Refill friction stir spot welding of AA6016-T4 aluminum alloy: study of new load-controlled process

Abstract

Aiming at structural weight reduction and energy efficiency, aluminum alloys have been considered as potential materials for automobile manufacturing. The development of technologies able to join aluminum alloys more effectively contributes to increasing their application in this transport industry sector. AlMgSi alloys have a highlight in selecting light materials for vehicle assembly due to its properties such as low density, high strength-to-weight ratio, and good corrosion resistance. Refill friction stir spot welding (Refill FSSW) is a solid-state joining technology suitable for joining lightweight materials. This process consists of a few steps performed within seconds, with low energy consumption, and without consumables addition. Thus, it is being considered as the most promising technology to replace traditional joining techniques as resistance spot welding (RSW) and self-piercing riveting (SPR) processes. This work presents the innovative Refill FSSW robotic process controlled by load. This new process was used to produce welds of AA6016-T4 sheets. Results obtained through the conventional position-controlled Refill FSSW process was used for comparing and validating the effectiveness of the new welding control system. Process parameters optimization was carried out in order to obtain a process condition that maximizes the mechanical performance (lap shear strength) of the joint performed using the load-controlled Refill FSSW. Statistical methods, such as Box-Behnken design, were applied to optimize the process parameters. Microstructure analyses were performed to characterize the features of the joint. Hardness and thermal characterization helped to understand different weld regions, the mechanical properties, and the fracture behavior of the welds. Furthermore, the dynamic mechanical property was analyzed by fatigue tests for the condition of the highest lap shear strength and the L-N curve was drawn. The results indicated a satisfactory static and dynamic performance of the welds. Finally, failure mechanisms of static and dynamic tests seemed to be strongly related to microstructural regions.