Process development and enabling high-speed friction stir welding of 25 mm AA2139-T8

Joining thick plates (≥ 12 mm) of aluminum (Al) alloys is challenging due to high tool forces, uneven material flow, and non-uniform heat distribution through the material’s thickness. Recent research on tool design and welding parameters for friction stir welding (FSW) in thick plate butt joining has reached a plateau. This study sheds light on the systematic development of a process via single-pass and double-pass FSW for high-strength aluminum alloy AA2139-T8, aiming to enhance joint efficiency and enable high-speed welding. The study presents experimental evidence of a novel tool design featuring opposing pin threads, which facilitates high-speed (178 mm/min) single-pass friction stir butt welding of 25 mm thick AA2139-T8. A series of FSW trials were performed both in-air and with a trailing water spray using steel backing plates (BP) to understand the impact of quenching and cooling rate on process response and joint performance. A joint efficiency of 83% was attained using the novel tool features and effectively controlling process forces and thermal boundary conditions.