The technological objective of the present research is related to a broader integration of Selective Laser Melting (SLM) into industry and society through development of a reliable method of quality control based on thermal emission from the laser impact zone recorded by a pyrometer and by a CCD – camera. The sub-task is to evaluate the performance of the applied optical diagnostic tools in SLM and to define the field of their application. The scientific objective is to better understand the thermal phenomena in SLM, to define the main influencing factors, and to optimize the process parameters. Among the latter, only several were selected for the present analysis: hatch distance between individual tracks, powder layer thickness and influence of the manufacturing strategy. Single tracks in SLM are often considered as a kind of "bricks" by superposition of which an object is build. That is why a particular attention is paid to a single track analysis. Powder layer thickness is one of the main influencing parameters on SLM quality and productivity, but it is difficultly controllable. Optical diagnostic tools could be used to find a solution.
To carry out permanent monitoring of the thermal radiation from the zone of powder melting and heat affected zone (HAZ), observations must be carried out coaxially with the laser beam. In this case, for any position of the laser beam and for any scanning speed, the diagnostic system will register thermal signal from the beam-powder interaction zone and around it.
Temperature was measured in the laser impact zone by an originally developed bi-colour pyrometer with the following performance: temperature range is 1200-2900 K, two InGaAs photodiodes with optical filters, transmission spectrum at central wavelength 1.26 µm with 100 nm bandwidth (this wavelength was applied for further analysis), 560 µm diameter of the zone of temperature measurements.
Brightness temperature distribution in the HAZ was acquired by the CCD camera with a resolution of 560x760 pixels. The brightness temperature measurements were realized with the exposure time of 3 ms.
To obtain the brightness temperature, the CCD camera was calibrated using a W - halogen lamp with a transmitting diffuser. Note that calibration was carried out through optical system of the SLM machine. The W - halogen lamp was calibrated with a black body in the temperature range from 1200 to 1800K.
Transversal and longitudinal profiles of brightness temperature captured by the CCD camera were obtained after treatment and averaging of 5 consecutive images.
It was found that the pyrometer signal from the laser impact zone and 2D temperature mapping from HAZ were rather sensitive to the variation of the operational parameters (as powder layer thickness, shift between consecutive laser beam passes, manufacturing strategy), and could be used for on-line control of manufacturing quality.