Effect of Magnesium and Silicon on Temperature and Mechanical Properties in Refill Friction Stir Spot Welding of Aluminum to Titanium

Optimizing the mechanical properties of aluminum to titanium welds is crucial to establish applications for dissimilar lightweight structures in the aerospace industry. In this context, solid-state welding technologies have proven effective in terms of short joining cycles, allowing the combination of cost-effective production and structural weight optimization. However, metallurgical effects between aluminum and titanium in the joint interface are still not completely understood due to differences in physical as well as chemical characteristics. In this study, aluminum alloy 6013 was welded to Ti6Al4V by refill Friction Stir Spot Welding, including systematic variations of Mg and Si alloying element content in the used AA6013 sheets. In total five different Al alloys were welded to the titanium to investigate the influence of Mg and Si during processing. Apart from the material selection, the weld strength is mainly influenced by the intermetallic compound thickness at the interface, which in turn primarily depends on the exposed temperature cycle. Consequently, major interest during this study was given on the temperature evolution, interfacial features and the global mechanical properties. It could be concluded that the temperature evolution was only slightly influenced by the process parameters, but systematic differences were found depending on the content of alloying elements of the Al alloy. In terms of mechanical properties, all material combinations generated solid joints, reaching between 〜4 and 8.5 kN of ultimate lap shear force and failing partially in the interface or in the aluminum sheet.