The Study On Scale Complex Phases TiB₂-TiC Coating On Spot Welding Electrode By Electro-spark In Situ Deposition

To improve the service life of the coated electrode in welding process and overcome theshortcoming of single phase TiC or TiB₂coating (TiC is poor conductivity, TiB₂is poorwettability with matrix.), the composite TiB₂-TiC ceramic coating technology wasproposed in this study. The research focuses on how to obtain the scale-like TiB₂-TiCcoatings by mechanical alloying, vacuum sintering and electrode sparking depositionprocess (ESD). The origin powders containing Ti, B₄C, Ni, C were ball-milled and thenanalyzed by X-ray diffraction technology (XRD). The results showed that the in situTiB₂-TiC composite coating was obtained as the milling time was larger than6h. At thesame time, the ball-milled powders had transformed into the transient phases, whichavailed for the formation of TiB₂-TiC composite coating. The differential thermalanalysis was carried out for the ball-milled powders. The DTA experiment showed thatthe alloying powders milled for25h could react and transform into TiB₂-TiC compositeat relatively low temperature (about1200℃), which provided an importance referencefor the vacuum sintering parameters. The powders (25h) were shaped and semi-sintered.The results showed that the rods sintered at1150℃have high strength, which couldsatisfy the requirement for the in situ electron sparking deposition process. In presentwork, based on the factors effecting coating, the orthogonal experiments were employedin order to obtain the desired TiB₂-TiC coatings. The desired operation parameters areas follow: the voltage is24V, the capacity is2000μF and the deposited time is120s.With the above parameters, the desired TiB₂-TiC coatings were obtained. The XRDanalysis shows that the coatings consist of TiB₂and TiC phase. The microstructureobserved by field emission microscopy showed that TiB₂was lath-like and TiC washexahedron. In order to determine the service life of the TiB₂-TiC coating electrode, thewelding experiment was employed with single-phase AC spot welding machine. Theresults showed that the life of the coated electrode was2500spots, which was muchlarger than that of the non-coating electrode (500spots). The analysis for the failureTiB₂-TiC coating electrodes showed that the improvement in service life was ascribed tothe following reasons. Firstly, the coating on the electrode inhibited the alloying processbetween the electrode matrix and galvanized steel. Therefore, the plastic deformationdegree of the electrode would slow down due to the higher surface hardness. Secondly,in service process, the TiB₂-TiC coating of the coated electrode produced cracks due tothe thermal mechanical force and welding heat, which propagated into net structure,leading to the increase of the alloying degree between the electrode matrix andgalvanized steel. Finally, because of the effect of local zone alloying on the electrode,the coatings were peeled, which resulted in the increase of the alloying degree. TheTiB₂-TiC coating electrode failed during welding process.