Ur Rehman Asif, Mahmood Muhammad Arif, Pitir Fatih, Salamci Metin Uymaz, Popescu Andrei C, Mihailescu Ion N
ERMAKSAN, Bursa 16065, Turkey.
Department of Mechanical Engineering, Gazi University, Ankara 06570, Turkey.
Nanomaterials (Basel). 2021 Dec 3;11(12):3284. doi: 10.3390/nano11123284.
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.
在激光粉末床熔融（LPBF）工艺中，操作条件对于基于热分布确定激光诱导的匙孔状态至关重要。这些状态分为浅匙孔和深匙孔，它们控制着LPBF工艺中缺陷形成的概率和强度。为了研究和控制LPBF工艺中的匙孔，提出了数学模型和计算流体动力学（CFD）模型。对于CFD，使用了带有离散元建模技术的流体体积法，同时通过考虑粉末床孔隙和表面对激光束的吸收来建立数学模型。详细探讨了动态熔池行为。对实验结果、CFD模拟结果和分析计算结果进行了定量比较，结果显示出良好的一致性。在LPBF中，由于粉末层中粉末颗粒之间的高热阻和空气，激光辐照区域周围的温度相对于周围环境迅速升高，导致激光横向热波传播缓慢。在LPBF中，匙孔可分为浅匙孔模式和深匙孔模式，由能量密度控制。增加能量密度时，浅匙孔模式会转变为深匙孔模式。深匙孔中的能量密度较高，这是由于匙孔内二次反射光束的多次反射和集中，导致材料迅速汽化。由于深匙孔模式下温度分布升高，当液态材料接近汽化温度时，形成气孔的概率比浅匙孔高得多。当温度迅速升高时，材料密度迅速下降，因此，由于比热容和熔化潜热，流体体积增加。反过来，这会降低表面张力并影响熔池均匀性。
