Thomas Tröster

Highly Anisotropic Steel Processed by Selective Laser Melting

For additive manufacturing of metals, selective laser melting can be employed. The microstructure evolution is directly influenced by processing parameters. Employing a high energy laser system, samples made from austenitic stainless steel were manufactured. The microstructure obtained is characterized by an extremely high degree of anisotropy featuring coarse elongated grains and a 〈001〉 texture alongside the build direction during processing. Eventually, the anisotropy of the microstructure drastically affects the monotonic properties of the current material.

Fatigue Strength Prediction for Titanium Alloy TiAl6V4 Manufactured by Selective Laser Melting

Selective laser melting (SLM), as a metalworking additive manufacturing technique, received considerable attention from industry and academia due to unprecedented design freedom and overall balanced material properties. However, the fatigue behavior of SLM-processed materials often suffers from local imperfections such as micron-sized pores. In order to enable robust designs of SLM components used in an industrial environment, further research regarding process-induced porosity and its impact on the fatigue behavior is required. Hence, this study aims at a transfer of fatigue prediction models, established for conventional process-routes, to the field of SLM materials. By using high-resolution computed tomography, load increase tests, and electron microscopy, it is shown that pore-based fatigue strength predictions for a titanium alloy TiAl6V4 have become feasible. However, the obtained accuracies are subjected to scatter, which is probably caused by the high defect density even present in SLM materials manufactured following optimized processing routes. Based on thorough examination of crack surfaces and crack initiation sites, respectively, implications for optimization of prediction accuracy of the models in focus are deduced.

Optical Studies of Non-Metallic Compounds under Pressure

Publisher Summary This chapter presents a review of the high pressure optical studies on rare earth ions in non-metallic compounds. The chapter demonstrates that the problem of host lattice structural dependence of 4f N states can be effectively tackled by high pressure techniques and hopefully, the interest for further more refined high pressure studies of this problem can be stimulated. The rare-earth compounds that have been studied by optical means under pressure are also reviewed in this chapter. Then, after a brief introduction of the most commonly used high pressure device, the diamond anvil cell is discussed in the chapter. The chapter also discusses the pressure induced changes of the crystal-field levels and their interpretation. Some aspects of the dynamical effects under pressure are discussed. These include lifetime and intensity measurements, the influence due to excited configurations and charge transfer bands, and the electron–phonon coupling.

Untersuchung alternativer Erwärmungsverfahren für den Presshärteprozess

Kurzfassung Die Platinenerwärmung ist ein wesentlicher Teil der Prozesskette des Warmformens, die neben der zu erreichenden Bauteilqualität noch wesentlich die Prozesskosten beeinflusst. Stand der Technik ist die Strahlungserwärmung in Rollenherdöfen. Diese Öfen ermöglichen eine gleichmäßige, aber langsame Erwärmung, sodass ein vergleichsweise hoher Platzbedarf besteht. Als Folge dessen rücken alternative Erwärmungstechnologien in den Fokus aktueller Forschungsarbeiten, u. a. die Widerstandserwärmung [1], die Wirbelbetterwärmung [2] und die Erwärmung durch Induktion [3]. Diese Erwärmungsmethoden zeichnen sich durch eine im Vergleich zum Rollenherdofen deutlich schnellere Erwärmung der Platine aus, was auf unterschiedlichen Wärmeerzeugungs- und Wärmeübertragungsmechanismen beruht, die bei den vorgestellten Technologien deutlich effektiver sind als die Erwärmung durch Strahlung und Konvektion im Rollenherdofen. Dieser Beitrag stellt Ergebnisse der induktiven Erwärmung und der Wirbelbetterwärmung des Warmformstahls 22MnB5 vor.

Forming limit curves determined in high-speed Nakajima tests and predicted by a strain rate sensitive model

Material characteristics such as yield strength and failure strain are affected by the loading speed. Even the start of instability and necking depends not only on the strain hardening coefficient but also on the strain rate sensitivity parameter. Therefore, the strain rate dependence of materials for both plasticity and the failure behavior is taken into account in crash simulations for strain rates up to 1,000 s−1. The current standard experiment for investigation of strain rate dependence is the high speed tensile test as described in a FAT guideline. Moreover, the need of material characterization at multi-axial loadings and high strain rates is pointed out in FAT guideline. Forming limit diagrams (FLD) can be used for the description of the material`s instability behavior at multi-axial loading. Usually, the FLD are determined quasi-statically at 1.5 mm/s. The usage of experimentally determined, quasi-static FLD also at high strain rates leads to great uncertainties and thus can be hardly used in crash simulations. A possibility for experimentally recording FLD at high forming rates > 100 s−1 offers the present described high speed Nakajima test. The results for the deep drawing steel DC01 illustrate the need of the determination of dynamic FLD. In this context, due to the strain rate dependence of the material behavior an extrapolation of quasi-static FLD is not feasible. Alternatively, the prediction of forming limit curves (FLC) at high strain rates is possible with the extended modified maximum force criterion. This new and extended model includes the strain rate dependence and therefore predicting forming limits at dynamic forming gets possible. The new approach is described and the accordance of experimental determined and predicted results for the begin of instability is presented.Material characteristics such as yield strength and failure strain are affected by the loading speed. Even the start of instability and necking depends not only on the strain hardening coefficient but also on the strain rate sensitivity parameter. Therefore, the strain rate dependence of materials for both plasticity and the failure behavior is taken into account in crash simulations for strain rates up to 1,000 s−1. The current standard experiment for investigation of strain rate dependence is the high speed tensile test as described in a FAT guideline. Moreover, the need of material characterization at multi-axial loadings and high strain rates is pointed out in FAT guideline. Forming limit diagrams (FLD) can be used for the description of the material`s instability behavior at multi-axial loading. Usually, the FLD are determined quasi-statically at 1.5 mm/s. The usage of experimentally determined, quasi-static FLD also at high strain rates leads to great uncertainties and thus can be hardly used in cra...

Verhalten von lasergeschmolzenen Bauteilen aus der Titan-Aluminium-Legierung TiAl6V4 unter zyklischer Beanspruchung∗

Abstract Selektives Laserschmelzen (SLM) gehört zu den additiven Fertigungsverfahren (AM) und gewinnt zunehmend an Bedeutung. Im Fokus dieses Beitrags stehen Ergebnisse der Untersuchungen an zyklisch belasteten SLM-Bauteilen aus der Titan-Aluminium-Legierung TiAl6V4. Hierzu wurden Experimente zur Bestimmung der Schwingfestigkeit sowie zur Charakterisierung des bruchmechanischen Materialverhaltens durchgeführt. Die Ergebnisse verdeutlichen, dass z. B. die Dauerfestigkeit und der Schwellenwert des Materials gegen Ermüdungsrissausbreitung beim SLM-Verfahren niedriger sind als die des Grundmaterials. In dieser Arbeit wird jedoch gezeigt, wie durch gezielte Wärmebehandlungsmethoden die Materialkenndaten der SLM-Bauteile verbessert werden können.

Einsatz von Simulationswerkzeugen in der Entwicklungskette von ultrahochfesten warmgeformten Strukturkomponenten im Fahrzeugleichtbau

Kurzfassung Um dem Zielkonflikt zwischen Fahrzeugleichtbau und steigenden Crashanforderungen bei der Auslegung von Karosseriestrukturen zu begegnen, werden zunehmend ultrahochfeste Stahlwerkstoffe eingesetzt. Aufgrund deren begrenzter Umformfähigkeit bietet die Technologie des Warmumformens eine leistungsfähige Alternative. Hierbei werden komplexe Bauteile bei hoher Temperatur einstufig geformt und im Werkzeug gehärtet. Dabei stellt die Vorhersage der Herstellbarkeit und der resultierenden Wandstärkenverteilung eine neue Herausforderung für die Umformsimulation dar, der mithilfe der thermisch-mechanisch gekoppelten Umformsimulation begegnet werden kann. Zusätzlich können die berechneten Bauteileigenschaften wie Wandstärkenverteilung und Werkstoffeigenschaften für eine nachfolgende Crashsimulation übernommen werden.

Forming limit curves of DP600 determined in high-speed Nakajima tests and predicted by two different strain-rate-sensitive models

Determination of forming limit curves (FLC) to describe the multi-axial forming behaviour is possible via either experimental measurements or theoretical calculations. In case of theoretical determination, different models are available and some of them consider the influence of strain rate in the quasi-static and dynamic strain rate regime. Consideration of the strain rate effect is necessary as many material characteristics such as yield strength and failure strain are affected by loading speed. In addition, the start of instability and necking depends not only on the strain hardening coefficient but also on the strain rate sensitivity parameter. Therefore, the strain rate dependency of materials for both plasticity and the failure behaviour is taken into account in crash simulations for strain rates up to 1000 s−1 and FLC can be used for the description of the material’s instability behaviour at multi-axial loading. In this context, due to the strain rate dependency of the material behaviour, an extrapolation of the quasi-static FLC to dynamic loading condition is not reliable. Therefore, experimental high-speed Nakajima tests or theoretical models shall be used to determine the FLC at high strain rates. In this study, two theoretical models for determination of FLC at high strain rates and results of experimental high-speed Nakajima tests for a DP600 are presented. One of the theoretical models is the numerical algorithm CRACH as part of the modular material and failure model MF GenYld+CrachFEM 4.2, which is based on an initial imperfection. Furthermore, the extended modified maximum force criterion considering the strain rate effect is also used to predict the FLC. These two models are calibrated by the quasi-static and dynamic uniaxial tensile tests and bulge tests. The predictions for the quasi-static and dynamic FLC by both models are presented and compared with the experimental results.Determination of forming limit curves (FLC) to describe the multi-axial forming behaviour is possible via either experimental measurements or theoretical calculations. In case of theoretical determination, different models are available and some of them consider the influence of strain rate in the quasi-static and dynamic strain rate regime. Consideration of the strain rate effect is necessary as many material characteristics such as yield strength and failure strain are affected by loading speed. In addition, the start of instability and necking depends not only on the strain hardening coefficient but also on the strain rate sensitivity parameter. Therefore, the strain rate dependency of materials for both plasticity and the failure behaviour is taken into account in crash simulations for strain rates up to 1000 s−1 and FLC can be used for the description of the material’s instability behaviour at multi-axial loading. In this context, due to the strain rate dependency of the material behaviour, an extrap...

Additively manufactured acoustic diffuser structures for ultrasonic measurement applications

To prevent errors in ultrasonic measurement systems (e.g. for sound velocity measurements), stray signals have to be absorbed or dispersed. While absorbers are commercially available, they are typically limited to a specific fluid, for example water, and to only a small temperature range. There are, however, some applications where more flexibility regarding the examined fluid (e.g. compatibility with gases and liquids) and the temperature is required. To achieve compatibility with a wide variety of fluids, structured diffusers are to be preferred over absorbers which require a matched acoustic impedance with the adjacent fluid. However, the structure of diffusers has to be in the scale of the wavelength of incident acoustic waves, which are often in the range of only a few millimeters, or below. We therefore propose using additive manufacturing techniques to create diffusive structures.Utilizing metals as materials for the diffuser allows for applicability over a wide temperature range. In an empirical s...

Separation of Hybrid Structures for the Reclaim of their Single Components

The main objective for an economic and ecological use of raw materials is the achievement of closed raw material cycles. Because of that, not only the manufacturing procedures are important during the development of new materials but also the recycling processes. Within the increased use of lightweight construction in recent years, the application of multi-material or hybrid structures reach high significance for the automotive industry. In this development, especially the carbon fibre reinforced plastics (CFRP) gained its importance. However, currently there are no recycling strategies available for hybrid structures; complete recycling processes for CFRP are still expandable. This work presents methods for separation of hybrid structures made of metal and CFRP, as well as the corresponding process windows and the boundary conditions. The separation is performed by introduction of thermal heat and the behaviour of these bonded compounds is analyzed based on shear tensile tests. The results of these studies are used to develop a complete recycling process for reclamation of hybrid structures.