Microstructural degradation control and dissimilar joint optimization through friction stir processing

• Friction stir processing above 500 rpm produced a defect-free nugget zone. • Higher tool speed at 1.66 mm/s yielded sound zones with clean interfaces. • Low travel speed (0.83 mm/s) at 1500 rpm caused cracks and structure loss. • Peak tensile shear load achieved at 1500 rpm and 1.66 mm/s. The need for hybrid-welded joints has continued to increase in different industrial sectors, especially for electrical applications. In this study, the effect of the friction stir welding process parameters on the microstructure and mechanical properties of hybrid aluminum-copper lap joint was investigated. The investigation focused on the impact of friction stir process on the mechanical characteristics, interface, and macro- and microstructure of base metals. A tool inserted from the copper side was subjected to two traveling speeds of 0.83 mm/s and 1.66 mm/s and three rotation speeds of 500, 1000, and 1500 rpm. Friction stir processing at rotational speeds over 500 rpm forms the nugget zone with minimal interface flaws. Traveling speed of 1.66 mm/s shows defect free interface. At lower traveling speed and high rotational speed, worst welding interface occurred. A critical transition in intermetallic compound (IMC) formation was observed: lower heat inputs favored the formation of Al 2 Cu, whereas higher rotational speeds (1500 rpm) and lower traveling speeds (0.83∼mm/s) promoted the growth of Cu-rich brittle phases, specifically AlCu and Al 4 Cu 9 , appearing as elongated dendritic structures. At a travel speed of 1.66 mm/s and a rotation speed of 1500 rpm, the maximum tensile shear force of approximately 140 kN was achieved. However, reducing the traveling speed to 0.83 mm/s at 1500 rpm triggered microstructural degradation characterized by numerous transverse cracks and a reduction in load-bearing capacity to approximately 105 kN. These findings establish a processing window to optimize bonding while controlling deleterious phase transitions in dissimilar joining.