Direction-independent temperature monitoring for Laser Metal Deposition with coaxial wire feeding

Among the Directed Energy Deposition (DED) processes, Laser Metal Deposition with wire (LMD-w) combines the advantages of a high precision and a high deposition rate. Recently, optical systems have been developed that allow to coaxially feed the wire through the center of a hollow laser beam into the induced melt pool. When a pyrometer is coupled to such an optical system, the measurement spot features the same annular shape as the laser spot. In this work, the temperature signal of such a setup was studied and the system was qualified for inline process monitoring. The size of the measurement spot was systematically varied, and the temperatures that resulted from the two modalities of a one-color as well as a two-color measurement were compared. It was shown that the size of the annular measurement spot significantly influences the temperature signal for both modalities. A better understanding of the measurement is essential to monitor the process reliably.