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Publication

In-situ Laser Based Subtractive Manufacturing for Increased Precision of Metal Parts Produced by Laser Powder Bed Fusion

Book - Dissertation

Laser powder bed fusion (LPBF) is an Additive Manufacturing technique which is currently in the center of focus of both academia and industry. Despite its numerous advantages, insufficient surface quality and dimensional accuracy remain major drawbacks. Furthermore, extensive post-processing is typically needed to meet the industrial requirements, which is further increasing the already high manufacturing time and cost. This work addresses these challenges by increasing the LPBF productivity and by in situ improving the surface quality of up-facing surfaces. The productivity is improved by enlarging laser spot size via "defocusing" (working outside of the focal plane). Larger spot size leads to higher productivity, at the expense of lower geometrical accuracy. This can be overcome by using a "hull-core" building strategy, consisting in a high-precision parameter set with a small laser spot size for the outer (hull) zone, and a high-productivity parameter set with a larger spot for the inner (core) zone. The results are promising, the estimated productivity increase based on bulk samples was by a factor of three (+193%). However, the exact effect of the difference in melt pool size on the microstructure is yet to be determined. The main part of this dissertation deals with geometrical challenges in LPBF. The roughness of horizontal up-facing surfaces can be improved by in-situ remelting. On the other hand, it often leads to an increase in sample edge height (edge effect). In this work, the geometrical accuracy in this region is improved using in situ laser ablation. Furthermore, a connection has been found between the inclined surface texture and the edge effect, allowing a better scan strategy optimization for different surface inclinations. Additionally, a novel method for in situ quality improvement of inclined surfaces has been developed. Typically, inclined surfaces cannot be in situ remelted, because they are covered with powder. The presented method uses laser-induced shock waves (LISW) generated by a nanosecond pulsed laser. In the first step, the powder is removed using LISW, enabling a direct access to the inclined surface, which can be remelted using a continuous wave laser. In the as-built state, inclined surfaces show a higher surface roughness compared to horizontal surfaces. The hybrid processing results in a significant surface quality improvement, the surface quality of both horizontal and inclined surfaces being comparable after remelting. The method can be applied to various component shapes with inclination angles up to 35-45º. First tests of these samples show a significantly improved fatigue life due the absence of notches preventing early crack initiation.
Publication year:2021
Accessibility:Open