Estimation of Melt Pool Dimensions, Thermal Cycle, and Hardness Distribution in the Laser-Engineered Net Shaping Process of Austenitic Stainless Steel

Abstract

Laser engineered net shaping (LENS) and other similar processes facilitate building of parts with freeform shapes by melting and deposition of metallic powders layer by layer. A-priori estimation of the layerwise variations in peak temperature, build dimension, cooling rate, and mechanical property is requisite for successful application of these processes. We present here an integrated approach to estimate these build attributes. A three-dimensional (3-D) heat transfer analysis based on the finite element method is developed to compute the layerwise variation in thermal cycles and melt pool dimensions in the single-line multilayer wall structure of austenitic stainless steel. The computed values of cooling rates during solidification are used to estimate the layerwise variation in cell spacing of the solidified structure. A Hall-Petch like relation using cell size as the structural parameter is used next to estimate the layerwise hardness distribution. The predicted values of layer widths and build heights have depicted fair agreement with the corresponding measured values in actual deposits. The estimated values of layerwise cell spacing and hardness remain underpredicted and overpredicted, respectively. The slight underprediction of the cell spacing is attributed to the possible overestimation of the cooling rates that may have resulted due to the neglect of convective heat transport within the melt pool. The overprediction of the layerwise hardness is certainly due to the underprediction of corresponding cell spacing. The application of Hall-Petch coefficients, which is strictly valid for wrought and annealed grain structures, to estimate the hardness of as-solidified cellular structures may have also contributed to the overprediction of the layerwise hardness.