The effect of pin profiles on the microstructure and mechanical properties of underwater friction stir welded AA2519-T87 aluminium alloy

Background: AA2519-T87 is a new armour grade aluminium alloy employed in the fabrication of light combat military vehicles. Joining of this material using fusion welding processes, results in the formation of solidification defects like porosity, alloy segregation and hot cracking. In order to overcome the solidification related problems, solid state welding processes such as friction stir welding (FSW) can be used. Though the joining takes place below the melting temperature of the material, the thermal cycle experienced by the thermo-mechanical-affected zone (TMAZ) and heat-affected zone (HAZ) is causing grain coarsening and precipitates dissolution in the age-hardenable aluminium alloys, which deteriorate the joint properties. To get rid of this problem, underwater friction stir welding (UWFSW) process can be employed. The water cooling reduces the heat, and thus, the thermal softening required taking place in TMAZ and HAZ. Therefore, the material flow is entirely different in FSW and UWFSW. Methods: In this investigation, a comparative study is made to understand the influence of tool pin profiles, namely straight cylindrical (STC), taper cylindrical (TAC), straight threaded cylindrical (THC) and taper threaded cylindrical (TTC) on stir zone characteristics and the resultant tensile properties of both FSW and UWFSW joints. Results: From this investigation, it is found that the joint made by taper threaded pin profiled tool underwater cooling medium exhibited higher tensile properties of 345 MPa and joint efficiency of 76 %. Conclusion: The enhancement in the strength is attributed to the precipitation hardening, grain boundary strengthening and narrowing of lower hardness distribution region (LHDR).