Effect of Welding Speed on the Prediction Accuracy of Residual Stress in Laser Welded 1.2 mm Thick Dual Phase Steel

Abstract

Post-weld residual stress and distortion are significant issues in laser welding as they reduce fatigue strength and toughness. In the present work, cold rolled dual phase steel sheets of 1.2 mm thickness were laser welded in both bead on plate and butt configurations. The optimum heat input was observed to be approximately 25 kJ/m for visually defect free weld with narrow weld bead and through-thickness penetration. A novel attempt was made to increase the welding speed whilst maintaining the optimum heat input. It was observed that, high welding speed led to high residual stress, resulting in longitudinal cracks, whereas the speed at a limiting value of 0.08 m/s was found to be beneficial for welding without longitudinal cracks. Literature suggests researchers have already proposed different mathematical models on keyhole mode laser welding that can predict the residual stress and deformation. However, most of them failed to predict it repetitively, especially for thin sheets. This work tried to find out the rationality of this argument. For low-speed condition, the range of residual stress values was substantially wide, making it difficult to predict actual residual stress with the aid of mathematical models. The range of residual stress values was found to be much lower for the optimum heat input and high-speed condition, resulting into better prediction accuracy. Residual stress values were also validated using kernel average misorientation (KAM) study.