Regulation of the microstructure and mechanical properties of the immiscible Fe/Mg dissimilar metal joints using MIG-TIG double-sided arc welding-brazing

Abstract

The immiscible and non-reactive Fe/Mg (304SS/AZ31B) dissimilar metals were butt joined by MIG-TIG double-sided arc welding-brazing (DSAWB) with AZ31 magnesium alloy welding wire. The brazing interface was metallurgically joined through the mutual diffusion of elements (Al, Fe and Mg) with the formation of α-Fe(Mg, Al) solid solution and FeAl intermetallic compound (IMC) at the interface. The joint tensile strength reached a maximum value of 235 MPa, which was 93.98 % of the base metal-Mg alloy. The joint tensile strength was influenced by the combination of joint forming and interface strength, with the latter being the main contributing factor. When the welding heat input was low, increasing the heat input by adjusting the welding parameters were advantageous for the wetting and spreading of liquid metal on the steel base material, ultimately improving the joint forming. The diffusion of elements in the brazing interface area was enhanced, resulting in improved interface strength when the diffusion distance of elements was maintained within the range of 1 μm–2.6 μm. The tensile strength remained high due to the excellent joint forming and interface strength. As the welding heat input continued to increase, excessive molten metal in the weld seam caused uneven joint forming, particularly on the back side. The diffusion of elements in the brazing interface area led to an increase in the formation of brittle intermetallic compounds, resulting in a decrease in interface strength. The overall tensile strength decreased due to the combined effect of both factors.