Experimental and numerical study of process parameters effects towards part quality and porosity during powder bed additive manufacturing processes.

Abstract

Powder bed additive manufacturing (PB-AM) process utilizes an electron beam or a laser as a heat source to melt the metallic powder particles. These processes have the capability of freeform fabrication, however certain defects such as porosity, high surface roughness, etc. would hinder its application. It is important to understand the effect of the process parameters and the underlying physical phenomena, which lead to the formation of such defects. In this regard, a three-dimensional (3D) thermo-fluid model is developed to study the effect of beam speed on the surface morphology during powder bed electron beam additive fabrication (PB-EBAF). Besides, the surfaces of PB-EBAF fabricated Ti-6Al-4V parts are analyzed using a white-light interferometer. The results show that in general, the build surface roughness along the beam moving direction slightly increases with the scanning speed. On the other hand, the hatch spacing noticeably affects the surface roughness in the transverse direction. In addition, the numerical model was modified to incorporate powder particles and study the effect of powder distribution towards the single-track formation during the laser powder bed fusion (LPBF) process. The numerical results show that the single-track morphology and density depend on the process parameters: scanning speed and laser power. Besides, micro-computed tomography (micro-CT) is utilized to characterize the pores formed during the LPBF process. Single tracks were fabricated with linear energy density (LED) ranging from 0.1 J/mm to 0.98 J/mm, and the samples were then scanned using micro-CT to measure keyhole porosity. The results show that the severity of the keyhole porosity increases with the increase of the LED. By keeping the LED constant in another single-track scanning experiment, different combinations of the power and the speed were tested to investigate the individual effect. The results show that for the same LED, the pore number and volume increased with increasing the power to a certain critical level, beyond which, the further increase and power resulted in fewer pore number and lower pore volume. The experimental results suggested that the dynamic phenomenon of a melt pool during the LPBF process is complex and sensitive to process parameters. Hence, a discrete element method (DEM) is utilized to obtain a powder distribution, which is then used to perform a thermo-fluid simulation using FLOW-3D software. The numerical results indicated that for a constant LED, the keyhole size increases with the increase in the laser power. The keyhole becomes stable at a higher power, which