Microstructure and properties of 5A06 aluminum alloy T-joints welded by stationary shoulder friction stir welding

The 5A06 aluminum alloy T-joints with 10 mm thick were welded by stationary shoulder friction stir welding. Through different welding parameter selection of low rotation speed matching high welding speed and high rotation speed matching low welding speed, combined with tensile testing, macroscopic and microscopic metallographic analysis, electron backscatter diffraction analysis and fracture analysis by scanning electron microscopy, the effect of heat input on mechanical properties and microstructures of T-joints was studied. The results show that the two sets of welding parameters can both obtain full penetration T-joints without any cavity defects, the weld surfaces are smooth and flat without weld-thinning. The different heat input changes the friction mode between the stir pin and the surrounding materials and then cause kissing-bonding defects in the welding joint, consequently affect the tensile strength of the joint. Under the parameter of high rotation speed matching low welding speed, the top center of the weld overlap zone is prone to cause weak bonding defects, resulting in a lower tensile strength of 198 MPa, tensile specimen fracture in the middle plate. Under the parameter of low rotation speed matching high welding speed, the weld is free of defects and the tensile strength of the joint is 287 MPa, with the tensile specimen fractured at the bottom plate.