Influence of heat treatments on the defect tolerance of additively manufactured AlSi10Mg

P. Lehner, B. Blinn, T. Beck

Additive manufacturing (AM) enables an enormous degree of freedom to design structural components. In combination with light weight alloys, this manufacturing technique consequently offers a high potential for reducing the weight of technical systems. To exploit this potential, a sound knowledge of the fatigue behavior of additively manufactured metallic materials is required, as this is a prerequisite for a reliable design. In this context, many research works have shown that the fatigue strength of additively manufactured components is limited because of process-induced notch effects. These notch effects result on the one hand from defects, e. g. lack of fusion, which occur more frequently at the surface or in the subsurface area, and on the other hand from the relatively high surface roughness. To reduce the notch effects localized at the surface and in the subsurface area, and thus, increase the fatigue lifetime, surface finishing can be performed. However, due to the complex geometries of additively manufactured components, surface finishing often is challenging or even impossible. Hence, the influence of the “as-built” surface on the fatigue behavior must be analyzed and considered. Furthermore, besides the defects and the surface roughness themselves, the ability of the material to counteract these notch effect, i.e., the defect tolerance, must be taken into account.

Based on that, the aim of this work was to analyze the influence of different heat treatments on the defect tolerance and the resulting fatigue behavior of additively manufactured AlSi10Mg. Therefore, specimens with layer planes perpendicular to the loading direction were manufactured by laser powder bed fusion (L-PBF). To modify the properties of the material volume, and in particular the defect tolerance, specimens were stress-relieved (300°C, 2h) or precipitation hardened (T6). Additionally, specimens in “as-built” condition were investigated. Note that all specimens were tested with the “as-built” surface in the gauge length to determine the interaction of the different material conditions and the “as-built” surface.

To determine the influence of the defects and the “as-built” surface on the fatigue behavior, S-Nf curves of the different batches were determined in fatigue tests and the respective fracture surfaces were analyzed. Based on this, the defect tolerance of the material conditions investigated was evaluated using the √area approach established by Murakami [1], whereby the modification for non-ferrous metals proposed by Noguchi et al. [2] was utilized. Additionally, instrumented cyclic indentation tests (CIT), which enable an analysis of the cyclic hardening potential and thus, the defect tolerance [3], were performed. Both, the √area approach as well as CIT demonstrate substantial differences in the defect tolerance, which is highest in the precipitation hardened condition. As all specimen exhibit fatigue crack initiation at process-induced defects and surface imperfections. A higher defect tolerance correlates with an increased fatigue strength, which underlines the relevance of the defect tolerance of additively manufactured metals.

[1] Y. Murakami. Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions. 2002, Kyushu, Japan: Elsevier

[2] H. Noguchi et al.: Proposal of method for estimation stress intensity factor range on small crack for light metals. 2007, Proc. 56th JSMS Annual Meetings, pp.137-138

[3] D. Görzen et al.: Influence of Cu precipitates and C content on the defect tolerance of steels. 2021 International Journal of Fatigue 144, 106042.