Mutual comparison of acoustic, pyrometric and thermographic laser powder bed fusion monitoring

Abstract

This paper compares three distinctive sensors for laser powder bed fusion metal additive manufacturing process monitoring. A microphone for airborne acoustic emissions, an on-axis two-colour pyrometer for melt pool temperature measurement and an off-axis thermographic camera are simultaneously applied. They are challenged with a large build area to investigate their robustness and sensitivity. This paper does not assess the sensors’ ability to detect specific process flaws, but instead gives a common ground comparison of general sensor characteristics. The camera provides a descriptive result in form of a heat-map, while it exhibits a lack of sensitivity. In contrast, the microphone presents a sensitivity up to 40 times higher than the camera and is still 15 times more sensitive than the pyrometer. However, with this comes increased susceptibility; its signal strength is strongly dependent on the distance to the melt pool as a result of frequency dependent dissipation. The pyrometer’s signal is sensitive enough for relevant process deviations to be uncovered, while being robust towards different sensing distances. Recommendations are given for successful implementation of the sensors. Additionally, novel process phenomena were uncovered: an interaction of the scanning direction with the shielding gas is discussed, plus insights regarding overhang scanning are acquired.