Abstract

In the laser powder bed fusion (LPBF) process, the operating conditions are essential in determining laser-induced keyhole regimes based on the thermal distribution. These regimes, classified into shallow and deep keyholes, control the probability and defects formation intensity in the LPBF process. To study and control the keyhole in the LPBF process, mathematical and computational fluid dynamics (CFD) models are presented. For CFD, the volume of fluid method with the discrete element modeling technique was used, while a mathematical model was developed by including the laser beam absorption by the powder bed voids and surface. The dynamic melt pool behavior is explored in detail. Quantitative comparisons are made among experimental, CFD simulation and analytical computing results leading to a good correspondence. In LPBF, the temperature around the laser irradiation zone rises rapidly compared to the surroundings in the powder layer due to the high thermal resistance and the air between the powder particles, resulting in a slow travel of laser transverse heat waves. In LPBF, the keyhole can be classified into shallow and deep keyhole mode, controlled by the energy density. Increasing the energy density, the shallow keyhole mode transforms into the deep keyhole mode. The energy density in a deep keyhole is higher due to the multiple reflections and concentrations of secondary reflected beams within the keyhole, causing the material to vaporize quickly. Due to an elevated temperature distribution in deep keyhole mode, the probability of pores forming is much higher than in a shallow keyhole as the liquid material is close to the vaporization temperature. When the temperature increases rapidly, the material density drops quickly, thus, raising the fluid volume due to the specific heat and fusion latent heat. In return, this lowers the surface tension and affects the melt pool uniformity.

Highlights

  • Additive manufacturing (AM) enables customized design, rapid tooling and complicated shape development

  • The recoil pressure and Marangoni-induced flow are responsible for this shallow melt pool formation; the recoil pressure has the highest impact among all the previously described phenomena [53]

  • When the melt pool has sufficiently penetrated downwards, it will continue its way to the back of the melt, owing to the liquid’s high deformability

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Summary

Introduction

Additive manufacturing (AM) enables customized design, rapid tooling and complicated shape development. It has attracted considerable interest from sophisticated technological applications [1,2], aviation [3,4,5], biomedical research [6,7] and architecture [8,9]. Laser Powder Bed Fusion (LPBF) is a well-known AM technology that offers many ad- 4.0/). Nanomaterials 2021, 11, 3284 vantages, including significantly reduced structural restrictions, high reproducibility and on-time delivery [2,10,11,12,13,14,15]. It is necessary to understand the deformations and the impact of input factors on the melt pool [19,20] dynamics

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