Modeling and experimental verification of transient/residual stresses and microstructure formation in multi-layer laser aided DMD process

Despite enormous progress in laser aided direct metal/material deposition (LADMD) process many issues concerning the adverse effects of process parameters on the stability of variety of properties and the integrity of microstructure have been reported. Comprehensive understanding of the transport phenomena and heat transfer analysis is essential to predict the thermally induced residual stresses and solidification microstructure in the deposited materials. Traditional solidification theories as they apply to castings or related processes, assume either no mass diffusion in the solid (Gulliver-Scheil) or complete diffusion in the solid (equilibrium lever rule) in a fixed arm space. These are inappropriate in high energy beam processes involving significantly high cooling rates. The focus of this paper is the solute transport in multi-pass LADMD process, especially the coupling of the process scale transport with the transport at the local scale of the solid-liquid interface. This requires modeling of solute redistribution at the scale of the secondary arm spacing in the dendritic mushy region. This paper is an attempt toward a methodology of finite element analysis for the prediction of solidification microstructure and macroscopic as well as microscopic thermal stresses. The computer simulation is based on the metallo-thermo-mechanical theory for uncoupled temperature, solidification, phase transformation, and stress/strain fields. The importance of considering phase transformation effects is also verified through the comparison of the magnitudes of residual stresses with and without the inclusion of phase transformation kinetics. The simulation has been carried out for H13 tool steel deposited on a mild steel substrate.