Dissimilar friction stir welding between AZ31 magnesium alloy and pure copper: Evolution mechanism from macro to micro scales

High-quality joining between Mg and Cu based alloys was still a huge challenge due to excessive formation of detrimental intermetallic compounds (IMCs) by most of the conventional welding and joining methods. In this study, with controlling heat input and intensifying dissimilar material deformation and intermixing, friction stir welding (FSW) technique was employed for the joining between AZ31 Mg and T2 copper, and the process mechanism was elucidated at both macro and micro scales by combining experimental and numerical approaches. First, a process-based contact boundary was proposed for a precise description of the condition at the interface between the tool and the redistributed dissimilar Mg/Cu materials. Second, defect-free Mg/Cu FSW joints were obtained by using Mg-RS/Cu-AS configuration. With the tool offset of 1.0 mm to Cu-AS, the maximum temperature in the stirring zone was 710 K, which was lower than the minimum eutectic temperature of Mg-Cu binary system. Third, over 5-times difference of flow stresses resulted in upward and downward materials transfer, and also different flow-deposition behaviors of Mg and Cu, respectively, with effect of the rotating tool, which finally caused the elongation of Mg/Cu interface. The thickness variation of IMCs layer ranging 0.5–2.5 µm was originated from the inconsistent evolution of temperature, velocity and strain rate at different locations. Both Mg₂Cu and MgCu₂ IMC phases were observed, and the Mg₂Cu layer was first generated, grown faster and finally much thicker than the MgCu₂ layer. Moreover, an elongated and continuous IMCs layer was advantageous for achieving Mg/Cu joining strength over 124 MPa, which was greatly improved comparing with the published results.