Refill friction stir spot welding: investigation of the similar and dissimilar welding of AA5754-H22 and AA6061-T6 sheets

Abstract

The automotive industry is shifting to a new paradigm of material utilization in vehicles. Different parts of the car structural body are increasingly being made of materials selected to have a customized performance according to its position in the car. For that reason, materials are welded in similar and dissimilar combinations beyond the capabilities of traditional welding methods. Refill Friction Stir Spot Welding (RFSSW) is an emerging technology able to weld similar and dissimilar combination of materials in a consistent manner and delivering the quality requirements for car industry. The present research work focused on the RFSSW of similar AA5754-AA5754 and dissimilar AA5754- AA6061, which commonly occur in formed sheets of car door panels. The investigation of internal defects, mechanical properties, metallurgy and material flow were the main objectives of this work. Design of Experiments, Response Surface Methodology and Analysis of Variance were the statistical tools to investigate the window of process parameters where welds were optimized and to find parameter combinations where defects could possibly be present. Mechanical testing was performed with Lap-Shear Strength tests and microhardness measurements. Microstructural analysis was performed with polarized light microscopy. Material flow was analyzed with Stop-Action technique. Statistical analysis revealed a very linear model for the similar combination depending mostly of the individual contribution of parameters, specially Plunge Depth. For dissimilar welding, the model is more dependent on the square and two-way contributions. It was identified internal pores and lack of filling defects that explained the poor performance for certain combinations of parameters.