Online monitoring and quality control of Selective Laser Melting using optical sensors

Selective Laser Melting (SLM) is a layer-wise material additive production process for the direct fabrication of functional metallic parts. Despite the progress made in improving the SLM process and part quality, an important issue still needing more investigation is online quality monitoring and control of the process. In fact, many problems can occur during the process, for example the drastic variations of the border conditions around the laser spot zone, due to changes of the local part geometry. Most problems that occur during the build can only be detected after the process is finished, which leads to a substantial loss of machine hours. In order to overcome this lack of online quality control, this paper presents an optical monitoring system consisting of two coaxial process sensors, a high speed CMOS camera and a photodiode, and a visual camera for the inspection of the scanned layer and the powder bed First, the hardware and the data processing steps are described Next, the use of the system for process monitoring purposes is illustrated and the use of the system for feedback control is demonstrated. It is shown that monitoring of the behaviour of the melt pool delivers very valuable process information on the quality of the part.