Effect of multi-pass friction stir processing on microstructure and mechanical properties of Al3Ti/A356 composites

In this paper, Al3Ti/A356 composites fabricated by in situ casting were subjected to multi-pass friction stir processing (FSP) with 100% overlap. The microstructure and mechanical properties before and after FSP were investigated. The as-cast specimen, consisting of coarse matrix grains, high porosity, and interwoven acicular Si particles and Al3Ti clusters, exhibited the lower tensile properties. After multi-pass FSP, both the strength and ductility of the composite samples gradually enhanced. This was attributed to the combined effect of grain refinement, porosity removal, and refined Al3Ti and Si particles' uniform dispersion. The maximum strength and elongation achieved in 4-pass FSPed composite were substantially improved to approximate 1.64 and 2.52 times those of the as-cast composite. Quantitative analysis of the strengthening mechanism in the FSPed samples indicated that grain refinement, instead of Orowan strengthening, contributed more and more to the yield strength of the composites as the number of FSP passes was increased.