Evaluation of the diffuse reflectivity behaviour of the melt pool during the laser metal deposition process

Laser metal deposition is an additive manufacturing process that allows the production of near net shape structures. Moreover the process can also be applied for the addition of material to an existing component for repair. In order to obtain structures with reproducible and accurate geometrical dimensions, it is necessary to understand the geometrical melt pool behaviour of the process better and to monitor and control the process. One of the critical parameters in this process is the wetting angle and the interfacial free energies of the melt pool. The varying specular reflectivity in space and time of the melt pool surface forms a technical challenge for the measurement of the 3D shape of the melt pool. This prevents the implementation of a straightforward laser triangulation approach. The specular behaviour also forms a problem for a standard stereo vision solution due to the fact that there are not enough points that can be recognized on the melt pool surface. The usage of higher illumination laser power and the evaluation of the small diffuse reflectivity of the melt pool can form a key enabling element in the measurement of the 3D shape of the melt pool surface. In the future this information can be used to develop a measurement system for a controlling system in order to prevent excessive dilution, heat transfer towards the substrate and to reduce the amount of residual stress. In this paper an optical setup with a high speed camera and specific illumination system will be described for the evaluation of the diffuse reflectivity behaviour. The images were obtained during the laser metal deposition process of stainless steel (316L). The measured diffuse reflectivity behaviour of different locations on the melt pool surface will be presented and explored in order to evaluate different approaches in the future to define the 3D shape of the melt pool.