Melt pool dynamics and microstructural growth prediction of additively manufactured thermoelectric material

Abstract

Laser powder bed fusion (L-PBF) method of additive manufacturing is typically applied to metallic alloys for structural applications. It may be applied to fabricate components of functional materials such as the binary Mg₂Si alloys with appropriate parameter optimization for energy harvesting applications. The L-PBF processing of such a material requires critical control over the input laser power and scan speed to minimise the structural defects in the end product. We developed a Computational Fluid Dynamics (CFD) model in MATLAB to simulate the thermal profiles of the dynamic molten pool to modulate the applied laser power and the scan speed. The material’s melting point and the conventional sintering temperature were used as the baseline numbers to scrutinize the suitable laser parameters. The CFD modeled laser parameters were tested to fabricate L-PBF processed Mg₂Si and the conventional sintering temperature of 1173 K was found to be a better limiting criterion than the melting point to prevent excessive balling and Mg vaporization. The fluidic flow velocity and its effect on the melt pool dynamics were observed experimentally. The microstructural variations in such materials are known to affect their thermoelectric performance, and the high cooling rate during the solidification in the L-PBF process favors dendritic growth, which differs from the conventionally fabricated samples possessing equiaxed grains. The microstructure evolution from the rapid solidification of the melt pool was predicted using the phase field analysis considering the effects of perturbation and the strength of anisotropy. A computational domain was simulated with an initial nucleation site of diameter 0.35 µm, and the directional dendritic growth patterns for different laser power and scan rates were obtained and analyzed. A comparable interdendritic spacing between the simulated primary dendritic arms (0.46 µm) and that measured within the L-PBF processed sample (0.57 µm) as seen under a Scanning Electron Microscope (SEM) is obtained, indicating the model’s efficacy in simulating the possible growth mechanism. The phase evolution of the L-PBF processed Mg₂Si component with the implication on its thermoelectric performance and the challenges in its fabrication are discussed.