Bonding mechanism of CMT fusion-brazed joints between magnesium and galvanized steel

Fusion-brazing of AZ31B magnesium alloy to HDG60 galvanized mild steel sheets was carried out by Cold Metal Transfer (CMT) technique.The optimized weld appearance was obtained by adjusting welding parameters.The bonding mechanism and mechanical properties of welding joints were studied by scanning electron microscope (SEM 6700F) equipped with energy dispersive X-ray spectrometer (EDS), electron probe microanalysis (EPMA),X-Ray analysis and universal tensile testing machine.Based on the experimental results,it was found that satisfied welded joints can be produced between magnesium and galvanized steel sheets.The welded joints were composed of weld metal,brazed interface and fusion zone.The brazed interface between the magnesium weld metal and galvanized steel primarily consisted of Al,Zn,Mg intermetallics and solid solution,such as Mg solid solutions,MgFeAlO₄,Al₇Zn₃ and Mg₂Zn₁₁.Zn and Al played key roles in bonding of Mg/steel joints,and Zn had flow effect on the formation of welded joints.During the tensile test of welded joints,the specimens fractured through the fusion zone,and the tensile-shear strength reached 218 MPa.