Laser Cladding With Combined NIR and Blue Diode Laser Including In-Line Atomic Emission Spectroscopy

Abstract

For Directed Energy Deposition processes (DED) lasers in the near infrared (NIR) as well as in the infrared (IR) range are predominantly used. Recent developments have also made high-power lasers in the visible spectrum available. As the DED process is used for cladding of surfaces, repairing and additive manufacturing of components, process monitoring and control methods are necessary to ensure a consistent manufacturing quality. Optical emission spectroscopy (OES) of the process radiation can provide information on process conditions and the deposition layer during DED processes. However, DED processes are in the heat conduction regime and superimposed broad spectral emissions dominate the wavelength specific signals. The object of this work is to compare the process behavior using a NIR and blue diode laser separately as well as in combination. The influence of the laser wavelength on the cladding result as well as on the emitted process radiation is to be determined. Therefore, single tracks of Co-based powder (MetcoClad21) were clad on an S235 base material by using each laser source separately as well as in combination. Both laser beams were combined within a single hybrid optic. While the scan speed and powder feed rate remained constant, the laser power was varied. Single spectra have been recorded from the process using a spectrometer. Single spectra are sorted and element lines were identified. Only non-ionised elements could be detected, with chromium appearing frequently. It was shown that comparable results in terms of cladding quality can be produced independently from the laser wavelength. In fact, less laser power (app. 30% less, 1 kW at 980 nm (NIR) compared to 0.7 kW at 450 nm (blue)) was needed aiming for comparable results in geometrical factors (as dilution, height, depth, width) and homogeneity (chemical composition distribution) by using blue laser irradiation. Furthermore, more spectrometric signals (approx. 2–28 times more) were detected compared to experiments using only NIR irradiation with the same laser power. This effect is particularly high at low laser powers and decreases with increasing power. Hence, it is possible to enable in-line process analysis by adding blue laser irradiation to the mix of the beam.