Interfacial Microstructure Produced during Dissimilar AA6013/Ti-6Al-4V Friction Stir Lap Welding under Zero-Penetration Condition

The purpose of this study was to investigate the interfacial microstructure that was produced during dissimilar friction stir lap welding (FSW) of 6013 aluminum alloy and Ti-6Al-4V. FSW was conducted under a zero-penetration condition, i.e., the welding tool was plunged exclusively into the upper (aluminum) plate of the dissimilar lap joint. To facilitate the interpretation of microstructural processes, finite element modeling (FEM) was applied to evaluate the temperature field within the weld zone. The FEM simulation revealed a very sharp temperature gradient across the dissimilar interface. This effect was attributed to the generation of FSW heat exclusively within the aluminum part and a relatively low thermal conductivity of titanium. The abrupt temperature drop on the titanium side imposed a strict limitation on the diffusion penetration of aluminum and thus resulted in a relatively thin (~0.5 μm) and discontinuous intermetallic compound. Due to the complex chemical composition of the FSWed aluminum alloy, the diffusion processes also involved alloying elements. Consequently, the evolved intermetallic compound had a complicated chemical composition, with the principal elements being aluminum, titanium, silicon, manganese, and magnesium.