Process observation in selective laser melting (SLM)

In additive manufacturing, the quality of products can be traced by observation of process variables track by track and layer by layer. The stacking of layer wise information can be used to consolidate the entire build up history of a product thus leading to a truly three dimensional quality histogram. The first step that is necessary to achieve such a quality histogram is the acquisition of process measurands that are related to product quality. Successful acquisition of measurements for thermal radiation has been reported in several publications. The authors of such papers report the detection of changes in boundary conditions of the process by observing the thermal radiation of the process. It has been reported that for example a change in laser power has an influence on the thermal emission and that different readings are received for processing a thin powder layer on a solid work piece compared to scanning pure powder in the situation of an overhang structure. A correlation to the underlying reason for the increase in thermal radiation however is mostly related to the experimental setup rather than to in process measurements. This report demonstrates an approach of acquiring and combining synchronous measurements of different physical properties of the process. The coaxial observation system used in the experiments enables the synchronous acquisition of measurements of the thermal emission and the acquisition of images that visualize the surface of the powder bed in the vicinity of the interaction zone. The images are used to monitor the motion of powder particles as they are influenced by the melting process. This amount of particle motion is then correlated to areas of different powder thicknesses. The combination of this information with excessive readings in thermal emission classifies the event to be a situation of noncritical deviation of thermal emission. In fact, this combination of extracted features establishes a first key criterion for an unequivocal event mapping