Effect of pre-strain on the strength of spot-welds

This investigation primarily aims to reveal the effect of pre-strain of base metals on the strength of spot-welds. Strength of spot-welds have been determined using tensile-shear specimens made on as-received and uniaxially pre-strained sheets of extra interstitial free (EIF) and high strength interstitial free (HIF) commercial steels used for automobile applications. These experiments are supplemented by characterization of microstructure, substructure, hardness profiles along the welds and post-failure fractographic examinations. The major results infer that: (i) the strength of spot-welds increases with increasing pre-strain, (ii) the location of failure is commonly at the interface of heat-affected zone and base metal except for the spot-welds on as-received HIF steel, where failure occurs at the base metal, and (iii) the remnant dislocation density around the spot-weld increases with increasing pre-strain. The increase in strength of the spot-welds of pre-strained specimens has been attributed to the amount of remnant dislocation density around the weld zone. The location of failure has been discussed in terms of microstructure, substructure characterized by TEM and hardness profile along the weldment.