Ontwikkeling van nieuwe strategieën om de precisie en de mechanische performantie van metalen onderdelen geproduceerd via selectief laser smelten te verhogen

Laser Powder Bed Fusion (LPBF) is an Additive Manufacturing (AM) technique that allows the fabrication of metal parts with unparalleled design freedom. Despite its advantages, the fatigue strength of LPBF-produced parts is often limited by their poor surface quality. Post-AM surface finishing is commonly an essential step for enhancing the parts' surface quality and thus fatigue strength. However, this increases the production time and costs, and is sometimes not feasible for the complex geometries of LPBF parts. This thesis addresses these challenges by developing an in-process remelting strategy to improve the surface quality - and thereby the fatigue strength - of parts with up-facing inclined surfaces directly during printing. The research explores the use of pulsed laser-induced shock waves to remove powder from inclined surfaces in order to enable subsequent remelting. The first part of the thesis deals with a dual laser PBF (DLPBF) strategy, which employs remelting as a single surface finishing step once the part is built. Laser remelting allows significant improvements in fatigue life compared to reference samples with as-built, electrical discharge machined, and mechanically polished surface conditions by up to a factor of 36, 10 and 3, respectively. Moreover, the strategy can be applied for improving the fatigue life of parts made from various AM materials (Ti64, M300, M789). Furthermore, simultaneous improvements in productivity, surface quality and fatigue life can be achieved by combining high speed LPBF using a high layer thickness of 120 μm with remelting. Despite these advances, the DLPBF strategy is considered impractical for treating larger parts. The second part of the thesis addresses these limitations by advancing the strategy towards remelting at defined height steps during printing. This so-called incremental dual laser PBF strategy (IDLPBF) enables fully automated remelting during printing and is applicable for theoretically any part height, inclinations up to 60°, and for surfaces located in the shadow zone. Significant fatigue life improvements can be achieved compared to as-built samples via surface quality improvement, subsurface pore healing, and leveraging of phase change induced compressive surface stresses.