Effect of welding time on the strength of WC-10Co/RM80 dissimilar metal resistance welded joints

To clarify the influence of welding time on the performance of WC–10Co cemented carbide/RM80 high-strength steel resistance-spot-welded joints, butt joints were produced in the range of 10 ~ 50 ms by resistance-spot welding. The joint strength, macro-morphology, microstructure and elemental distribution were systematically investigated by shear testing, SEM, EDS and optical microscopy. The results show that with increasing welding time the shear fracture force first increases and then decreases, reaching a peak value of 1 466.1 N at 40 ms. When the time is further extended to 50 ms, spatter becomes severe and the strength drops to 1 123.8 N. Both the heat-affected-zone width and the volume of the extruded nugget exhibit a similar trend, attaining their maxima at 40 ms. Microstructural observations indicate that an unduly short time leads to insufficient interfacial bonding, whereas an excessive time causes metal spattering and dissolution/loss of the Co binder phase. The diffusion distances of Fe, Co and W increase monotonically with welding time. The Ni interlayer is completely molten as the time is prolonged, yet at 50 ms excess energy induces porosity and cracks. Taking all factors into account, 40 ms is identified as the optimum welding time that provides the highest shear strength together with the fewest defects.