Impact of single structural voids on fatigue properties of AISI 316L manufactured by laser powder bed fusion
Introduction
In recent years, additive manufacturing (AM) evolved to a promising manufacturing technique for light-weight components, spare parts or optimized functional components since the layer-by-layer manufacturing offers a high geometrical design freedom [1] and a sustainable use of resources. Since this manufacturing process is still relatively new compared to conventionally and well-studied processes, certain challenges are present which have to be addressed to enhance the scope of applications for additively manufactured parts. Regarding metallic materials, powder bed fusion techniques (PBF) such as laser (PBF-LB/M) or electron beam powder bed fusion (PBF-EB) are most established processes both in research and industrial application. Until now, PBF-LB/M was already successfully used to process different material classes such as Al-based [2], Ti-based [3], [4], Ni-based [5] and Fe-based [6], [7] alloys. In terms of Fe-based alloys preferably the austenitic steel AISI 316L was investigated due to its high mechanical strength and corrosion resistance, which makes it suitable for various applications in the marine, biomedical and even automotive industry [8]. However, many of the components produced suffered from an increased surface roughness and process-induced defects which can negatively affect the corresponding mechanical properties [9], [10]. Due to the cycle of heating and cooling during the build-up of layers, the solidified metallic material experiences a heat flow that deviates significantly from the thermal history of conventional forming or casting processes. The resulting microstructure tends to be finer and has a cellular dendritic or columnar morphology depending on the alloy constitution [11], [12]. The melting and solidification process is also characterized by a complex superposition of different physical processes, such as Marangoni convection, thermo-capillar effects, Plateu-Rayleigh instability and evaporation effects [13]. These can lead to the development of defects depending on the process boundary conditions and the selected process parameter window.
In general, it is differentiated between surface or near-surface defects, such as notch-like defects or plate-pile like stacking defects [14], [15], and inner defects such as gas porosity or lack of fusion defects [16]. Both, surface and inner defects can result from non-ideal process parameters [16], [17]. Especially surface defects are difficult to prevent and often require an additional post treatment such as chemical etching [18] or conventional machining. For reducing the internal porosity, parametric studies are often carried out to optimize process parameters. In case of the austenitic steel AISI 316L, Choo et al. [19] investigated the effect of changing laser power on the formation of inner defects and the corresponding microstructure. It could be demonstrated that porosity can be controlled by the laser power whereby a minimal porosity of 0.13% was achieved. Leicht et al. [8] showed that a decreased energy density during the process can increase the amount of inner defects which negatively effects the fracture strain in quasi-static tensile tests. A parameter study was performed by Röttger et al. [20] comparing the achieved microstructures with hot-isostatic pressed (HIP) and cast material. They identified suitable processing parameters for high density specimens by using high exposure time and small point distance, which means a high line energy density. The higher yield stress of the PBF-LB/M specimens compared to the HIPed and cast material was explained by a smaller grain size. Simultaneously, fatigue properties are affected by microstructure and defects as stated by Zhang et al. [21], [22]. Within their investigations they directly correlated the fatigue strength with the material’s ductility because both are found to be sensitive to porosity. Next to optimizing process parameters, additional heat treatments can be used to reduce the amount of process-induced defects [23], [24]. Regarding this, Zhang et al. [25] stated that thermally-induced defects might occur due to recrystallization which become new sites for crack initiation. As can be seen, inner defects can be positively influenced by optimizing process parameters and using additional heat treatments, although they cannot be eliminated completely [26]. However, HIP of the PBF-LB/M processed material leads to reduced crack density but has a neglectable effect on the porosity [20]. With regard to a fatigue life estimation, even the effect of build direction on the mechanical properties of AISI 316L [27], [28] has to be taken into account, whereby Yu et al. [29] stated that upright manufactured specimens show decreased yield and ultimate tensile strength compared to horizontal manufactured specimens. Thus, during cyclic loading, a contrary tendency was visible. It was proposed that the differences in fatigue behavior are attributed to the solidification microstructure since fatigue crack propagation is deflected due to the different grain boundaries which leads to an enhanced fatigue strength. An extensive study of the relationship between building direction and tensile behavior was performed by Hitzler et al. [30]. It was stated that next to horizontal and vertical build orientations even the azimuth angle can change the mechanical properties.
The mechanical properties, especially the cyclic properties, can be influenced by various parameters which complicate a potential fatigue life prediction. Zhang et al. [31] have shown that even different processing windows can lead to altering failure mechanisms resulting in an either microstructure- or porosity-driven failure. For both mechanisms, different fatigue prediction models were proposed. Another approach for the model-based description of the fatigue behavior of AM 316L steel was presented by Blinn et al. [32]. They used a combination of cyclic indentation testing and physical based lifetime calculation to characterize the anisotropic fatigue behavior caused by different building directions (vertical, horizontal and under 45°). This method allowed to describe various fatigue properties of the building orientations and identify different defect tolerances caused by the anisotropic material behavior. They also found out that the fatigue life was significantly influenced by microstructural defects. Different authors used the approach of Murakami to calculate the area of process-induced defects not only for PBF-LB/M 316L steel [32], [33] but also Ti6Al4V [34] and AlSi10Mg [35] to gain an increased understanding of the effect of defects on fatigue behavior of AM materials. In a recent study, Wilson-Heid et al. [36] investigated the influence of artificial internal pores on the tensile behavior of AISI 316L. Thereby, specimens with various pore diameters ranging from 150 to 4800 µm were manufactured by PBF-LB/M. It could be stated that the ultimate tensile strength was severely impacted when the pore diameter was bigger than 2400 µm. Furthermore, specimens with a pore diameter <450 µm showed similar mechanical strength compared to dense samples during quasi-static testing. Similar results were obtained by Kleszczynski and Elspaß for PBF-LB/M samples made of the austenitic alloy X5CrNiCuNb16-4 [37]. Kim et al. [38] did comparable research with 17-4 PH stainless steel made by PBF-LB/M and created octahedron-shaped cavities or a region with lack of fusion defects by locally changing the process parameters in the gauge length of dog-bone samples. They used in-situ tensile tests in a CT to reveal the failure process during loading. For the octahedral defect’s failure initiated from the inner surface a high surface roughness was present. Using CT-data-based FE simulation an identification of possible failure locations was feasible. Andreau et al. [39] investigated the influence of artificial defects and their position on the HCF-behavior of PBF-LB/M 316L steel. Based on their results, they stated that the critical defect size is 20 µm for surface defects and 380 µm for internal defects, respectively. Furthermore, Molaei et al. [40] recently presented results from fatigue testing with notched specimens which were fabricated by PBF-LB/M. They achieved multiaxial loading conditions by introducing notches into a hollow specimen geometry. It was found that the effect of notches on the fatigue performance is strongly influenced by the metallurgical and mechanical properties of the specimen. This was noticed when the results differed in dependence to the surface roughness or the residual porosity of the specimen. Additionally, it was found that heat treatments have only a minor effect on the fatigue performance of notched specimens.
While for notches and artificial surface defects a lot of research has been done, there is a lack of research for artificial internal defects as those with a specific geometry can only be realized by AM techniques. Within the scope of this work, specimens with various defect sizes ranging from 300 to 1500 µm were manufactured by PBF-LB/M based on the austenitic steel AISI 316L. Next to microstructural and non-destructive defect characterization, a combination of quasi-static and cyclic testing is used to investigate the mechanical behavior and correlate it with the size of the artificial defect. Additionally, common fatigue life approaches are adapted to enable a fatigue life prediction as a function of the present defect size.
Section snippets
PBF-LB/M manufacturing of fatigue specimens
Within these investigations, the specimens were manufactured with AISI 316L metal powder material distributed by EOS GmbH (Krailing, Germany) which corresponds to 18Cr-14Ni-2.5Mo (1.4441) and nitrogen as a process gas. The chemical composition is given in Table 1.
An EOS M290 PBF-LB/M system, also distributed by EOS GmbH, was used to produce 20 cylinders with a diameter of 10 mm and a length of 100 mm, as shown in Fig. 1, with the parameter set shown in Table 2. We used an alternating stripes
Microstructure
Fig. 3 shows two exemplary images from the microstructural analysis carried out by light microscopy. Within the microstructure no indicators for residual porosity, lack-of-fusion defects or impurities are visible, which is also underlined by the results from µCT investigations (see Section 3.3). When comparing Fig. 3a and b a different mesoscopic arrangement of melt traces can be noticed. Within the xz-cross sections (parallel to building direction) a scale-like structure can be detected, which
Conclusions and outlook
Defined structural defects were implemented by Boolean operations into the CAD data of 316L specimens, which were fabricated by PBF-LB/M. µCT measurements and fractography after mechanical testing of these specimens confirmed that only slight deviations were noticed concerning the resulting shape and accuracy of the implemented defects. The method used is therefore well applicable to introduce defined structural defects into PBF-LB/M components. In the future, the metrological quantification of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thank the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) for its financial support within the research project No. 379213719 “Damage tolerance evaluation of electron beam melted cellular structures by advanced characterization techniques” (WA 1672/32-1) and No. 372290567 “Mechanism-based assessment of the influence of powder production and process parameters on the microstructure and the deformation behavior of SLM-compacted C + N steels in air and in corrosive
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2022, Theoretical and Applied Fracture MechanicsCitation Excerpt :Biswal et al. [29] suggested that the shapes and locations of internal pores have a greater influence on fatigue performance than the pore size does, as determined by FEM calculations and analysis of stress concentration factors of porosity defects. Kotzem et al. [30] introduced artificial defects to characterize the influence of defects on fatigue properties of AM AISI 316L. The material is proven to have higher defect tolerance against internal defects than surface defects.