Resistance spot welds of dual phase (DP) steels are prone to low fracture toughness due to the formation of brittle martensitic structure in the fusion zone (FZ). In-process tempering of martensite via applying second pulse current is considered a new pathway to improve mechanical performance of the welds. The success of in-process tempering depends upon precise controlling the amount of heat input and uniform temperature distribution which in turn influenced by postweld tempering pulse parameters. This paper aims to investigate the effect of three postweld tempering pulse parameters such as welding current, welding time and cooling time applied after main pulse current on microstructure and mechanical properties of DP590 steel resistance spot weld. Mechanical properties in terms of peak load and failure energy were determined after performing cross tension (CT) test. Taguchi quality design based on L16 orthogonal array has been used to determine the optimum conditions for maximum peak load and failure energy. Moreover, microstructure-property relationship is also studied. The results show that at optimum conditions maximum improvement of 62% in peak load and 62.3% in failure energy is achieved in double pulse welds compared with conventional single pulse weld. It was found that improvement in peak load and failure energy resulted from (i) enhanced weld nugget size (WNS) and (ii) tempering of martensite in FZ and heat affected zone (HAZ). These factors are influenced by heat input (Q) during postweld heating cycle (PWHC) which in turn increased with increasing second pulse current and time.

由于在熔合区（FZ）中形成脆性的马氏体组织，因此双相（DP）钢的电阻点焊易于产生较低的断裂韧性。通过施加第二脉冲电流对马氏体进行工艺回火被认为是改善焊缝机械性能的新途径。过程中回火的成功取决于对输入热量的精确控制和均匀的温度分布，这又受焊后回火脉冲参数的影响。本文旨在研究三个焊接后回火脉冲参数，例如焊接电流，焊接时间和主脉冲电流后施加的冷却时间，对DP590钢电阻点焊的组织和力学性能的影响。在进行交叉拉伸（CT）测试后，确定了根据峰值载荷和破坏能的机械性能。Taguchi基于L16正交阵列的质量设计已被用来确定最大峰值负载和故障能量的最佳条件。此外，还研究了微结构与性能的关系。结果表明，与传统的单脉冲焊相比，在最佳条件下，双脉冲焊最大可提高62％的峰值载荷和62.3％的故障能量。发现峰值载荷和破坏能量的提高是由于（i）熔核大小（WNS）的增加和（ii）FZ和热影响区（HAZ）中马氏体的回火。这些因素受焊后加热周期（PWHC）期间热量输入（Q）的影响，而热量输入又随着第二脉冲电流和时间的增加而增加。