Spectroscopic monitoring and melt pool temperature estimation during the laser metal deposition process

Laser metal deposition is an additive manufacturing process that allows the production of near net shape structures. Moreover the process can also be applied for the addition of material to an existing component for repair. In order to obtain structures with reproducible and excellent material properties, it is necessary to understand the thermal behaviour of the process better and to monitor and control the process. One of the critical parameters in this process is the measurement of the melt pool temperature and its distribution. The varying emissivity in space and time for the melt pool forms a fundamental physical problem. This also prevents a correct temperature measurement of the melt pool temperature distribution with a thermal camera. The usage of the spectral information within the emitted light of the melt pool can form a key enabling element in the estimation of the emissivity and as such reveal the temperature information. In the future this information can be used in a controlling system in order to prevent excessive heat transfer towards the substrate and to reduce the amount of the residual stress. Another key criterion for the additive manufacturing process is the prevention of oxidation of the deposited layers in order to eliminate the formation of brittle zones within the components. The appearance of specific discrete spectral lines can reveal essential information of the stability of the process such as oxidation. In this paper an optical setup with an optical spectrometer will be described for the measurement of the radiated spectrum ranging from 400nm up to 850nm. The spectra were obtained during the laser metal deposition process of the stainless steel (316L). The measured spectroscopic data of different locations on the melt pool surface will be presented and explored in this paper. Based on the measured spectra different temperature estimation algorithms will be presented and evaluated. The appearance of specific discrete spectral lines will also be examined.