Metallic materials and material fatigue

Brief description

Depending on the material and the selected process parameters, cut surfaces produced by high-speed shear cutting (HGSS) can contain adiabatic shear bands (ASB). These are homogeneous zones that are clearly microstructurally separated from the base material and have a significantly different property profile compared to the surrounding material, such as a higher hardness. However, the structure-property relationships of HGSS interfaces have so far been largely unexplored. Sub-project 1 therefore focuses on answering the question of how HGSS interfaces behave under tribological, corrosive and cyclic mechanical loads. This is directly linked to the need to gain a fundamental understanding of the mechanisms involved and thus the identification of process-microstructure-property relationships. The prerequisite for this is the interpretation of the results of the wear, corrosion and fatigue tests, i.e. the correlation of the determined cut surface properties with the process parameters and the resulting microstructure, which must be carried out in coordination with the research group partners. The research results obtained in SP1 are the basis for evaluating the performance characteristics and thus the practical suitability of HGSS cutting surfaces. Through feedback with the other sub-projects, the results also enable direct influence on the design of the HGSS process and thus a targeted adjustment of the properties of the functional surface.

Brief description

Many components have to withstand periodic load changes during their life cycle. Of great technical and economic interest are long service lives with high bearable loads. For components subjected to this type of cyclic loading, it is necessary to select materials and adapt the geometric design with regard to high realisable fatigue strengths. Application-specific, specified minimum vibration cycles must be endured without cracks. As the component surface is the favoured location for crack initiation under high cycle fatigue (HCF) loading, the geometric and material properties have a decisive influence on the cyclic load capacity and therefore the fatigue strength. This means that in addition to strength, hardness and the macroscopic surface topography, the notch effects resulting from the microstructure components also have an influence on the microscale. However, these microstructure-based influencing factors have not yet been sufficiently researched in terms of their effect on fatigue strength. The aim of the project is to record the influence of the near-surface microstructure on the fatigue strength not only qualitatively but also quantitatively in comparison to the macroscopic, production-related surface topography in order to be able to take all relevant factors into account when designing a component for improved application properties.

Brief description

The process of incremental sheet metal forming (IBU) offers the possibility of manufacturing components in small batch sizes in line with requirements and economically. The application behaviour of manufactured components is largely determined by the prevailing residual stress state. The aim of the research project is to improve the application behaviour of incrementally formed high-strength components through the targeted adjustment of the residual stress state in the industrial manufacturing process. The residual stresses are introduced into the workpiece in a locally defined manner through the process control of incremental sheet metal forming in order to improve the component properties. In the transfer project with Faurecia Autositze GmbH, the findings of the priority programme are transferred to industrial requirements. In addition to component performance under static and cyclical loads, the focus is on the geometric accuracy of the manufactured components and the reduction of process time.

Brief description

As part of this project, the mechanical and corrosive material behaviour of heat-resistant steels in H2/CH4 combustion atmospheres under industry-relevant boundary conditions in the context of hydrogen admixture to natural gas through to pure hydrogen use in thermal processing plants is being investigated. The results of these investigations are available to plant manufacturers, service companies and material suppliers. This enables a more precise design of future plants as well as a targeted adaptation of existing plants when using hydrogen. The knowledge gained can be used to extend cost-intensive maintenance intervals and avoid unplanned system failures. Overall, this project makes an important contribution to the safe and efficient utilisation of hydrogen in thermal processing plants. The data and methods provided will help companies and SMEs to promote the introduction of hydrogen as a fuel in industry and enable the transition to a low-carbon and sustainable energy supply.

Brief description

New drive technologies and energy storage systems must be developed to significantly reduce CO2 and NOx emissions. The topics of hydrogen and lightweight construction are moving to the centre of attention here. Additive manufacturing processes, such as powder bed-based laser melting (LPBF) and laser metal deposition (LMD) – each in combination with high-performance materials, are the key technologies for achieving this goal. LPBF opens up unique possibilities for the designer to optimise the design of system components to the prevailing loads. This design freedom allows the generation of the highest functional packing densities, which significantly increases the degree of lightweight construction. This process has already been tried and tested for the titanium alloy Ti-6Al-4V, particularly with regard to high-performance materials, and is also qualified for aerospace applications. However, this alloy is very sensitive to hydrogen embrittlement in the α+β-state. It should be possible to significantly improve this property by adapting the process control in the LPBF process depending on the geometry. A complementary link is also being sought with the development of processing parameters for the LMD process. The locally limited generative material application allows the constructive realisation of a layered composite design. The functional separation of surface and base material also offers load-collective solutions. The high degree of design freedom of the locally limited material application also allows the specific requirements of a repair to be taken into account.

Brief description

In over 90 % of cases, cyclic loading is the cause of the failure of structural components. In addition to fatigue strength, crack propagation resistance is of decisive importance for the technical applicability of a material. In order to further develop materials, it is necessary to intervene conceptually in the microstructure. In classic alloys with a property-determining base element, however, this approach is increasingly reaching its limits. The situation is different with high-entropy alloys (HEAs) and medium-entropy alloys (MEAs), whose development potential is largely untapped. MEA CrCoNi is a promising representative of this alloy concept for use under cyclic stress. On the one hand, it has one of the highest measured fracture toughnesses compared to established alloys, and on the other, CrCoNi is characterised by a high threshold value for crack propagation. This is significantly influenced by the chemical composition of the alloy as well as by the targeted introduction of microstructural defects. The latter method in particular allows the microstructure of a material to be customised to the respective application.

Brief description

Plasma electrolytic oxidation (PEO) is an innovative process for producing ceramic protective coatings. Complex-shaped aluminium components can be qualified for areas of application that require high wear and corrosion resistance, so that a contribution to resource efficiency can be ensured through lightweight material construction. A limiting factor for the areas of application is the reduction in fatigue strength resulting from the PEO due to the brittle nature of the coating. However, since cyclic stresses are present in most areas of application, particularly in moving systems, it is necessary to increase the damage tolerance of the coatings and thus the service life of the coated component. Ceramic bulk materials that meet high requirements in terms of cyclic load-bearing capacity are generally based on zirconium oxide, as this has a higher fracture toughness compared to aluminium oxide. The aim of the proposed project is therefore to improve the fatigue strength of plasma-electrolytically oxidised aluminium substrates by forming crack-resistant, damage-tolerant composite ceramic layers consisting of Al₂O₃/ZrO₂. In order to achieve this goal, stable REACh-compliant Zr-containing electrolytes are being developed that enable the production of these layers using PEO. The focus is on a comprehensive understanding of the incorporation mechanisms of ZrO₂ phases into the aluminium oxide layer as a function of the PEO process regime as well as the evaluation of the influence of the zirconium oxide phases on crack initiation under cyclic loading and the local crack toughness in correlation with the layer microstructure and the damage tolerance of the composite ceramic layers.

Brief description

The aim of the project is to develop and validate a model for predicting the development of nitriding zones and the property profile of steels after plasma nitriding. Special focus is placed on the diffusion-dependent nitrogen distribution and the reciprocal influence of the carbon present in the steel. By using diffusion-reaction and cross-diffusion approaches, existing models for the simulation of plasma nitriding processes are extended in such a way that, in addition to the nitrogen concentration, the carbon concentration and the microstructure development within the compound layer are also calculated. The required diffusion coefficients and solubility limits of the occurring iron nitride phases and the matrix are determined by means of derivation and numerical approximation of inverse problems for carbon and nitrogen. The aim is to describe the diffusion coefficients and solubility limits as a function of temperature and alloy. All input and validation data required for the modelling are determined experimentally on a full-factorial test matrix with variation of the process parameters temperature, time and atmospheric composition and the resulting nitriding zones are characterised structurally, chemically and mechanically. The overarching aim of this project is to predict the complex dependencies between process parameters and the resulting layer properties during plasma nitriding by applying the nitriding model that has been developed. This will eliminate the need to evaluate large parameter windows for steels with different chemical compositions.

Brief description

The use of cost-effective ferritic stainless steels, which conserve raw materials, is to be investigated for water electrolysis. This is necessary because bipolar plates (BPP) for electrolysers are significantly thicker and larger than BPPs for conventional fuel cells (FC). Compared to the frequently used austenitic stainless steels, ferritic steels do not contain nickel, but much cheaper chromium. However, the problem is that without nickel, formability and corrosion resistance are reduced. So far, ferritic BPPs can therefore only be used in fuel cells with flat and therefore inefficient flow fields. In addition, the medium and the high electrical voltages are critical in water electrolysis and lead to rapid degradation of the steels without further surface adaptation. Studies have shown that, due to the chromium content in ferritic steels, self-passivation layers with very high corrosion resistance and sufficiently high electrical conductivity can be produced. The aim of the project is to investigate this approach in depth and transfer it to ferritic BPPs for electrolysis. The surface of the ferritic BPPs is to be treated or coated using different processes. The formability of the steels is to be significantly enhanced through adapted forming process strategies so that flux fields with sufficiently deep channel structures are generated, thereby increasing the efficiency of the novel bipolar plates.

• Zoz Simoloyer CM08-8l high-energy ball mill
• High-energy ball mill Zoz Simoloyer CM01-2l / SiN
• Planetary ball mill Fritsch Pulverisette 5

• Powder atomisation system
• AMC casting device
• Electric arc furnace (utilisation via PVW professorship)

• Spark plasma sintering plant (SPS) (utilisation via the PVW professorship)

• Various muffle and protective gas furnaces and quenching baths

• Prusa MK3S+ filament/granule printer

• Materialographic preparation technique
• Electron microscopic preparation technique
• Light microscopic examination technique
• Quantitative microstructure analysis OLYMPUS a4i
• DURAMIN microhardness tester
• Recording hardness measurement FISCHERSCOPE HM2000 XYm
• Scanning electron microscope LEO1455VP with X-ray micro-range analysis EDAX GENESIS
• NEON40EsB field emission scanning electron microscope with EDXS and EBSD
• HITACHI H8100 transmission electron microscope with EDAX GENESIS X-ray micro-range analyser
• Nanoindenter UNAT

• Glow discharge spectrometer SPECTRUMA GDA750
• X-ray analysis (XRD)

• Netzsch STA 409 C
• Netzsch DIL 402 E
• Netzsch TMA 402

• RUMUL Testronic resonance testing machine - Fatigue tests
• SincoTec Power Rotabend rotating bending test machine - Fatigue tests
• Servohydraulic testing machine MTS Landmark - Fatigue tests
• RUMUL Testronic resonance testing machine - crack propagation tests

• MatLab
• Mathcad
• Deform
• Fluent
• JMatPro
• ABAQUS
• ANSYS
• MemBrain

• Model tests in the laboratory
• Boiling test (intergranular and stress corrosion cracking test)
• Corrosion test chamber HK 400
• Potentiostat PS6 (determination of current density potential curves)
• Electrochemical MiniCell (ECMC)
• Pitting and crevice corrosion test, ASTM G48
• Climate test chamber
• Micro corrosion cell
• Corrosion test chamber HK 430

• 3-D profilometer
• pH value measuring device
• X-ray fluorescence material analyser
• Colour and gloss meter BYK Gardener spectro-guide

• Miller test
• Rubberwheel
• Universal tribometer
• Vibration wear tribometer
• Taber burner
• Scratch test
• Touch probe
• SRV tribometer
• Pin disc / ball disc Test bench

• 3-D profilometer
• Coating thickness gauge
• Coating thickness tester according to the calotte grinding method (e.g. DIN EN 1071-2)
• X-ray fluorescence material analyser