Characterisation of Surface Roughness and its Amplification During Multilayer Laser Powder Bed Fusion Additive Manufacturing

Abstract

Surface roughness controls the mechanical performance and durability (e.g., wear and corrosion resistance) of laser powder bed fusion (LPBF) components. The evolution mechanisms of surface roughness during LPBF are not well understood due to a lack of in situ characterisation methods. Here, we quantified key process and defect dynamics using synchrotron X-ray imaging and ex situ optical imaging and explained the evolution mechanisms of surface roughness during multi-layer LPBF of Ti-6AI-4V. We found that (I) the average surface roughness alone is not an accurate representation of surface topology of an LPBF component and (II) the surface topology is multimodal (e.g., roughness and waviness) and multiscale (e.g., 25 µm sintered powder to 250 µm molten pool wavelength). Both roughness and topology are significantly affected by the formation of pre-layer humping, spatter, and rippling defects. We developed a surface topology matrix that accurately describes surface features by combining 6 different metrics: average roughness, root mean square, maximum profile peak height, maximum profile valley height, skewness, and melt pool size ratio. This matrix provides a guide to determine the appropriate linear energy density to achieve the optimum surface finish of Ti-6AI-4V thin-wall builds. This work lays a foundation for surface texture control which is critical for build design, metrology, and performance in LPBF.

Keywords: additive manufacturing, surface roughness, Laser Powder Bed Fusion, Rayleigh Taylor instability, Lack of fusion