Optimizing the mechanical properties of Al-4.5Cu-xSi alloys through multi-pass friction stir processing and post-process aging

The effect of post-process aging on the microstructure and mechanical properties of multi-pass friction stir processed (FSPed) Al-4.5Cu alloy containing Si (1, 3, and 5 wt.%) was studied. According to the results, adding Si improved the fluidity and decreased the porosity content of the alloy. The addition of Si up to 3 wt.% also enhanced the mechanical properties. However, further addition of Si up to 5 wt.% impaired the tensile properties. Applying the first pass of FSP improved the tensile strength and fracture strain of the alloy containing 3 wt.% Si by 25 and 125%, respectively. However, the second and fourth pass of FSP substantially improved the fracture strain, but deteriorated the hardness and tensile strength of the alloy containing 3 wt.% Si. Post-FSP aging at 180 °C for 8 h significantly improved the mechanical properties. For instance, compared to the as-cast condition, the hardness, tensile strength, fracture strain, and toughness of post-aged four-pass FSPed Al-4.5Cu-3Si alloy increased by 107, 108, 175, and 310%, respectively. According to the fractography results, the fracture surface morphology of Al-4.5Cu-3Si alloy changed from a quasi-cleavage mode in as-cast condition to a ductile-dimple fracture mode after post-FSP aging.