Weld Metal Microstructure Prediction in Laser Beam Welding of Austenitic Stainless Steel

Featured Application The proposed modelling of the thermal field allows the evaluation of the fusion zone composition and the solidification mode. With reference to laser beam welding in a single pass of AISI 304L austenitic steel plates, the approach outlines a lean tool for the assessment of the thermal effects on the microstructure resulting from the setup of the main welding parameters, such as the heat source power and welding speed, facilitating the selection of the optimal process conditions, and allowing a proactive welding process control to avoid hot cracking formation. In the present work an approach to weld metal microstructure prediction is proposed, based on an analytical method that allows the evaluation of the thermal fields generated during the laser beam travel on thick plates. Reference is made to AISI 304L austenitic steel as a base material, with the aim to predict the molten zone microstructure and verify the best condition to avoid hot cracking formation, which is a typical issue in austenitic steel welding. The "keyhole" full penetration welding mode, characteristic of high-power laser beam, was simulated considering the phenomenological laws of conduction by the superimposition of a line thermal source along the whole thickness and two point sources located, respectively, on the surface and at the position of the beam focus inside the joint. This model was fitted on the basis of the fusion zone profile, which was experimentally detected on a weld seam obtained by means of a CO2 laser beam, in a single pass on two squared edged AISI 304L plates, that were butt-positioned. Then the model was applied to evaluate the thermal fields and cooling rates, the fusion zone composition and the solidification mode.