Showing papers by "Carolin Körner published in 2022"

Actual state-of-the-art of electron beam powder bed fusion

Abstract: Abstract As a relatively young additive manufacturing technology, Electron Beam Powder Bed Fusion (EB-PBF) attracts increasing attention in academics and industry. Especially, the last five years have seen an explosion in machine and process development as well as material research for EB-PBF. Compared to other additive manufacturing approaches, e.g., the more widely used laser powder bed fusion, EB-PBF shows various unique features and advantages for processing high-performance metallic components. In this article, recent advancements in the realm of EB-PBF over the past five years are reviewed.

Very high cycle fatigue durability of an additively manufactured single-crystal Ni-based superalloy

Abstract: A single crystalline (SX) nickel-based superalloy additively manufactured (AM) by electron beam-based powder bed fusion (PBF-E) was investigated under very high cycle fatigue (VHCF) at 1,000 °C in fully reversed conditions (Rε = −1). Specimens processed using a classical Bridgman solidification route and the impact of a hot isostatic pressing (HIP) treatment were also considered. It is shown that the fatigue lifetime of the AM specimens is higher or in the same range of the Bridgman processed ones with the same chemical composition. All defect-free AM samples fail by surface initiation with very long VHCF lives. In the absence of metallurgical defects such as grain boundaries or pores, the superalloy chemical stability against oxidation governs VHCF failure.

Surface topographies from electron optical images in electron beam powder bed fusion for process monitoring and control

Abstract: Electron optical imaging is a process monitoring method in electron beam powder bed fusion, which enables porosity detection in situ for every layer of a part. The two main defect types occurring in a build process are porosity, due to insufficient local energy input, and surface bulging, when it is too high. This work adds the ability to measure surface topographies quantitatively and in situ to electron optical imaging. To this end, electron optical images are recorded with a four detector system from melt surfaces fabricated on a Ti–6Al–4V plate. A complete computation chain is proposed, based on a developed model of the imaging process. The model explains the correction of image distortions and the calibrated calculation of the gradient information of the respective surfaces. It becomes possible to reconstruct height maps of melt surfaces by the usage of a suitable normal integration algorithm. The computation chain is validated by the comparison of the resulting reconstructions with laser scanning microscope measurements. Electron optical imaging is able to measure porosity and bulging simultaneously, which allows to gain important process information for process control and the future implementation of feedback control loops.

Impact of the Power-Dependent Beam Diameter during Electron Beam Additive Manufacturing: A Case Study with γ-TiAl

Abstract: The development of process parameters for electron beam powder bed fusion (PBF-EB) is usually made with simple geometries and uniform scan lengths. The transfer to complex parts with various scan lengths can be achieved by adapting beam parameters such as beam power and scan speed. Under ideal conditions, this adaption results in a constant energy input into the powder bed despite of the local scan length. However, numerous PBF-EB machines show deviations from the ideal situation because the beam diameter is subject to significant changes if the beam power is changed. This study aims to demonstrate typical scaling issues when applying process parameters to scan lengths up to 45 mm using a fourth generation γ-TiAl alloy. Line energy, area energy, return time, and lateral velocity are kept constant during the additive manufacturing process by adjusting beam power and beam velocity to various scan lengths. Samples produced in this way are examined by light microscopy regarding lateral melt pool extension, melt pool depth, porosity, and microstructure. The process-induced aluminum evaporation is measured by electron probe microanalysis. The experiments reveal undesired changes in melt pool geometry, gas porosity, and aluminum evaporation by increasing the beam power. In detail, beam widening is identified as the reason for the change in melt pool dimensions and microstructure. This finding is supported by numerical calculations from a semi-analytic heat conduction model. This study demonstrates that in-depth knowledge of the electron beam diameter is required to thoroughly control the PBF-EB process, especially when scaling process parameters from simply shaped geometries to complex parts with various scan lengths.

Microvascular development in the rat arteriovenous loop model in vivo-A step by step intravital microscopy analysis.

Abstract: The arteriovenous (AV) loop model is a key technique to solve one of the major problems of tissue engineering-providing adequate vascular support for a tissue construct of significant size. However, the molecular and cellular mechanisms of vascularization and factors influencing the generation of new tissue in the AV loop are still poorly understood. We previously established a novel intravital microscopy approach to study these events. In this study, we implanted our observation chamber filled with two types of hydrogels such as fibrin and methacrylate gelatin (GelMA) and performed intravital microscopy (IVM) on days 7, 14, and 21. Initial microvessel formation was observed in GelMA on day 14, while the vessel network showed clear indicators of network rearrangement and maturation on day 21. No visible microvessels were observed in fibrin. The chambers were explanted on day 21. Histological examination revealed higher numbers of microvessels in GelMA compared to fibrin, while the AV loop was thrombosed in all fibrin constructs, possibly due to matrix degradation. GelMA proved to be an ideal matrix for IVM studies in the AV loop model due to its slow degradation and transparency. This IVM model can be employed as a novel tool for live and thus faster comprehension of crucial events in the tissue regeneration process, which can improve tissue engineering application.

Evolution of an industrial-grade Zr-based bulk metallic glass during multiple laser beam melting

Abstract: • The crystallization behavior of a Zr-based bulk metallic glass - AMZ4 - during multiple laser beam melting is systematically investigated. • A better understanding of the changes in the microstructure, composition, and micromechanical properties of AMZ4 during laser beam processing. • The effect of oxygen content on the laser beam-induced crystallization behavior of AMZ4 is revealed. Selective laser melting (SLM), taking advantage of its inherent rapid cooling rates and near-net-shape forming ability, has been employed to fabricate bulk metallic glasses (BMGs). However, crystallization is frequently triggered during the SLM process, which results in the loss of advantageous properties of BMGs, such as extremely high hardness and near-theoretical yield strength. Although many studies have been conducted to investigate SLM of BMGs, there is still a lack of knowledge about the microstructural and compositional evolution during the laser beam processing, particularly the micromechanical property response upon crystallization. In the present work, a systematic investigation is performed to gain a much better understanding about the evolution of microstructure and composition as well as the corresponding micromechanical property change during multiple laser beam melting. The material used in this study is an industrial-grade Zr-based BMG Zr 59.3 Cu 28.8 Al 10.4 Nb 1.5 (AMZ4) with two different oxygen levels. AMZ4 demonstrates its good thermal stability by the fact that observable crystalline structure appears around the melt pool only after more than once laser beam treatment. The compositional stability of AMZ4 is manifested by the homogeneous elemental distribution on the melt pool area after even twenty-five laser beam remelting. The laser-metal interaction, melting and subsequent solidification are not effectively influenced by the emerging and expanding of crystallization zone (or heat affected zone, HAZ). Higher oxygen content results in not only a larger HAZ but also more quenched-in nuclei at the melt pool bottom. The HAZ does not exhibit a fully crystallized structure, but rather has a mixture of amorphous and crystalline phases. Crystallization of AMZ4 leads to an increase in hardness and Young’s modulus of the material.

Predictive simulation of bulk metallic glass crystallization during laser powder bed fusion

Abstract: Laser powder bed fusion (L-PBF) has been employed to fabricate bulk metallic glass (BMG) parts. However, traditional experimental trial-and-error methods to determine process parameters for specific materials and L-PBF machines are time-consuming and expensive. In this paper, a phenomenological crystallization model, namely the Nakamura model, is coupled with L-PBF process simulation. A convenient approach for the crystallization parameter determination and a two-step Euler method for the numerical implementation has been developed. Numerical simulations are performed using the material parameters of a Zr-based BMG Zr59.3Cu28.8Al10.4Nb1.5 (at.%, trade name: AMZ4). The numerical results are validated by comparing with experimental results from different perspectives. Based on the numerical findings, a comprehensive understanding of BMG crystallization behavior during L-PBF is gained. In the end, the crystallization model is implemented in our in-house developed software SAMPLE2D. SAMPLE2D simulation results are presented, whereby the L-PBF process window for fully amorphous AMZ4 parts is explored. Thereby, it is believed that the developed numerical software can be applied to aid process development for BMGs by taking the crystallization phenomenon into account.

A novel approach for powder bed-based additive manufacturing of compositionally graded composites

Abstract: In this feasibility study, a novel selective powder raking approach is used for Electron Beam Powder Bed Fusion (EB-PBF) to produce fully dense and compositionally graded composites. The adaption of the chemical composition along the thickness direction of the printed parts is based on the application of a powder blend composed of powders with different particle sizes combined with varying layer thicknesses during EB-PBF. The microstructure of EB-PBF-processed samples with a composition gradient is investigated. The distribution of different phases in the as-built samples is discussed. Based on the results derived in this work, this approach can be easily extended to other powder bed-based AM technologies as well as to other material systems to produce functionally graded materials.

Microstructure analysis and mechanical properties of electron beam powder bed fusion (PBF-EB)-manufactured γ-titanium aluminide (TiAl) at elevated temperatures

Abstract: Abstract Additively manufactured γ-titanium aluminide has a high specific strength and temperature resistance. This opens new possibilities for future lightweight constructions for aerospace applications. The objective of this work was to characterize additively manufactured Ti–48Al–2Cr–2Nb alloy specimens, which were successfully manufactured by electron beam powder bed fusion. For microstructural characterization, the as-built state was investigated with light and scanning electron microscopy. In the electron backscatter diffraction analysis, the size and the orientation of the grains were observed. The pore size and distribution were examined in computer tomographic scans, which showed a near fully dense material with a relative density of >99.9%. Furthermore, the hardness curve over the building height was examined in hardness mappings. Thereby, a strong decrease in hardness could be observed with an increase in part height. To evaluate the reliability of the manufactured alloy, quasi-static compression tests were carried out at temperatures up to 650 °C. Within these tests, a high compression strength (σc,p,0.2,650 °C = 684 MPa) was determined, which implicated a potential substitution of nickel-based superalloy components in aerospace applications under compressive loads.

Revealing dynamic processes in laser powder bed fusion with in situ X-ray diffraction at PETRA III.

Abstract: The high flux combined with the high energy of the monochromatic synchrotron radiation available at modern synchrotron facilities offers vast possibilities for fundamental research on metal processing technologies. Especially in the case of laser powder bed fusion (LPBF), an additive manufacturing technology for the manufacturing of complex-shaped metallic parts, in situ methods are necessary to understand the highly dynamic thermal, mechanical, and metallurgical processes involved in the creation of the parts. At PETRA III, Deutsches Elektronen-Synchrotron, a customized LPBF system featuring all essential functions of an industrial LPBF system, is used for in situ x-ray diffraction research. Three use cases with different experimental setups and research questions are presented to demonstrate research opportunities. First, the influence of substrate pre-heating and a complex scan pattern on the strain and internal stress progression during the manufacturing of Inconel 625 parts is investigated. Second, a study on the nickel-base superalloy CMSX-4 reveals the formation and dissolution of γ' precipitates depending on the scan pattern in different part locations. Third, phase transitions during melting and solidification of an intermetallic γ-TiAl based alloy are examined, and the advantages of using thin platelet-shaped specimens to resolve the phase components are discussed. The presented cases give an overview of in situ x-ray diffraction experiments at PETRA III for research on the LPBF technology and provide information on specific experimental procedures.

Basic Mechanism of Surface Topography Evolution in Electron Beam Based Additive Manufacturing

Abstract: This study introduces and verifies a basic mechanism of surface topography evolution in electron beam additive manufacturing (E-PBF). A semi-analytical heat conduction model is used to examine the spatio-temporal evolution of the meltpool and segment the build surface according to the emerging persistent meltpool domains. Each persistent domain is directly compared with the corresponding melt surface, and exhibits a characteristic surface morphology and topography. The proposed underlying mechanism of topography evolution is based on different forms of material transport in each distinct persistent domain, driven by evaporation and thermocapillary convection along the temperature gradient of the emerging meltpool. This effect is shown to be responsible for the upper bound of the standard process window in E-PBF, where surface bulges form. Based on this mechanism, process strategies to prevent the formation of surface bulges for complex geometries are proposed.

Laser powder bed fusion of FeCoBSiNb-Cu bulk metallic glass composites: Processing, microstructure and mechanical properties

Abstract: Fe–Co–B–Si–Nb bulk metallic glasses are prone to the formation of micro-cracks during laser powder bed fusion (LPBF) additive manufacturing. Introducing ductile Cu into Fe–Co–B–Si–Nb bulk metallic glass alloy system rich with Fe and Co could be a promising solution to reduce or even eliminate micro-cracks since there is almost no miscibility of Cu in Fe and Co. In this work, processing, microstructure and mechanical properties of Cu-containing {(Fe 0 . 6 Co 0.4 ) 0.75 B 0 . 2 Si 0.05 } 96 Nb 4 (at.%) bulk metallic glass composites (BMGCs) under different laser power and scanning speed by LPBF are investigated in detail. A moderate area energy density facilitates the fabrication of highly dense (relative density: up to ∼99.5%) and almost crack-free (crack density: ∼0.1 mm −1 ) FeCoBSiNb–Cu bulk samples. These bulk samples possess an interpenetrating composite microstructure mainly composed of an amorphous Fe(Co)-rich phase and a crystalline Cu-rich phase. A crystalline Cu-rich phase is distributed relatively homogeneously in an amorphous phase and helps to reduce and even eliminate micro-crack formation during LPBF processing. Also, the Cu-rich phase has the effect of a heat sink and thus speeds up heat dissipation, facilitating the formation of an amorphous phase. The microhardness of the ductile Cu-rich phase and the hard amorphous Fe(Co)-rich phase in the interpenetrating composite microstructure was evaluated. Compressive tests at room temperature indicate that the dense bulk samples exhibit a relatively high fracture strength and a large fracture strain. Additionally, bulk samples with lack-of-fusion pores show a larger fracture strain although their fracture strength is lower than that of dense bulk samples. It is concluded that the LPBF-processed FeCoBSiNb–Cu BMGCs constitute a promising wear-resistant material to be used for example as bearing materials. • Dense and almost crack-free Cu-containing Fe–Co–B–Si–Nb bulk metallic glass composites (BMGCs) are prepared successfully under a moderate area energy density using laser powder bed fusion (LPBF) additive manufacturing. • An interpenetrating composite microstructure mainly composed of an amorphous Fe(Co)-rich phase and a crystalline Cu-rich phase is observed. • An amorphous fraction of ∼80% in the Fe(Co)-rich region is retained and attributed to a heat sink effect of the Cu-rich phase although a high area energy density can cause partial crystallization in the amorphous phase. • Dense and almost crack-free FeCoBSiNb–Cu BMGCs exhibit a balanced mechanical property profile including a relatively high fracture strength and a good plasticity. • Compared with dense and almost crack-free bulk samples, porous FeCoBSiNb–Cu BMGCs under a low area energy density by LPBF show an improved compressive plasticity despite partial decrease of their fracture strength.

Impact of Endothelial Progenitor Cells in the Vascularization of Osteogenic Scaffolds

Abstract: The microvascular endothelial network plays an important role in osteogenesis, bone regeneration and bone tissue engineering. Endothelial progenitor cells (EPCs) display a high angiogenic and vasculogenic potential. The endothelialization of scaffolds with endothelial progenitor cells supports vascularization and tissue formation. In addition, EPCs enhance the osteogenic differentiation and bone formation of mesenchymal stem cells (MSCs). This study aimed to investigate the impact of EPCs on vascularization and bone formation of a hydroxyapatite (HA) and beta-tricalcium phosphate (ß-TCP)–fibrin scaffold. Three groups were designed: a scaffold-only group (A), a scaffold and EPC group (B), and a scaffold and EPC/MSC group (C). The HA/ß–TCP–fibrin scaffolds were placed in a porous titanium chamber permitting extrinsic vascularization from the surrounding tissue. Additionally, intrinsic vascularization was achieved by means of an arteriovenous loop (AV loop). After 12 weeks, the specimens were explanted and investigated by histology and CT. We were able to prove a strong scaffold vascularization in all groups. No differences regarding the vessel number and density were detected between the groups. Moreover, we were able to prove bone formation in the coimplantation group. Taken together, the AV loop is a powerful tool for vascularization which is independent from scaffold cellularization with endothelial progenitor cells’ prior implantation.

Correlation of powder degradation, energy absorption and gas pore formation in laser-based powder bed fusion process of AlSi10Mg0.4

Abstract: Gas pore formation observed during laser-based powder bed fusion is yet not fully understood. This study is a comprehensive investigation of the mechanisms of hydrogen pore formation in the powder bed fusion process of AlSi10Mg0.4 via laser beam. The influence of varying process parameters such as hatch distance, layer thickness and build plate temperature as well as the impact of virgin and long-term reused powder on part porosity and melt pool characteristics is investigated in detail. A novel, very efficient method to characterize powder aging by the correlation of powder gray value measurements with the oxygen content is presented. The relation between powder aging and the change of the absorption coefficient reveals the origin of gas formation as being intimately related to the energy input. Vacuum heat treatment of as-built parts indicates the potential of hydrogen removal in a postprocessing step with the prospect of welding additively manufactured aluminum parts.

3D-Printed Raney-Cu POCS as Promising New Catalysts for Methanol Synthesis

Abstract: Simultaneous generation and activation of Raney-type periodic open cellular structures (POCS) is a highly promising approach for generating novel structured methanol synthesis catalysts. In detail, we produced stable and highly active POCS from a Cu50Al50 alloy by additive manufacturing via Powder Bed Fusion by Electron Beam (PBF-EB) and activated them via selective leaching of aluminum in a sodium hydroxide/sodium zincate solution. The Raney-type Cu structures possessed catalytic methanol productivities of up to 2.2 gMeOHgnp-Cu h−1 (PBF-EB sticks) and 1.9 gMeOHgnp-Cu h−1 (PBF-EB POCS), respectively. Moreover, it was found that besides the nanoporous layer thickness, an optimum Zn/Cu ratio of 0.3–0.4 can also by adjusted by the leaching conditions.

Miniature mechanical testing of LMD-fabricated compositionally & microstructurally graded γ titanium aluminides

Abstract: The design freedom in Laser Metal Deposition provided by the absence of a powder bed enables the fabrication of Functionally Graded Materials through Additive Manufacturing. For the first time, two suitable γ-TiAl alloys (TiAl48Cr2Nb2, TiAl45Nb4C) are combined in direct and gradual transitions to generate different microstructure morphologies and, consequently, different mechanical properties within a component after an identical heat treatment. The influence of alloy composition, microstructure type, and material transition on the tensile properties and fracture toughness is analyzed through miniature testing. Miniature tensile tests show no orientation dependency in regard to the build direction and the composition/microstructure transition is not found to be a preferred fracture site. The miniature fracture toughness tests reveal that already small composition changes—insufficient to alter the microstructure configuration—can have a significant effect on the cracking behavior. Graphical abstract

Numerical Design of CoNi-Base Superalloys With Improved Casting Structure

Abstract: Abstract Numerical methods can accelerate the design of alloys with improved material properties. One approach is the coupling of multi-criteria optimization with CALPHAD-based models of alloy properties. While this technique has already yielded promising new Nickel-base superalloys, the applicability to CoNi-base alloys has not yet been investigated. These alloys show promising properties for application as wrought high-temperature materials. We designed three CoNi-base superalloys, which were optimized for either high strength or high chemical homogeneity. The alloys were cast, and mechanical and thermophysical properties were characterized. The alloy optimized for strength showed creep performance inferior to a conventionally designed CoNi-alloy but had a much lower density. For developing highly homogeneous alloys, Scheil calculations were implemented in the optimization routine to quantify the severity of segregation. Non-equilibrium phases could be predicted successfully, resulting in a degree of homogeneity that rivaled that of a low-segregation ternary Co-base alloy. A comparison of elemental partitioning behavior and phase transition temperatures with CALPHAD calculations showed that trends are well represented for the most part. Finally, the applicability of the alloy design approach for Co-rich superalloys is evaluated, and possible applications for the optimized alloys are discussed.

Correlative Micro-Compression and 3D X-ray Nanotomography Study of the Fracture Behavior of TCP Phases in an Additively Manufactured Ni-Base Superalloy

Abstract: 1. Institute of Microand Nanostructure Research (IMN) & Center for Nanoanalysis and Electron Microscopy (CENEM), Interdisciplinary Center for Nanostructured Films (IZNF), Department of Materials Science and Engineering, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Erlangen, Bavaria, Germany. 2. Materials Science & Engineering Institute I, Interdisciplinary Center for Nanostructured Films (IZNF), Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Erlangen, Bavaria, Germany. 3. Chair of Materials Science and Engineering for Metals (WTM), Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Erlangen, Bavaria, Germany. * Corresponding authors: michi.sommerschuh@fau.de, erdmann.spiecker@fau.de