Spectroscopic monitoring of metallic bonding in laser metal deposition

A new approach is presented in this paper to link optical emission spectrum analysis to the quality of clad layers produced with laser metal deposition (LMD). A Nd:YAG laser (lambda=1.064 mu m) was used to produce clad tracks with Metco 42C powder on AISI 4140 steel substrate. The-laser power was ramped in steps of 220 W from 800 W to 2780 W focusing the laser beam into a 4.8 mm diameter spot. A constant cladding speed of 10 mm s(-1) is used with a powder feeding rate of 0.36 g s(-1). The extent of metallic bonding is evaluated from dilution data obtained by measuring the melt depth in the substrate from cross sections of the dads. The spectra generated during the deposition process were collected with an optical spectrometer attached to the laser head. The discrete spectral lines from the collected spectra were identified from the NIST database as Fe I and Cr I lines. Two different spectral analysis methods were used, namely the calculation of the electron temperature T-e and the intensity ratio IR. The electron temperature T-e is computed from four discrete Cr I lines at 526.415, 529.827, 532.834, and 534.044 nm wavelength. The intensity ratio IR is a measure of the absorption of a discrete Fe I line at 588.91 nm. The analysis of the collected spectra indicates that onset and extent of metallic bonding can be detected with a spectrometer during the deposition process. A sudden decrease of the IR signal during the LMD process indicates the onset of metallic bonding. A further increase of the laser power mainly contributes to an increase in melt depth, leading to a higher dilution. In this region, the T-e signal shows a better correlation with the dilution. (C) 2015 Elsevier B.V. All rights reserved.