Handbook Committee, p 123-151 All rights reserved. DOI: 10.1361/asmhba0001061 www.asminternational.org Aluminum Foundry Products Revised by A. Kearney, Avery Kearney & Company Elwin L. Rooy, Aluminum Company of America ALUMINUM CASTING ALLOYS are wrought alloys. Aluminum casting alloys cast aluminum alloys are grouped according the most versatile of all common foundry must contain, in addition to strengthening to composition limits registered with the alloys and generally have the highest cast- elements, sufficient amounts of eutectic- Aluminum Association (see Table 3 in the ability ratings. As casting materials, alumi- forming elements (usually silicon) in order article "Alloy and Temper Designation Sys- num alloys have the following favorable to have adequate fluidity to feed the shrink- tems for Aluminum and Aluminum Al- characteristics: age that occurs in all but the simplest cast- loys"). Comprehensive listings are also • Good fluidity for filling thin sections ings. maintained by general procurement specifi- The phase behavior of aluminum-silicon • Low melting point relative to those re- cations issued through government agencies compositions (Fig. 1) provides a simple quired for many other metals (federal, military, and so on) and by techni- • Rapid heat transfer from the molten alu- eutectic-forming system, which makes pos- cal societies such as the American Society sible the commercial viability of most high- minum to the mold, providing shorter for Testing and Materials and the Society of casting cycles volume aluminum casting. Silicon contents, Automotive Engineers (see Table 1 for ex- • Hydrogen is the only gas with apprecia- ranging from about 4% to the eutectic level amples). In addition, aluminum casting al- ble solubility in aluminum and its alloys, of about 12% (Fig. 1), reduce scrap losses, loys are sometimes grouped according to permit production of much more intricate and hydrogen solubility in aluminum can their quality level or intended end-use ap- be readily controlled by processing meth- designs with greater variations in section plication (see the section "Selection of thickness, and yield castings with higher ods Casting Alloys"). Of these various meth- • Many aluminum alloys are relatively free surface and internal quality. These benefits ods, the grouping of aluminum casting al- derive from the effects of silicon in increas- from hot-short cracking and tearing ten- loys in terms of chemical compositions is dencies ing fluidity, reducing cracking, and improv- discussed below. • Chemical stability ing feeding to minimize shrinkage porosity. The Aluminum Association designation The required amounts of eutectic formers • Good as-cast surface finish with lustrous system attempts alloy family recognition by surfaces and little or no blemishes depend in part on the casting process. Al- the following scheme: loys for sand casting generally are lower in Aluminum alloy castings are routinely eutectics than those for casting in metal • lxx.x: Controlled unalloyed compositions • 2xx.x: Aluminum alloys containing cop- produced by pressure-die, permanent-mold, molds because sand molds can tolerate a per as the major alloying element green- and dry-sand, investment, and plas- degree of hot shortness that would lead to ter casting. Aluminum alloys are also readi- extensive cracking in nonyielding metal • 3xx.x: Aluminum-silicon alloys also containing magnesium and/or copper ly cast with vacuum, low-pressure, centrif- molds. Resistance to cracking during cast- ugal, and pattern-related processes such as ing is favored by a small range of solidifica- • 4xx.x: Binary aluminum-silicon alloys • 5xx.x: Aluminum alloys containing mag- lost foam. Total shipments of aluminum tion temperature, which drops from about nesium as the major alloying element foundry products (all types of castings ex- 78 °C (140 °F) at 1% Si to zero at about 12% • 6xx.x: Currently unused clusive of ingot) in the United States for Si. Good feeding characteristics to mini- • 7xx.x: Aluminum alloys containing zinc 1988 were about 10 6 Mg (10 6 tons), of which mize shrinkage porosity are benefited by a as the major alloying element, usually about 68% was accounted for by die cast- profile of volume fraction solidified versus also containing additions of either cop- ings (see the shipment statistics in the arti- temperature that is weighted toward the low- per, magnesium, chromium, manganese, cle "Introduction to Aluminum and Alumi- er portion of the temperature range--that is, or combinations of these elements num Alloys" in this Volume). toward increased eutectic concentration. • 8xx.x: Aluminum alloys containing tin as In the binary aluminum-silicon system, the major alloying element Alloy Systems under the nonequilibrium conditions of casting, the volume fraction of eutectic in- • 9xx.x: Currently unused Aluminum casting alloys are based on the creases linearly from about 0 to 1 as silicon Designations in the form xxx.l and xxx.2 same alloy systems as those of wrought content increases from 1 to 12%. include the composition of specific alloys in aluminum alloys, are strengthened by the remelt ingot form suitable for foundry use. same mechanisms (with the exception of Alloy Groupings and Designations Designations in the form xxx.O in all cases strain hardening), and are similarly classi- Although the systems used to identify and define composition limits applicable to cast- fied into non-heat-treatable and heat-treat- group aluminum casting alloys are not inter- ings. Further variations in specified compo- able types. The major difference is that the nationally standardized, each nation (and in sitions are denoted by prefix letters used casting alloys used in the greatest volumes many cases individual firms) has developed primarily to define differences in impurity contain alloying additions of silicon far in its own alloy nomenclature. In the United limits. Accordingly, one of the most com- excess of that found (or used) in most States and North America, for example, mon gravity cast alloys, 356, has variations 124 / Specific Metals and Alloys 3000 2600 jf 2200 ~-'"/ 99.95% AI ~ solutionLiquid / L +si 100% Si 1ooo / 1400 --L +(AO-- / 1000 . "(,4/) (.41) + Si , \'7 600 ~ \~ 200 /'/ " ' " " "'~.. 40 50 60 70 80 90 100 ~ ~ ~'~ ~" % Silicon ~ Si . • - %~" = 8% Si 12% Si 20% Si 50% Si Aluminum-silicon phase diagram and cast microstructures of pure components and of alloys of various compositions. Alloys with less than 12% Si are referred Fig, 1 to as hypoeutectic, those with close to 12% Si as eutectic, and those with over 12% Si as hypereutectic. A356, B356, and C356; each of these alloys remelted for making castings, iron and zinc When available, a second phase will aid the has identical major alloy contents but has contents tend to increase and magnesium required feeding of shrinkage areas and will decreasing specification limits applicable to content tends to decrease. help compensate for solidification stresses. impurities, especially iron content. General Composition Groupings. Al- When these alloys, and other single-phase Composition limits of casting alloys reg- though the nomenclature and designations alloys, are cast using permanent mold or istered with the Aluminum Association are for various casting alloys are standardized other rigid mold casting methods, special given in Table 3 in the article "Alloy and in North America, many important alloys techniques are required to relieve solidifica- Temper Designation Systems for Aluminum have been developed for engineered casting tion stresses. Careful techniques are also usu- and Aluminum Alloys." production worldwide. For the most part, ally needed to promote the progress of the In designations of the lxx.x type, the each nation (and in many cases the individ- metal solidification from the remote areas of second and third digits indicate minimum ual firm) has developed its own alloy no- the casting to the hotter and more liquid aluminum content (99.00% or greater); menclature. Excellent references are avail- casting areas, to the risers, and then to the these digits are the same as the two to the able that correlate, cross reference, or riser feeders. When these necessary and more right of the decimal point in the minimum otherwise define significant compositions in exacting casting techniques are used, the aluminum percentage expressed to the near- international use (Ref 1, 2). aluminum-copper alloys can and have been est 0.01%. The fourth digit in lxx.x desig- Although a large number of aluminum successfully used to produce high-strength nations, which is to the right of the decimal alloys has been developed for casting, there and high-ductility castings. More exacting point, indicates product form: 0 denotes are six basic types: casting techniques are also helpful and re- castings (such as electric motor rotors), and quired when casting other single-phase alumi- • Aluminum-copper 1 denotes ingot. num casting alloys or alloy systems. • Aluminum-copper-silicon In 2xx.x through 8xx.x designations for In terms of alloy additions, manganese in • Aluminum-silicon aluminum alloys, the second and third digits small amounts may be added, mainly to • Aluminum-magnesium have no numerical significance but only combine with the iron and silicon and re- • Aluminum-zinc-magnesium identify the various alloys in the group. The duce the embrittling effect of essentially

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