Chapter 10 - Mesoscale understanding of particle melting behavior, thermodynamic mechanism, and porosity evolution of randomly packed powder-bed using finite volume method

Abstract

A transient three-dimensional powder-scale model was established for investigating the thermodynamics, heat and mass transfer and surface quality during laser powder bed fusion (LPBF) of Inconel 718, Ti6Al4V, and TiC/Ti6Al4V predicted by finite volume method (FVM). The influence of processing parameters (laser volumetric energy density, hatch spacing, scan speed) on the thermal behavior, material evaporation, surface morphology, residual porosity evolution, and densification behavior in the molten pool and processed parts was studied. It was shown that the powder material underwent the transformation from the partial melting state to the complete melting state and finally to the overheating state with the applied laser volumetric energy density (VED) increased. The solidified track ranged from the discontinuous tracks with the rough surface to the continuous and dense tracks, and finally to the fluctuated tracks, with the increase in VED. As an appropriate hatch spacing was settled, a reasonable temperature gradient and resultant surface tension tended to drive the molten liquid with a steady velocity, favoring the formation of a flat surface of the component and attendant low average surface roughness. At a high scan speed, the top surface was primarily dominated by open porosity, accompanied by the large-sized interlayer porosity due to the limited energy input penetrated into the powder-bed and the attendant incomplete melting of powder material. Laser scan speed is required to be controlled to obtain the top surface free of metallurgical porosity and interlayer porosity and the sound metallurgical bonding of the neighboring layers in the building direction.