Effects of Ni coating thickness on the microstructure and mechanical properties of resistance-welded WC-Co/B318 steel joints

WC-Co pellets electroplated with different Ni coating thicknesses and B318 steel were resistance welded for carbide-tipped saw blade applications. The effects of the Ni coating thickness on the welding process, interfacial microstructures, and mechanical properties of the joints were investigated. The results showed that the shear force increased before decreasing as the Ni coating thickness increased. When the coating thickness was 40 mu m, expulsions began to appear during welding, while a thicker coating resulted in longer expulsion times. Voids and macro cracks were generated at the joints because of the expulsions, which reduced the shear force of the joint. The XRD results of the welding slag and the EDS results of the interface indicate that the microstructures can be divided into five phase categories: light gray (Fe3W3C), gray (Co3W3C), white (WC), fishbone eutectic structure (Fe6W6C), and dark [(gamma Fe, Ni) solid solutions]. As a thicker Ni coating hinders reactions between molten Fe and dissolved WC, the joint interface contained more Fe6W6C at 20 mu m and 30 mu m but had more bulk Fe3W3C at 40 mu m and 50 mu m. The upper part of the joint showed brittle fractures on the WC-Co side, while most of the lower part fractured at the interface. Only the joints at 20 mu m and 30 mu m had a small fracture area on the steel side. Moreover, the fracture on the steel side at 30 mu m showed elongated shear dimples, which explains the maximum shear force of the joint at 30 mu m.