Heat distribution in welds by short-time high-current post-heating and its improving effect on cross tension strength: development of resistance spot welding with pulsed current pattern for ultrahigh-strength steel sheets

In order to achieve high joint strength in resistance spot welding of ultrahigh-strength steel, the effect of adding a ‘pulsed current pattern’ consisting of a 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 980 N mm^?2 grade steel sheets. FEA shows that the short-time high-current post-heating leads to rapid heating in the nugget and heat-affected zone (HAZ) compared to a conventional temper pattern sheet interface higher than the centre of the nugget. The pulsed current pattern utilizes the effect of this high-current post-heating to properly reheat the nugget and HAZ, which prevents brittle fracture through the nugget without remelting it, even in a short cool time of eight cycles. The experimental results show that the pulsed current pattern improves the failure mode from partial plug failure to plug failure and increases cross tension strength (CTS). The pulsed current pattern does not decrease the hardness of the nugget and results in retention of sufficient tensile shear strength (TSS), while the softened nugget by the conventional temper pattern causes lowering of TSS. A wider proper current range with high CTS over 10 kN and plug failure can be obtained in pulsed current pattern than in the conventional temper pattern.