Electrode lifetime in resistance spot welding of coated sheets: Experiments and modeling

One of the most important challenges associated with resistance spot welding of coated sheets is short electrode life time. It has been established that the two factors-electrode tip growth and surface alloy formation-each affect negatively on electrode life when considered separately. This study's objectives were to examine the combined effects of these factors and ascertain how they relate to electrode life. Due to this, two electrodes were used to investigate the electrode life when resistance welding galvanized steel: a conventional Cu-Cr electrode with a compressive strength of 800 MPa and a dispersion strengthened copper electrode with a high volume percentage of alumina particles (3.5%) with a compressive strength of 500 MPa. Electrode life time tests showed that the life of composite electrode (700 weld number) was reduced by about 20% compared to Cu-Cr electrode (900 weld number). Electrode tip features showed that cracking in Cu-Cr electrode formed at about 200 weld number while in composite electrode this occurred at 600 weld number. The electrode tip cross section revealed the formation of four separate alloy layers at the tips of both electrodes, however the Cu-Cr electrode had significantly higher crack densities and a lower average thickness during life test (45 µm) than the composite electrode (55 µm). It was caused by the higher rate of electrode tip growth in Cu-Cr electrodes, which increased welding current and extended electrode life. A simple model was purposed for electrode life based on Joule heating effect that takes the effects of both electrode tip growth and electrode tip alloy formation in to account. According to the model and experiments when significant interaction occurs at the electrode tip, the some tip growth rate not only isn’t detrimental but also increases the electrode lifetime.