Microstructure and micro-hardness analysis of friction stir welded bi-layered laminated aluminum sheets

Abstract

Friction Stir Welding (FSW) is an economical and environment friendly solid-state welding technique. Properties such as high specific strength, low crack propagation rate and high resistance to impact and fatigue loadings enhance the use of aluminum laminates in the modern aircraft and automotive industry. In this study, FSW process was employed for butt joining of bi-layered Al-1350 laminated sheet. Preliminary experimentation, microhardness and microstructure evolution of the FSWed laminates were examined. The microscopic investigations revealed that the FSW process parameters greatly affect the dissolution of strengthening particles, material mixing and formation of defects. The Scanning Electron Microscopy (SEM) revealed that increasing in the tool rotation rate led to increase in the grain size and the size of second phase particles. However, with increasing tool traverse speed a very small rise in the grain size was observed. The increase in the density of second phase particles promoted a small rise in the hardness. Moreover, the microhardness of bi-layered aluminum weldments decreased with increase in grain size hence, following the Hall–Petch relationship. A comparative analysis of FSW of multilayered and monolithic sheet was also performed. Finally, a set of best suited parameters was suggested for the welding of bi-layered laminated sheets of aluminum.