In-situ dimensional metrology of hybrid manufactured parts

An important roadblock for the further diffusion of laser-based metal additive manufacturing (AM) into the manufacturing industry is the part-to-part consistency in terms of geometrical dimensions. The dimensional accuracy and surface roughness of the manufactured parts are still inferior to that of components that are manufactured by conventional machining methods due to several phenomena that occur during the build-up process, such as shrinkage and warpage. However, it is expected that the hybrid approach (i.e. combining additive with a subtractive process) can enhance the dimensional accuracy of the manufactured component if combined with dedicated measurement feedback loops.

Hence, in order to quantify and even control this improvement, there is a clear need to integrate a non-destructive metrological measuring system within the AM process chain. This will substantially enhance the likelihood to achieve the first-time-right principle. In this project we will develop a metrology measuring system which is integrated inside the processing area (in-situ). Hitherto, AM in-process monitoring has been limited to monitoring the electromagnetic radiation emitted by the melt pool with a high-speed camera.

Recently, first concepts have been proposed to integrate geometrical metrology solutions in-situ for laser based manufacturing. For example, Schmitt et al. developed a frequency domain low-coherent interferometer (FD-LCI) based on low coherence interferometry, and applied the technique to laser structuring and laser machining. Fratz et al. developed digital holography system for in-line measurement of production defects. The main disadvantage of these in-line focused light techniques is that point measurements are generated with a lateral resolution depending on the focal length of the F-Theta lens. Secondly, when used in-process, these measurements are obviously not occurring at standard conditions, and thermal stresses and influences of subsequent layer consolidation will influence dimensional stability of the workpiece. Experimental verification of in-situ measurements will therefore be performed in order to investigate the representativeness for meeting component specifications, and to develop and validate measurement compensation strategies.