Influence of Multi-pass Friction Stir Processing on Microstructure and Mechanical Properties of Die Cast Al-7Si-3Cu Aluminum Alloy

The influence of overlap multi-pass friction stir processing on the microstructure and the mechanical properties, in particular, strength, ductility and hardness of die cast Al-7Si-3Cu aluminum alloy was investigated. It was observed that increase in the number of overlap passes friction stir processing resulted in significant refinement and redistribution of aluminum silicon eutectic phase and elimination of casting porosities. The microstructural refinement by the friction stir processing not only increases the ultimate tensile strength from 121 to 273 MPa, but also increases the ductility as observed by the increase in fracture strain from 1.8% to 10%. Analysis of the fractured surface reveals that the microstructural refinement obtained by friction stir processing plays a vital role in transforming the fracture mode from completely mixed mode to the ductile mode of the fracture with increasing number of passes. The change in the size, shape, morphology and distribution of eutectic silicon particles and elimination of the porosities are the main reasons for the increases in tensile strength and ductility due to friction stir processing.