Tensile and fatigue behavior of resistance spot-welded HSLA steel sheets: Effect of pre-strain in association with dislocation density

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.