Showing papers by "Carolin Körner published in 2021"

Human Umbilical Vein Endothelial Cell Support Bone Formation of Adipose-Derived Stem Cell-Loaded and 3D-Printed Osteogenic Matrices in the Arteriovenous Loop Model

Abstract: Introduction: For the regeneration of large volume tissue defects, the interaction between angiogenesis and osteogenesis is a crucial prerequisite. The surgically induced angiogenesis by means of a...

A multivariate meltpool stability criterion for fabrication of complex geometries in electron beam powder bed fusion

Abstract: Process parameters for manufacturing of complex parts in electron beam powder bed fusion are usually derived from material-specific process windows, which are established by fabrication and evaluation of standardized cuboid specimen. The mechanisms defining low- and high-energy boundaries of the process window are well understood. The boundary emerging at high scan speeds and beam power however, was observed but the underlying mechanism is not further discussed. In addition, appropriate methodologies to transfer process parameters from standardized process windows to complex geometries are not readily available, as the meltpool geometry is significantly affected by the scan length. This work introduces and verifies a characteristic scan length dependent process parameter limit for the fabrication of complex geometries. Electron-optical process monitoring enables the surface characterization for a wide range of process parameter and the subsequent identification of the process parameter limit as a linear function of scan length. A semi-analytical heat conduction model is used to examine the corresponding meltpool geometries. The underlying mechanism is determined as the meltpool stability limit, which occurs, when the aspect ratio of the meltpool reaches the threshold for a liquid film instability. Based on the meltpool geometries for different scan lengths, an analytical relationship for the process parameter limit as a function of scan length is proposed. This relationship may be used as a guideline for the selection of process parameters and the development of new scan strategies for the fabrication of complex geometries.

Electron beam wire cladding of nickel alloys and stainless steel on a reactor pressure vessel steel

Abstract: Claddings made of nickel-base alloys or stainless steel are frequently used in the oil, chemical, and power generation industries to protect low-alloy steels from corrosion. Arc weld overlays, which are commonly deposited, often exhibit microstructures near the fusion line that are susceptible to cracking. Due to its very localized and controlled heat input, electron beam welding has the potential to produce claddings with a more resistant microstructure. In this study, multi-layer nickel-base alloy and stainless steel claddings were fabricated by wire-feed electron beam additive manufacturing. Dilution was high because of the specific wire melting technique. It was found that the degree of dilution had a considerable influence on the precipitation of secondary phases, which, however, did not impair the integrity of the claddings. The influence of dilution on the formation of precipitates could be rationalized with thermodynamic calculations. In the nickel-base alloy claddings, the fusion line was sharp without any interfacial martensite. Only a few potentially detrimental type-II grain boundaries were found. Instead, epitaxial solidification was prevalent. This was attributed to metastable melting that was caused by high heating rates. Thus, electron beam-based manufacturing could produce claddings that are more resistant to disbonding than standard arc weld overlays.

Personalized medicine for reconstruction of critical-size bone defects - a translational approach with customizable vascularized bone tissue.

Abstract: Tissue engineering principles allow the generation of functional tissues for biomedical applications. Reconstruction of large-scale bone defects with tissue-engineered bone has still not entered the clinical routine. In the present study, a bone substitute in combination with mesenchymal stem cells (MSC) and endothelial progenitor cells (EPC) with or without growth factors BMP-2 and VEGF-A was prevascularized by an arteriovenous (AV) loop and transplanted into a critical-size tibia defect in the sheep model. With 3D imaging and immunohistochemistry, we could show that this approach is a feasible and simple alternative to the current clinical therapeutic option. This study serves as proof of concept for using large-scale transplantable, vascularized, and customizable bone, generated in a living organism for the reconstruction of load-bearing bone defects, individually tailored to the patient’s needs. With this approach in personalized medicine for the reconstruction of critical-size bone defects, regeneration of parts of the human body will become possible in the near future.

Isothermal crystallization kinetics of an industrial-grade Zr-based bulk metallic glass

Abstract: Bulk metallic glasses (BMGs), due to their amorphous structure, exhibit remarkable mechanical properties, and there is an increasing interest in their commercialization. For the industrial fabrication of BMG, knowledge about the isothermal crystallization kinetics of industrial-grade BMG is required. Previous investigations on isothermal crystallization kinetics are mainly based on high-purity samples with very good glass forming ability and/or mainly limited to the low temperature regime. In the present study, a systematic investigation on the isothermal crystallization kinetics of an industrial-grade Zr-based BMG (Zr 59.3 Cu 28.8 Al 10.4 Nb 1.5 at.%, trade name: AMZ4) has been performed using conventional and flash differential scanning calorimetry. We report the time-temperature-transformation (TTT) diagrams of the AMZ4 with two different oxygen levels. The diagrams cover the temperature range from glass transition temperature up to liquidus temperature, that have the typical “C-shaped” noses. Faster crystallization of the higher oxygen level AMZ4 was observed, and the underlying mechanisms were investigated. The universal isothermal Johnson-Mehl-Avrami-Kolmogorov (JMAK) model was employed to model the isothermal crystallization kinetics. Satisfactory match was achieved between the experimental facts and the JMAK model, and the interfacial energies between the crystalline phase and liquid were determined as ∼ 0.04 J/ m 2 for the industrial-grade AMZ4. The crystallization fraction dependence of Avrami index and activation energy is studied and found to be neglectable in the JMAK modeling. The critical casting thicknesses were estimated based on the TTT diagrams.

A Single Crystal Process Window for Electron Beam Powder Bed Fusion Additive Manufacturing of a CMSX-4 Type Ni-Based Superalloy.

Abstract: Using suitable scanning strategies, even single crystals can emerge from powder during additive manufacturing. In this paper, a full microstructure map for additive manufacturing of technical single crystals is presented using the conventional single crystal Ni-based superalloy CMSX-4. The correlation between process parameters, melt pool size and shape, as well as single crystal fraction, is investigated through a high number of experiments supported by numerical simulations. Based on these results, a strategy for the fabrication of high fraction single crystals in powder bed fusion additive manufacturing is deduced.

Improving the Effectiveness of the Solid-Solution-Strengthening Elements Mo, Re, Ru and W in Single-Crystalline Nickel-Based Superalloys

Abstract: Tailoring the partitioning behavior of solid solution strengtheners is a crucial design strategy for advanced Ni-based superalloys. The goal of this study was to maximize the enrichment of Mo, Re, Ru and W in the γ-matrix phase. To determine the composition dependency of the partitioning behavior, a set of polycrystalline superalloys with varying contents of these solid solution-strengthening elements and a W-containing single-crystalline alloy series with varying concentrations of the γ′-forming elements Ta and Ti was produced. Assessed properties include phase compositions by electron-probe micro-analysis, phase transformation temperatures by differential scanning calorimetry and creep behavior. Re exhibits the most pronounced enrichment in the γ-matrix, followed by Mo, Ru and W. Due to the preference of the Al-sites in the γ′-phase in the order Ta > Ti > W > Mo > Re, the solid solution strengthening elements Mo, Re and W are displaced from the γ′-phase by increasing Ti and Ta contents. The investigated solutes do not directly influence the partitioning behavior of Ru as it prefers Ni-sites in the γ′-phase. Compressive creep experiments reveal a correlation between the content of solid solution strengtheners in the γ-phase and creep performance.

In-situ electron optical measurement of thermal expansion in electron beam powder bed fusion

Abstract: Thermal expansion plays an important role during additive manufacturing by electron beam powder bed fusion because it strongly affects the quality of the manufactured parts. However, prediction and control of thermal expansion is a difficult task due to the high complexity of part geometries and the corresponding temperature distribution. Therefore, powerful process monitoring tools are required to improve insight into the process. In the current study, the capabilities of in-situ acquisition of electron optical images are assessed with respect to measurement of thermal expansion. For this purpose, a large series of electron-optical images was recorded during cooling down of a PBF-EB build job. The subsequent image analysis revealed that the image quality is high enough to monitor thermal shrinkage of the part and to quantify thermal strain with remarkable accuracy. These findings are very promising and give rise to new applications of in-situ electron-optical observation with regard to temperature management and validation of numerical frameworks.

Thermoelastic properties and γ’-solvus temperatures of single-crystal Ni-base superalloys

Abstract: The present work shows that thermal expansion experiments can be used to measure the γʼ-solvus temperatures of four Ni-base single-crystal superalloys (SX), one with Re and three Re-free variants. In the case of CMSX-4, experimental results are in good agreement with numerical thermodynamic results obtained using ThermoCalc. For three experimental Re-free alloys, the experimental and calculated results are close. Transmission electron microscopy shows that the chemical compositions of the γ- and the γʼ-phases can be reasonably well predicted. We also use resonant ultrasound spectroscopy (RUS) to show how elastic coefficients depend on chemical composition and temperature. The results are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted.

New Grain Formation Mechanisms during Powder Bed Fusion

Abstract: Tailoring the mechanical properties of parts by influencing the solidification conditions is a key topic of powder bed fusion. Depending on the application, single crystalline, columnar, or equiaxed microstructures are desirable. To produce single crystals or equiaxed microstructures, the control of nucleation is of outstanding importance. Either it should be avoided or provoked. There are also applications, such as turbine blades, where both microstructures at different locations are required. Here, we investigate nucleation at the melt-pool border during the remelting of CMSX-4® samples built using powder bed fusion. We studied the difference between remelting as-built and homogenized microstructures. We identified two new mechanisms that led to grain formation at the beginning of solidification. Both mechanisms involved a change in the solidification microstructure from the former remelted and newly forming material. For the as-built samples, a discrepancy between the former and new dendrite arm spacing led to increased interdentritic undercooling at the beginning of solidification. For the heat-treated samples, the collapse of a planar front led to new grains. To identify these mechanisms, we conducted experimental and numerical investigations. The identification of such mechanisms during powder bed fusion is a fundamental prerequisite to controlling the solidification conditions to produce single crystalline and equiaxed microstructures.

A Novel Approach to Predict the Process-Induced Mechanical Behavior of Additively Manufactured Materials

Abstract: The grain structure and texture of additively manufactured materials depend strongly on the local temperature gradients during the solidification of the material. These grain structures and textures influence the mechanical behavior, ranging from isotropy to transversal and orthotropic symmetry. In the present contribution, a cellular automaton is used to model the grain growth during selective electron beam melting. The resulting grain structures and textures serve as input for a mesoscopic mechanical model. The mechanical behavior on the mesoscale is modeled by means of gradient-enhanced crystal plasticity, applying the finite element method. Computational homogenization is applied to determine the resulting macroscopic elastic and plastic properties of the additively manufactured metals. A general orthotropic yield criterion is identified by means of the initial yield loci computed with mesoscopic simulations of representative volume elements. The numerical results are partly validated with experimental data.

Electron-Optical In Situ Imaging for the Assessment of Accuracy in Electron Beam Powder Bed Fusion

Abstract: The current study evaluates the capabilities of electron-optical (ELO) in situ imaging with respect to monitoring and prediction of manufacturing precision in electron beam powder bed fusion. Post-process X-ray computed tomography of two different as-built parts is used to quantitatively evaluate the accuracy and limitations of ELO imaging. Additionally, a thermodynamic simulation is performed to improve the understanding of ELO data and to assess the feasibility of predicting dimensional accuracy numerically. It is demonstrated that ELO imaging captures the molten layers accurately (deviations <100 μm) and indicates the creation of surface roughness. However, some geometrical features of the as-built parts exhibit local inaccuracies associated with thermal stress-induced deformation (deviations up to 500 μm) which cannot be captured by ELO imaging. It is shown that the comparison between in situ and post-process data enables a quantification of these effects which might provide the possibility for developing effective countermeasures in the future.

Numerical Alloy Development for Additive Manufacturing towards Reduced Cracking Susceptibility

Abstract: In this work, we investigated the viability of established hot cracking models for numerically based development of crack-resistant nickel-base superalloys with a high γ′ volume fraction for additive manufacturing. Four cracking models were implemented, and one alloy designed for reduced cracking susceptibility was deduced based on each cracking criterion. The criteria were modeled using CALPHAD-based Scheil calculations. The alloys were designed using a previously developed multi-criteria optimization tool. The commercial superalloy Mar-M247 was chosen as the reference material. The alloys were fabricated by arc melting, then remelted with laser and electron beam, and the cracking was assessed. After electron beam melting, solidification cracks were more prevalent than cold cracks, and vice versa. The alloys exhibited vastly different crack densities ranging from 0 to nearly 12 mm−1. DSC measurements showed good qualitative agreement with the calculated transition temperatures. It was found that the cracking mechanisms differed strongly depending on the process temperature. A correlation analysis of the measured crack densities and the modeled cracking susceptibilities showed no clear positive correlation for any crack model, indicating that none of these models alone is sufficient to describe the cracking behavior of the alloys. One experimental alloy showed an improved cracking resistance during electron beam melting, suggesting that further development of the optimization-based alloy design approach could lead to the discovery of new crack-resistant superalloys.

Watching the Vessels Grow: Establishment of Intravital Microscopy in the Arteriovenous Loop Rat Model.

Abstract: Tissue engineering in reconstructive surgery seeks to generate bioartificial tissue substitutes. The arteriovenous (AV) loop allows the generation of axially vascularized tissue constructs. Cellula...

Free Transplantation of a Tissue Engineered Bone Graft into an Irradiated, Critical-Size Femoral Defect in Rats.

Abstract: Healing of large bone defects remains a challenge in reconstructive surgery, especially with impaired healing potential due to severe trauma, infection or irradiation. In vivo studies are often performed in healthy animals, which might not accurately reflect the situation in clinical cases. In the present study, we successfully combined a critical-sized femoral defect model with an ionizing radiation protocol in rats. To support bone healing, tissue-engineered constructs were transferred into the defect after ectopic preossification and prevascularization. The combination of SiHA, MSCs and BMP-2 resulted in the significant ectopic formation of bone tissue, which can easily be transferred by means of our custom-made titanium chamber. Implanted osteogenic MSCs survived in vivo for a total of 18 weeks. The use of SiHA alone did not lead to bone formation after ectopic implantation. Analysis of gene expression showed early osteoblast differentiation and a hypoxic and inflammatory environment in implanted constructs. Irradiation led to impaired bone healing, decreased vascularization and lower short-term survival of implanted cells. We conclude that our model is highly valuable for the investigation of bone healing and tissue engineering in pre-damaged tissue and that healing of bone defects can be substantially supported by combining SiHA, MSCs and BMP-2.

Comparison of Transmission Measurement Methods of Elastic Waves in Phononic Band Gap Materials.

Abstract: Periodic cellular structures can exhibit metamaterial properties, such as phononic band gaps. In order to detect these frequency bands of strong wave attenuation experimentally, several devices for wave excitation and measurement can be applied. In this work, piezoelectric transducers are utilized to excite two additively manufactured three-dimensional cellular structures. For the measurement of the transmission factor, we compare two methods. First, the transmitted waves are measured with the same kind of piezoelectric transducer. Second, a laser Doppler vibrometer is employed to scan the mechanical vibrations of the sample on both the emitting and receiving surfaces. The additional comparison of two different methods of spatial averaging of the vibrometer data, that is, the quadratic mean and arithmetic mean, provides insight into the way the piezoelectric transducers convert the transmitted signal. Experimental results are supported by numerical simulations of the dispersion relation and a simplified transmission simulation.

Secondary Recrystallization of Nickel-Base Superalloy CM 247 LC After Processing by Metal Injection Molding

Abstract: Fabrication of parts by metal injection molding (MIM) results in very fine grain sizes. In the present investigation, heat treatments were employed to gain a larger grain size that is more creep resistant. The alloy under investigation was CM 247 LC. The composition was slightly modified to facilitate grain growth. Secondary recrystallization was observed to occur during post-sintering annealing treatments approximately 50 °C below the sintering temperature. The grain size increased from 25 µm to > 2 mm. The increase of grain size was found to improve creep strength significantly. The samples in the as-sintered condition exhibited a bimodal grain size distribution. The grain size in a thin surface zone after sintering was slightly increased because of a lower C and O content in this zone that promoted normal grain growth. Secondary recrystallization did not occur in the surface zone. This is attributed to a lack of driving force in this area.