Interface evolution and mechanical properties in lightweight Mg–Li/Al dissimilar joints via dynamic support friction stir lap welding

This study investigates the impact of rotation speed on the interface evolution and mechanical properties of dissimilar joints between AA6061-T6 aluminum alloy and LA103Z magnesium-lithium alloy via dynamic support friction stir lap welding (DSFSLW). The introduction of a support shoulder, replacing the conventional backing plate, effectively minimizes backside heat dissipation and reduces thermal gradients at the Mg–Li/Al interface. Microstructural analysis reveals a pronounced transition in interfacial morphology with increasing rotation speed. At lower speeds, the interface is characterized by a mechanically mixed zone consisting of alternating fragmented strips. As rotation speed increases, enhanced thermal input and material flow drive the formation of a diffusion-dominated interface, comprising continuous layers of intermetallic compounds (IMCs). Thermodynamic calculations integrated with weld thermal cycle revealed competitive Li/Mg diffusion driven by chemical potential gradients. This diffusion behavior governs the formation of Mg 17 Al 12 (γ phase) and Al 2 MgLi (τ phase), accompanied by an amorphous-to-crystalline transition. The complex interlayer structure results in a significant hardening effect, with microhardness values reaching 163.2 HV near the interface. The peak lap shear strength of 197 N/mm occurs at 600 rpm, with a 42% reduction in strength at 900 rpm due to excessive IMC growth. Fracture analysis confirmed interfacial failure governed by brittle IMC, highlighting the critical role of interfacial structure in joint performance.