THREE-DIMENSIONAL TRANSIENT FINITE ELEMENT ANALYSIS FOR MICROSTRUCTURE FORMATION AND RESIDUAL STRESSES IN DOUBLE-PASS LASER AIDED DMD PROCESS

摘要

Despite immense advances in Laser Aided Direct Metal/Material Deposition (LADMD) process many issues concerning the effects of process parameters on the stability of variety of properties and the integrity of microstructure have been reported. Modeling of heat flow seems to be a standard practice to couple heat flow calculations to related macroscopic phenomena such as fluid flow in the melt and solid-liquid mushy region, macrosegregation and thermal stresses. A key component in these models is the coupling between thermal and solute fields. Like macrostructural phenomena even microstructural features such as phase appearance, morphology, grain size or spacing are certainly no less important. The focus of this paper is the solute transport, in particular the manner in which process scale transport is coupled to transport at the local scale of the solid-liquid interface which requires a modeling of the redistribution of solutes at the scale of the secondary arm spaces in the dendritic mushy region. Basic microsegregation models which assume either no mass diffusion in the solid (Gulliver-Scheil) or complete diffusion in the solid (equilibrium lever rule) in a fixed arm space are inappropriate in high energy beam processes involving significantly high cooling rates. This paper aims at incorporating a model that accounts for finite mass diffusion and coarsening of the arm space. Due to the complexity and nonlinearity of LADMD process, analytical solutions can rarely address the practical manufacturing process. Consequently, this is an attempt towards a methodology of finite element analysis to predict solidification microstructure and thermal stresses. The simulation has been carried out for H13 tool steel deposited on a mild steel substrate. However, the program can easily be extended to a wide variety of steels.