The microstructure and elements distribution of the deep cryogenic treatment electrodes and non-cryogenic treatment electrodes for spot welding hot dip galvanized steel are observed by a scanning electrical microscope. The grain sizes, the resistivity and the hardness of the electrodes before and after deep cryogenic treatment are measured by X-ray diffraction, the DC double arms bridge and the Brinell hardness testing unit respectively. The spot welding process performance of hot dip galvanized steel plate is tested and the relationship between microstructure and physical properties of deep cryogenic treatment electrodes is analyzed. The experimental results show that deep cryogenic treatment makes Cr, Zr in deep cryogenic treatment electrodes emanate dispersedly and makes the grain of deep cryogenic treatment electrodes smaller than non-cryogenic treatment ones so that the electrical conductivity and the thermal conductivity of deep cryogenic treatment electrodes are improved very much, which make spot welding process performance of the hot dip galvanized steel be improved.
With the squeeze of electrode tips, the oxide film on aluminum (Al) alloy surface is broken and numbers of micro-gaps are formed randomly. The micro-gaps act as conducting spots at the beginning of welding, so the contact resistance is extremely high and unstable in spot welding of Al alloy. In this paper, a new contact resistance model is adopted to simulate the nugget forming process. This model describes the random distribution characteristic of conducting spots. The simulation results indicate that, within the first 5 ms of welding current (AC, 50 Hz), the temperature distribution at the workpieces interface is seriously irregular. In addition, the nugget does not nucleate from the weld center and grow continuously, however, it nucleates randomly from several points almost instantaneously and then merges into an entity quickly. Experimental results agreed with the numerical simulation.
