The Study On ZrB₂-CrB₂ Composite Coatings On Surface Of Spot Welding Electrode By Electric-spark Deposition

Spot welding electrode widely used in automotive industry is consumable part.In the process of spot welding,the end of the electrode is subjected to the joint action of the force and the heat,which can easily lead to the coarse deformation of the electrode.During spot welding the coated steel,the alloying reaction leads to an earlier failure.In order to solve the above problems and improve the service life of spot welding electrode,The method of surface strengthening is adopted in this paper.The ZrB₂-CrB₂ coating was deposited on the surface of the spot welding electrode by electric spark.First,the ZrB₂-CrB₂ discharge electrodes of different components are prepared by mechanical ball milling and pressing sintering.The two groups of88.4ZrB₂-6.6CrB₂-5Ni and 79.7ZrB₂-5.3CrB₂-15Ni,which have the best deposition effect,are selected as discharge electrodes for electric spark.The ball-milling parameters were as follows:ball-to-powder ratio:3:1;ball milling time:12h;and rotational speed of ball mill:400rpm.The dosage of liquid paraffin is 1%.In the process of pressing,the pressure of the hydraulic press is 3MPa,and the state of 1²min needs to be maintained.The sintering temperature is 1450?.Next,the effect of electro-sparking deposition on the organization and structure of ZrB₂-CrB₂ coating was investigated.In a certain spot welding electrode speed?700rpm?and deposition time?70s?,88.4ZrB₂-6.6CrB₂-5Ni coating the best deposition parameters:voltage:20V,capacitor:3000?F;79.7ZrB₂-5.3CrB₂-15Ni coating the best deposition parameters:voltage:20V,capacitor:2000?F.Finally,the life of the coating electrode group was tested.The experimental data show that average life span of the common electrode is 500,and the largest increase in electrode life is the?88.4ZrB₂-6.6CrB₂-5Ni?/Ni coating in the Zr B₂-CrB₂/Ni coating system,and the life of the electrode reaches the maximum of 1800 points.The life of the electrode was not increased after the atmosphere protection,but the phenomenon of splash and adhesion decreased significantly during the spot welding process.The lifetime of?79.7ZrB₂-5.3CrB₂-15Ni?/Ni coating electrode is 1100.After atmosphere protection,the life of the electrode increases to 1300.This is because the oxidation phenomenon of the coating decreases after the atmosphere is protected,and the internal defects of the coating are improved.The reason for the increase of life of ZrB₂-CrB₂/Ni coating electrode is that the coating prevents the direct contact between the spot welding electrode matrix and the galvanized steel surface coating Zn,and slows down the degree of alloying of the electrode end surface.At the same time,the coating can slow down the plastic deformation of the electrode.The main reasons for the failure of coating electrode are pit erosion,alloying and plastic deformation.