Interface evolution during refill friction stir spot welding and post-weld heat treatment of AA6013/Ti6Al4V with varying Mg and Si content

The welding of dissimilar material combinations such as aluminum and titanium alloys is challenging due to their distinct mechanical and thermal properties. In particular, the formation of brittle intermetallic compounds can negatively affect the mechanical properties. Refill friction stir spot welding is a solid-state joining method that can reduce the thermal influence compared to fusion welding. This study analyzes the effect of systematic Mg and Si additions in AA6013 on the intermetallic compound formation for Al-Mg-Si / Ti6Al4V welds after processing and artificial aging heat treatments. The thermal cycles during the refill friction stir spot welding of five Al-Mg-Si variations are analyzed. Microscopic investigations showed fragments up to discontinuous layers of intermetallic compounds (IMCs) after welding and highly columnar Ti(Al,Si)₃ and Al₁₈Mg₃Ti₂ IMCs after the post-weld heat treatment, depending on the specific alloy composition. Al₁₈Mg₃Ti₂ is formed as mixed structure with Ti(Al,Si)₃ or as separate layer between the Ti(Al,Si)₃ and the aluminum, depending on the amount of solute Mg which did not form Mg₂Si. The IMCs thickness after heat treatment showed significant differences depending on the Mg and Si amount and element accumulations were found in the interfaces. Thermodynamic calculations revealed the phases during heat treatment and proved a liquid phase for alloys containing high amount of solute Mg. This liquid phase solidified as Al₁₈Mg₃Ti₂ after the heat treatment, showing a fragile characteristic, including cracks. To minimize the liquid and therefore Al₁₈Mg₃Ti₂ formation, a limited amount of Mg in solid solution must be respected by an appropriate ratio of Mg to Si.