Study On Service Life Of Spot Welding Electrode Of Deep Cryogenic Treatment CuCrZr Alloy

Galvanized steel sheet has good corrosion resistance and is widely used in automobile body structure.In the process of automobile manufacture,galvanized steel sheet is usually welded by resistance spot welding in welding workshop.At present,CuCrZr alloy is the main material of spot welding electrode.In the process of resistance spot welding galvanized steel sheet,CuCrZr electrodes are easy to cause softening deformation,spatter,electrode adhesion,wear,alloying and other problems and failures.Therefore,surface modified coated electrodes,composite electrodes and other new electrodes are one of the research hotspots in this field.Compared with these new types of electrodes,deep cryogenic treatment of copper alloy electrodes can further extend the life of the electrodes,reduce wear and tear of the electrodes,make them easy to grind,and have simpler preparation process and lower cost.In this paper,the effect of technological parameters on spot welding process and nugget shape was studied,and the electrode life at different deep cryogenic treatment time was measured.The micro-morphology and failure mechanism of resistance spot welding electrode were analyzed by means of ultra-depth-of-field microscopy,SEM,EDS and XRD diffractometer.The galvanized steel sheet was spot welded with a new type of electrode and CuCrZr electrode with deep cryogenic treatment time of 12h and 48h respectively.The welding current had the greatest influence on the diameter of the nugget,followed by the welding time and the electrode pressure.When the electrode pressure remains unchanged and the welding current or welding time increases,the spatter and electrode adhesion become more and more serious,and the diameter of the nugget,the diameter of the plasticity circle and the indentation rate also increase.When the welding current or time remains unchanged and the electrode pressure increases,the effect is just the opposite.Compared with CuCrZr electrodes,the spatter and adhesion of the new electrodes treated by deep cryogenic treatment were significantly reduced during spot welding.Under the experimental conditions,the optimum spot welding parameters of deep cryogenic treatment electrode are welding current 9.62 kA,welding time 10 CYCS and electrode pressure 2.8 kN.The electrode life of resistance spot welding galvanized steel sheet was measured by nugget diameter judgment method and shear strength judgment method.The test results showed that the life of CuCrZr electrode was 800 points;the life of DCT-12h electrode was2000 points,which was 2.5 times longer than that of CuCrZr electrode;and the life of DCT-48h electrode was 3 300 points,which was about 4 times longer than that of CuCrZr electrode.Deep cryogenic treatment can increase the surface hardness of copper alloy electrodes.The longer the deepcryogenic treatment time,the harder the electrode is,and the slower the diameter of the electrode surface increases.With the increase of spot welding times,the mushroom-like deformation at the end of the electrode increases and the surface hardness decreases.The wear of the upper electrode is more serious than that of the lower electrode,and the increase speed of the surface diameter of the spot welding electrode is faster.The increase speed of the surface diameter of the DCT-48h electrode is only 1/2 of that of the CuCrZr electrode.The main reason of electrode failure in resistance spot welding of galvanized steel sheet is caused by Cu-Zn alloying.Deep cryogenic treatment of the electrode can inhibit the diffusion of zinc into the electrode matrix,which is conducive to preventing copper spalling at the end of the electrode.The main products of Cu-Zn alloying at the end of the failure electrode are CuZn,（CuZn）and Cu₅Zn₈ metal compounds.