In-situ determination of micro-hardness in laser cladding of stellite using optical emission spectroscopy

Offline measurements in Laser Cladding (LC) processes, such as micro-hardness measurement, are destructive and time-consuming tests. Therefore, in-situ monitoring of clads parameters is of significant importance. In this study, we investigate the plasma plume emission produced during the LC process of Stellite 6 powder on a lowcarbon steel substrate and its correlation with the micro-hardness of the clads over a wide range of LC parameters. It is demonstrated that a strong correlation between the spectral features in the UV region of 280-320 nm and micro-hardness exists, so that, the micro-hardness of samples can be determined by the total intensity of light in this range, within an average tolerance of 10 %. An exponential relation between hardness and UV radiation is empirically obtained that works very well for a wide range of dilutions, from 20 % to 70 %. It means that the UV spectra even determine the micro-hardness of samples with dilutions above 60 %, where the process shifts towards laser powder welding. This radiation is suggested to originate from the collisions of electrons with nanosized clusters, which are produced during the laser interaction with the melt pool. The produced nanoparticles are the source of UV radiation and scatters the driving laser light, at the same time, processes that qualitatively explain the exponential relationship between the hardness and the UV light intensity.