Effect of Defocusing Distance on Microstructure and Properties of 5052 Aluminum Alloy/T2 Copper Laser Welding-Brazing Joint

5052 aluminum alloy and T2 copper was joined by laser welding brazing with Zn-Al flux-cored welding wire as the filler metal under a laser power of 2 400 W, a welding speed of 6 mm·s⁻¹, and a wire feeding speed of 2 m·min⁻¹. The effect of defocusing distance （0, +2, +4, +6, +8 mm） on the wetting and spreading performance of welding wire, and on the microstructure, phase composition and mechanical properties of joints was studied. The results show that with the increase of defocusing distance, the spreading area of Zn-Al flux-cored wire on the surface of copper decreased, indicating that its wetting and spreading performance decreased. When the defocus distance was 0, +2, +4, +8 mm, the spreading performance of weld metal was poor, resulting in defects such as weld beads and cracks in the weld. When the defocus distance was +6 mm, the weld metal spread well, and no obvious defects appeared on the weld surface; the forming quality was the best. When the defocus distance was 0, the brazing interface reaction layer on the copper side was mainly composed of Al₄Cu₉ and Al_4.2Cu_3.2Zn_0.7 phases. When the defocus distance was +6 mm, it was mainly composed of AlCu and CuZn₅ phases. When the defocus distance was +8 mm, it was mainly composed of AlCu and Al_4.2Cu_3.2Zn_0.7 phases. With the increase of defocusing distance, the hardness change in different areas of the joint was not significant, and the tensile strength first increased and then decreased. The tensile strength was the largest at +6 mm defocusing distance, reaching 212 MPa; at this time, there were small-sized dimples distributed in the tensile fracture, presenting a typical ductile fracture morphology.