The appropriate heat treatment of the composites will de- crease the compositional inhomogeneity and yield higher strength by precipitation hardening of the matrix alloys.12,13 The solute elements brought into solution during the solu- tion heat treatment process, locked into solid solution dur- ing quenching and then uniformly dispersed and precipitated out during aging. The solution treatment times of an alloy is governed by the diffusion rate of alloying elements into solution which is governed by the initial microstructure of the castings, i.e. grain size and distribution of the secondary phases. In the base alloys typically used in MMCs, the ele- ments are tied up in phases such as silicon rich phases and other intermetallics that take a significant amount of time to break up and be dissolved in the solid solution during solu- tion heat treatment.
Today, the heat treatment parameters used for Al MMCs are typically the same as those developed for the matrix alloy. These may not be suitable for the composites be- cause of the finer microstructure generally developed in the composites and the existence of the large number of Al/ reinforcement interfaces in MMCs.14
The finer grain size
in matrix alloys (due to the higher cooling rate in pres- sure infiltration) along with the higher internal residual stress (introduced by the coefficient of thermal expansion mismatches between reinforcement and matrix materials) will significantly reduce the time required for solutionizing because of the enhanced diffusivity through the combina- tion of grain boundary diffusion, dislocation diffusion and lattice diffusion.15
This is expected to shorten the solution
treatment time which will (1) postpone the grain growth, (2) decrease the risk of recrystallization, and (3) minimize
the reaction occurring between the metal matrix and the ceramic reinforcement particles. Ultimately, this reduction in solution time will improve MMC mechanical properties while decrease the heat treatment cost.
Technical Approach
Existing studies focus on general effects of under-aging and over-aging the composites and offer little insight as to what the optimal heat treatment parameters should be for different ceramic loading of selectively reinforced composites. The current study aims to optimize the heat treatment process (temperature & time) parameters for a few representative MMCs made via pressure infiltration of porous ceramic pre- forms. Through the present studies, it is expected to illus- trate the following:
1. Determine if/how the reinforcements interact with the casting alloy
2. Determine what the optimal solution treatment pa- rameters are for different type (composition and volume fraction) composites. The current study investigates the effect of the heat treatment param- eters on the mechanical properties of the composite
This relationship will help determine if the solution time of the composite can be reduced without a significant effect on the mechanical properties of the part, thereby, resulting in significant time and energy savings. It will also provide a much needed reference of the effects of heat treatment on mechanical effects of aluminum MMCs with different rein- forcement loads.
Table 1. Comparison of Casting Techniques for Al MMCs3,4,10,11
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International Journal of Metalcasting/Winter 11
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