Optimizing Properties of Automotive Cylinder Heads Cast of Aluminum 940153
An analysis of the long range potential for improvement of aluminum castings for use in automotive applications was made. Comparisons of lost foam, semi-permanent (SPM), and sand casting were done using cylinder heads as the test configuration. The casting processes were compared on the basis of tensile properties, as well as their basic capabilities to provide the highest strength.
It was found that, while all three processes are very useful, sand casting can be used to make the highest properties in a wide variety of alloys. Countergravity casting sand molds at low metal temperatures with controlled amounts of hydrogen yielded castings with excellent properties and required minimum risering. This same effect could not be obtained in heads cast in the semi-permanent mold process, due to segregation of hydrogen caused by the metal molds. Hot isostatic pressing (HIPping) was required to produce the optimum properties, and sand castings with controlled hydrogen HIPped very well. A new low cost HIPping process developed should increase the use of HIPping in the near future. The ultimate in stiffness was shown to be provided by aluminum metal matrix composites (MMCs). Metallurgy of castings made with up to 40% particulate silicon carbide is discussed.