The effects of coating parameters on the quality of TiB2–TiC composite phase coating on the surface of Cu–Cr–Zr alloy electrode

Abstract

To improve the lifetime of electrodes used for the resistance spot welding of Zn-coated steel sheets, a TiB2–TiC composite coating was fused onto the surface of spot welding electrodes using an electro-spark deposition (ESD) process. Different processing parameters were studied in order to improve the quality of the electrode coating. The microstructure, elemental composition, phase structure, and mechanical properties of the coatings were characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and microhardness testing. When TiB2–TiC is deposited onto the electrodes in air, extensive coating defects occur within the bulk TiB2–TiC coating and at the interface with the electrode; however, coating defects are significantly reduced when the TiB2–TiC is deposited in argon. Delamination and oxidation of the TiB2–TiC coating were prevented and the number of cracks diminished. When TiB2–TiC was deposited in air on an electrode pre-coated with Ni, almost all coating defects were eliminated. The TiB2–TiC coated directly onto electrodes in air was found to have the lowest average microhardness of HV50 650, and the average hardness values obtained in argon and with Ni-coated electrodes were about HV50 1050 and HV50 1080, respectively. A heat-affected zone (HAZ) of about 20μm in depth within the substrate was observed near the interface between the coating and the substrate for all coating conditions.