Improving tensile-shear properties of friction stir lap welded dissimilar Al/Mg joints by eliminating hook defect and controlling interfacial reaction

To improve tensile-shear properties of fiction stir lap welded (FSLW) dissimilar Al/Mg joints, pin-tip profiles were innovatively designed and welding speed was optimized, and effects of them on formation, interface microstructure and mechanical properties of different FSLW joints were investigated. With increasing the welding speed, the tensile-shear load of FSLW joints produced by three pins presents an increasing firstly and then decreasing trend. Compared with R-pin, the hook and hole defect in the joints made by S-pin and T-pin are eliminated owing to additional eccentric force. Moreover, the joints obtained by T-pin at 75 mm/min have the highest tensile-shear load, and a maximum value of 3.425 kN is produced, which increases by 96.8%. Meanwhile, the pin-tip profile improves significantly the interface reaction depending on the welding temperature. For R-pin, thick brittle intermetallic compounds of about 6.9 μm Al₃Mg₂ and 13.3 μm Al₁₂Mg₁₇ layers at the welding interface derived from diffusion reaction are formed, resulting in continuous cracks. However, using T-pin can raise the interface temperature, and which makes the interface liquefy locally to generate only 2.2 μm Al₃Mg₂ layer and dispersive (Al₁₂Mg₁₇ + Mg) eutectic structure. This can release high residual stress and remove welding crack, consequently enhancing the interface properties of T-pin joints.