Refill Friction Stir Spot Welding: evaluation of the welding of AA2198-T8 sheets and preliminary tool wear investigation

Abstract

Refill Friction Stir Spot Welding (RFSSW) is an innovative method for joining similar and dissimilar lightweight metallic materials or thermoplastics. This technique has the advantage of issuing energy savings, producing welds with suitable mechanical properties and having the possibility of industrial scalability with the use of robots and automation. The use of aluminum alloys is widespread in aircraft industry. However, some of these alloys present poor weldability with conventional methods, an issue which can be overcome with the use of friction-based processes such as RFSSW. Furthermore, the substitution of riveting (today used as joining method for these alloys) can cause a reduction of aircraft weight, saving energy and potentially reducing costs. This work presents the evaluation of overlapped joints performed on AA2198-T8 1.6 and 3.2 mm-thick sheets produced by RFSSW. The optimization and correlation between process parameters and weld performance was studied by employing design of experiment (Taguchi) and analysis of variance (ANOVA) by measuring the welds’ response to the variation of welding time, plunge depth and rotational speed in order to verify the technical feasibility of the process. Moreover, the stir zone area was quantified and a positive correlation of this area was found with lap shear strength (LSS) performance. Finally, a preliminary study of the tool wear effects on the welds’ properties and microstructure was performed. Although results show a trend in reduction of lap shear strength along the 2350 studied joints, all the tested welds presented good mechanical properties, which greatly exceed the minimum standardized requirements for aircraft industry. However, more studies need to be carried out in order to determine the wear effects on the tool which affect the welds’ properties and tool lifecycle and might influence the economic and technical feasibility of using this technique in large scale industrial setups.