Effect of friction stir process on the microstructure and corrosion behavior of AZ91 Mg alloy

Abstract

Friction stir process (FSP) has been utilized to modify the surface microstructure of AZ91 alloy in three levels of rotation speeds and pass numbers. Microstructural and electrochemical analyses were performed on as-received and FSPed alloys, and their correlation was investigated. Optical and electron microscopy showed a significant grain refinement, accompanied by aluminum's dissolution into the matrix phase. The highest aluminum over-saturation occurred at 1200 rpm, which led to eutectic phase discontinuity at the grain boundaries. Additionally, the basal plane texture was studied using pole figures of the surface, which revealed extreme basal texture after the FSP. Utilization of X-ray diffraction (XRD) to calculate the residual stress before and after the FSP indicated substantial residual stress build-up. Accumulation of the eutectic phase at the grain boundaries led to the shift of the open circuit potential (OCP) toward more positive values in a 3.5% NaCl solution. Moreover, according to the electrochemical analyses, the corrosion rate increased after the FSP. At 1200 rpm of the rotation speed, more severe aluminum dissolution into the matrix phase increased surface layer resistance to breakdown. It was also observed that the accumulation of higher eutectic phases at the grain boundaries induces more galvanic microcells, which caused a drop of charge transfer resistance.