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

Abstract

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".

Effect of Cooling Rate on The Microstructure And Cryogenic Impact Toughness of HAZ in 9% Ni Steel

“…In general, it is known that the M-A phase can be continuously distributed along the prior austenite grain boundary in IC CGHAZ, and cracks easily propagate along this continuous distribution [31,36,37]. However, since a continuous distribution of the M-A phase was not observed in this study, its effect on impact toughness was not significant.…”

Correlating Electrode Degradation with Weldability of Galvanized BH 220 Steel during Electrode Failure Process of Resistance Spot Welding

“…It can be realized that the degraded electrode surface is mainly composed of ZnO and α-Fe(Zn). As demonstrated in Table 4, the distribution of elements in the centre and the edge of the electrode is different, indicating that the centre temperature during welding is lower than the edge temperature [32]. Through the analysis of element distribution, it is confirmed that due to the reaction among the Cu electrode, Zn alloy coating and the BM, there are various alloy layers with certain Fe-Zn-Cu phases on the electrode surface.…”

“…This is in good accordance with the ratio of nugget height and steel thickness value resulting in close to 1 according to Figure10 (b). The mechanism of forming the nugget with high height is 1) in the process of the nugget formation, due to high current density, the electrode surface melts, 2) part of the molten metal is transferred to the electrode through fusion phenomenon, forming thicker nugget, 3) finally, because the molten pool extends toward the electrodes, it further deepens the electrode surface alloying process [32]. In addition, in the welding process after 616 points (Figure 10 (b)), different from Figure 10 (a), there is almost no HAZ between the electrode and the welded steel, and the nugget is exposed near the surface.…”

Correlating Electrode Degradation with Weldability of Galvanized BH 220 Steel during Electrode Failure Process of Resistance Spot Welding

“…This is in good accordance with the ratio of nugget height and steel thickness value resulting in close to 1 according to Figure10 (b). The mechanism of forming the nugget with high height is 1) in the process of the nugget formation, due to high current density, the electrode surface melts, 2) part of the molten metal is transferred to the electrode through fusion phenomenon, forming thicker nugget, 3) finally, because the molten pool extends toward the electrodes, it further deepens the electrode surface alloying process [32]. In addition, in the welding process after 616 points (Figure 10 (b)), different from Figure 10 (a), there is almost no HAZ between the electrode and the welding steel, and the nugget is exposed near the surface.…”