Lehrstuhl für Werkstoffprüftechnik

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Mechanistic machine learning for metamaterial fatigue strength design from first principles in additive manufacturing
(2024-04-08) Awd, Mustafa; Saeed, Lobna; Münstermann, Sebastian; Faes, Matthias; Walther, Frank
Digital control in manufacturing processes produces significant amounts of metadata. The production process metadata, such as thermal and optical measurements, enables a higher degree of property grading than uninstrumented manufacturing and feedback for fault detection. This study explores how metadata can design fatigue-resistant structures using physically grounded models such as density functional theory, cyclic plasticity, and fracture mechanics that train machine learning algorithms. Machine learning models work very efficiently in their trained physical space. In comparison, mechanistic models are computationally costly for complex phenomena such as fatigue. We show how fatigue can be administered consistently at all scales by energy-based criteria and how a mechanistic function can be built based on this concept. The energy mechanistic function allows exact quantification of the effect of the existing flaws from manufacturing on fatigue lifetime under certain load boundary conditions. Since the mechanistic function is local and subscale to the prediction scale of the machine learning model, it can be used to build density functions for probabilistic regression of the fatigue property on the scale above. The analysis is applied to the selective laser melting process due to the availability of digital control and metadata generation during deposition.
Fatigue and short crack assessment of powder bed fusion laser-based fabricated AlSi10Mg miniature specimens under alternating bending load
(2024-10-28) Stammkötter, Sebastian; Tenkamp, Jochen; Teschke, Mirko; Donnerbauer, Kai; Koch, Alexander; Platt, Timo; Biermann, Dirk; Walther, Frank
Al-Si alloys are commonly used in the automotive and aircraft industry because of their excellent strength-to-weight ratio. Due to the laser powder bed fusion manufacturing process, inhomogeneous cooling affects the microstructure as well as defect distributions. Within this paper, the uniform fatigue damage tolerance assessment was further qualified for (miniature) bending specimens with different loaded volumes based on the concepts according to Murakami (√area) and Shiozawa for an initial defect-based model. These approaches were used to calculate defect-related fatigue life curves, in which the cyclic stress intensity factor (ΔK) at the initiating defect (√area) was used to represent local stress concentration at the crack tip instead of nominal stress-based S-N curves. Results of S-N curves did not allow a precise lifetime prediction due to increasing effect of manufacturing-related defect distributions, while fracture mechanical approaches enable a uniform fatigue lifetime description of different testing volumes. The calculated fatigue limit and short crack threshold value suggested by Noguchi based on the extended approach of Murakami need to be compared and validated experimentally. Furthermore, the effects of miniaturization and crack propagation have been identified and considered. Uniform fatigue life predictions and efficient materials testing have been combined and show potential for future research.
Scale-bridging micromagnetic evaluation of surface integrity and conditioning mechanisms in deep hole drilled AISI 4140 steel
(2024) Strodick, Simon; Walther, Frank; Biermann, Dirk
In the face of global warming, modern production engineering is confronted with the pressing challenge of developing methods for a more sustainable production. A key strategy in manufacturing with vast potential for climate change mitigation is to improve material and resource efficiency by enhancing the surface integrity of components through adapted manufacturing processes. As numerous surface and subsurface properties are closely linked to performance, tailoring surface integrity enables the production of components with significantly improved capability, reliability and lifetime. This general connection between the design of machining processes, the resulting surface integrity of components and their performance also applies to the field of deep hole drilling. A widely employed process for machining large diameter bores with high length-to-diameter ratios is Boring Trepanning Association (BTA) deep hole drilling. This process is commonly used in fields like aerospace engineering or petroleum industry to machine components, such as landing gears, drill collars, or hydraulic cylinders. A major obstacle in assessing the surface integrity at the critical inner surface of deep drilled components is the hindered accessibility of the borehole wall, as it complicates or fully prevents the use of many conventional approaches, such as X-ray diffraction. In addition to this, some of the most common methods for assessing aspects of surface integrity like microhardness testing or optical microscopy of cross-sections are either fully-destructive or semi-destructive and thus their application is restricted to a limited amount of spot checks, while damaging the workpiece. In contrast, magnetic Barkhausen noise (MBN) analysis is an entirely nondestructive technique, based on relatively compact sensors that can be positioned inside of deep drilled boreholes. Consequently, this method is a particularly promising approach for a holistic evaluation of surface integrity in ferromagnetic deep drilled components. On this account, this thesis aims at qualifying MBN for the nondestructive, efficient, and reliable assessment of the surface integrity in deep drilled components. Since MBN is affected by multiple aspects of surface integrity which overlap and interfere, the mechanisms governing the MBN analysis need to be elucidated and separated. Therefore, in a first step, a thorough evaluation of surface integrity in BTA deep hole drilling is performed. This includes interconnected analyses of the microstructure, the microhardness gradients and the residual stress states in subsurfaces of deep drilled specimens. To analyze the impact of cutting parameters on surface integrity, the cutting speed and the feed in drilling are varied. It is observed that white etching layers (WEL) form at the surface of bores, when using relatively high cutting speeds and feed rates. These WEL bring about a significant increase in hardness. For instance, it is found that the microhardness inside the WEL can exceed the microhardness of the bulk material by more than three times. To investigate further into the microstructural evolution in the subsurface during deep hole drilling, electron backscatter diffraction analyses are performed. The results indicate a strong refinement of the grains resulting from severe plastic deformation as well as swept grains in the material adjacent to the WEL. As the nanocrystalline structures observed are close to the limit of the lateral resolution in electron backscatter diffraction, transmission Kikuchi diffraction is carried out additionally to validate the findings obtained by electron backscatter diffraction. In addition to this, electron channeling contrast imaging and scanning transmission electron microscopy are used to evaluate grain size gradients, grain morphologies, voids, and lattice defects such as dislocation structures and stacking faults. Building on these fundamental findings on surface integrity, scale-bridging micromagnetic approaches are combined in the next step, to identify interrelations between the different aspects of surface integrity and the micromagnetic properties. The evolution of the magnetic microstructures in external magnetic fields is evaluated by magneto-optic Kerr effect microscopy. In addition to this, magnetic domain structures are evaluated by the Bitter technique. Magnetic force microscopy is employed for an analysis of the magnetic microstructure with particularly high spatial resolution. Inside of white etching layers, no distinct magnetic domains are observed by any of the approaches. In specimens free of WEL, domain patterns are found, which expand to the very surface. These multi-instrumental investigations provide the scientific basis for the application of MBN. In MBN analysis, it is observed that WEL lead to significantly lower maximum MBN amplitudes, due to the inherent structure and properties of WEL. The high number of dislocations, stacking faults and grain boundaries inside the WEL, evidenced by the microstructural analyses, all act as pinning sites for domain wall motion. Based on these observations, it is assumed that the WEL barely contributes to the generation of Barkhausen noise in MBN analysis and that this results in the particularly low MBN amplitudes for specimens with WEL. Based on these findings, a multi-sensory MBN-based test stand is developed, which allows for the rapid characterization of surface integrity in BTA deep drilled components. It is used to compare the results of different systems for MBN analysis, employing both multipurpose sensors and a sensor designed to fit a drill head for BTA deep hole drilling. To analyze the impact of WEL on the mechanical properties of deep drilled components a procedure is developed, inspired by the tube flattening test according to ISO 8492. Results are provided for the compression of components when applying quasi-static and cyclic loads.
Validating the performance of direct fastening (PAF) into concrete with high-speed measuring technology
(2023-09-25) Scholz, Ronja; Franck, Pascal; Yousef, Alhussain; Spyridis, Panagiotis; Walther, Frank
In recent years, attention has been driven to the direct fastening technology by use of power actuated nail-type fasteners (PAFs) set in concrete. By means of the series of tests shown in this paper and the high-speed measuring equipment used, a first approach to the possibility of evaluating the setting performance is demonstrated. The difficulty is to judge whether the operation was successful and whether the fastening can bear the required force, since it is not possible to look into the concrete. For example, it remains hidden whether and, if so, how much the nail was deflected and bent by the concrete. Furthermore, it is unknown how the surrounding concrete was affected by occurring strains and heating as a result of friction. Since the setting process is very fast, it is necessary to use high-speed measurement technology. In the case of the studies conducted, both digital image correlation (DIC) and thermography were used to record material responses. Following the setting process, the nails and the surrounding concrete were separated and examined with computer tomography (CT). The CT provides the information on the inside of the concrete and one can see cracks and other reactions to the setting process.
In vitro-Kurzzeit-Prüfmethode zur Prädiktion des Langzeit-Eigenschaftsprofils von Magnesiumimplantaten
(2024) Wegner, Nils; Walther, Frank; Maier, Petra
Infolge des demografischen Wandels in der westlichen Welt und einer immer aktiver werdenden Gesellschaft steigt der Einsatz von Biomaterialien und damit die Anforderungen an deren Leistungsfähigkeit, Effizienz und Kosten. Die gängigen permanenten Implantatmaterialien können dabei die Anforderungen bei einem temporären Einsatz im menschlichen Körper nicht erfüllen, sodass für derartige Anwendungen zunehmend an biodegradierbaren Metallen geforscht wird. Diese ermöglichen eine zeitlich begrenzte Unterstützung der menschlichen Körperfunktion und korrodieren aufgrund der geringen Beständigkeit unterdessen kontinuierlich, sodass nach einer vollständigen Auflösung eine Zweitoperation zur Entfernung und die einhergehenden Risiken und Kosten obsolet werden. In diesem Feld fokussieren sich die Forschungsaktivitäten auf Magnesiumsysteme, die aufgrund der mechanischen und biokompatiblen Eigenschaften sowie des natürlichen Vorkommens im menschlichen Körper eine Reihe an Vorteilen mit sich bringen. Dennoch erfüllen viele Magnesiumlegierungen nicht die hohen Anforderungen in Bezug auf eine geringe Korrosionsrate mit einer einheitlichen Korrosionsmorphologie zur Einhaltung einer ausreichenden Lebensdauer, um im Rahmen der Funktionsphase des Implantats dem heilenden Knochen ausreichend Zeit zu geben. Im Bereich der in vitro-Prüfmethoden zur Prädiktion dieser Langzeit-Eigenschaften besteht eine Lücke zum Ausschluss ungeeigneter neuer Materialien vor zeit- und kostenintensiven präklinischen Studien. Um diese Lücke zu schließen, wird in dieser Arbeit eine in vitro-Kurzzeit-Prüfmethode zur Prädiktion und Bewertung des Langzeit-Eigenschaftsprofils anhand der beiden etablierten Magnesiumlegierungen WE43 und ZX10 sowie einer Oberflächenmodifikation mittels plasma-elektrolytischer Oxidation (PEO) entwickelt und validiert. Durch die Berücksichtigung mehrerer Magnesiumsysteme umfasst die Methodenentwicklung unterschiedliche mikrostrukturelle Charakteristika sowie modifizierte Oberflächen. Dabei liegen für die beiden Legierungen unterschiedliche Legierungselemente, deren -anteile und dementsprechend eine variierende Ausscheidungsmorphologie und Korngröße vor. Da es sich infolge des Abbauprozesses um ein zeitabhängiges Eigenschaftsprofil aus Korrosionsrate, Korrosionsmorphologie und der wechselwirkenden mechanischen Stabilität handelt, wird in einem ersten Schritt die reproduzierbare Quantifizierung von Korrosionsrate und -morphologie durch einen gekoppelten Ansatz aus Wasserstoffmessung sowie zwei- und dreidimensional bildgebenden Verfahren genutzt. Die Messung des Wasserstoffs erlaubt direkte Rückschlüsse auf die Korrosionsrate durch Berücksichtigung der chemischen Reaktionsgleichung, sodass im Zuge der Methodenentwicklung für die unterschiedlichen Belastungskollektive und die einhergehenden Anforderungen Prüfstände zur Wasserstoffmessung entwickelt werden. Im Rahmen der gekoppelten analytischen Methoden gilt es die Ausgangsmikrostruktur hinsichtlich ihrer Heterogenität und elektrochemischen Stabilität zu charakterisieren, da beide Größen einen dominanten Einfluss auf die Korrosionsrate und -morphologie besitzen, was wiederum durch die Kurzzeit-Prüfmethode abgebildet wird. Zur Quantifizierung der Korrosionsmorphologie wird ein Ansatz aus Rasterelektronenmikroskopie und Mikro-Computertomografie angewandt, wobei für letzteres ein Skript zur automatisierten Quantifizierung der gemessenen Korrosionsnarben hinsichtlich ihrer geometrischen Ausprägung nach Korrosion und Korrosionsermüdung verfasst wird. Den eigentlichen Grundstein zur Umsetzung der Kurzzeit-Prüfmethode legt die galvanostatische Polarisation zur Erhöhung der Korrosionsrate, sodass während einer Kurzzeit-Immersion der absolute Massenverlust der ursprünglichen Langzeit-Immersion am freien Korrosionspotential simuliert und dessen Einfluss auf die Korrosionsermüdungseigenschaften bestimmt werden kann. Die Quantifizierung des Einflusses und damit die Validierung der Prüfmethode wird durch ein bruchmechanisches Modell vorgenommen. Hierbei werden die Korrosionsnarben als Oberflächendefekt mit dem Modell nach Murakami und Endo betrachtet und ausgewertet, sodass durch die Anwendung des Modells der generelle Einfluss der Korrosionsnarben auf die Lebensdauer bestimmt sowie die Abhängigkeit von der Kurzzeit-Prüfmethode und der Immersionszeit quantifiziert wird. Auf dieser Basis wird die Prüfmethode abschließend validiert sowie weiterführend ein Modell aufgestellt, das die Lebensdauer in Abhängigkeit der Immersionszeit und der vorliegenden Belastung bestimmt und somit die Anwendbarkeit der in vitro-Kurzzeit-Prüfmethode sowie eine Erweiterung mit dem Modell nach Murakami und Endo bei biodegradierbaren Magnesiumlegierungen zeigt. In Abhängigkeit der Mikrostruktur ermöglicht die Methode, gekoppelt mit dem bruchmechanischen Ansatz, die defektbasierte Bewertung des Korrosionsermüdungsverhaltens unter Langzeit-Immersion mehrerer Wochen und führt zu einer signifikanten Zeit- und Kostenersparnis bei der Charakterisierung. Damit kann über die in vitro-Kurzzeit-Prüfmethode eine Abschätzung zur Einhaltung oder Verfehlung der Funktionsphase des Implantats getroffen werden.
Fatigue condition monitoring of notched thermoplastic-based hybrid fiber metal laminates using electrical resistance measurement and digital image correlation
(2023-05-17) Mrzljak, Selim; Trautmann, Maik; Blickling, Philipp; Wagner, Guntram; Walther, Frank
In this work, the monitorability of fatigue damage in notched thermoplastic-based hybrid fiber metal laminate, containing AA6082-T4 sheets and glass and carbon fiber-reinforced polyamide 6, is investigated using constant amplitude tests. Electrical resistance measurement and digital image correlation were combined to determine the initiation and evolution process of fatigue damage. Preliminary to the application of the electrical resistance measurement during fatigue load, basic investigations regarding necessary measurement accuracy and conditions, e.g. temperature and cross-section influence, were conducted to achieve reliable measurement results. Via digital image correlation fatigue crack growth was determined and correlated with the change in electrical resistance for two metal/fiber-reinforced polymer layer configurations (2/1 and 3/2) and notch geometries (drilling hole and double-edge notch). The results show that reliable detection of fatigue-related damage states is possible independent of aluminum sheet treatment (mechanically blasted or anodized surface), with earlier crack initiation and faster propagation for higher metal volume fraction (layer configuration 2/1). For the two investigated notch geometries an overall similar crack behavior was found. The electrical resistance values directly correlate to varieties of crack formation and growth, representing the aluminum sheet damage progress of the laminate well, and enabling the possibility of e.g. a limit value-based failure criterion. However, geometry and crack-related changes in electric current flow and thus current density must be taken into account for targeted monitoring of the laminate condition, as they cause significant changes in electrical resistance.
Mechanical in vitro fatigue testing of implant materials and components using advanced characterization techniques
(2021-11-30) Wegner, Nils; Klein, Martin; Scholz, Ronja; Kotzem, Daniel; Macias Barrientos, Marina; Walther, Frank
Implants of different material classes have been used for the reconstruction of damaged hard and soft tissue for decades. The aim is to increase and subsequently maintain the patient's quality of life through implantation. In service, most implants are subjected to cyclic loading, which must be taken particularly into consideration, since the fatigue strength is far below the yield and tensile strength. Inaccurate estimation of the structural strength of implants due to the consideration of yield or tensile strength leads to a miscalculation of the implant's fatigue strength and lifetime, and therefore, to its unexpected early fatigue failure. Thus, fatigue failure of an implant based on overestimated performance capability represents acute danger to human health. The determination of fatigue strength by corresponding tests investigating various stress amplitudes is time-consuming and cost-intensive. This study summarizes four investigation series on the fatigue behavior of different implant materials and components, following a standard and an in vitro short-time testing procedure, which evaluates the material reaction in one enhanced test set-up. The test set-up and the applied characterization methods were adapted to the respective application of the implant with the aim to simulate the surrounding of the human body with laboratory in vitro tests only. It could be shown that by using the short-time testing method the number of tests required to determine the fatigue strength can be drastically reduced. In future, therefore it will be possible to exclude unsuitable implant materials or components before further clinical investigations by using a time-efficient and application-oriented testing method.
Improving the defect tolerance of PBF-LB/M processed 316L steel by increasing the nitrogen content
(2022-08-02) Stern, Felix; Becker, Louis; Cui, Chengsong; Tenkamp, Jochen; Uhlenwinkel, Volker; Steinbacher, Matthias; Boes, Johannes; Lentz, Jonathan; Fechte-Heinen, Rainer; Weber, Sebastian; Walther, Frank
Nitrogen (N) in steels can improve their mechanical strength by solid solution strengthening. Processing N-alloyed steels with additive manufacturing, here laser powder bed fusion (PBF-LB), is challenging as the N-solubility in the melt can be exceeded. This degassing of N counteracts its intended positive effects. Herein, the PBF-LB processed 316L stainless steel with increased N-content is investigated and compared to PBF-LB 316L with conventional N-content. The N is introduced into the steel by nitriding the powder and mixing it with the starting powder to achieve an N-content of approximately 0.16 mass%. Thermodynamic calculations for maximum solubility to avoid N outgassing and pore formation under PBF-LB conditions are performed beforehand. Based on the results, a higher defect tolerance under fatigue characterized by Murakami model can be achieved without negatively influencing the PBF-LB processability of the 316L steel. The increased N-content leads to higher hardness (+14%), yield strength (+16%), tensile strength (+9%), and higher failure stress in short time fatigue test (+16%).
Effect of microstructural heterogeneity on fatigue strength predicted by reinforcement machine learning
(2022-08-25) Awd, Mustafa; Münstermann, Sebastian; Walther, Frank
The posterior statistical distributions of fatigue strength are determined using Bayesian inferential statistics and the Metropolis Monte Carlo method. This study explores how structural heterogeneity affects ultrahigh cycle fatigue strength in additive manufacturing. Monte Carlo methods and procedures may assist estimate fatigue strength posteriors and scatter. The acceptable probability in Metropolis Monte Carlo relies on the Markov chain's random microstructure state. In addition to commonly studied variables, the proportion of chemical composition was demonstrated to substantially impact fatigue strength if fatigue lifetime in crack propagation did not prevail due to high threshold internal notches. The study utilizes an algorithm typically used for quantum mechanics to solve the complicated multifactorial fatigue problem. The inputs and outputs are modified by fitting the microstructural heterogeneities into the Metropolis Monte Carlo algorithm. The main advantage here is applying a general-purpose nonphenomenological model that can be applied to multiple influencing factors without high numerical penalty.
A 2D and 3D segmentation-based microstructure study on the role of brittle phases in diffusion brazed AISI 304L/NiCrSiFeMoB joints
(2023-10-14) Otto, Johannes Leon; Sauer, Lukas M.; Brink, Malte; Schaum, Thorge; Lingnau, Lars A.; Macias Barrientos, Marina; Walther, Frank
Nickel-based filler metals are frequently used in high temperature vacuum diffusion brazing for austenitic stainless-steel joints when components are subjected to high static or dynamic loads, corrosive environments and elevated temperatures. Due to melting point depressing metalloids such as silicon and boron, hard and brittle intermetallic phases are formed during the brazing process depending on the diffusion mechanisms. These brittle phases significantly affect mechanical and corrosive properties of the compounds. To quantify the influences of their amount, morphology and distribution, deep learning image segmentation was applied to segment these phases of the athermal solidification zone and the diffusion zone. Subsequently, characteristic microstructure parameters were calculated from these. The parameters of six different brazed joint variations were compared with their experimental characterization of mechanical and corrosive properties so that several correlations could be identified. Finally, a layer-by-layer removal of a brazed joint was performed using a focused ion beam, and a 3D model was reconstructed from the generated images to gain a mechanism-based understanding beyond the previous 2D investigations.
Characterization of the high-temperature behavior of PBF-EB/M manufactured γ titanium aluminides
(2022-02-24) Teschke, M.; Moritz, J.; Telgheder, L.; Marquardt, A.; Leyens, C.; Walther, F.
Due to their high specific strength and temperature resistance, γ-titanium aluminides (γ-TiAl) have a growing importance for automotive and aerospace applications. However, conventional processing is very challenging due to the inherent brittleness of the material. Therefore, new manufacturing techniques and methods have to be established. Additive manufacturing techniques such as electron powder bed fusion (PBF-EB/M) are favored, since they enable near net shape manufacturing of highly complex geometries. The high preheating temperatures, which typically occur during PBF-EB/M, can significantly improve the processability of TiAl and facilitate the fabrication of complex parts. In this study, a previously optimized material condition of the β-solidifying TNM alloy TNM-B1 (Ti-43.5Al-4Nb-1Mo-0.1B) was manufactured by PBF-EB/M. The resulting microstructure, defect distribution and morphology, and mechanical properties were characterized by means of characterization methods, e.g., CT, SEM, light microscopy, hardness measurements, and tensile tests. A special focus was on the mechanical high-temperature behavior. The pronounced sensitivity of the material to defects and internal notches, e.g., due to lack of fusion defects (misconnections) which were found in the as-built condition, was identified as a main cause for premature failure below the yield point due to the low ductility. This failure was analyzed and potential improvements were identified.
Effect of carbon nanofibre orientation on fatigue properties of carbon fibre-reinforced polymers
(2023-01-06) Mrzljak, Selim; Zanghellini, Benjamin; Gerdes, Lars; Helwing, Ramon; Schuller, Reinhard; Sinn, Gerhard; Lichtenegger, Helga; Walther, Frank; Rennhofer, Harald
Nano-reinforcements in carbon fibre-reinforced polymer (CFRP) have proven to enhance the mechanical properties considering quasi-static, as well as fatigue load and, are a promising option with regard to CFRP performance optimisation. While general knowledge about the nanofiller content and its influence in CFRP is well documented, the use of alignment techniques for a specific orientation of the nano-reinforcements is still insufficiently studied. In this work, the influence of oriented carbon nanofibres (CNF) on the mechanical properties of bidirectional CFRP is investigated. CFRP was produced CNF-reinforced with and without orientation using a hot press, where an electric field was applied during curing. The laminates were characterised with respect to dispersion quality, pore volume, quasi-static properties (tensile and bending tests) and dynamic properties (fatigue tests). Electrical resistance measurement was applied together with digital image correlation and in situ computed tomography to generate knowledge about the fatigue-related damage evolution and evaluate the sensors for viable use of condition monitoring. Results show that the orientation of CNF has a significant impact on both quasi-static and fatigue properties, increasing the strength while reducing and slowing down the introduced damage. Orientation of nanofillers thus shows large optimization potential of mechanical properties of CFRP components.
Position-dependent mechanical characterization of the PBF-EB-manufactured Ti6Al4V alloy
(2021-10-12) Kotzem, Daniel; Höffgen, Alexandra; Raveendran, Rajevan
By means of additive manufacturing, the production of components with nearly unlimited geometrical design complexity is feasible. Especially, powder bed fusion techniques such as electron beam powder bed fusion (PBF-EB) are currently focused. However, equal material properties are mandatory to be able to transfer this technique to a wide scope of industrial applications. Within the scope of this work, the mechanical properties of the PBF-EB-manufactured Ti6Al4V alloy are investigated as a function of the position on the building platform. It can be stated that as-built surface roughness changes within building platform whereby highest surface roughness detected by computed tomography (Ra = 46.0 ± 5.3 µm) was found for specimens located in the front of the building platform. In contrast, no significant differences in relative density could be determined and specimens can be assumed as nearly fully dense (> 99.9%). Furthermore, all specimens are affected by an undersized effective diameter compared to the CAD data. Fatigue tests revealed that specimens in the front of the building platform show slightly lower performance at higher stress amplitudes as compared to specimens in the back of the building platform. However, process-induced notch-like defects based on the surface roughness were found to be the preferred location for early crack initiation.
Ti6Al4V lattice structures manufactured by electron beam powder bed fusion - microstructural and mechanical characterization based on advanced in situ techniques
(2022-12-17) Kotzem, Daniel; Arold, Tizian; Bleicher, Kevin; Raveendran, Rajevan; Niendorf, Thomas; Walther, Frank
Powder bed fusion (PBF) processes enable the manufacturing of complex components in a time- and cost-efficient manner. Especially lattice structures are currently focused since they show varying mechanical properties, including different deformation and damage behaviors, which can be used to locally tailor the mechanical behavior. However, the present process-structure-property relationships are highly complex and have to be understood in detail in order to enable an implementation of PBF manufactured lattice structures in safety-relevant applications. Within the present work Ti6Al4V lattice structures were manufactured by electron beam powder bed fusion of metals (PBF-EB/M). Based on the classification of bending- and stretch-dominated deformation behavior, two different lattice types, i.e. body-centered cubic like (BCC-) and face-centered cubic like (F2CCZ) structures were selected. Microstructural features were detected to evaluate if potential different microstructures can occur due to different lattice types and to answer the question if microstructural features might contribute to the mechanical behavior shown in this work. Furthermore, X-ray microfocus computed tomography (μCT) analysis were carried out to enable a comparison between the computer-aided designed (CAD) and as-built geometry. For mechanical characterization, quasi-static and cyclic tests were used. In particular, the BCC lattice type showed a more ductile material behavior whereby higher stiffness and strength was determined for the F2CCZ lattice type. Additionally, different in-situ measurement techniques such as direct current potential drop system and digital image correlation could be deployed to describe the damage progress both under quasi-static and cyclic loading.
Innovative X-ray diffraction and micromagnetic approaches for reliable residual stress assessment in deep rolled and microfinished AISI 4140 components
(2022-08-03) Strodick, Simon; Vogel, Florian; Tilger, Meik; Kipp, Monika; Baak, Nikolas; Biermann, Dirk; Walther, Frank; Denstorf, Marie; Kukui, Dimitri; Barrientos, Marina Macias
The residual stress state in the subsurface is a key element of surface integrity. It is well known to have a significant impact on a component's properties in terms of fatigue behavior and resistance to wear and corrosion. For this reason, adjusting residual stresses during manufacturing is a major challenge in modern production engineering, to improve and ensure a component's fatigue strength. In this context, hydrostatic deep rolling of the workpiece surface using adapted parameters enables the targeted induction of compressive residual stresses into subsurface layers. Due to specific properties regarding subsurface and topography for functional components in tribological applications, a further machining operation by microfinishing following deep rolling seems to be purposeful. In particular with regard to the production of components exposed to periodic load changes when used, the process combination can enable a substitution of the typically required conventional subsurface zone hardening. With the aim of economical process design, the corresponding parts can be manufactured with significantly reduced time and costs. Efficient and well-founded methods for monitoring the resulting influence on the subsurface zone properties are essential for a reproducible and target-oriented process design. The prevailing method for the non-destructive assessment of residual stresses in both academia and industry is X-ray diffractometry using the sin2 ψ-method. However, this method is time-intensive and requires complex instrumentation. Thus, efforts have been undertaken in past decades to develop alternative methods for the efficient and reliable characterization of residual stresses. In this research, the applicability of the cos α-method in X-ray diffractometry and a micromagnetic approach for residual stress assessment was investigated, analyzing deep rolled and microfinished AISI 4140 specimen conditions. In addition to the diffractometric and micromagnetic measurements, metallographic and topographic analyses of machined surfaces were carried out. Deep rolling was found to induce significant compressive residual stresses of up to −1100 MPa. After microfinishing of the deep rolled surfaces, favorable compressive residual stresses remain in the subsurface, reaching approximately up to −750 MPa. Based on this, the production of tailored surfaces with respect to a suitable combination of topography and subsurface is possible. For all surface states investigated, a good agreement between the two approaches in X-ray diffraction was found. Magnetic Barkhausen noise (MBN) measurements prove to be well applicable for an efficient and holistic assessment of surface integrity in the subsurface of deep rolled and microfinished AISI 4140 specimens.
Assessing the lightweight potential of additively manufactured metals by density-specific Woehler and Shiozawa diagrams
(2022-08-05) Merghany, Mohamed; Teschke, Mirko; Stern, Felix; Tenkamp, Jochen; Walther, Frank
Additive manufacturing (AM) using the powder bed fusion (PBF) process is building up the components layer by layer, which enables the fabrication of complex 3D structures with unprecedented degrees of freedom. Due to the high cooling rates of the AM process, fine microstructures are generated. This leads to an improvement in quasistatic properties such as tensile strength, whereas the fatigue strength is comparable to that of conventionally manufactured metal or even reduced. This is due to the presence of process-induced defects formulated during the manufacturing process in combination with the increased notch stress sensitivity of high-strength metals. In this work, the fatigue damage assessment using different approaches like those of Murakami and Shiozawa for three AM alloys (AlSi10Mg, 316L, and TNM-B1) containing defects is studied for better understanding of capability and mechanisms. Moreover, the effect of the lightweight potential is investigated, and how the specific material density can be considered when the fatigue damage tolerance is characterized.
Impact of solar radiation on chemical structure and micromechanical properties of cellulose-based humidity-sensing material Cottonid
(2021-04-06) Scholz, Ronja; M. Langhansl, F.; Hemmerich, M.; Meyer, J.; Zollfrank, C; Walther, F.
Renewable and environmentally responsive materials are an energy- and resource-efficient approach in terms of civil engineering applications, e.g. as so-called smart building skins. To evaluate the influence of different environmental stimuli, like humidity or solar radiation, on the long-term actuation behavior and mechanical robustness of these materials, it is necessary to precisely characterize the magnitude and range of stimuli that trigger reactions and the resulting kinetics of a material, respectively, with suitable testing equipment and techniques. The overall aim is to correlate actuation potential and mechanical properties with process- or application-oriented parameters in terms of demand-oriented stimuli-responsive element production. In this study, the impact of solar radiation as environmental trigger on the cellulose-based humidity-sensing material Cottonid, which is a promising candidate for adaptive and autonomously moving elements, was investigated. For simulating solar radiation in the lab, specimens were exposed to short-wavelength blue light as well as a standardized artificial solar irradiation (CIE Solar ID65) in long-term aging experiments. Photodegradation behavior was analyzed by Fourier-transform infrared as well as electron paramagnetic resonance spectroscopy measurements to assess changes in Cottonid’s chemical composition. Subsequently, changes in micromechanical properties on the respective specimens’surface were investigated with roughness measurements and ultra-micro-hardness tests to characterize variations in stiffness distribution in comparison to the initial condition. Also, thermal effects during long-term aging were considered and contrasted to pure radiative effects. In addition, to investigate the influence of process-related parameters on Cottonid’s humidity-driven deformation behavior, actuation tests were performed in an alternating climate chamber using a customized specimen holder, instrumented with digital image correlation (DIC). DIC was used for precise actuation strain measurements to comparatively evaluate different influences on the material’s sorption behavior. The infrared absorbance spectra of different aging states of irradiated Cottonid indicate oxidative stress on the surface compared to unaged samples. These findings differ under pure thermal loads. EPR spectra could corroborate these findings as radicals were detected, which were attributed to oxidation processes. Instrumented actuation experiments revealed the influence of processing-related parameters on the sorption behavior of the tested and structurally optimized Cottonid variant. Experimental data supports the definition of an optimal process window for stimuli-responsive element production. Based on these results, tailor-made functional materials shall be generated in the future where stimuli-responsiveness can be adjusted through the manufacturing process.
In situ characterization of polycaprolactone fiber response to quasi-static tensile loading in scanning electron microscopy
(2021-06-24) Delp, Alexander; Becker, Alexander; Hülsbusch, Daniel; Scholz, Ronja; Müller, Marc; Glasmacher, Birgit; Walther, Frank
Microstructural responses to the mechanical load of polymers used in tissue engineering is notably important for qualification at in vivo testing, although insufficiently studied, especially regarding promising polycaprolactone (PCL). For further investigations, electrospun PCL scaffolds with different degrees of fiber alignment were produced, using two discrete relative drum collector velocities. Development and preparation of an adjusted sample geometry enabled in situ tensile testing in scanning electron microscopy. By analyzing the microstructure and the use of selected tracking techniques, it was possible to visualize and quantify fiber/fiber area displacements as well as local fractures of single PCL fibers, considering quasi-static tensile load and fiber alignment. The possibility of displacement determination using in situ scanning electron microscopy techniques for testing fibrous PCL scaffolds was introduced and quantified.
Testing procedure for fatigue characterization of steel-CFRP hybrid laminate considering material dependent self-heating
(2021-06-18) Mrzljak, Selim; Schmidt, Stefan; Kohl, Andreas; Hülsbusch, Daniel; Hausmann, Joachim; Walther, Frank
Combining carbon fiber reinforced polymers (CFRP) with steel offers the potential of utilizing the desired characteristics of both materials, such as specific strength/stiffness and fatigue strength of fiber reinforced polymers (FRP) and impact resistance of metals. Since in such hybrid laminates multiple material layers are combined, a gradual failure is likely that can lead to changes in mechanical properties. A failure of the metal partner leads to an increase in stress on the FRP, which under fatigue load results in increased self-heating of the FRP. Therefore, a suitable testing procedure is required and developed in this study, to enable a reproducible characterization of the mechanical properties under fatigue load. The resulting testing procedure, containing multiple frequency tests as well as load increase and constant amplitude tests, enabled characterization of the fatigue performance while never exceeding a testing induced change in temperature of 4 K. In addition to the development of the testing procedure, an insight into the manufacturing induced residual stresses occurring in such hybrid laminates, which impacts the load-bearing capacity, was established using finite element simulation. The gathered data and knowledge represents a basis for future in-depth investigations in the area of residual stress influence on the performance of hybrid laminates and highlights its importance, since not only the used testing procedure determines the measured fatigue performance.
Influence of different alloying strategies on the mechanical behavior of tool steel produced by laser-powder bed fusion
(2021-06-17) Chehreh, Abootorab Baqerzadeh; Strauch, Anna; Großwendt, Felix; Röttger, Arne; Fechte-Heinen, Rainer; Theisen, Werner; Walther, Frank
Additive manufacturing is a high-potential technique that allows the production of components with almost no limitation in complexity. However, one of the main factors that still limits the laser-based additive manufacturing is a lack of processable alloys such as carbon martensitic hardenable tool steels, which are rarely investigated due to their susceptibility to cold cracking. Therefore, this study aimed to expand the variety of steels for laser powder bed fusion (L-PBF) by investigating an alternative alloying strategy for hot work tool steel powder. In this study, a comprehensive investigation was performed on the powder and L-PBF processed specimen properties and their correlation with the existing defects. Cubical specimens were created using the following two alloying strategies by means of L-PBF: conventional pre-alloyed gas-atomized powder and a mixture of gas-atomized powder with mechanically crushed pure elements and ferroalloys. The influence of the particle parameters such as morphology were correlated to the defect density and resulting quasi-static mechanical properties. Micromechanical behavior and damage evolution of the processed specimens were investigated using in situ computed tomography. It was shown that the properties of the L-PBF processed specimens obtained from the powder mixture performs equal or better compared to the specimens produced from conventional powder.

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