In Situ, Parallel Monitoring of Relative Temperature, Material Emission, and Laser Reflection in Powder-Blown Directed Energy Deposition

Abstract

In situ monitoring is critical for developing new control methods, advanced materials and toolpath planning strategies in laser beam directed energy deposition (DED-LB). Coaxial melt pool monitoring has typically been performed with cameras [e.g., infrared, two-color pyrometer, charge-coupled device, or complementary metal-oxide semiconductor], which have focused on melt pool morphology and temperature distribution. While these techniques capture critical deposition information, they do not capture other important phenomena such as the unique coupling between the laser and melt pool, which limits the design and generality of open-loop and closed-loop process control. We establish in situ, parallel signals by monitoring multiple process phenomena at the same time through different wavelength bands and thermal correlation. Increased laser coupling was observed using in situ, parallel monitoring, where lower reflectivity/higher absorption of the laser light within a vapor depression led to an increase in thermal emission in the visible region. Ultimately, a relationship between each change in process parameter and the relative absorption of the laser was established. In situ monitoring of the laser coupling phenomena not only provides insight into material processing conditions but will also enable more complex control in DED-LB processes.