Role of heat treatment in optimizing microstructure of Al-Si cast alloys for elevated temperature applications

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The properties of aluminum casting alloys are controlled by the microstructure which depends on the alloy composition and thermal conditions during solidification. The as-cast microstructure may be further altered if the casting is subjected to thermal exposure during a heat treatment process or when exposed to high temperature service conditions. In the latter case, minimum change in properties is desirable to maintain the performance and ensure sustainability of the component over its lifetime.   The majority of aluminum castings are derived from Al-Si based alloys, as silicon enhances the castability. The Si content controls the volume fraction of the eutectic Si phase formed in the alloy, which in turn constitutes a sizeable part of the microstructure, and thus influences the fracture behavior. While the eutectic Si phase has been studied over many years, its contribution to the elevated temperature properties when subjected to the high temperatures under actual service conditions is an aspect that has received little attention. It is important to explore this aspect in order to fully understand the elevated temperature behavior of Al-based alloys. In particular, it is important to determine how seriously the Si particle strength would be affected when the particles have undergone coarsening during thermal treatment. From recent studies, we have observed that the coarsened Si particles display certain defects like pinholes and grooves even after prolonged solution treatment times. However, the effect of these defects on the Si particle strength, premature failure and its consequences on casting quality has yet to be established. Besides using Al3X type dispersoids to maintain high temperature alloy strength, and controlling the negative effects of intermetallic phases, determining the optimum heat treatment parameters is crucial to promote maximum dissolution of strengthening Cu and Mg phases at the same time avoid incipient melting-related defects which would increase rejection rate and manufacturing costs. To stay ahead in the global market, Canada must place emphasis on high-quality products and lower production costs.   Thus, the research program will focus on the effects of Si particle coarsening on elevated temperature properties, using as examples 396 (11.5%Si), and 220 (1.2%Si) alloys recently developed for automotive applications, in addition to a low Mg-319 (6.5%Si) alloy for comparison. Both 220 and 396 alloys have the same level of Mg. Thus, the alloys cover Si content in the range 1.2-11% with two levels of Mg (~0.1, 0.35%) & Cu (~1.5, 4.5%). By carrying out a fundamental study of all aspects of enhancing the high temperature strength of these alloys, the main goal is to show how, through proper heat treatment, selection of alloy composition & additives, critical components with sustainable high temperature performance could be commercially produced at affordable cost, using such alloys to replace the currently used alloys.