Low-temperature aging treatment to optimize interface joining for dissimilar friction stir lap welded aluminum and titanium alloys

Abstract

The increasing demand for high-strength and lightweight structural components has driven the development of reliable joining for aluminum and titanium alloys. However, defects and brittle intermetallic compounds at Al/Ti interface limit the mechanical performance of joint. To address this issue, the post-weld low-temperature aging (AG) for dissimilar friction stir lap welding of 6061-O aluminum alloy and Ti-6Al-4V titanium alloy is systematically investigated. The results indicate that appropriate AG treatment effectively eliminates interface cracks and voids, smooths the sharp geometric features of hook region, and optimizes the thickness and continuity of interfacial reaction layer. After 8 h AG at 160°C, the joint exhibits a maximum lap shear strength of 470 N/mm, indicating a 26.4% increase over as-welded joint and corresponding to 98.7% of the load bearing capacity of aluminum base metal. Furthermore, the fracture mode of joint transforms to notably ductile pattern with fracture location shifting from the interface to aluminum base metal. Microstructural analysis reveals that the Gibbs-Thomson effect drives the dissolution of Ti fragments and Al/Ti mixtures during AG. The dissolved species diffuse toward larger particles, inducing Ostwald ripening and forming a more continuous and denser interface layer, thereby enhancing metallurgical bonding of both metals. However, the lap shear strength of joint drops to 370 N/mm when the AG time is further extended to 12 h, owing to the excessive interfacial-layer coarsening under over-AG condition and the induced lowering of interface's ability to accommodate deformation. This study provides an engineering-feasible post-weld treatment strategy for optimizing Al/Ti dissimilar joint.