Role of melt flow dynamics on track surface morphology in the L-PBF additive manufacturing process

Abstract

Laser powder bed fusion (L-PBF) is an additive manufacturing process used to fabricate intricate metallic parts by melting successive layers of metallic powder. However, the fabricated part's poor surface roughness limits the extent to which the process can be used. The process involves rapid melting and solidification, and the unsteady flows within the melt pool dictate the surface morphology of the solidified tracks, which governs the surface smoothness of the end product. This work reports an experimental and computational investigation on the role of melt flow dynamics on the track's surface morphology. Single tracks were experimentally built by varying laser spot radius and scanning speed, and the corresponding melt pool and track surface morphology were characterized using metallographic and 3D optical profilometry techniques. Track formation is numerically simulated using an integrated 3D Discrete Element- Computational Fluid Dynamics (DEM-CFD) model. DEM is used to determine the spatial arrangement of the particles in the powder layer. In the CFD model, a free surface (VOF) approach is employed to precisely capture the interface between the gas and the metal phase. The interaction of laser with the powder bed and substrate, the physics of melting, solidification, and evaporation, and the melt flow due to interfacial forces (surface tension and recoil pressure) are accounted. With the help of simulation results, the role of melt flow dynamics on track morphology is described. Track roughness in the experimental samples is predicted with the help of multivariate polynomial regression analysis. The experimental and numerical data of track surface roughness and melt pool characteristics show decent agreement. Based on computational and experimental results, it was found that there exists an optimum scanning speed and laser spot radius which provide the minimum track surface roughness.