Surface Roughness Considerations in Design for Additive Manufacturing: A Space Industry Case Study

Abstract: Additive Manufacturing (AM), commonly known as 3D printing, is the name given to a series of manufacturing technologies that make objects from 3D model data via a layer-upon-layer process. This process provides design engineers with new design opportunities outside the design constraints of traditional subtractive manufacturing. The space industry is seeking to capitalise on these new design freedoms of AM due to the bespoke and often complex nature of parts designed for use in space applications. The Laser Powder Bed Fusion (LPBF) AM technology has been identified as capable of producing components with the part properties required for space applications. Hence, additionally, the precision of the laser brings a high degree of design freedom, enabling the production of near-net shape innovative weight-saving part designs. However, due to the powdered metal material, the LPBF process is categorised with rough surfaces in the “as built” state. The more intricate one’s design, the more difficult it could be to process the removal of this roughness and thus the costlier the production. There are many variables that affect the degree of surface roughness, such as build orientation, overhangs, support structure and process-related parameters. Consequently, the as-built surface for most applications is too rough and could lead to adverse effects on proprieties such as fatigue. Hence there lies an intrinsic material-machine-geometry relationship of parts made through LPBF. Thus, there is a need to develop Design for AM (DfAM) supports that provide an understanding of designs' relationship to surface condition and resultant material properties. To address this, this thesis presents a systematic literature review of research related to LPBF surface roughness and explores trends in managing surface roughness. Additionally, through a space industry case study, a proposed process that uses Additive Manufacturing Design Artefacts (AMDAs) is used to investigate the relationship between design, surface roughness, fatigue, and material characteristics. From the review, it was found that, in general, research focuses on the relationship between surface roughness and LPBF build parameters, material properties, or post-processing. There is very little support for design engineers in the design process who want to consider how surface roughness from an AM process affects the final product (less than 5% of the review articles). In investigating surface roughness, the AMDA process enabled the identification of process characteristics that impact roughness levels and geometric adherence to part design. However, iterations of AMDAs can be required to clarify product-specific design uncertainties, though the designer obtains a better understanding of their design and the AM process with each iteration. Material analysis suggests that post-processing requirements must be understood if required, as inadequate processing may leave residuals that impact performance. Additionally, the same geometry on an LPBF part will have variations in mechanical and material properties dependent on the geometry orientation of the part, independent of the surface condition.

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