Study On ZrB₂-TiB₂Multiphase Coating Formed By In-situ Electro-Spark Deposition On Spot-welding Electrode And Control Of Coating Defects

Resistance spot welding is a very important technical method in modern auto mobile manufacture industry. It increases the producing cost and debases the productivity due to the electrodes degradation prematurely when resistance spot welding coated steel, so it is an urgent and important research to prolong the life of the electrodes in automobile manufacturing.To increase the service life of spot-welding electrodes for zinc-coated steel plates, a ZrB2-TiB2multiphase coating was successfully deposited on the surface of spot-welding electrode by electro-sparking deposition (ESD). A discharge electrode for in-situ ESD of the ZrB2-TiB2multiphase coating was obtained by mechanical ball milling of Zr-Ti-B (molar ratio1:1:4) particles and a semi-sintering process. The ideal ball-milling parameters were as follows:ball milling time:20h; ball-to-powder ratio:20:1; and rotational speed of ball mill:500rpm. The forming pressure for the discharge electrode was300MPa.1wt%of a forming agent (liquid paraffin) and5wt%of a Ni metal binding agent were added to ensure a suitable compact strength and excellent compactness of the discharge electrode after sintering. The sintering temperature was set to1450℃to meet the need for in-situ deposition of the desired coating.The ZrB2-TiB2multiphase coating was obtained by in-situ ESD and the ideal deposition parameters were as follows:capacitance:2000μF; voltage:12V; rotational speed of spot-welding electrode:700rpm; deposition time:120s; and vibration frequency of discharge electrode:30Hz. Irrespective of the changes in the deposition parameters, there was an issue with the quality of the coating due to defects, particularly crack defects.With respect to the ZrB2-TiB2multiphase coating defects due to the preparation process and the inherent material properties, argon protection, a transition layer structure, ultrasonic-assisted ESD, and a friction stir post-treatment process were used in this study to control the coating defects. Argon protection can be used to effectively control oxidation coating; pre-coating with nickel before the ESD of the ZrB2-TiB2multiphase coating caused a reduction in the columnar crystallization zone and controlled the formation of lamination defects at the interface between the coating and the substrate. Ultrasonic-assisted ESD was used to effectively refine the crystal grain size and eliminate the columnar crystallization zone, and friction stir process with cold specification helped improve the surface quality of the coating.After the optimization of the deposition parameters and control of ZrB2-TiB2and ZrB2-TiB2/Ni coating defects, the weldability lobe test for the spot-welding electrode indicated that the ZrB2-TiB2/Ni-coated electrode had the widest weldability lobe, followed by the ZrB2-TiB2-coated electrode; the uncoated electrode had the narrowest weldability lobe. The main mechanisms of spot-welding electrode failure included plastic deformation, alloying, and pitting. The average service life of the ZrB2-TiB2/Ni-coaed was approximately3800spots, the ZrB2-TiB2-coated electrode was approximately2800spots, and the service life of the uncoated electrode was as low as approximately600spots. The improved service life of the coated electrode is primarily attributed to the contributions of the coating in improving the reduction of plastic deformation and alloying of the spot-welding electrode tips. The weak adhesion strength between the coating and the substrate was the main factor that led to shorter service life of the ZrB2-TiB2-coated electrode than the ZrB2-TiB2/Ni-coated electrode. For the uncoated electrode and the ZrB2-TiB2-coated electrode, plastic deformation and alloying were the main factors that caused electrode failure. For the ZrB2-TiB2/Ni-coated electrode, the coating effect remained until electrode failure and the primary form of electrode failure was plastic deformation.