Surface modification of resistance welding electrodes by electro-spark deposited composite coatings Part II. Metallurgical behavior during welding

Electrodes with monolithic TiCP/Ni coatings and multi-layer Ni/(TiCP/Ni)/Ni coatings were used to resistance spot weld Zn-coated sheet steel to investigate metallurgical behaviour of the coatings during welding. Scanning electron microscopy, energy-dispersive X-ray analysis and X-ray diffraction were employed to characterize the microstructure of coatings, reactions of electrodes with Zn-coating and alloy layer formation. The results showed that molten Zn penetrated TiCP/Ni coatings via the cracks that were present within as-coated TiCP/Ni coating, starting from the first weld. Additional cracks continually formed in the coating during welding due to action of the welding force on the low toughness coating, resulting in formation of a granular loose overlay at the outer surface which were easily detached and stuck onto the work sheet. On the contrary, cracks could be rarely found within Ni/(TiCP/Ni)/Ni coating until 100welds or more were made, and much fewer cracks formed up to 400welds, compared to the TiCP/Ni coating. With Ni/(TiCP/Ni)/Ni coating on the electrode surface, alloying between copper alloy and molten Zn as well as pitting (erosion) of the electrode tips were remarkably reduced, and hence, a slower growth rate of tip diameter was observed.