Estimation of melt pool size by complementary use of external illumination and process emission in coaxial monitoring of selective laser melting

Coaxial imaging of melt pool dynamics provides several advantages over other monitoring methods in SLM. The ability to track the processing zone ensures the possibility to observe defect formation dynamics mainly related to melting and solidification. Commonly, the melt pool dynamics are observed by means of process emission. In process emission images, geometrical information of the melt pool are not directly available and their extraction would require the use of a calibrated sensor in order to measure the temperature levels; as a consequence commonly an arbitrary threshold is applied to the image. The use of external illumination for monitoring purposes allows for suppressing the process emission and observing the melt pool geometry by means of the reflected light. On the other hand, the obtained images show lower contrast and can be difficult to process by means of image processing algorithms. Accordingly, this work proposes the complementary use of external illumination to calibrate the melt pool geometry. Afterwards the process emission and melt pool dynamics in SLM are characterized. For the purpose, an open SLM platform with an inhouse designed coaxial monitoring module is used. Images with external illumination were used to estimate the melt pool size for AISI 316L. The information was used to set a threshold value for determining the melt pool size observed at the near infrared emission band. The proposed strategy proved promising for real time monitoring and control applications and can represent a feasible solution for industrial systems.