Improvement of Microstructures and Mechanical Properties of Resistance Spot Welded DP600 Steel by Double Pulse Technology

Abstract

The effects of the second pulse in resistance spot welding (RSW) on the mechanical properties of dual phase (DP600) steel were investigated. The experimental results show that the addition of the second pulse can give rise to the formation of acicular ferrite, retained austenite and chromium carbides, which can markedly improve the cross-tensile strength (CTS) comparing to that treated by the single pulse technology. Meanwhile, the formation mechanism of acicular ferrite, bearing the cubic to cubic orientation relationship with respect to the surrounding martensite lath, was revealed by the analysis of the thermal history of DP600 steel under different RSW processes. The effect of microstructure on the fracture behavior was also discussed.

Effect of pulse scheme on the microstructural evolution, residual stress state and mechanical performance of resistance spot welded DP1000-GI steel

“…As illustrated, the average maximum load and absorbed energy for the single pulse welds decreases from 7.7 kN and 43.1 J to 6.2 kN and 23.8 J for the double pulse welds, respectively. These results are in contrast to previous reports which showed that double pulse-welded or post-treated samples always show a better mechanical performance [4,5,[18][19][20]. Figure 3(b) depicts the measured Vickers hardness distribution across the different microstructural zones of the welds.…”

Characterization of Microstructure and Mechanical Properties of Resistance Spot Welded DP600 Steel

“…In the current case of 1.5 mm thickness, the cooling rate is estimated to be above 600 °C/s. These cooling rates are much higher than those needed to form martensite (which are around 40-120 °C/s) in the weld and HAZ in DP steels [27,28]. There is insufficient time for carbon diffusion at such high cooling rates.…”

Microstructures of magnetically assisted dual-phase steel resistance spot welds

“…For a higher strength grade of DP steels, tempering can be performed by including in situ postweld current pulsing. 25 The extent of tempering depends on the composition and processing of the steel. The addition of a second pulse of current can lead to the formation of acicular ferrite, retention of austenite, and precipitation of carbides, all of which can improve the toughness of the FZ.…”