Cast Aluminum Alloys
The desirable characteristics of wrought alloys are also generally applicable to cast alloys, but in fact the choice of one casting alloy or another tends to be more often made on the basis of their relative abilities to meet one or more of the four following characteristics:
• Ease of casting
• Quality of finish
• High strength, especially at high temperatures
• High toughness
1. Ease of Casting.
Many aluminum casting alloys have relatively high silicon contents that provide excellent flow characteristics during casting, enabling them to be utilized for large and complex castings (e.g., even complete engines). Relatively minute details in the shape of the casting can be accurately and reliably replicated.
2. Quality of Finish.
By proper selection of aluminum casting alloy, extremely fine surface quality can be achieved. While such alloys typically require more attention to casting practice, they are widely used in applications where the finished casting surface mirrors the finish needed in surface.
3. High Strength.
Many aluminum casting alloys respond to heat treatment following casting and achieve relatively high levels of strength and excellent strength–weight ratios. With careful design of molds, high chill rates can be assured and both high strength and high toughness can be achieved.
4. High Toughness.
With careful selection of alloy and heart treatment combined with process technology often referred to as premium engineered casting, optimizing metal flow and chill rated, the toughness of castings may be comparable to that of wrought alloys at comparable strength levels. Techniques such as hot isostatic pressing (HIP) are available to further reduce porosity and improve performance.
Unfortunately few casting alloys possess all four characteristics, but some generalizations useful in alloy selection for specific applications may be made:
1. Ease of Casting. The high-silicon 3xx.x series are outstanding in this respect as the relatively high silicon contents lend a characteristic of good flow and mold-filling capability. As a result, the 3xxx.x series are the most widely used and are especially chosen for large and very complex casting.
2. Finish. The 5xx.x and 7xx.x series are noteworthy for the fine finish they provide, but they are more difficult to cast than the 3xx.x series and so are usually limited to those applications where that finish is paramount. A good example is the use of 7xx.x alloys for bearings.
3. Strength and Toughness. The 2xx.x alloys typically provide the very highest strengths, especially at high temperatures, but are among the more difficult to cast and lack good surface characteristics. Therefore their use is usually limited to situations where sophisticated casting techniques can be applied and where strength and toughness are at a premium, as in the aerospace industry. Relatively high strengths and superior toughness can also be achieved with some of the higher purity 3xx.x alloys (e.g., A356.0, A357.0) in heattreated conditions.
J. G. Kaufman
Kaufman Associates
Columbus, Ohio
Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright 2006 by John Wiley & Sons, Inc.
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