Surface modification of resistance welding electrode by electro-spark deposited composite coatings: Part I. Coating characterization

To improve electrode life during resistance spot welding of Zn-coated steel sheets, a monolithic TiCP/Ni composite coating was deposited onto the surface of a Cu–Cr–Zr electrode by an electro-spark deposition process. The coating was designed as a barrier to prevent electrodes from alloying with the Zn-coating leading to degradation by pitting or erosion. Some coated electrodes were also subjected to laser treatment in order to eliminate cracks formed in the as-coated monolithic TiCP/Ni composite coating. In addition, a multi-layer deposition process has also been proposed to improve the coating quality by using Ni and TiCP/Ni composite as deposition materials alternately. The microstructure and mechanical properties of the coatings have been characterized by scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction and micro-hardness tests. The results showed that extensive cracking occurred within the monolithic TiCP/Ni coating and at the interface. It was found possible to reduce or eliminate cracks and delamination of the monolithic TiCP/Ni coating via the multi-layer deposition process using Ni, TiCP/Ni. The multi-layer coatings of Ni/(TiCP/Ni)/Ni showed higher Ni content and lower hardness. Although post laser treatment of monolithic TiCP/Ni coatings could eliminate cracks and improve coating quality, the softening of the copper alloy substrate limited the performance of laser treated electrodes.