Experimental and numerical investigation of single-pass laser welding of 20 mm-thick high-strength steel under reduced ambient pressure

The currently available high-power laser presents significant opportunities for welding of thick section components in single-pass. However, weld defects frequently occur and depth of penetration is usually lower than expected in this situation. In this work, a process of laser welding under reduced ambient pressure was investigated to achieve a single-pass complete-joint-penetration weld of 20 mm-thick high-strength steel. An experimental and theoretical program of investigation was undertaken to evaluate the characteristic of this welding process. The weld appearance, porosity inside the weld metal, microstructure and mechanical properties of weld joint were examined experimentally. The temperature field was calculated numerically using a 3D heat transfer and fluid flow model. A defect-free sound weld joint of 20 mm-thick plate was obtained just using an 8-kW laser power. The weld joint showed a very high aspect ratio characteristic of electron beam welding. From base metal (BM) to fusion zone (FZ) through heat-affected zone (HAZ), the microhardness varied significantly duo to their different microstructures resulted from the various peak temperatures and cooling rates for varying distances from weld center. All the tensile test samples fractured at the BM away from HAZ and FZ, which suggested that the welded metal was stronger than BM. The calculated weld geometry agreed with the corresponding experimental results through considering the role of ambient pressure. Their good agreement indicated the validity of the potential causative variables considered in the simulations. The modelling results also showed that the weld pool had a lower peak temperature and thinner liquid weld metal around keyhole under reduced ambient pressure.