Thermal dynamics and torque-force interplay in friction stir welding of advanced aluminium composites

This study presents a comprehensive analysis of the thermal history and force-torque dynamics in the friction stir welding (FSW) of advanced high-strength structural aluminium matrix composites. Using AA6092/17.5 SiCp-T6 plates, the research explores the intricate interplay between thermal profiles, torque-force behaviour, and weld integrity, with a key focus on tool revolution pitch (TRP), which significantly influences heat flow patterns and material deformation. The results reveal a gradual decrease in peak temperature from 509 °C to 444 °C as TRP increases from 0.017 to 0.069 mm/rev, followed by a sharp drop to 310 °C at 0.12 mm/rev. A consistent thermal asymmetry is observed, with the advancing side exhibiting higher temperatures than the retreating side, reaching a maximum temperature difference of 54 °C at 0.12 mm/rev. Cooling rates initially increase from 4.81 °C/s at 0.017 mm/rev to 8.15 °C/s at 0.069 mm/rev before declining to 5.54 °C/s at 0.12 mm/rev. A steady increase in spindle torque and Z-force with rising TRP is identified, attributed to reduced heat input and insufficient plastic deformation at elevated TRP levels. Microstructural analysis reveals that defect-free welds are obtained up to a TRP of 0.094 mm/rev, whereas insufficient material flow and plasticisation result in tunnel defect formation at 0.12 mm/rev. Grain size decreases from 4.88 μm at TRP1 to 4.31 μm at TRP3 due to reduced heat input but increases to 7.36 μm at TRP5 due to incomplete recrystallization. Additionally, coarser reinforcement particles emerge at higher TRP values, resulting from reduced tool rotations within a specific area.