Effects of nickel layer on interfacial micro-structure distribution and macro-mechanical properties for vacuum-brazed dissimilar steel joints

In this paper, nickel-plated Q235 carbon steel and 304 stainless steel were vacuum brazed together using pure copper foil as the filler metal. The effects of nickel plating at the carbon steel surface on the shear strength of the brazed joints were investigated and verified by the finite element analysis method. The microstructure and shear strength of the brazed dissimilar steel joints were characterized by SEM and tensile experiments. The results found that the pure copper brazing material exhibited excellent wettability to the base metal. With the increase of heat treatment temperature and holding time, the plated Ni particles fuse with each other and the boundary of the Ni particles gradually disappears, which increases the diffusion distance of Fe, Ni, and P elements and the width of the interface transition zone. The maximum shear strength of the brazed dissimilar steel joint without the nickel layer is 265.9 MPa for the optimized brazing parameters with 0.06-mm thickness of pure copper foil. After nickel plating on the carbon steel surface with heat treatment of 500 celcius for 1 h, the shear strength of the brazed dissimilar steel joint reduces to 213.4 MPa, which may be due to the formation of brittle intermetallic compounds by Ni and P elements on the surface of the layer at high temperatures.