Improvement of Cross-tension Strength Using Concave Electrode in Resistance Spot Welding of High-strength Steel Sheets

In the spot welding of ultrahigh-strength steel sheets, the generation of expulsion could be suppressed and large-diameter nuggets could be formed by using a concave electrode. The expulsion was suppressed because the clearance for expansion was formed when using the concave electrode. Thus, nuggets with a diameter of >= 4 root t (lowest for securing joint strength, t = 1 mm) were obtained without expulsion in a greater welding current range than the range when using a conventional electrode. Furthermore, nuggets with a diameter of approximately 6.5 mm were formed with a large current of 8 kA, which could not be set when using the conventional electrode. In addition, a transition from partial interfacial fracture to pullout fracture occurred as the nugget diameter increased, and the cross-tension strength (CTS) tended to increase. A value of about 8 kN was obtained for a nugget with a diameter of 6.5 mm, which is approximately 1.5 times larger than the value (approximately 5.5 kN) obtained with the conventional electrode. The electrode life test indicated little deterioration in the CTS after 1400 welds. Thus, we propose using a concave electrode to improve the joint strength in the spot welding of high-strength steel sheets.