How to choose the alloy, casting process and thermal treatment

How to choose the aluminum casting alloy, casting process and thermal treatment?

This requires a knowledge of the service conditions of the part under consideration. Many different casting alloys are in use today, with up to five different thermal treatment options. This results in a large number of alternatives to choose from to satisfy individual requirements. Because of these many alloy and thermal treatment combinations, the possible range of typical mechanical properties varies widely. Since commercial castings often do not have critical service requirements, the foundry should be consulted as to the most economical alloy and production method for the job.

In most cases, aluminum castings are designed for maximum efficiency. In such cases, the alloy and heat treatment must be selected carefully. Designers, with their knowledge of the service requirements for the casting, must confine the alloy choice to those that provide the necessary properties and then must be guided by the foundry for the final choice.

Sometimes, the alloy that shows the best properties on paper may have production characteristics that make it less desirable on an overall basis than other eligible alloys. The foundry is in the best position to advise on such factors as availability, relative ingot costs, production costs and reproducibility of results. When this is coordinated with the designer’s knowledge of service requirements, such as strength, hardness, corrosion resistance, impact strength and machinability, the best possible selection will result.

Because of this coordination, changes from the initial design may be indicated to improve design efficiency and/or lower production costs. For instance, a casting with sound design from other standpoints may have a size or shape conducive to distortion in heat treating, which could be minimized through design changes.

Production and service requirements have a large bearing on the casting method, as do the size and shape of the part. For example, castings required in large numbers must be made either by the permanent mold, diecasting or automated sand casting processes, provided the size and design features of the casting and available alloys are suitable.

Sand casting often is used to produce parts with hollow cavities and a complex arrangement of ribs, pockets, etc., and for parts unsuitable for casting in metal molds. In some cases, it is advantageous to redesign a casting for either permanent mold or diecasting methods. Sand casting usually requires minimum tooling charge, but the unit price of the castings and the finished part can be high. Permanent mold casting requires a higher tooling charge, but the unit price is lower, particularly for longer runs. Diecasting usually requires the highest tooling charge but also the lowest piece price on large quantities.
Once the casting method is determined, the alloy choice is narrowed (because not all alloys can be used with all casting methods). The next considerations are the service requirements. If high strength is required, heat-treatable alloys must be used. Alloy choice can be narrowed further when remaining requirements, such as pressure tightness, corrosion resistance and machinability, are considered.

In some instances, it may be required to maximize one certain property—for example, highest possible yield strength. This limits the alloy and heat treatment choices, as well as the casting method, to one or two choices. In addition, compromises will have to be made on the other requirements, such as ductility.

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