Modification of Microstructure and Enhancing Mechanical Properties of as-cast AZ91 Magnesium alloy via Friction Stir Processing

Magnesium alloys, as the lightest structural alloys, due to their high strength-to-weight ratio offer significant potential for improving energy efficiency of various transportation systems. This paper addresses the influence of friction stir processing (FSP) treatment on the microstructure and mechanical properties of cast AZ91 Mg alloy. It is demonstrated that FSP treatment enables elimination of dendritic structure, significant grain refining, break-up and partial dissolution of coarse β and formation of ultra-fine sub-micron Mg17Al12 particles. These microstructural modifications resulted in enhancement of mechanical properties in terms of tensile strength and energy absorption by 48% and 283%, respectively. It is shown that FSP treatment altered the failure mechanism of the alloy from brittle cleavage-dominant mode to ductile dimple-dominant mode which can increase the potential of Mg alloys to use in safety-critical application. Therefore, it can be concluded that FSP, as a process of sever plastic deformation at high temperature, has a great potential to tailor the microstructure and enhancing the mechanical properties of cast Mg alloys.