Process modeling for solidification microstructure and transient thermal stresses in laser aided DMD process.

摘要

Despite enormous progress in Laser Aided Direct M aterial Deposition (DMD) process adverse effects of process parameters on variety of properties and microstructure have been reported. De-bonding at substrate-deposition interface and cracking in deposited layers are a few that occur due to excessive stress build-up. Very high heating and cooling rates are inherent to this process. Consequently, the effects of solid state phase transformations cannot be neglected. A complete model that provides a quantitative relationship between process parameters, phase transformation kinetics, solidification parameters, thermal stresses and microstructure is highly desirable. This research deals with four key aspects of laser aided DMD process. First, effect of solid state phase transformation on thermal stresses. Second, influence of high cooling rates on solidification microstructure at the scale of primary and secondary dendrite arm spacing. Third, effect of deposition patterns on both thermal stresses and solidification microstructure. And fourth, development of a fully coupled temperature-stress/strain field model. Finite Element (FE) results indicate a drastic change in stress states with the inclusion of phase transformation. FE modeling of various deposition patterns (raster zigzag, spiral in-to-out and spiral out-to-in) illustrates their significant effect on both thermal stresses and solidification microstructure. Two different models are proposed as per metallo-thermo-mechanical theory. While the first model is based on sequentially coupled temperature-phase transformation-stress fields, the second model is based on fully coupled temperature-stress fields. In a study of single layer deposition of H13 tool steel on mild steel substrate, nodal points at the top of deposited material that had highest element deformations showed a marginal disparity between the two models.