Spectroscopic monitoring of metallic bonding in laser metal deposition

Abstract

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 (λ=1.064μm) was used to produce clad tracks with Metco 42C powder on AISI 4140 steel substrate. The laser power was ramped in steps of 220W from 800W to 2780W focusing the laser beam into a 4.8mm diameter spot. A constant cladding speed of 10mms−1 is used with a powder feeding rate of 0.36gs−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 clads. 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 Te and the intensity ratio IR. The electron temperature Te is computed from four discrete Cr I lines at 526.415, 529.827, 532.834, and 534.044nm wavelength. The intensity ratio IR is a measure of the absorption of a discrete Fe I line at 588.91nm. 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 Te signal shows a better correlation with the dilution.