Material flow and microstructure evolution in friction stir welding of AA6111 alloy sheets

Material flow and microstructure evolution are the two important aspects of the Friction stir welding (FSW) technique, which control the properties and performance of the friction stir welds. The present work investigates these two aspects of the FSW with main emphasis on the friction stir lap joints (FSLJs) on the AA6111 sheets. Due to the characteristic material flow during FSW, the faying surfaces in FSLPs are deposited in and around the weld nugget and create unavoidable defects. These defects reduce the load carrying thickness of the joints by providing an easy path for the weld failure. Therefore, to control the faying surface defects, a clear understanding of the material flow in FSW and its effect on the FSLJs is needed. With this objective, a comprehensive study of the material flow during FSLJs and the effects of various process parameters on the faying surface defects were conducted. This study not only revealed the mechanism of the formation of the faying surface defects but also explained the detailed material flow in the FSW. This investigation was further extended to study the role of FSW tool features on the material flow and the tool features to control the faying surface defects were proposed.;The microstructure evolution during the FSW of AA6111 was investigated by differential scanning calorimetry (DSC), microhardness and transmission electron microscopy (TEM) techniques. This study showed that the increase in the heat input during FSW resulted in better aging response of the joints. The microhardness and strength variation across the FSW weld were explained from the DSC and TEM observations.