Weldability of AA6061-T6 and AISI 304 by underwater friction stir welding

Abstract

Quality of friction stir welded structures made of thin sheets of aluminium alloy and steel in air medium is affected by several welding defects, inter-metallic compounds and weak heat affected zone, which causes poor weld strength. The present experimental investigation has been carried out to study the feasibility of underwater friction stir welding (UwFSW) between AA6061-T6 and AISI 304 at different values of tool rotational speed and plunge depth. In UwFSW, less heat inhibits the formation of thick inter-metallic compounds, welding defects. In addition, faster cooling rate in UwFSW produces fine microstructure in the weld zone. In the present study, authors have attempted to characterize the various types of weld defects, microstructural changes occurring in conventional friction stir welding in air medium (CFSW) and UwFSW. These results have been used to understand the variation in the weld strength and the microhardness obtained in CFSW and UwFSW at different values of tool rotational speed and plunge depth. Results have revealed the presence of various types of defects such as void, micro-cavity, kissing bond, and Lazy S in the weld region. Further, X-ray micro computed tomography (X-ray micro CT) test on the CFSWed and UwFSWed samples revealed the presence of welding defects (porosity) in the stir zone. UwFSW reduced the weld porosity, thickness of intermetallic compound and yielded fine microstructure which collectively improved the weld strength.