Novel strategy for automated quality enhancement of up-facing inclined surfaces by incremental dual laser powder bed fusion

Abstract

The low quality of up-facing inclined surfaces, combined with a lack of in-situ treatments, is a crucial challenge in Laser Powder Bed Fusion of metals. A recently proposed strategy, so-called Incremental Dual Laser Powder Bed Fusion (IDLPBF), aims to enhance the quality of up-facing inclined surfaces of metal parts directly during building. This is achieved through automatically alternating between laser remelting and building in height steps of 1.5 mm. Laser remelting involves selectively removing powder that is covering the inclined surface through nanosecond pulsed laser-induced shock waves and subsequently remelting the uncovered surface using a continuous wave laser. Treated surfaces are hence composed of laser remolten zones separated by elevated ridges in the overlap region, referred to as stitching zones. These stitching zones can compromise the geometrical accuracy and, consequently, pose a serious risk to fatigue performance. This work focuses on the enhancement of stitching zones and resulting quality of inclined surfaces through IDLPBF of a novel tool steel (M789). The paper covers the effect of various scan strategies for laser remelting on the stitching zone size. By optimizing the orientation of consecutive remelting passes and the position of the remelting fields, the stitching zone width could be reduced from 830 µm to 70 µm. Moreover, the optimized IDLPBF process was successfully applied to a large range of surface inclinations with respect to the horizontal build plate (5°-60°). For the first time, a significant quality improvement for 60° inclined surfaces was achieved, reducing the stress concentration factor by 35 %. The significantly reduced maximum stress concentrations were largely observed at stitching zones, in contrast to multiple high stress concentration sites randomly scattered across as-built surfaces. As a proof of concept, 60 mm high demonstrator parts including typically inaccessible surfaces located in the shadow zone were fully automated manufactured by IDLPBF.