Quantification of surface-near porosity in additively manufactured aluminum brackets using X-ray microcomputed tomography

Abstract

The combination of additive manufacturing (AM) and topology optimization facilitates the design and production of lightweight structural components, e.g. using AlSi10Mg, while reducing cost and enhancing product performance in aerospace applications. Nevertheless, the relatively larger surface area of topology-optimized parts also favors an increased occurrence of defects like surface-near pores. Pores in load-carrying areas may profoundly influence the component´s mechanical performance, hence extensive non-destructive evaluations (NDE) are mandatory to predict the effect of defects in aluminum AM components. In this paper we non-destructively analyze cutout specimens of topology-optimized engine brackets and in-process test coupons that were manufactured from AlSi10Mg by selective laser melting (SLM) using X-ray microcomputed tomography (XCT). We investigate the respective parts in high scanning resolutions between 1.25 µm and 20 µm voxel size to extract pore size distributions and distance from surface. Using a standard clustering approach we were able to differentiate between small, spherical gas and larger, irregular Lack-of-Fusion (LOF) pores that show a clear spatial distribution from the part surface. Smaller gas pores show an average diameter of ca. 40 µm while LOF pores are larger (ca. 160 µm). Importantly, LOF pores in the analyzed sample are found in a wide range of distances from the surface (ca. 10 - 430 µm). This distribution is the consequence of the applied contour scan mode to minimize surface roughness while increasing the level of surface-near porosity. Since surface-near pores play a major role in fatigue performance of structural parts, this study provides valuable microstructural information for subsequent analyses concerning the effect of defects in aluminum AM components.