Absorbance measurement for in situ process regime identification in laser processing

Abstract

Laser melting can be conducted in two different process regimes, the conduction and the keyhole mode, which exhibit significantly different characteristics, dynamics, and stability and are highly sensitive to a magnitude of process parameters. Despite these differences and the resulting high relevance of the prevailing process regime for process development, the regime is commonly deduced after specimen testing. An identification of the regime parallel to the process could speed up the process development of, for example, laser beam welding or laser-based powder bed fusion of metals. Therefore, the possibility of an in situ regime identification under process-near conditions is the aim of these investigations. For this, the absorbance is measured in situ by using an integrating sphere on an in-house-developed test rig. This test rig can mimic real production process conditions to detect the characteristic change in the degree of absorption when switching between the process regimes. These measurements were conducted during experiments in which only the laser power was varied. A significant change in absorption was detected at a threshold laser power of 100 W, which correlates with the transition between the process regimes’ conduction and keyhole regime. This threshold was proven by subsequent identification analysis of micrographic cross sections. This correlation promises the possibility of fast in situ process regime identification under near-real production process conditions with the potential of accelerating process development.