Contributions to special issue: “Additive Manufacturing at Montanuniversität Leoben” (Advanced engineering materials)
Sabine Bodner, Sepide Hadibeik and Alexander Jelinek recently contributed to a special issue of Advanced Engineering Materials with the focus on “Additive Manufacturing at Montanuniversität Leoben” released in April 2023. The studies dealt with the production of specimens by two additive manufacturing techniques, which could not be more different.
Alexander Jelinek and co-authors explore the possibilities of two-photon lithography (TPL) for fabricating mechanical specimens. TPL is a sophisticated polymer resin-based technique that employs ultra-short pulsed laser light (e.g. femtosecond lasers) to very locally cure the photoresist on a highly focused spot, and therefore enables printing in the (sub-)micrometer range. The main topic was to develop printing strategies for the fabrication of micro-mechanical specimens, which can later be tested with an appropriate mechanical test setup, such as in-situ SEM or common nanoindentation devices. For largely overhanging structures (as a cantilever specimen geometry, shown in subfigures 1(a), (b)) a slight taper was introduced on facing downwards surfaces as shown to avoid printing artefacts and not influencing the overall mechanical behavior of the specimen. In subfigure 1(c) multiple cantilever specimens are shown, printed side-by-side as used for mechanical testing.
A more complex shape, namely a push-to-pull device incorporating a double edge notched tension specimen (shown in subfigures 1(d) and (e)), enables complex tension testing via a more convenient compression setup. Design considerations are described that finally lead to testable specimens. Finite element modelling and simulation were performed for both final geometries to accomplish geometry factors crucial for physical experiments. Further, a small-scale demonstration of a potential use case along future experiments is performed. Here, many of these complex shaped specimens can be placed side-by-side and tested in an automated manner to maximize the throughput during the test. This experimental approach is enabled by the high precision and versatility, combined with the automation capabilities of the TPL technology and would not be possible with any other technique in this size regime.
Figure 1: a) Optimized notched cantilever geometry fabricated via direct laser writing without printing artifacts. b) Magnified detail of the cantilever tip. Large printing artifacts are absent, just minor features remained. c) Twelve notched cantilever specimens printed side by side on a preprepared substrate demonstrating the high-throughput testing possibility. d) Optimized PTP DENT specimen, printed with slightly increased laser writing power without any printing artifacts visible. e) Higher magnification detail of the notched region, with just minor deviations from the ideal shape at the resolution limit of the printing device.
Alexander Jelinek and co-authors further contributed the cover of the special issue showing a photomontage of an iconic building of Leoben in the foreground and an actually printed array of the above-described push-to-pull specimen as potentially used for a future work.
Sabine Bodner and co-authors worked on the laser-based powder bed fusion (PBF-LB) of a commercially available stainless steel 316L used for metal based additive manufacturing. This technology uses a laser to locally melt the metal feedstock and therefore finally build up a 3D geometry. The mechanical performance of these structures is strongly influenced by the process parameters used for PBF-LB manufacturing, and thus of particular interest for safety relevant applications. Thus, it is crucial to understand the complex correlation between (i) process parameters, (ii) microstructure and (iii) mechanical performance of the parts. Within the presented work, the authors investigated the influence of three different hatching strategies (plane, stripe, chess) on the crystallographic texture and mechanical properties of the printed material. anisotropic deformation of the test-cross-sections during tensile tests. Pronounced textures were characterized by electron backscatter diffraction and synchrotron radiation experiments and could be correlated with the anisotropic deformation behavior of the material during tensile tests (subfigures 2(a) to (c)). The presented results contribute to understand and improve the tailoring of metal’s microstructures during the PBF-LB by applying different hatching strategies, which leads to control of the local mechanical performance of powder bed-fused metallic components.
Figure 2: (a-c) Representative tensile engineering stress–strain curves for the three different hatch styles plane, stripe, and chess, respectively. The stripe samples b) exhibit a higher ductility due to the increased work hardening at larger plastic strains, which is in contrast to the plane a) and chess samples c). The insets in each diagram represent the fracture surfaces of the samples and the respective test cross sections. The stripe samples exhibit pronounced anisotropic x-y (in-plane) deformation leading to elliptical necking.
Sepide Hadibeik and co-authors explored the feasibility of in-situ alloying of Zr-based bulk metallic glass (BMG) during PBF-LB. Fabricating complex and large BMG geometries via conventional manufacturing techniques, such as melt spinning and suction casting, is confined by limited cooling rates. Thus, it is not possible to produce BMG components with larger cross-sections. PBF-LB, however, provides significantly higher cooling rates due to the short interaction of laser and BMG materials. It is therefore possible to produce 3D parts using this technique. As the commercial availability of amorphous powders suitable for the PBF-LB process is very limited, the material in this study was thus alloyed directly during manufacturing in the PBF-LB device. The authors used a powder blend consisting of 40 wt.% Zr, 20 wt.% Al, 20 wt.% Cu and 20 wt.% Ti to fabricate BMG through PBF-LB (subfigure 4 a to c). In order to optimize the relative density, the process parameters including laser power and scanning speed were optimized within wide ranges of 50 to 200 W and 50 to 800 mm s−1, respectively (subfigure 4 e). In all printed samples, microscopic and micromechanical examinations revealed no glass formation, but a fine-grained microstructure consisting of CuTi grains and ZrCu nanocrystals. The main reason for the absence of the amorphous phase in the matrix is that a substantial proportion of the Zr raw powder remained unmolten throughout the structure. Consequently, glass formation was hindered by a considerable amount of compositional deviation (subfigure 4 e).
Subfigure 4: a) blending the elemental powder process, b) SEM micrograph of the feeding powder and its corresponding chemical composition, c) PBF-LB process to manufacture 3D bulky parts, d) printed cubes and cylinders via in-situ alloying and manufacturing, e) optimizing the process parameters, energy density according to relative density measurements, f) microhardness measurement showing non-homogenous compositional microstructure.
Links to Publications
High-Throughput Micromechanical Testing Enabled by Optimized Direct Laser Writing