Abstract: An innovative and accurate method for controlling the interfacial structure of Mg-alloy/steel direct lap joints using a hybrid laser-gas tungsten arc (GTA) flexible control heat source was proposed. The study investigated the impact of spatial regulation of a flexible hybrid heat source on the thermal gradient distribution across the joint geometry and the resulting stress states in the joints, revealing the respective bonding and strengthening mechanisms. The findings indicate that the malposition alters the thermal gradient distribution within the joint, influencing the metallurgical bonding area, interfacial structure, fracture mode, and fracture path. Furthermore, the results showed that varying the laser pulse frequency affected the frequency of the laser-induced arc and the number of keyholes generated per unit length of the joint, which effectively altered the joint geometry and interfacial stress state, and could be used to enhance the load-bearing capacity of the joints. The joint's maximum load was measured to be 325.2 N/mm, which is approximately 88% of the Mg-alloy matrix and the highest reported strength for Mg alloy-steel dissimilar lap joints. A composite interfacial layer structure was achieved, consisting of Al₁₁(Mn, Fe)₄ and a small amount of Mg₂Al₃ intermetallic compounds (IMCs) at the weld front, along with Al₁₁(Mn, Fe)₄ and Fe₃Al IMCs in the weld middle. The key factors to achieving a high bonding strength of Mg-alloy/steel lap joints have been summarized. This study provides a technical and theoretical reference for the precise collaborative control of joint geometry, interfacial structure, and mechanical properties of dissimilar material lap joints.

Key words: Hybrid laser-GTA flexible heat source, Thermal gradient distribution, Stress state, Composite interface, Magnesium alloy, Steel