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Öğe Analyzing the influence of the investment casting process parameters on microstructure and mechanical properties of open-pore Al–7Si foams(2023) Firoozbakht, Mahan; Blond, Aurelien; Zimmermann, Golo; Kaya, Ali Can; Fleck, Claudia; Bührig-Polaczek, AndreasAluminum alloy foams with high specific compressive strength and good energy absorption capacity are widely used as structural materials and shock absorbers. The investment casting process offers an interesting opportunity for the production of open-pore aluminum foams. However, foam casting differs from casting of solid parts, and there is a lack of practical knowledge about the microstructure tuning of the ultra-thin cast struts. Our study focuses on the influences of the casting conditions such as mold temperature on the microstructure and consequently on the mechanical properties of aluminum-silicon alloy foams. We investigated microstructural features using various metallography techniques and defined mechanical properties under uniaxial compression test. Here, we observed improvements in the strut microstructure together with decrease of the mold filling by reduction of the mold temperature. Microstructure improvements involve transformation of a single dendritic aluminum grain in each strut to globular grains along with a more homogeneous distribution of the silicon particles across the strut cross-section. These changes bring higher ductility and energy absorption efficiency to the foams, which are evident from the smoother plateau of the stress-strain curves. Decline of the mold filling, on the other hand, has negative effect on the overall mechanical properties.Öğe Fatigue and corrosion-fatigue behavior of the ?-metastableTi-5Al-5Mo-5V-3Cr alloy processed by laser powder bedfusion(2024) Kaya, Ali Can; Alcântara, Erika Gabriele Alves; Meinke, Reinhard; Selve, Sören; Fleck, ClaudiaWe performed rotating bending tests and axial (tension-compression) loadincrease and constant amplitude high-cycle fatigue tests in air and Hanks' balanced salt solution (HBSS) on the ?-metastable titanium alloy Ti-5Al-5Mo-5V3Cr, processed by laser powder bed fusion (LPBF-M), solution-treated and aged, and shot-peened. Rotating bending loading in air revealed a strong influence of process-induced flaws on fatigue endurance. Especially in the highcycle fatigue range and the transition region, the stochastic distribution of the flaws and flaw sizes led to a high scatter of the number of cycles to failure. The axial load-increase tests yielded a good fatigue life estimation, with a negligible difference between air and HBSS. The cyclic deformation behavior in HBSS was also strongly influenced by the local microstructure and defect distribution, and, thus, by crack formation and propagation. Plastic deformation and microcrack growth interact, and their relative amount resulted in different progressions of the plastic strain amplitude over the number of cycles for different specimens. Changes in the free corrosion potential and the corrosion current were highly sensitive indicators for fatigue-induced damage on the rough surfaces, which was correlated to the microscopic examination, fracture surface features, and the fatigue crack development.Öğe Foams of gray cast iron as efficient energy absorption structures: a feasibility study(Wiley-V C H Verlag Gmbh, 2019) Kaya, Ali Can; Zaslansky, Paul; Rack, Alexander; Fischer, Sebastian F.; Fleck, ClaudiaGray cast iron foams, were produced using a reticulated polyurethane by using a modified investment casting process. To evaluate the attributes of the produced micro-geometries, foam segments and single struts are investigated by light and electron microscopy in 2D, and synchrotron micro-computed tomography in 3D. Mechanical properties are determined by macro/micro-mechanical testing and nanoindentation. In the microstructure of the gray cast iron struts, both flake-like coarse type A and locally fine undercooled type D graphite particles are observed. The distribution of the two graphite types is heterogeneous and is the likely cause for the large scatter of the mechanical properties of the single struts. The high graphite content and the resulting brittle behavior of the struts lead to strong serrations in the stress-strain curve of the foams with a negative effect on energy absorption. We found a relatively low energy absorption efficiency of below 50% as compared to 75% in 316L austenitic steel struts. The small specimen size results in scale effects which strongly influence the mechanical properties of the foams. Further improvement in the fabrication of gray cast iron foams is needed to tailor graphite distributions and optimize performance of cast iron-based foams.Öğe Influence of anisotropy on the deformation behaviour in microtensile 316L steel specimens fabricated by laser powder bed fusion (PBF-LB/M)(2023) Kaya, Ali Can; Fleck, Claudia; Salamci, Metin U.Laser powder bed fusion of metals (PBF LB/M) allows fabrication of intricate structures without any need of tools and moulds. In this process the building orientation becomes a significant issue for the complex structures. Here we fabricated small size 316L steel tensile specimens aligned with their loading axis perpendicular and parallel to the building direction by PBF LB/M. Microstructure and mechanical properties were characterized by colour etching, in-situ tensile testing in a scanning electron microscope, nanoindentation and digital image correlation. The orientation of the melt pools has a significant impact on the deformation behaviour of the PBF LB/M specimens. In parallel specimens (polar angle = 90o ) the semi-circular melt pools are aligned perpendicular to the loading axis resulting in an accommodation of larger amount of plasticity as compared to the perpendicular specimens (polar angle = 0o ). While strain is localized in the case of perpendicular specimens because primary slip lines are aligned perpendicular to the loading axis, strain is homogenously distributed along the longitudinal axis of the parallel specimens because primary slip lines are aligned parallel to the loading axis. Twin-inducedplasticity and transformation induced plasticity are the deformation mechanisms observed on the fractured specimens. Strain hardening rate in stage IV extended to a higher stress value for the perpendicular specimens as compared to parallel specimens because cross slip is confined in the fusion boundaries. The building direction has shown significant influence on the deformation mechanisms of the PBF LB/M steels which may be a significant issue to overcome for the complex shape specimens.Öğe Micro and nano damage observations of martensitic and austenitic open cell cast foams(Elsevier Science, 2022) Kaya, Ali Can; Fromert, Jan; Jost, Norbert; Fleck, ClaudiaIn current study a detailed investigation of the 304 austenitic and 420 martensitic foams damage has been carried out at all hierarchical levels. In-situ compression testing of foams and microtensile testing of single struts have been conducted in scanning electron microscope (SEM). Strain field analysis has been carried out by Digital image correlation (DIC) method. Nano damage has been applied by Nanoindentation on the unloaded and compressed foams. 304 foams depicted a ductile failure at each scale, while 420 foams failed with a tremendous brittleness, such that no real plateau regime has been observed. At microscale 420 struts ruptured with an intergranular cracking, while 304 struts underwent transformation induced plasticity (TRIP) effect with signif-icant ductility. Although 304 steel struts have higher strength, 420 steel foams exhibited a higher strength due to their high hardness. Since there is not a plastic bending of the 420 struts, load is distributed homogenously to all struts. For this reason, it was concluded that the hardness and ductility of struts are the dominating factors in the foam deformation.Öğe Modeling of complex gray cast iron open-cell foams revealing insights on failure and deformation on different hierarchical length-scales(Wiley-VCH, 2021) Kaya, Ali Can; Zaslansky, Paul; Fleck, ClaudiaOpen-cell gray cast iron foams are a class of porous materials of increasing interest, with great potential to be used in energy damping applications and for sound isolation. We created finite element (FE) models from 3D reconstructed data of foams and of isolated struts imaged with resolutions spanning from 0.65 to 10.5 mu m. Representative volume elements (RVE) are loaded in tension to simulate and analyze foam mechanical behavior. A ductile damage model is used for tensile testing of single struts and compression testing of foams. RVEs loaded along three axes demonstrate important effects of orientation of graphite particles in the microstructure. Only simulation results that take failure into consideration are consistent with experimental findings. Strut fracture initiation strongly depends on the cross-sectional area and its circularity and foam simulation results are heavily influenced by damage modeling. The complexity of the open-cell foam behavior is revealed at several hierarchical levels. Simulations in which material properties are assigned excluding damage overestimate the experimental results of the foam; conversely, a very good agreement is observed if damage is considered. The models exhibiting anisotropic mechanical properties fully reproducing large fluctuations in the mechanical properties of the struts are observed in in situ experiments.Öğe Modification of open-cell cast aluminum-silicon foams with strontium(2024) Kaya, Ali Can; Firoozbakh, Mahan; Buehrig-Polaczek, Andreas; Blond, Aurelien; Fleck, ClaudiaThe mechanical properties of open-cell aluminum foams can be influenced by enhancing the microstructure of the struts. The foams produced by investment casting face slow cooling rates, which makes it challenging to improve the morphology of the phases. In the case of aluminum silicon cast foams, the silicon phase accumulates on the surface of the struts, which leads to brittle fractures. In the present study, we successfully modified the silicon phase in open-cell AlSi7Mg and AlSi10Mg cast foams by adding strontium and investigated the influence of the strontium content on the microstructure and mechanical properties at the foam and strut levels. Despite the cooling rates of less than 0.5 ?C/s during solidification, the strontium addition of 200–800 ppm effectively decreased the size of the silicon particles and improved their distribution in the micrometer-sized struts. Improvements in the compressive properties of the foams and the tensile properties of the struts only occurred at the strontium levels of 200 and 400 ppm. The effective modification in this casting condition is due to the limited solidification space, which favors the formation of the atomic clusters responsible for the modificationÖğe Relation between Tensile Strut and Compressive Foam Deformation Behaviour: Failure Mechanisms and the Influence of Dendritic Versus Globular Grain Structure in an AlSi7Mg0.3 (A356) Precision?Cast Open?Cell Foam(2024) Kaya, Ali Can; Blond, Aurelien; Firoozbakht, Mahan; Bührig-Polaczek, Andreas; Fleck, ClaudiaOpen-cell aluminum foams are gaining importance for the design of lightweightstructures and as electrodes in lithium-ion batteries. AlSi7Mg0.3 foams areproduced by a modi?ed investment casting process. By tuning the mold tem-perature, a change from the usual nearly monocrystalline dendritic to a poly-crystalline globular grain structure is achieved. Tension and compression tests onsingle struts and foam specimens, respectively, are combined with digital imagecorrelation, scanning electron microscopy, and phase contrast-enhancedmicrocomputed tomography in a synchrotron facility to correlate the mechanicalproperties and the failure mechanisms with the microstructure. The “globular”foams exhibit a lower strength and a less pronounced subsequent stress dropthan the “dendritic” foams and the deformation mechanism changes from shearband-dominated failure to a layer-by-layer collapse, because of the lower strengthand higher ductility of the “globular” struts. The “dendritic” struts have a morehomogeneous microstructure, while the “globular” struts often contain siliconagglomerates in their central region. Accordingly, the latter struts exhibit a higherdegree of scatter for the fracture strain. Thus, the arrangement of the siliconparticles and the eutectic determines the mechanical properties on the strut leveland thereby the failure behavior on the foam level.Öğe Single steel strut mechanical testing: challenges and future research directions(Taylor & Francis Ltd, 2019) Kaya, Ali Can; Zaslansky, Paul; Fischer, Sebastian; Fleck, ClaudiaOpen-cell stainless steel foams, composed of hollow struts, are excellent candidates for energy absorption and thermo-mechanical applications. The basic mechanical element responsible for the function of these foams is the single strut. However, testing and characterisation of single foam struts to predict the foam strength have stirred up a new debate about approaches to micro-tensile testing of such elements. In this paper, we present a protocol for in-situ micro-tensile testing of hollow steel struts using a custom-made grip system. The adapted grips make it possible to perform analysis of the deformation of multiple sintered struts. Here we present and discuss challenges encountered during such micro-tensile testing of hollow steel struts.Öğe Strain hardening reduces energy absorption efficiency of austenitic stainless steel foams while porosity does not(Elsevier Sci Ltd, 2018) Kaya, Ali Can; Zaslansky, Paul; İpekoğlu, Mehmet; Fleck, ClaudiaStrain hardening significantly affects the mechanical function of steel foams. We compare hardening and failure of two commercially available austenitic stainless steel foams (316L and 310) spanning strut porosities of 9.4 to 144%. Damage is correlated with strut microstructure and geometry, combining in-situ quasi-static compression testing in the SEM, 3D-evaluation by synchrotron mu CT and bending simulations. We provide an analytical model for the experimentally observed strain hardening. Upon compression, 316L steel foams exhibit a plateau regime of continuously increasing stress due to the hardening effect, whereas 310 steel foams show almost constant plateau stress This is explained by the much less ductile behaviour of the 310 steel foam struts as compared to 316L steel foam struts. Finite element modelling suggests that significant stress concentrations develop around micro porosities in the 310 struts. Due to its finer and less porous microstructure, the 316L foam exhibits a larger energy absorption capacity than the 310 foams. This results in distinctly different efficiency-strain curves. However, up to about 25% strain, the efficiency values are surprisingly similar. Thus, modification of microstructure and/or pore micro-geometry can be used to optimise the stress-strain response to achieve the desired energy absorption property of steel foams. (C) 2018 Elsevier Ltd. All rights reserved.
