2019 年 37 巻 4 号 p. 215-223
Advantageous effect on cross tension strength (CTS) of resistance spot welds by “pulsed current pattern” has been evaluated assuming nugget diameter variation which unexpectedly causes deterioration of joint strength of ultra-high strength steel (UHSS) in auto assembly lines. Pulsed current pattern is newly developed resistance spot welding method consisting of main welding for forming nugget and multiple combinations of short-time cool time and short-time high-current post-heating (pulsed current) to realize time-effective change in brittle nugget. For quantitative evaluation of nugget diameter variation and post-heating effects, various welding experiments and numerical simulations are conducted by various main welding current without or with 1- or 2-time pulsed current using 1180MPa grade steel sheets. The experimental results clarify 2-time repetitions of pulsed current ensures plug failure and high CTS over 10kN at wider range of nugget diameter than without and even with 1-time pulsed current. Numerical calculation and cross-sectional microstructures suggest that post-heating with too small nugget diameter overheats nugget and partially remelts phosphorus-rich area due to solidifying segregation leading to low CTS, but repetitive pulsed current can lower peak temperature to appropriate range to avoid deterioration of CTS. Therefore, pulsed current pattern is expected to realize high quality joint even when nugget diameter is not insufficient due to instability of welding processes.