Modeling of Direct Energy Deposition of Austenitic Stainless Steels

This work concerns the modeling of heat transfer and microstructure evolution in laser metal deposition of austenitic stainless steels through an Integrated Computational Framework. Three main modules, namely thermal, primary phase selection and micro-segregation were developed for the prediction of the microstructure achievable by additive processes, particularly by Direct Energy Deposition (DED). A 3D thermal model by finite difference was implemented to predict the thermal field of a cube of approximate size 10 times the laser spot. The calculated temperature history is provided as input for microstructure simulation during solidification, upon rapid cooling and thermal cycling. The thermal module provides Thermal Gradient, Solidification Rate and Cooling Rate values needed by the primary phase selection module. Primary Cellular Arm Spacing and Secondary Dendrite Arm Spacing are calculated for primary and secondary solidification phases. The micro-segregation module integrates a diffusion model with a thermodynamic equilibrium calculation of the stable phases, to predict the phase fractions evolution (austenite, ferrite and liquid) and the main alloying elements concentrations (C, Cr, Mn, Mo, Ni, Si, Cu and N) within the phases. A graphic user interface for input management and for visualizing the outputs of the three modules was designed and set-up. A post-processing module allows to redirect the 3D outputs of the thermal model and to display cutting planes on the 3D model. The thermal model was calibrated and verified on samples built with AISI 316L powder by varying laser power and speed, characterized in terms of phase type and structure size. From the microstructure, thermal gradients and cooling rates were traced and compared with the calculated values. Model predictions were in agreement with experimental evidence. Although the model has been verified and tested primarily for DED deposition in AISI 316L steel, it is flexible and adaptable to other deposition modes and other alloys.