New Possibilities for High Strength Aluminum

The U.S. Department of Energy funded a number of proposals in 2004 under the Energy Saving Melting and Revert Reduction Technology (E-SMARRT) program, which included a project lead by Canmet Materials (CMAT), Hamilton, Ontario, Canada, on permanent mold casting light metal alloys. CMAT focused specifically on alloys that were considered difficult to pour in a permanent mold but offered significant energy savings by reducing revert (or returns).

Fig. 1. The bracket casting was selected because its geometry was typical of structural components.

A206 is one of the highest strength structural alloys. Yield strengths in excess of 45 ksi are routinely achieved in the T71 temper. It has very good high temperature properties with little reduction in strength up to 350F (177C), making it an ideal alloy for cylinder heads and other elevated temperature applications.

When poured into a permanent mold, however, A206 has a tendency for hot tearing, which is caused by thermal contraction in the late stages of solidification. Because of this difficulty, A206 most often is cast in green or chemically bonded sand molds. A recent research project aimed to establish processing parameters for A206 in permanent mold castings that prevent hot tearing for structural components in automotive and other applications.

Overcoming Obstacles in Simulation

When working with the 200 series of aluminum alloys, chemistry control is more important than with 300 series aluminums. Iron and silicon levels need to be minimized to achieve the expected properties. Impurities from insufficiently cleaned crucibles or improperly mixed alloys can lead to unsuitable results. Metalcasters must ensure tools and ladles are clean. Process controls to minimize changes in chemistry also are essential.

In 200-series alloys, mechanical properties depend on controlling the iron and silicon levels in the alloy. In A206, iron levels need to be controlled to less than 0.1% and silicon levels to less than 0.05%. Castings made with specified chemistries and solidification rates typical of the permanent mold process can develop yield strengths of 50 ksi coupled with double digit elongation.

To research the ability to cast A206 in permanent mold, a case study on a mounting bracket casting used in medical equipment was conducted. The component, shown in Fig. 1, is currently produced in A356 in a 4140 steel permanent mold. The current design ensures the A356 alloy can be cast without hot tearing. The A356 component also needed to pass grade C radiographic quality.

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