Formation mechanism of copper/nickel-coated steel dissimilar metals joint by micro spot resistance welding

Dissimilar metals of pure copper and nickel-coated steel were joined by micro spot resistance welding. Tensile shear test, optical microscopy (OM), scan electron microscopy (SEM) and energy spectrum analysis (EDS) were used to study the influence of coated metal Ni on formation and shear strength of the joint. The results show that formation mechanism of copper/nickel-coated steel dissimilar metals joint included solid state bonding and fusion bonding. In formation of joint, Cu and coated metal Ni bonded at solid state under the forging force and welding heat. The coated metal Ni and Fe began to melt. Then coated metal Ni was extruded to the edge by the melted metal. New structure formed between Ni, Fe and Cu at the edge. Finally, the nugget formed. Parabolic and elongated dimples were observed at the fracture surface in both joints under two different mechanisms.