An open-architecture laser powder bed fusion system and its use for in-situ process measurements

Abstract

Design and development of an open-architecture laser powder bed fusion (LPBF) system for in-situ process measurements of the build process during additive manufacture is described. The aim of this work is to create new knowledge and contribute towards further understanding of complex laser powder interaction through in-situ process monitoring. The designed system is sufficiently automated to enable single tracks and high density multiple layer components to be built. It is easily transportable to enable measurements at different measurement facilities and its modular design enables straightforward modification for specific measurements to be made. The system produces components with >99% density, hence, the build conditions are representative to observe process fundamentals and to develop process control strategies. Open-architecture design enabled access to the build area allowing a range of insitu measurements such as high energy flash x-ray imaging, camera-based high-speed imaging, schlieren imaging and temperature measurements. High speed imaging of the LPBF process results reveal that the process is more dynamic than is generally appreciated and can involve considerable motion of powder particles and agglomerates in and above the powder bed. Many critical process regimes were observed for the first time, such as changes in inclination of the laser with varying power and scan speed; and denudation became less severe with respects to an increase in layer number. Schlieren imaging results enabled the visualisation of the argon gas flow and laser plume propagation in the atmosphere above the powder bed. In-situ monitoring has been extended to study the effect of ambient pressures from a high vacuum to 5 bar positive pressure on the LPBF. Considerable disruption to the powder bed is observed at pressures below 20 mbar. As the pressure decreases, the expansion of the laser plume prevents particles reaching the melt pool: profiles and crosssections of the track reveal a drastic reduction in its cross-sectional area. At above atmospheric pressure, argon (up to 5 bar), the process was further disrupted by severe plasma formation along with an increase in size and number of spatter. The particle entrainment and resulting denudation was reduced, and single-track continuity was enhanced. In further study, it has been found that helium, used as shielding gas at higher pressure, mitigates negative effects of argon; generates smooth and uniform tracks and islands. The smoothness and continuity of built layers at 5 bar in helium was comparable to argon at atmospheric pressure, with considerable increase in scan speed.