Additive manufacturing (AM) enables the production of topologically optimized structures, offering a high potential for lightweight design. However, AM components reveal a poor surface quality and a high number of process-induced defects, especially in the surface region, leading to a significant reduction of the fatigue strength. Considering the influence of these notch effects, not only their extent, but also the sensitivity of the material to the stress concentrations, i.e., the defect tolerance, is crucial. In addition, process-induced residual stresses can also reduce the fatigue strength. Thus, a sound knowledge of the influence of the complete surface morphology on the fatigue life must be considered for a reliable design of AM components.

In the presented work fatigue specimens made of AlSi10Mg were manufactured by laser powder bed fusion (L-PBF). To analyse the influence of the surface morphology on the fatigue behaviour, specimens with “as-built” or a machined and polished surface were analysed. To realize four different surface morphologies, both kind of specimens were investigated in a non-heat-treated and an artificially aged (T6) condition.

For each condition, S-Nf curves were determined, which revealed, as expected, an increase in fatigue strength for the specimens with polished surface, which was less pronounced for the heat-treated condition. However, based on fracture surface analyses, the surface roughness was found to have no significant influence on the fatigue strength. The increased fatigue strength after surface finishing is related to changes in the residual stresses since the machining and polishing introduces compressive residual stresses. As the non-heat-treated specimens revealed tensile residual stresses at the “as-built” surface, the increase in fatigue strength caused by finishing is more pronounced for the not heat-treated condition.

Using the √area approach established by Murakami [1] and modified by Noguchi et al. [2] for nonferrous materials, a substantially higher defect tolerance for the T6 heat-treated variants were indicated. This results in higher fatigue strength for both surface conditions.

In summary, this work emphasizes the importance of residual stresses and defect tolerance on the fatigue strength of L-PBF manufactured AlSi10Mg.

[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