Abstract
In order to achieve high joint strength in resistance spot welding of ultra high strength steel (UHSS), the effect of adding "pulsed current pattern" consisting the combination of short cool time and short-time high-current post-heating was investigated. Finite element analysis (FEA) for post-heating patterns and experimental evaluation for joint strength were conducted using 980N/mm2 grade steel sheets. FEA shows that the short-time high-current post-heating leads to rapid heating in nugget and heat affected zone (HAZ) compared to conventional temper pattern consisting long-time low-current post-heating. In particular, the high-current post-heating reheats HAZ near the electrodes and that near sheet-sheet interface higher than the center of the nugget. Pulsed current pattern utilizes the effect of this high-current post-heating to properly reheat nugget and HAZ, which prevents brittle fracture through nugget without remelting the nugget even in a short cool time of 8 cycles. The experimental results exhibit the pulsed current pattern improves failure mode from partial plug failure to plug failure and increase cross tension strength (CTS). The pulsed current pattern does not decrease hardness of the nugget and results in retaining sufficient tensile shear strength (TSS), while softened nugget by the conventional temper pattern causes lowering TSS. Wider proper current range with high CTS over 10kN and plug failure can be obtained in pulsed current pattern than in conventional temper pattern.
