Dissimilar joining of AZ31B Mg alloy to Ni-coated Ti-6Al-4V by laser heat-conduction welding process

To improve the metallurgical bonding of immiscible and nonreactive Mg/Ti system, Ni electro-coating was employed acting as an intermediate element by reacting with both Mg and Ti in laser heat-conduction welding process. Influence of laser power on welding quality was investigated and interfacial reaction with participation of Ni coating was analyzed. The elemental diffusion mechanism was clarified with the assistance of numerical simulation. Reliable joints were obtained with suitable laser powers ranging from 1200 W to 1800 W. Continuous reaction layer of Ti2Ni and Ti solid solution formed at the interface. The reaction layer thickness was not monotone increasing with increase of laser power. Mg-Al-Ni ternary phase dispersed in fusion zone at Mg side. Diffusion of electroplated Ni was influenced by a bidirectional mechanism to Mg side and Ti side, which resulted in the variation of reaction layer thickness. The maximum tensile-shear load could reach 144 N/mm (about 53.3% of joint efficiency of base metal AZ31B Mg alloy) when laser power was 1500 W. Joint strength and fracture mode were associated with two aspects: interfacial reaction and weld appearance of Mg side. As a result, reliable joining was realized based on appropriate interfacial metallurgical bonding and no weld defects at Mg side.