Study On The Hybrid Laser-TIG Butt Double-side Fusion Welding Of Dissimilar Magnesium Alloy And Steel With Ni As Interlayer

Dissimilar structural materials are designed for improving comprehensive performances of the key parts by domestic and foreign designers as it could take advantages of the excellent properties of both materials. Especially in recent years, the development of hybrid magnesium alloy/steel structural materials, which has the synergetic advantages of overall weight loss and local load bearing enhancement, has a wide prospect of application. To solve the problem of joining dissimilar magnesium alloy to steel becomes the premise and key to achieve a wide range of industrial applications.At present, butt fusion welding dissimilar metals of magnesium alloy and steel is less employed by researchers. Our previous work shows that the Mg/Fe joint fractures at the welding seam by using Cu-Zn as interlayer. However, this paper mainly concentrates on improving the tensile strength of welding seam as breakthrough point to further improve the mechanical property of the joint. Ni could easily dissolve with Fe to form a solid solution at high temperature. So Ni is chosen as the interlayer for welding magnesium alloy and steel to expect reducing the content of Ni in the welding seam, thus reducing the degradation of the welding seam and further improving the tensile strength of the Mg/Fe joint. The experiments of single-side welding process and double-side welding process are performed in this paper. The microstructure of the joint, the characteristic of the Mg/Fe interface and the bonding mechanism of the joint are also studied in this paper.By adding pure Ni interlayer, It is found that single-side welding of the Mg/Fe interface bonding is not suitable because of the easily formed crack or hole by hybrid laser-TIG butt fusion welding process. So double-side butt fusion welding process of Mg/Fe by hybrid laser-TIG welding is put forward, the tensile strength of the joint could reach232MPa, which is about90%of the magnesium alloy base metal, the fracture location of the joint is near the Mg/Fe interface. The butt fusion joint is mainly composed of welding seam, Mg/Fe interface and Fe-Ni solid solution zone with the wide of about300μm. The welding seam is mainly comprised of two phases, Mg and AINi. The AlNi particles with micro-nano size (about500nm) distribute uniformly in the welding seam. Mg/Fe interface is a thin transition layer with a thickness of about1μm, Fe-Ni solid solution zone is mainly a solid solution zone of Ni in Fe.In the experiment of hybrid laser-TIG double-side butt fusion welding magnesium alloy to steel with Cu-Zn as interlayer with different heat input, it shows that the change of joint geometry is not the main reason for the reliable bonding of Mg/Fe, but the change of microstructure of the welding seam, which reduces the degradation of the seam with Ni as interlayer, is the key to obtain high tensile strength of the joint.During the butt double-side fusion welding process of magnesium alloy to steel, most Ni enters into Fe substrate to react with each other to form the clear Fe-Ni solid solution zone, only less Ni enters into molten pool to react with magnesium alloy and AlNi particles with micro-nano size are prior to be formed by the following arc melt the magnesium alloy. The plenty of small white particles identified as AlNi phase with a size of about500nm are formed and distribute uniformly in the welding seam, which could bring dispersion strengthening effect for the welding seam. So it could reduce the degradation of the welding seam ascribe to the dispersion strengthening effect of AlNi phase in the seam to significantly improve the tensile strength of the joint. It promote the formation of Mg/Fe interface while the molten Mg fill and spread to the steel side. Obvious concentration of Al is detected at the interface, Mg, Fe and Ni are also detected at the interface, we speculate that some compounds comprised of element Fe、Al are probably formed to promote the realiable joining magnesium alloy to steel. Therefore, Ni plays a excellent role on the reliable bonding of butt fusion welding magnesium alloy and steel.