Interfacial Characterization and Cracking Phenomenon in Dissimilar Friction Stir Welded T-joints between AA6061 and CuZn36

This study focused on the interfacial characterization and cracking phenomenon in dissimilar friction stir welded (FSWed) T-joints between aluminum alloy AA6061 and brass CuZn36 under the various welding speeds from 75–200 mm/min. The results revealed that welding speed strongly affected the interface morphology and diffusion behavior. At low welding speeds, a wider bonding region and stronger diffusion between Al and Cu were achieved, resulting in the formation of several intermetallic compounds (IMCs), including AlCu, Al₂Cu, and Al₄Cu₉ along the interface. However, excessive diffusion and IMC thickening led to interfacial embrittlement and crack initiation approximately 74–240 µm away from the interface toward aluminum side. In contrast, higher welding speeds reduced mixing and diffusion, producing poor metallurgical bonding. The hardness distribution along the skin exhibited a typical “W shape”, with the lowest hardness of 50 HV at the heat affected zone of AA6061, whereas the stringer showed a maximum hardness of about 630 HV near the interface due to the presence of a thick and brittle IMC layer. Overall, the results highlight the challenge of obtaining a defect-free joint between AA6061 and brass CuZn36, where controlling welding speed is critical to balancing interfacial bonding and crack prevention.