Emissivity calibration method for pyrometer measurement of melting pool temperature in selective laser melting of stainless steel 316L

Selective laser melting (SLM) is an additive manufacturing (AM) technique for producing arbitrary work pieces, in which a laser beam is controlled to melt specific regions of a metal powder bed layer by layer so as to build up the required geometric form. In the present study, a method is proposed for calibrating the measurements obtained by a pyrometer for the melting pool temperature in the SLM of stainless steel 316L powder using the estimated values of the emissivity coefficients obtained from finite element heat transfer simulation and experimental tests. The accuracy in temperature prediction by heat transfer simulation is also confirmed by embedding a thermocouple into the powder bed. As a result, the calibration process is applicable to both one-color and two-color pyrometry methods. It is shown that the average error between the temperature measurements obtained from the calibrated pyrometer and the simulated temperature is just 1%. In other words, the feasibility of the proposed emissivity-based calibration method is confirmed. In the author's knowledge, this is the first proposed idea to calibrate the emissivity of the pyrometer based upon the simulation model for accurately extracting the true melting pool temperature.