Galvannealed (hot dip galvanized and annealed) dual phase (DP600) steel and interstitial free (IF) steel are two of the widely used materials in automotive industries owing to their high specific strength, toughness, formability and corrosion resistance. This inevitably requires their joining to form larger components. Resistance spot welding has emerged as the predominant welding technique for joining of sheet metals in automotive industries. There are complexities regarding microstructural variation of the fusion zone and across heat affected zones during spot welding of fundamentally dissimilar steels. This is further aggravated by the prospect of zinc redistribution during the welding process. Current study presents the evolution of microstructure in different zones of welding in IF and DP600 steel. Micro-hardness profiles and electron back scattered diffraction studies were performed. Furthermore, redistribution of elements in different welding zones is studied using electron probe micro analysis. The effect of welding current and time on the nugget size and the strength of the weld during tensile–shear is determined. It was observed that there was an increase in nugget diameter and maximum load bearing capacity of the joint with an increase in heat input, until the occurrence of expulsion. Furthermore, critical nugget diameter for pull-out failure was determined and existing empirical models were verified for the applicability in this case. None of the existing empirical models could predict the critical nugget diameter correctly and thus a new empirical relationship between sheet thickness and critical nugget diameter for zinc coated dissimilar steels has been proposed. Failure behaviour of the spot-welded sheets were studied under tensile–shear configuration under quasi-static and fatigue mode of loading and it showed substantial difference in location of crack initiation and fractographic features. Tensile–shear specimen at optimum welding parameter failed from the base metal of the IF steel side with dimples on the fracture surface, whereas fatigue specimen failed from the heat affected zone of the IF steel side with transgranular striations on the fracture surface.

Graphic abstract

中文翻译：

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电阻点焊镀锌双相和间隙自由钢的组织，拉伸和疲劳行为

镀锌（热浸镀锌和退火）双相（DP600）钢和无间隙（IF）钢由于其高的比强度，韧性，可成形性和耐腐蚀性而成为汽车工业中的两种广泛使用的材料。不可避免地需要将它们连接起来以形成更大的组件。电阻点焊已成为汽车工业中钣金连接的主要焊接技术。在基本异种钢的点焊过程中，熔合区和整个受热区的微观结构变化存在复杂性。焊接过程中锌重新分布的前景进一步加剧了这种情况。当前的研究提出了IF和DP600钢在不同焊接区域的显微组织的演变。进行了显微硬度分布和电子背散射衍射研究。此外，使用电子探针显微分析研究了元素在不同焊接区域中的重新分布。确定了焊接电流和时间对拉伸剪切过程中熔核尺寸和焊缝强度的影响。可以观察到，随着热输入的增加，直到发生排出之前，接头的熔核直径和最大承载能力都有所增加。此外，确定了拔出失败的临界熔核直径，并验证了现有的经验模型在这种情况下的适用性。现有的经验模型都无法正确预测临界熔核直径，因此，提出了镀锌异种钢的板厚与临界熔核直径之间的新的经验关系。研究了点焊板在准剪切和疲劳加载模式下的拉伸剪切结构下的失效行为，结果表明，裂纹萌生位置和断裂特征均存在很大差异。处于最佳焊接参数的拉伸剪切试样从IF钢一侧的母材中破裂，并在断裂表面出现凹痕；而疲劳试样从IF钢一侧的热影响区破裂，在断裂表面上出现横纹的条纹。研究了点焊板在准剪切和疲劳加载模式下的拉伸剪切结构下的失效行为，结果表明，裂纹萌生位置和断裂特征均存在很大差异。处于最佳焊接参数的拉伸剪切试样从IF钢一侧的母材中破裂，并在断裂表面出现凹痕；而疲劳试样从IF钢一侧的热影响区破裂，在断裂表面上出现横纹的条纹。研究了点焊板在准剪切和疲劳加载模式下的拉伸剪切结构下的失效行为，结果表明，裂纹萌生位置和断裂特征均存在很大差异。处于最佳焊接参数的拉伸剪切试样从IF钢一侧的母材中破裂，并在断裂表面出现凹痕；而疲劳试样从IF钢一侧的热影响区破裂，在断裂表面上出现横纹的条纹。

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