Study on the Effect of the Localized Electrode Degradation on Weldability During an Electrode Life Test in Resistance Spot Welding of Ultra-High Strength Steel

Evaluation of electrode degradation in resistance spot welding of 1180TRIP steel was carried out through an electrode life test, and weldability testing was performed by the measurement of the nugget size, the peak load analysis and the measurement of the electrode tip size at 30 intervals. Analysis of the change in the size of nuggets indicated a trend of rapid decrease. Moreover, the peak load showed that there was a rapid decrease until the 150th weld spot and then a slight increase until the 360th weld spot. The crack and cavity in the weld nugget and the protrusion of the weld pool were mostly observed after the 150th weld spot. Unlike the previous study, the electrode tip size steadily decreased during welding. The decreased electrode tip size could be explained by the narrow contact area between the electrode and sheet during welds, which would account for the continuous occurrence of sticking and expulsion between the electrode and the sheet. A geometric deformation occurred at the edge of the electrode owing to the deformation of the electrode without any alloying, whereas the region which was in contact with the sheet experienced Cu-Zn-Fe alloying. As a result, frequent electrode sticking onto the TRIP sheet caused the formation of protrusions and cavities on the electrode surface. Thus, the localized electrode degradation can be divided into three regions: region III where the electrode was undergoing wear and geometric deformation, region II where the protrusion occurred, and region I where the cavity was formed. Finally, the electrode degradation in the resistance spot welding of ultra-high strength steels is defined as “plateau formation” that is due to the geometric as well as metallurgical changes which contradict the existing “mushrooming effect”. Key words: resistance spot welding, ultra-high strength, trip steel, electrode degradation, plateau formation, electron probe micro analyzer