# Weld Metal Microstructure Prediction in Laser Beam Welding of Austenitic Stainless Steel
###### Abstract  
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 austeninite 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 CO\({}_{2}\) 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. laser beam welding; analytical modelling; thermal fields; austenitic stainless steel; fusion zone; solidification mode +
Footnote †: journal: Applied Sciences  
modes (A and AF); conversely, if the austenitic steel solidifies primarily as ferrite-\(\delta\) (FA), it results less susceptible to hot cracking, due to the higher solubility of the impurities in this phase. The \(\delta/\gamma\) interface shows also a better cracking resistance than \(\delta/\delta\) or \(\gamma/\gamma\) interfaces, because they have higher grain boundary wettability with respect to the eutectic liquid enriched by impurities, as experimentally characterized in [5] and more recently in [6] where the role of the microstructure of the mushy zone is highlighted. Therefore, the metallurgical features that accompany the transition between AF and FA solidification modes are crucial to establishing the material susceptibility to hot cracking, considering also other issues, such as sensitizing to intergranular corrosion and loss of toughness, due to carbide precipitation and to the formation of embrittling phases [7]. In this respect the ability to predict microstructures and properties of the austenitic stainless steels, according to their composition, has been the topic of many studies. Starting from the historical works of [PERSON] and [PERSON], [PERSON] et al. [8] investigated the effects of cooling rate, [PERSON] et al. [9] proposed a new ferrite diagram (WRC 1988) that shows, in the austenite/ferrite zone, the ferrite percentages, or ferrite number (FN); then the diagram was modified by [PERSON] and [PERSON] [10] by the inclusion of the solidification mode boundaries (WRC 1992), obtaining a very useful tool to predict weld microstructure on the base of the grade of dilution. This kind of diagram is still attractive for phase evaluations based on the fused zone composition, especially in the case of dissimilar welds [11; 12]. Other than composition, cooling rate [13] is a non-negligible parameter in determining the solidification modalities, which in turn results from the welding conditions. In recent years, numerous studies have been developed to simulate, by the finite element method (FEM), the thermal fields generated during welding: for a review on these numerical methods, see the articles by [PERSON] et al. [14] and [PERSON] et al. [15]). In particular, [PERSON] et al. carried out a work to evaluate the dilution grade in multi pass arc welding and predict the final microstructure of the weld [16]; more recently, [PERSON] [17] calibrated the heat source by selecting the finite element mesh in such a way that the calculated shape of the molten pool corresponded to that from real tests.