Microstructure and interface characteristics of laser penetration brazed magnesium alloy and steel

Based on the experiments of laser penetration brazing for Mg alloy and steel dissimilar materials, the microstructure and interface characteristics of butt joints were investigated by optical microscopy, scanning electron microscope with energy dispersive spectrometer and tensile testing. It was found that the strong diffusion reaction between molten Mg alloy and solid steel would lead to the formation of Mg/Fe transition-layer. The molten time and the absolute temperature of interface are the main factors to control the thickness of reaction layer. Moreover, the thickness of transition-layer at the upper part of joint, ∼4 μm, was twice that of the lower part. It is important to notice that the fracture of joint happened at the interface, and the average tensile strength of butt joints could reach ∼182 MPa, ∼81% that of AZ31B base material, showing the mixed rupture characteristics of quasi-cleavage and dimples. Meanwhile, the particles of Mg alloy remained on the fracture of steel side, which was evidence for the reliable joining of Mg alloy and steel dissimilar metals.