In the automotive applications, sheet metals are inevitably subjected to pre-straining during stamping operation prior to spot welding. The integrity of vehicles is governed by the static and fatigue strengths of the joints. However, the effect of pre-strain on the tensile and fatigue properties of the spot-welds on the high strength low alloy (HSLA) steel sheets has not been well studied. To abridge this gap, steel sheets of 1.2 mm thickness in as-received condition as well as after pre-straining to three different levels of 5%, 10%, and 15% were spot-welded in the form of lap-shear joints. With an increase in pre-strain, the load-bearing capacity of the joints monotonically increased, while there was no significant effect of pre-strain on the fatigue behavior and endurance limit of the joints. This difference in tensile-shear and fatigue behavior of the joints for pre-strained samples is attributed to the variation in the location of the failure, which has been explained in terms of the difference in failure mechanisms involved with the tests. The crack initiated at the base metal in case of tensile-shear tests, whereas under cyclic loading, fatigue cracks initiated between fine-grained heat affected zone (FGHAZ) and coarse grain heat affected zone (CGHAZ) located near the interfacial notch between the two welded sheets. While the location of failure in tensile-shear test is dictated by its lower strength across the joint, that in fatigue test is governed by the stress concentration. The magnitude of dislocation density has been evaluated quantitatively in the base metal, heat affected zone (HAZ), and fusion zone of weld joint using X-ray diffraction (XRD), and the dislocation substructures have been examined by transmission electron microscopy (TEM). The increase in tensile-shear load-bearing capacity is due to increased dislocation density in base metal with pre-strain. As most of these dislocations generated due to the pre-straining are annihilated at the HAZ during welding, there was no significant effect of pre-strain on the fatigue strength of the joints. These results were corroborated by microstructural examination and micro-hardness measurements across the weld joints in as-received and pre-strained conditions.

在汽车应用中，不可避免地要在冲压操作期间在点焊之前对钣金件进行预应变。车辆的完整性取决于接头的静态和疲劳强度。但是，还没有很好地研究预应变对高强度低合金（HSLA）钢板上点焊的拉伸和疲劳性能的影响。为了缩小该间隙，以搭接接头的形式点焊了1.2毫米厚的钢板，这些钢板在接收状态下以及在预应变至5％，10％和15％的三个不同水平之后进行了点焊。随着预应变的增加，接头的承载能力单调增加，而预应变对接头的疲劳行为和耐力极限没有显着影响。预应变样品的接头在拉伸剪切和疲劳性能方面的这种差异归因于失效位置的变化，这是根据测试所涉及的失效机理的差异来解释的。在拉伸剪切试验的情况下，裂纹在母材处产生，而在循环载荷下，疲劳裂纹在两者之间的界面缺口附近的细粒热影响区（FGHAZ）和粗晶粒热影响区（CGHAZ）之间产生。焊接板。拉伸剪切试验中的失效位置取决于其在接头处的较低强度，而疲劳试验中的失效位置则取决于应力集中。位错密度的大小已在贱金属，热影响区（HAZ），X射线衍射（XRD）分析了焊接接头的熔合区，并通过透射电子显微镜（TEM）检查了位错亚结构。拉伸剪切承载能力的增加归因于具有预应变的母材中位错密度的增加。由于在焊接过程中，由于预应变而产生的大多数这些位错在热影响区被消除，因此预应变对接头的疲劳强度没有显着影响。这些结果通过在接收和预应变条件下整个焊接接头的显微组织检查和显微硬度测量得到了证实。拉伸剪切承载能力的增加归因于具有预应变的母材中位错密度的增加。由于在焊接过程中，由于预应变而产生的大多数这些位错在热影响区被消除，因此预应变对接头的疲劳强度没有显着影响。这些结果通过在接收和预应变条件下整个焊接接头的显微组织检查和显微硬度测量得到了证实。拉伸剪切承载能力的增加归因于具有预应变的母材中位错密度的增加。由于在焊接过程中，由于预应变而产生的大多数这些位错在热影响区被消除，因此预应变对接头的疲劳强度没有显着影响。这些结果通过在接收和预应变条件下整个焊接接头的显微组织检查和显微硬度测量得到了证实。