Abstract
The present study reports an experimentally validated three-dimensional computational model for studying the transport phenomena associated with the molten pool produced by a laser melting based direct energy deposition process. In the present study, the substrate used is SS 316, while the powder feed material is Inconel 718, which is introduced co-spatially with the moving laser beam. The model considers alloy solidification, as the powder material is different from that of the substrate. An enthalpy-based technique is used for modelling the phase change processes so that the solid-liquid interface need not be tracked separately. The reference co-ordinate system is kept moving with the scanning laser beam while solving the mass, momentum, energy, and species conservation equations. The model considers differential melting points of various elements (major elements being nickel, chromium, and iron), their vaporization above the respective boiling points, Marangoni convection arising out of high temperature gradients on the top surface of molten-pool, and the momentum of powder feed particles (being carried by pressurized nitrogen as carrier gas) along with that of the shielding gas (nitrogen). For a fixed laser spot size and constant powder feed rate, other process parameters including the laser scanning speed and laser power have been varied for parametric studies. This transient simulation has been carried out till a steady state of the melt pool is attained, whereafter there is no significant change in the temperature and composition of the molten pool with time. The computational model predicts the velocity distribution, temperature profile with recognition of the melting isotherm, and species distribution in the melt pool, along with the geometry, i.e., width and depth, of the dilution zone. Corresponding experiments have been performed using the same process parameters for depositing IN 718 on a SS 316 substrate. The transient temperature profile in the melt-pool is captured using an IR Camera, whereas the deposited samples are processed and characterized to view the dilution zone. The results obtained from computations with respect to the temperature profile, particularly the melting isotherm, and the dimensions, viz., width and depth, of the dilution zone are validated against the experimental results. This model can then be utilized for designing and optimizing laser melting based direct energy deposition processes. The findings from this model including the local temperature at the solidification front, and concentration of alloying element in the melt-pool region can also serve as inputs for studying the micro-structure evolution of the deposited layers.
