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