Multifunctional Ceramic Filter Systems for Metal Melt Filtration

There exists an increasing pressure on the metal making and metal using industry to remove solid and liquid inclusions such as deoxidation products, sulfides, nitrides carbides etc. and thereby improve metal cleanliness. It is well known that size, type and distribution of non-metallic inclusions in metal exert considerable effects on the mechanical properties of the cast products. In terms of this contribution a new generation of metal qualities via melt filtration with superior mechanical properties for use in light weight structures and high demand construction materials are explored. The main target is an enormous reduction of non-metallic inclusions in the metal matrix by the use of intelligent filter materials as well as filter systems with a functionalizedFilter surface functionalization filter surface. Especially a new generation of combined refining filter systems will be illustrated. The metal melt comes first in contact with a reactive filter which generates gas bubbles in the melt as well as activates gas bubbles on the surface of the inclusions. As a result, a kind of flotation of the inclusions towards the slag on the surface of the melt takes place. Further the high reactivity as well as the gas bubbles contribute to the agglomeration of the fine inclusions to big clusters which flow due to buoyancy forces to the surface of the melt or are filtrated on the surface of active filters, which do not form gas bubbles but provide on their functionalizedFilter surface functionalization surfaces the same chemistry as the inclusions for a sufficient adhesion and as a result for a sufficient filtration of the inclusions. With this approach a purification higher than 95% can be achieved. Another topic is dealing with carbon-bonded filter materials based on environmentally friendly binder system based on lactose and tanninCarbon-bonded aluminalactose/tannin binder. Furthermore, functional calcium aluminate coatings in combination with carbon are studied with regard to their impact on the active/reactive filtration and flotation in steel melts, respectively. Another major focus is the investigation of water-soluble filter skeleton-templates, which are produced by 3D-hybrid-printing techniques and coated by flame sprayingFlame spraying technology. Subsequently, the filter skeleton-templates are removed in water, avoiding sharp-edged cavities inside the filter.

Investigations at the high-temperature confocal laser scanning microscopeHigh-Temperature Confocal Laser Scanning Microscope (HT-CLSM) (HT-CLSM) allow observing the interaction of non-metallic particlesInclusion in terms of potential filter material with endogenous particles of molten steel in the high-temperature rangeSteel melt. The respective particle velocities are determined from the particle movements and conclusions are drawn about the attractive forces of the particles. The interactions of exogenous Al2O3 particles, MgO and MgAl2O4 spinel particles, and CA6 calcium aluminate particles with endogenous constituents of molten steel X15CrNiSi25-20 are analyzed in the present work. Accompanying experiments were performed in a heating microscopeHeating microscope on the interaction between steel and MgO and steel and CA6. Scanning electron microscopy SEM/EDX/EBSD studies reveal not only the interactions of the non-metallic inclusions with each other, but also reactions of the molten steel with the exogenous particles that affect the agglomeration behavior. While exogenous and endogenous Al2O3 particles exhibit high attractive forces and almost no react with the molten steel, a liquid reaction layerLiquid layer forms around the magnesia particles, which leads to a reduction of the attractive forces. After dissolution of the reaction layer, the attractive forces increase. Spinel particles are surrounded by a strong meniscus in the observed steel melt. Endogenous particles moving toward the spinel do not adhere to the particle. Reactions were also observed when CA6 particles came into contact with molten steel. In this process, the calcium aluminate is depleted of calcium. Only loose connections of the exogenous Ca-depleted CA6 with endogenous Al2O3 particles have been detected.

The application of ceramic foam filters is state of the art in the casting of aluminum melt. Despite their industrial use since the 1960s, the filtration mechanisms are not fully understood. The Collaborative Research Center 920 investigated the influence of the filter surfaceFilter surface chemistry and wetting between filter and melt on the filtration efficiencyFiltration efficiency. The investigated filter surfaces based on Al2O3Al2O3, MgAl2O4MgAl2O4, 3Al2O3·2SiO23Al2O3·2SiO2 and TiO2TiO2 showed differences in the filtration efficiency and the wetting behavior whereby a good correlation for inclusions smaller than 110 µm was found–the larger the contact angleContact angles the higher the filtration efficiency. The results raised the question whether the intrinsic contact angle or the different roughnessRoughness of the materials caused the changes in the filtration efficiency. A filtration trial at the Hydro pilot filtration line revealed a strong influence of the filter surface roughness on the filtration efficiency. A filter coated with Nano-Al2O3, with a divergent intrinsic contact angle compared to be Al2O3 reference filter, showed no improvement of the filtration behavior indicating no influence of the intrinsic contact angle on the filtration.

The chapter will focus on the thermodynamic databaseMulticomponent thermodynamic databases development relevant to modeling the interactions between filter materials, coatings, and inclusions in steel and Al-alloy. The CALPHAD approach is applied to develop thermodynamic databases, i.e. the available phase diagram data and experimental thermodynamic values are used to optimize the parameters describing the Gibbs energy of phases which can exist in the system. Thermodynamic description of multicomponent systems is the basis for more advance simulation of technological processes. In the chapter, the fundamentals and theory of thermodynamic modeling will be discussed in detail, and the most important results obtained will be presented. Examples of the thermodynamic calculations applied for solution of technological problems will be discussed.

TheDensity Functional Theory (DFT) contribution focuses on the accurate prediction of heat capacities for intermetallicsIntermetallics, the estimation of reaction paths for coated and uncoated alumina foam filters in contact with metallic melts, and the investigation of thermally induced changes in various filters and filterFilter components. Density functional theory (DFTDensity Functional Theory (DFT)) was able to provide isobaric heat capacities for Al–Fe and Al–Fe-Si systems that outclassed the empirical Neumann–Kopp ruleNeumann-Kopp rule and matched the experimental values over a wide temperature range. Moreover, DFT calculations clarified that the formation of hercyniteHercynite at the interface between alumina filters and steel melt was the result of a solid-state reaction involving high concentrations of FeO. Ex-situ Raman spectroscopyRaman spectroscopy was used to compare carbon-bonded alumina filters using different bindersBinder from Carbores®PCarbores®P to environmentally friendly lactoseLactose/tanninTannin, as a function of heat treatment. For theseFiltercarbon-bonded carbon-bonded filtersFiltercarbon-bonded, the prominent D and G bands were used to confirm the existence of graphitization processes and determine the size of graphite clusters resulting from these processes. In order to investigate the pyrolysisPyrolysis processes occurring in selected binderBinder constituents of the lactose/tannin filters, the evolution of Raman spectra with temperature was analyzed via in-situRaman spectroscopyin-situ measurements. Wherever it was appropriate, experimental Raman data were compared with DFT-simulated spectra. Further, Raman spectroscopy was used to study the thermally induced formation of metastable alumina, helping to understand the structural changes that take place during the transformation of boehmiteBoehmite (γ-AlO(OH)) to corundum (α-Al2O3) via metastable transition phases: γ-Al2O3, δ-Al2O3, and θ-Al2O3.

The functionalization of ceramic foam filters aims typically at the enhancement of the thermal shock resistance and the reactivity of the filters with respect to specific inclusions and impurities. For this purpose, thermodynamically metastable phases are utilized that have a strongly defective crystal structure and/or nanocrystalline character. Such phases possess frequently better or even unique properties in comparison with their thermodynamically stable counterparts. However, the stability of metastable or defect-rich phases is usually impaired by microstructural changes, which occur during the contact of these phases with the metallic melt at high temperatures and which speed up finally the degradation of the functionalized filters. In general, the first step towards the stabilization of the thermodynamically metastable and/or defect-rich phases is the understanding of their microstructure and the microstructure changes accompanying the transition to the thermodynamically stable state. In this chapter, the thermally induced microstructure changes are illustrated on the examples of selected carbon containing binders and metastable alumina phases. In order to be able to describe the crystal structure and microstructure of these compounds in more details, which is required for the targeted development of the functional filter materials, complementary methods of crystal structure and microstructure analysis like X-ray and electron diffraction, X-ray and nuclear magnetic resonance spectroscopy and electron microscopy were combined and further developed.

The reactions between newly developed filter materials and metal melts containing various inclusions were analyzed under laboratory conditions. For the melt production, a Spark Plasma Sintering (SPSSpark Plasma Sintering) apparatus was utilized. The SPSSpark Plasma Sintering process provides very high heating rates, which emulate the thermo-shock during a real filtration process, and variable reaction times, which allows to simulate both, short and long filtration processes. The short-time filtration processes are relevant for die-casting, where one ton of a steel passes the filter in approx. 15 s, the long-term ones for continuous casting taking several hours. In the SPSSpark Plasma Sintering process, the convection of the steel meltSteel melt is suspended, which makes the interpretation of the interfacial reactions and reaction kinetics more straightforward and trustworthy. Furthermore, the SPSSpark Plasma Sintering process allows speeding up the reaction diffusionDiffusion kinetics by using tiny diffusion couples having the form of powder mixtures that contain the metal or alloy and the functional filter material under study. In such samples, the equilibrium state that is suitable for a direct comparison with the results of thermodynamic simulations can be achieved very quickly.

Fe is a detrimental impurity element in secondary, i. e. recycled, Al–Si cast alloysAl–Si cast alloys (Zhang et al. in Miner. Process. Extr. Metall. Rev. 33:99, 2012;Raabe et al. in Prog. Mater. Sci. 128, 2022;). It leads to decrease of castability and promotes crack formation due to formation of primary, Fe-containing intermetallicIntermetallic particles, e.g. plate-shaped β-Al–Fe–Si, coarse αh-Al–Fe–Si or αc-Al–(Fe,Mn,Cr)–Si in presence of further transition metal elements e.g. Mn and Cr. Successfully, dealing with such secondary Al–Si cast alloys contributes to sustainability, circular economy and reduction of energy consumption (Raabe et al. in Prog. Mater. Sci. 128, 2022;Taylor in Mater. Sci. Forum 689:429, 2011;). In the present chapter, a systematic understanding is provided for dealing with Fe impurities in secondary Al–Si alloys by. removal of FeRemoval of Fe on the basis of melt conditioningConditioning and metal melt filtration and modificationModification of Fe-containing phases into harmless microstructural components. In this context new insight is obtained into. the crystal structures of some relevant intermetallicIntermetallic phases occurring in secondary Al–Si alloys, thermodynamics and kineticsKinetics of phase formation during solidification and the interaction of different filter materials with the transition metal containing Al–Si alloys. The crystal structures of the β-Al–Fe–Si and δ-Al–Fe–Si phases and of their ordered variants were investigated. This allowed reliable distinction of occurring intermetallicIntermetallic phases, the αh-Al–Fe–Si, the αc-Al–(Fe,Mn,Cr)–Si, the β-Al–Fe–Si and the δ-Al–Fe–Si phase, especially by electron backscatter diffraction. While modification of the alloy composition by the Mn, Cr content and presence of other transition metal elements affect the thermodynamic properties of the phases, these elements also significantly affect the kineticsKinetics of phase formation during solidification at high cooling rates including the resulting phase morphology. The formation of primary, intermetallicIntermetallic phases during melt conditioning closely above the solidification temperature of the (Al)-solid solution can be utilized for the removal of FeRemoval of Fe by separating the primary, Fe-containing, intermetallic particles from the Fe-depleted Al melt. Removal of such particles by application of filters to increase the Fe-removal efficiency extends the filters’ use beyond the removal of nonmetallic inclusions, contributing to production of high-quality, recycled Al–Si alloys. Evaluation of wettability, chemical reactions and microstructure in the interaction region between the filter material and Al–Si melts and the Fe-depleted melt reveals a beneficial effect of C-bonded Al2O3 filter material.

This chapter presents the most important results of investigations on reactive filter materials for the purification of aluminum melts. Reactive filter materials were developed with the aim to remove impurities dissolved in the melt, such as hydrogen from liquid aluminum, by means of specific chemical interactions between the molten metal and the filter material. Selected ceramic foam filters, consisting of carbon-free and carbon-bonded ceramics, were used in their uncoated state as well as treated with various coatings. Numerous fundamental studies were carried out to evaluate the applicability of the new filter materials: sessile-drop-experiments, immersion and filtration tests, the metallographic evaluation of the used filters and the metal samples retrieved from these experiments. Interfacial reactions and the purity of the treated melts were determined with the help of these experiments, following microstructural analyses to obtain indications for the filtration properties and the potential chemical reactions between the filter material and the melt. As a result, it was possible to determine that spodumene, LiAl(Si2O6), positively influences the hydrogen porosity of aluminum castings when applied as a reactive filter material. Filtration alone already helps to prevent areas of increased macroporosity by calming the melt flow, but filter materials containing spodumene further affect microporosity in the castings in positive ways.

Cleanliness of magnesium alloy melts plays an important role in producing high-quality, lightweight products exemplary for the automotive or aircraft industry with high-quality standards. Therefore, the applicability of ceramic foam filters in the active and reactive filtration of magnesium alloys was explored as an option of metallurgical refining. Carbon-bonded alumina foams were selected in their uncoated as well as variously coated states. To evaluate the applicability of selected ceramic filter materials, various tests were applied, investigating filter properties, interface interactions, melt cleanliness and wetting behavior of the filter, such as sessile drop tests, immersion and filtration tests, and metallographic evaluation of AZ91. Immersion tests in AZ91 at 680 °C for up to 60 min showed the durability of coated and uncoated Al2O3-C and the formation of finely-structured MgO in-situ layers on any alumina- or spinel-containing surfaces in contact with molten AZ91. These layers, resulting from interface reactions with the melt, are regarded to have the potential to attract and bind oxidic inclusions from the metallic melt during filtration, as AZ91 samples showed reduced inclusion amounts after their contact with Al2O3-C filters. Another milestone was the synthesis of MgAlON as a filter coating, proven to be resistant towards the reactive magnesium alloy melt.

This chapter focuses on the application of a conventional attenuation based X-ray computed tomography for the investigation of porous and dense structural components in different stages of the manufacturing process and loading. Firstly, the image acquisition process, image processing and qualitative evaluation are introduced using reticulated foam filter and a nozzle component as examples. Secondly, the quantification strategies of the reconstructed volume data involving segmentation of targeted features and its geometrical characterization are presented. Thirdly, the issue of ex-situ investigations is outlined and discussed using differently sized carbon-bonded alumina filters subjected to thermomechanical loading. Fourthly, the interrupted in-situ compression testing of glass foam structures is demonstrated. Finally, the benefits and limitations of conventional X-ray computed tomography as analysis method for porous and dense materials are emphasized.

Even though ceramic foam filters for metal melt filtration have been used in the casting industry for many decades, the filtration mechanisms have not yet been satisfactorily determined. Due to the opaqueness of the melt and the need for high operating temperatures as well as the complexity of the aluminum casting process, filtration experiments are expensive and a detailed insight into the filtration process is hardly achievable. However, the analysis by means of a model system contributes to an essential understanding of the processes taking place. Metal melt systems are characterized by their high surface tension resulting in poor wettability of the solid surfaces in contact with the liquid melt. Therefore, the model system needs to exhibit both similar flow characteristics and wetting properties as the melt system to obtain reliable results. In this study, water was used as the model liquid and the wetting properties of the solid surfaces have been modified to mimic the characteristic wetting behavior of the melt-filter interface. The influence of filter and particle properties as well as process parameters on the filtration efficiency of ceramic foam filters have been investigated. In order to minimize possible overlapping effects in the determination of individual parameters influencing the separation efficiency, care was taken to vary only one parameter, so that the filtration is only dependent on one variable. Besides water-based filtration experiments, it is also possible to have a closer look on interaction forces between inclusion particles and the filter wall and how a higher filtration efficiency can be achieved. Here, too, a water-based model system is beneficial due to the same issues of available devices and costs.

In order to develop improved filters for metal melt filtration, different physical phenomena that take place during depth filtration of liquid metals need to be well understood. Due to the difficult accessibility of the process, the harsh process conditions and the randomness of the typically employed ceramic foam filters, representative experimental investigations are extremely difficult to perform and often provide only integral quantities or selective information. This chapter presents a numerical model for simulating the depth filtration of liquid metal at the pore-scale, i.e., fully resolving the complex filter geometry, which can also accurately handle the curved filter walls. In the model, the velocity and pressure distribution of the melt flow is obtained by the lattice-Boltzmann method and the temperature field is calculated using the finite volume method, while the transport and filtration of the inclusions are predicted by solving the equation of motion for particles in a Lagrangian reference frame. In order to obtain a consistent representation of the curved filter walls for both particle transport and fluid flow, the Euclidean distance field of the filter structures is employed. By comprehensive parametric studies, the sensitivity of the filtration process with respect to various geometric parameters and process conditions is investigated. Therefore, geometries of conventionally manufactured filters, acquired from 3D μCT scanning, as well as computer-generated filter structures are considered. Their performance is assessed by evaluating various effective properties, such as the viscous and inertial permeability and the filtration coefficient. The numerical predictions allow to draw conclusions with respect to the dominant physical mechanisms and are compared with those from simplified physical models, which are shown to be sufficiently accurate for the pre-screening of filters. On the basis of the detailed results, suggestions for improved filter geometries are made, depending on the considered filtration process. Further, simplified models for the prediction of the effective thermal conductivity of open-cell foams in presence and absence of radiation are presented and validated using the detailed numerical predictions.

This chapter contains a summary of thermodynamic investigations of ceramic filters, metal melts and their interaction, which can serve as a basis for the optimization of the filters as well as the casting and metal melt filtration process. First, the thermophysical properties of two different filter base materials are briefly discussed. Subsequently, after demonstrating measurement conditions and parameters, the effective thermal conductivities of filters with varying pore size, porosity and material measured by the Transient-Plane-Source method at temperatures up to 700 °C are presented. The experimental determination of radiative properties of the filters using a Fourier-transform infrared spectrometer with an external integrating sphere was compared to simple predictive methods. Finally, after performing experiments with air, a measurement section was created and further developed to determine the volumetric heat transfer coefficient during metal melt filtration. The first results obtained with aluminum melt are presented.In addition to the knowledge of heat transport processes, the understanding of the sorption and diffusion behavior of various gases in metal melts is important. Therefore, a thermogravimetric apparatus, which enables the direct determination of the mass changes caused by (ab-)sorption using a high-precision magnetic suspension balance, was modified especially for this measurement task.

In this chapter the fundamental principles of the interaction of poorly wetted particles with interfaces of particles and bubbles are investigated in a water-based model system in which the similarity of poor wettability of non-metallic inclusions by molten metal and the poor wettability of silanized metal-oxide-particles by water is utilized. Capillary forces, the presence of nanobubbles and absorption of gas layers accompany the decreased wettability and lead to strong attractive forces. The combined effect of wettability and surface roughness is analyzed in detail, employing a variety of Atomic Force Microscopy techniques, as well as theoretical modeling of capillary forces and retarded van der Waals Forces for layered substrates. These concepts are extended to investigate particle-bubble interactions at different approach velocities by Colloidal Probe Atomic Force Microscopy and analysis by the Stokes-Reynolds-Young–Laplace model. The influence of temperature effects on the particle–particle interaction is investigated by High Temperature Atomic Force Microscopy. Additionally, the suitability of different interaction potentials for the Molecular Dynamics simulation of sintering alumina nanoparticles is accessed. Macroscopic agglomeration and hetero-coagulation experiments in a baffled stirred tank provide an insight into the dynamics of agglomeration and hetero-coagulation at for the metal melt filtration typical inclusion concentrations and wettability states.

To capture and predict the chemo-thermo-mechanical behavior of ceramic foam filters, material models and simulation tools are required. The description of the thermo-mechanical inelastic behavior as well as the in-situ layer formation on reactive filters have been the aims of this subproject. Challenging aspects in the whole progress are the exact geometrical replication of the underlying foam structure of the filter and the lack of experimental data for many relevant loading cases. The software FoamGUI is developed to generate parametrized, periodic three-dimensional representative volume elements (RVE) of foam structures, which are used in continuum and fluid mechanical simulations as well as for 3D-printing. Calculation concepts are formulated to predict the inelastic deformation and failure behavior of ceramic open-cell foams under thermo-mechanical loading. First-order homogenization approaches are used to conclude from the mesoscopic behavior of the foam RVE to the macroscopic response of filter structures. A hybrid approach is developed in the established framework of rate-independent plasticity in combination with neural networks, which replace the plastic flow potential and the evolution equations of internal state variables. Another modeling aspect is motivated by the experimentally observed growth of an in situ layer during the so-called reactive phase of the filtration process. This phenomenon motivates the development of a model to describe diffusion, chemical reactions and phase transition processes of multi-phase/multi-component systems using the phase-field method. This allows the simulation of spatially and temporally resolved microstructure evolution leading to the layer formation.

Basic fluid dynamic processes of melt filtration have been investigated in order to increase the performance and efficiency of filtration systems in steelmaking, especially for continuous steel casting. Numerical simulations have been performed to investigate the interactions between filter structures and the mean melt flow, the development of endogenous non-metallic inclusion (NMI) populations in the flow, and inclusion removal from the melt. For this purpose, Euler–Lagrange models of the particle-laden flow have been developed. As a major finding, the reactive cleaning process of the melt has been proven to be a very efficient cleaning method. Here, inclusion removal is strongly improved by the lifting action of reactively generated gas bubbles at the NMI surfaces. Details of the reactive cleaning process and the combination of reactive cleaning and active filtration have been investigated, too. The prediction quality of the numerical models with regard to fluid flow and the efficiency of the employed filtration systems have been successfully examined by comparing numerical simulations with the results from experimental investigations in different water model experiments and the steel casting simulator.

The advent of additive manufacturing has vastly increased the design space of filters for metal melt filtration. In order assess the large amount of possible filter designs before they are actually manufactured, a virtual prototyping workflow based on HPC simulations is proposed. A major challenge are the large data volumes produced even by single CFD simulations of metal melt flow and even more so by virtual prototyping settings, where a large number of filter designs must be evaluated. The LITE-QA framework addresses the large data problem by providing data management methods supporting both the simulation and the analysis phase: In the HPC environment, simulation data are compressed and indexed. In the analysis phase, search engine-like capabilities allow, e.g., for focused visualizations of filter regions that meet the search queries of the analyst. Building on the LITE-QA framework, a virtual prototypingVirtual prototyping study involving 84 new filter designs for metal melt filtration is conducted. Statistical analyses and comparative visualizations provide insights into the effects of geometric modifications of a reference design. Promising filter designs are identified where simulation results indicate an improved filtration efficiency, while only having a moderate effect on melt flow pressure. These virtual prototypes are proposed as candidates for further testing as physical prototypes.

AFiltration efficiency special metal-casting simulatorSteel casting simulator allowed to investigate the behavior of nozzles and filters in contact with a steel melt under controlled atmosphere. First, the cloggingClogging of carbon-bonded alumina nozzles with different active or reactive coatingsReactive coatings was evaluated by introducing exogenous inclusions while monitoring the changes in melt flow with time. In addition, the microstructure of the nozzles was investigated by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy to assess the coating decomposition and phase transformations. In further approaches, endogenous inclusionsEndogenous inclusions were generated in situ by pre-oxidizing and deoxidizing the steel melt. Filters with different active and reactive coatings were immersed for different times in order to investigate the evolution of newly formed phases on the filter surface as well as characterize their cleaning performance based on the analysis of the solidified steel. The inclusions in the frozen steel melt were characterized with the aid of an automatic SEM – ASPEXASPEX-system, which identified the chemistry as well as the size and population of the inclusions. In the last approach, investigations of a new combined refining process based on the immersion of reactive filters and a subsequent filtration via carbon-free active filters was investigated, in order to remove the remaining clusters of inclusions.

The current contribution aims to find the optimal method for removing dramp element copper from molten steel scrap. A special reactive ceramic coating on filter ceramic substrates based on ZnAl2O4 was reported as an effective material for the decopperization process. Therefore, the interaction between the molten copper containing iron Fe-Cu alloys and ZnAl2O4 as well with pure alumina were investigated by applying the sessile drop method at low oxygen partial pressure atmosphere. Furthermore, selenium was found to have a great affinity to copper for forming copper selenide, later on selenium was added to liquid Fe-Cu alloy to verify its decopperizationDecopperization efficiency.

This chapter presents the interaction between carbon free active and carbon bonded reactive ceramic filter materials and molten iron by utilizing the sessile drop method. The most of carbon free ceramic filter materials showed non-reactive system during the interaction. After the interaction, the number, type, and the size of the non-metallic inclusions were registered with the aid of ASPEXAspex analysis. On the other hand, the carbon bonded materials showed a series of phenomenon during the interaction including the formation of oxide layers covered on the iron droplet and whiskersWhisker on the surface of the sample droplets and substrates. To explain these phenomena, the stability of the oxides was calculated. The calculated results indicate that the oxides that consisted of substrates could decompose under the present experimental conditions.

To prove the applicability of carbon-bonded refractories on basis of Al2O3-C for the filtration of metal melts, their mechanical properties such as compression and bending strength were investigated at elevated temperatures up to 1500 °C. The tests have been carried out on compact specimens and on real filter structures without and with functional coatings. Fracture mechanical tests were performed at room temperature and 1400 °C. In a further approach, the residual strength after contact of the filters with molten steel was determined at elevated temperatures. In addition, a new environmentally friendly binder system based on tannin and lactose has been evaluated.

Open cell ceramic foam filters are used to improve the quality of metallic cast products. They play a major role on reducing the number of inclusions within the microstructureMicrostructure of the cast product and restraining the liquid flow inside the mold. The newly developed carbon-bonded alumina ceramics are investigated considering the mechanical and thermal loads of the filtration process. The aim of this project is to assess the strength, the fracture mechanical behavior, and the damage properties of the filter material. Since the tiny struts of the foam have different properties than the common bulk material specimen, small specimens of millimeter size are needed. Within this work, a combination of experiments and numerical simulations are conducted for various mechanical tests at temperatures up to 1500 $${}^{\circ }\text {C}$$ ∘ C . The first test is the small punch test (SPT), where a small disk-like specimen is supported on a circular die and loaded with a spherical tipped punch until failure of the disk occurs. The small punch fracture stress is obtained from the experiments, as well as the corresponding parameters of the Weibull distribution necessary for the evaluation of the cold modulus of rupture (CMOR). Furthermore, a modified version of the SPT, the so called ball on three ball test (B3B), is carried out. In the B3B test, miniaturized disk-shaped specimens are supported with three spherical balls and loaded with a spherical tipped punch until failure. Hereby, the fracture mechanical parameters, such as the fracture toughness, are identified with the help of numerical simulations. Moreover, the Chevron notched beam test (CNB) is used to determine the fracture toughness of the ceramics. The final test is the Brazilian disc test (BDT), where compressive loads are exerted on the specimen leading to tensile stresses along the specimen diameter, perpendicular to the applied load direction. This test is employed to determine the Young’s modulus, the yield stress and the fracture toughness. In general the simplicity of all these tests, their setup, application, and results evaluation, in addition to the ease of specimens production, make them very attractive. The size of these specimens is about one order of magnitude smaller than that of common standard tests. The accompanying numerical simulations are inevitable to extract from the measurements the wanted mechanical properties.

This chapter concerns the influence of internal defects (i.e. nonmetallic inclusions, secondary phases and cast defects) on the fatigue lifetime of steel and aluminum alloys in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. The detrimental effect of internal defects depends on multiple factors such as size, morphology, chemical composition, test temperature or position in the material. Specimens were tested after active and/or reactive melt filtration processes of the materials which served to influence the amount and size distribution of internal defects. Fatigue experiments up to 109 cycles were carried out using ultrasonic fatigue testing equipment. In addition, in situ methods, as e.g. full surface view thermography and acoustic emission (AE), were applied to study the processes of crack initiation and propagation, which finally lead to fatigue failure. Furthermore, the cyclically strained samples were subjected to fractographic analysis and the S–N-curves were discussed according to the characteristics of the crack-initiating defects. Based on these investigations, an enhanced knowledge about the correlation of internal defects on the materials’ fatigue strength enables a specific melt filtration strategy adjusted to the materials’ service conditions.

This chapter presents results on the analysis of nonmetallic as well as intermetallic inclusions within a metal matrix. In both, steel and aluminum matrix these impurities cause detrimental effects during production as well as in service, e.g. under mechanical load. In steel, nonmetallic inclusions originate from the steelmaking process and range in the magnitude of ppm. In recycled aluminum alloys, iron-rich intermetallic phases exhibit a volume fraction in the range of percent caused by insufficient scrap separation. Both types of detrimental inclusions/precipitates were investigated within different materials such as case hardening steel, quenched and tempered steel as well as Al-Si cast alloy. In order to reduce the amount of impurities, the effects of appropriate crucible materials, reactive and active melt filtration and chemical composition of the used materials were studied. Therefore, extensive metallographic investigations on sections were conducted with optical microscopy, manual and automated scanning electron microscopy, focused ion beam preparation and transmission electron microscopy aiming to determine the compositions of inclusions and intermetallic phases. Focusing on the morphology of inclusions and intermetallic phases, experiments with electrolytic and chemical extraction as well as X-ray micro tomography were performed. The gained knowledge can be utilized to improve filtration and reduce volume fraction and size of nonmetallic inclusions and intermetallic phases. This enables the design of long-lasting and safe materials.

In ultrasonic fatigue testing of steels one can observe rapid local heating due to crack initiation and propagation caused by non-metallic inclusions and in addition also an overall heating of the gauge length portion of samples which is attributed to dissipational effects. The computations performed in this study are based on a three-dimensional, fully-coupled linear thermoelastic continuum model, where dissipation is included by employing a volumetric heat source. In the numerical computation the temperature distribution evolution in the geometry of interest is the result of a combination of initial conditions, boundary conditions and the heat source contribution. The heat source function's geometry and intensity are deduced by comparing computed temperature profiles to data obtained experimentally using a thermo-camera. It turns out that the modeling approach, making extensive use of thermography data, yields computational results that are in agreement with the experimental heat evolution, and additionally the amount of heat generated is in agreement with results found in literature.

This chapter presents results of investigations on the strength, deformation and toughness behavior of quenched and tempered 42CrMo4 steel. Intentional impurification and, afterwards, filtration by functionalized ceramic foam filters were applied in order to process cast steels with different amounts and distributions of non-metallic inclusions. As references, a hot-rolled steel batch and spark-plasma sintered materials were studied. The investigations focused on the loading rate and temperature effects. Both, tensile and fracture mechanics tests, were performed in order to investigate the damaging behavior due to non-metallic inclusions remaining after the melt processing of the steel. A further goal was to predict the fracture toughness of the material based on the combination of microstructural information on the inclusion distribution and the strain rate and temperature-dependent strength and deformation behavior. It was shown that the damaging effect of non-metallic inclusions, in particular agglomerated inclusions properties, is localized which leads to relatively low strain to fracture and fracture toughness, but also to crack path deflection. Furthermore, it could be observed that the small interparticle distances within agglomerated non-metallic inclusions determine the fracture toughness behavior of the materials. By analyzing the acoustic emissions, the onset of crack growth as well as the size of the plastic zone at the crack tip could be estimated.

Ceramic foam filtersFilter haveCasting been usedFiltration efficiency in aluminumAluminum foundries since 1974 to increase the purity of the melt. In practice, the implementation of the foam ceramic filterFilter in the castingCasting system is in most cases a question of space on the pattern plate. It does not follow any defined rules but is at best guided by design recommendations from the filter manufacturers. To increase the filtration efficiencyFiltration efficiency, the filterFilter position in the castingCasting system is examined. For this, 20 and 30 ppi filters were scanned with computer tomography (CT). The data from the CT were then loaded into the simulationSimulation program Flow 3D. With this program, four different filter positions as well as the influence of the filter length and roughness of the filter surface are investigated concerning their filtration effectiveness. The simulationSimulation results are subsequently evaluated with castingCasting trials. For this, four different molds for four different filterFilter positions were created. The same alloy (AlSi7) was used for the trials as for the simulation. To study the behavior of particles during the filtration process, impurities were added to the melt using 3 wt.% Al2O3 - Metal Matrix Composite. After the samples were casted, the filters are cut out and the Al2O3 particles in the filter are counted for each filter position. The comparability of the casting trials and the results of the simulation have been examined.

The filtrationFiltration of steel melts in the continuous castingContinuous casting of steel is of special interest in order to meet the ever-increasing purity requirements regarding the metal’s purity. Due to the high mass flow and the high casting duration, the application of in-built filters in the tundish system is rather challenging, especially regarding the filter capacity. In the framework of this contribution, a new concept involving the immersion of filters into the steel melt from above the tundish was investigated. This approach allows for the flexible exchange of used filters without interruption of the underlying process. At the Institute of Ceramics, Refractories and Composite Materials, carbon-bonded aluminaCarbon-bonded alumina filters on the basis of established slurry compositions and modified replication techniques were scaled up for the industrial application in continuous castingContinuous casting of steel. The best property profile was observed for a triple coating approach based on centrifugation, dip coating and spray coating. In the tundish of a steel casting simulator, a lab-scale casting test was conducted in order to check the thermos-mechanical behavior of the filter in contact with molten steel. After the successful evaluation in lab scale, the filters were tested in industrial trials in cooperation with the company thyssenkrupp Steel Europe AG. A ladle casting with immersed filter lasted approximately 45 min with a static casting speed of 8 to 10 t/min and a melt temperature of above 1550 °C. The filters were analyzed for inclusions and potential damage at the Institute of Ceramics, Refractories and Composite Materials. Dense clogging layers on the filter surface were identified as in-situ layers as usually observed in lab-scale steel casting simulator tests in consequence of reactive filtrationFiltration effects. Furthermore, inclusion clusters which were removed at the filter bottom due to the direct inflow in consequence of the buoyancy were observed as signs of active filtration. A further alternative to foam filtersFoam filters is presented by gel-cast spaghetti filter structures with mechanically robust full-strut structure. In the subproject T01K, the gel castingGel casting process was optimized in order to realize the necessary upscaling of the components. With the aid of alginate-based gel casting, spaghetti filter with organized lattice structures were manufactured and tested. Steel casting simulator trials showed favorable behavior in contact with the melt.

Research results of this chapter show a great potential to improve inclusion removal from steel melts using active and reactive exchangeable filtration systems in steelmaking. This contribution investigates numerically the performance and the efficiency of the reactive cleaning and active filtration in continuous casting tundishes. For this purpose, a Euler–Lagrange model of the disperse two-phase flow of steel melt and non-metallic inclusions has been developed. Here, implicit large eddy simulations have been employed to resolve the large-scale turbulent structures in the tundish flows. By means of multiphase flow simulations, two prototype tundish configurations (laboratory one-strand tundish, industrial-scale two-strand tundish) were researched with alumina-coated, carbon-bonded ceramic foam hybrid filters. The research aimed the investigation of the effect of the filtration system, e.g. filter position, filter shapes and filter size on inclusion removal. The results of the numerical simulations indicated the high cleaning efficiencies obtained by using a reactive filter system, where reactively generated carbon monoxide bubbles carried a high amount of inclusions to the slag. Moreover, it was concluded from the results that the contribution of active filtration to inclusion removal by the deposition of inclusions on filter surfaces was neglectable compared to the contributions of reactive cleaning.

Iron (Fe) provides a non-reactive dissolved impurity in aluminum (Al) alloys, which forms a coarse, plate-shaped intermetallic β-phase during solidification. This β-phase is detrimental to the mechanical and casting properties. Therefore, the reduction of Fe by binding in Fe-containing intermetal-lics (sludge phase) was realized via a two-stage procedure, which consisted of conditioning of the melt by manganese (Mn) and chromium (Cr) with subsequent-ly applied metal melt filtration. For this purpose, the formation characteristics of the Fe-rich intermetallic phases were investigated regarding the temperature, time, and initial chemical composition to separate these intermetallics from the residual melt. To evaluate the different process parameters of Fe removal for a potential implementation in lightweight metal foundries, a process technology on an indus-trial scale was developed in cooperation with an industrial partner. The examina-tion of samples in optical microscopy (OM) using image analysis were conducted to determine the area fractions of Fe-rich intermetallics. In addition, optical emis-sion spectrometer (OES) measurements were performed. Complementary investi-gations were achieved by scanning electron microscopy (SEM), with energy dis-persive spectroscopy (EDS), and electron backscatter diffraction (EBSD) to measure the partial chemical composition and for phase identification. The for-mation characteristics of the Fe-containing phases were investigated using DSC cooling curves and selective sampling in quenching experiments. In the experi-mental trials, a maximum reduction of iron of ≈50% was revealed compared to the unfiltered sample, whereby greater influence on the formation of α-intermetallics was inferred by temperature than by time. Moreover, the elements Mn and Cr were reduced by about 66% and 86% at 620 °C, respectively, thus, the element contents in the filtered samples approached the chemical composition of the standard alloy (EN-AC-AlSi9Cu3(Fe)).

Although continuous casting became the state of the art for the casting of ordinary steel grades, ingot casting by bottom teeming still has relevance in the steelmaking industry, especially for the manufacturing of specialty and alloy steels. As for every casting process, the ever-increasing quality requirements by customers lead to increased demand for new technologies to increase the purity of the cast steel melt regarding its inclusion content. Due to the special design of the bottom-teeming ingot casting facility and the discontinuous operation as batch process, the application of filters is a promising approach. Tailored foam geometries were prepared based on additive manufacturing via selective laser sinteringSelective laser sintering (SLS) and transformed into filters via modified replication techniques and flame spraying. Additionally to filter application, the functionalization and quality improvement of applied hollowware refractories has high potential to remove existing inclusions from the steel melt and avoid the formation of new inclusions during casting. The investigated hollowware components were manufactured by pressure slip casting on the basis of coarse-grained alumina compositions and subsequent functionalization by spray coating based on carbon-bonded alumina slurries. Simultaneous application of functionalized, “reactive” refractory components and flame-sprayed, “active” filters enables a combined filtration system which unites the advantages of the distinct filtration mechanisms. In the continuous casting of specialty steels, the conditions are more severe resulting in additional challenges regarding the application of filters. An approach investigated in this subproject is the use of extruded filter starter casting tubes above the tundish outlet. To achieve this, extrusion mixes based on cellulose derivatives and materials of the system Al2O3-ZrO2-C (AZC) were investigated for their suitability. The new concepts were tested in industrial casting trials in cooperation with the company Deutsche Edelstahlwerke Specialty Steels Europe GmbH (DEW). Post-mortem, the former melt-refractory interface of the applied components was investigated and steel samples from the ladle, the gating system and the ingot were analyzed in comparison to untreated samples.

The application of ceramic foam filters (CFF) ensures the achievement of the required mechanical properties of cast refractory products. Thereby, the present solid non-metallic impurities precipitate on the surface of the CFF struts and junctions. The Collaborative Research Center 920 deals with the removal of these impurities to increase the metal melt cleanliness in a basic scientific research approach. The findings are of high importance for metal processing industry and an application transfer aims to contribute towards an innovative and competitive casting industry. Subsequent, one promising approach is presented.

This chapterFatigue lives is focused on the fatigue life and damage mechanisms of steel 42CrMo4 in the high cycle fatigue (HCFHigh Cycle Fatigue (HCF)) and very high cycle fatigue (VHCFVery High Cycle Fatigue (VHCF)) regimes at temperatures up to 773 K. For this purpose, resonance fatigue testing was used at different test frequencies (90 Hz and 20 kHz). The influences of the manufacturing process (wrought or cast condition), as well as the core hardness (various heat treatment conditions), were investigated. Fractographic examinations of the fracture surfaces allowed the analysis of crack-initiating defects. Together with light microscopic observations of the defect distribution, the fatigue mechanisms of the steel 42CrMo4 were investigated at different temperatures (RT, 473 K and 773 K). A short crack model according to Chapetti applied to the present results was used to describe the change in the fatigue damage mechanisms operating at RT/473 K and 773 K, respectively. It is shown that high-temperature fatigue at 773 K was dominated by crack growth, whereas fatigue at RT and 473 K was dominated by crack initiation. These investigations complete the work presented in Chap. 24 , in which the influence of nonmetallic inclusions on the ultrasonic fatigue behaviour of steel 42CrMo4 is being analysed at room temperature. The present results provide important insights into the crack-initiating defects and their distributions as they are relevant in typical industrial applications.