Modelling and Simulation

Ziel dieses Projekts ist es, basierend auf prädiktiven numerischen
Simulationen die additive Herstellung von Bauteilen aus massiven
metallischen Gläsern durch selektives Laserstrahlschmelzen zu
ermöglichen. Es sollen geeignete Prozessstrategien erarbeitet werden,
die den amorphen Materialzustand möglichst ohne Alterungseffekte sowohl
im Volumen als auch für komplexe Geometrien gewährleisten. Dazu müssen
mittels der numerischen Simulation neben der Berechnung des
Temperaturfeldes und der Materialkonsolidierung während des
Fertigungsprozesses auch Aussagen über das Erstarrungsverhalten, die
Alterung und schließlich die Kristallisation möglich werden.

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The basic mechanisms that are essential in the powder based selective beam melting process are poorly understood. Most of the existing analytical and numerical models describing the process of consolidation in a homogenized image, i.e. individual powder particles are not resolved. This approach is suitable for information on averages, but cannot capture the local influence of the powder, i.e. the powder size distribution, the stochastic effect of the powder bed, the wetting of the powder by the melt and the formation of the melt. The actual selective melting process and thereby acting mechanisms can only be understood on the scale of the powder particles, with the help of numerical simulation on the mesoscopic scale. The aim of this project is to provide a numerical tool for mesoscopic simulation of selective beam melting and to use it to develop innovative process strategies. The mesoscopic scale allows the prediction of defects, surface quality and accuracy of the structure for different materials as a function of material parameters (powder form, bulk density, ...) and the process parameters (beam shape, energy per unit length, speed, ...).
In the first phase, a tool for the 2D simulation of selective electron beam melting was developed and validated with experimental results. The main task was the modeling of the entire build process with its different time scales (pre-heating, melting, applying new powder layer). Among other things, the complex coupling of the beam in the powder bed, radiation losses at the surface, mass and energy loss through evaporation and the deformation of the molten bath by the evaporation pressure is taken into account. The software is now able to simulate assembly processes, taking into account different scanning strategies on many layers. Such process strategies as the remelt strategy and the refill strategy are investigated. The verification of the numerical results is done in close cooperation with subproject B2.
In the second phase, the previous model is transferred to polymers. For this purpose, the absorption of the laser beam in the partially transparent stochastic powder bed and the highly viscous, viscoelastic material behavior must be described. Development and verification of the model is carried out in cooperation with subproject B3. In a further step, a method of 3D simulation of the grain structure in the selective beam melting of metals is implemented, in order to predict the texture of the materials as a function of process strategy.

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A new numerical tool will be explored that supports the experimental alloy developer in defining new compositions with potential for high strength. Starting with a composition space that is defined by the developer based on his metallurgical experience and his design goals, the numerical tool will propose the most promising compositions. The research program will on the one hand address open questions regarding the mathematical optimization in this application and on the other hand new models for predicting the relevant material properties.

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Beam-based additive manufacturing (AM) of metals in a powder bed not only offers the opportunity to build complex, custom-made components of high-performance materials, but also to adjust the local material properties by proficient processing. The variation of solidification conditions enables the modification of microstructure length scales. Additionally, latest research results indicate, that also the texture of the components is adjustable during manufacturing. Therefore, entirely new perspectives are opened regarding optimization of light weight components, because not only the topology, but also the texture of the material is adjustable to the local loads on the component. In order to comprehend and control the texture evolution, the hydrodynamic non-equilibrium solidification process (grain growth, selection and nucleation) needs to be fundamentally understood. Experimental investigations show that especially the mechanisms of nucleation under the extreme conditions of AM are insufficiently resolved and are not reproduced by classical models.The aim of this proposal is to identify, to fundamentally understand and to physically model the microstructure evolution, especially the nucleation under the special solidification. This model should be implemented in existing software, which is developed at our chair. Modeling and verification are experimentally substantiated basing on additively manufactured samples of IN718. At the end of the project the model should predict the solidification structure, grain structure and texture evolution during beam and powder bed-based AM.The project draws on our software for simulating the consolidation process during beam and powder bed-based AM. The software contains a lattice Boltzmann method to describe the hydro- and thermodynamics during melting and solidification. This method s coupled to a cellular automaton modeling the grain structure evolution during solidification neglecting currently grain nucleation. Our new theoretical ansatz contains besides the temperature gradient and the solidification front velocity for the first time additional information about the texture of the previous layers (orientation, spacing of cells/dendrites, segregation) and the local composition of the melt at the interface to the currently processed layer. It should be investigated, how orientation changes at the solidification front in combination with the present segregations in the rapidly melted material (memory of melt) induce grain nucleation by local undercooling. These findings are mathematically utilized for a grain nucleation model.

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The overarching goal of AMAZE is to rapidly produce large defect-free additively-manufactured (AM) metallic components up to 2 metres in size, ideally with close to zero waste, for use in the following high-tech sectors namely: aeronautics, space, automotive, nuclear fusion and tooling.

Four pilot-scale industrial AM factories will be established and enhanced, thereby giving EU manufacturers and end-users a world-dominant position with respect to AM production of high-value metallic parts, by 2016. A further aim is to achieve 50% cost reduction for finished parts, compared to traditional processing.

The project will design, demonstrate and deliver a modular streamlined work-flow at factory level, offering maximum processing flexibility during AM, a major reduction in non-added-value delays, as well as a 50% reduction in shop-floor space compared with conventional factories.

AMAZE will dramatically increase the commercial use of adaptronics, in-situ sensing, process feedback, novel post-processing and clean-rooms in AM, so that (i) overall quality levels are improved, (ii) dimensional accuracy is increased by 25% (iii) build rates are increased by a factor of 10, and (iv) industrial scrap rates are slashed to <5%.

Scientifically, the critical links between alloy composition, powder/wire production, additive processing, microstructural evolution, defect formation and the final properties of metallic AM parts will be examined and understood. This knowledge will be used to validate multi-level process models that can predict AM processes, part quality and performance.

In order to turn additive manufacturing into a mainstream industrial process, a sharp focus will also be drawn on pre-normative work, standardisation and certification, in collaboration with ISO, ASTM and ECSS.

The team comprises 31 partners: 21 from industry, 8 from academia and 2 from intergovernmental agencies. This represent the largest and most ambitious team ever assembled on this topic.

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Electron beam melting additive manufacturing is used to produce successive layers of a part in a powder bed and offers the ability to produce components closest to their final dimensions, with good surface finish. At this time the process is faster than any other technique of comparable quality, however the parts are not produced at sufficient rate to make them economically viable for any but very high value specific applications. One key output of the project will be the knowledge surrounding the use of the high powder electron beam gun, including the process control, and modeled and validated understanding of beam-powder bed interaction. The target objectives is the transfer of the 2D model to a 3D model and its parallel implementation. The outcome of the simulation will be compared with real experimental data and therefore the model parameters are adjusted in such a way that the resulting numerical melt pool sizes correspond to the experimental ones.

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Geschäumte Materialien stellen aufgrund ihrer zellularen Struktur eine interessante Materialklasse mit attraktiven Eigenschaften dar. Unabhängig vom Material ist die Schaumbildung im Allgemeinen wenig verstanden und die Schaumherstellung basiert im Wesentlichen auf dem Trial-and-Error-Prinzip. Die numerische Simulation eröffnet hier neue Wege, grundlegende Phänomene bei der Schaumbildung zu er-forschen und die daraus abgeleiteten Erkenntnisse praktisch umzusetzen. Basis für…

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A predictive software relies on exact physical and numerical models. The most important aspect is the correct modelling of the thermal conditions. Almost all modifications of process parameters have a direct influence on heat conduction, the coupling of the energy source or heat sinks by e.g. heat radiation or evaporation. Furthermore, many material parameters are temperature dependent and sensitive to a correct model. During melting a melt pool evolves, whose dynamics are mainly covered by capillarity, wetting, Marangoni convection and gravity. The temperature gradient and the solidification velocity mainly influence the final microstructure while solidification.

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The 2D simulation software SAMPLE^2D of selective electron beam melting bases on the software for modelling of foam formation. The base software is extended by certain modules comprising the electron beam absorption, phase transitions, (selective evaporation or grain structure evolution. After a careful experimental validation, the aim of this software is to predict process windows and explain process phenomena.

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Most process phenomena during selective electron beam melting are covered by a 2D simulation. A more realistic modelling of the melt pool dynamics and the grain structure evolution is reached by 3D simulations. Therefore, two different simulation tools for these purposes are developed at WTM.

The 3D hydrodynamics software SAMPLE^3D requires a massively parallel implementation, which has been developed in cooperation with the chair of system simulation. The melt pool dynamics and the material consolidation are investigated in full spatial dimension. Using this software, process windows for dense parts as well as innovative process strategy modifications are predicted.

The grain structure evolution is modeled by the separate software SAMPLE^3D_GS, which enables the grains to grow in all possible directions during processing. Here, a macroscopic approach is used, where the powder particles are approximated by a continuum. Additionally, only the thermodynamics are modelled. With these simplifications, domains on the scale of whole parts are possible to simulate.

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To study the selective laser melting of polymers, the current numerical model for metals is transferred for viscoelastic materials. Therefore, the energy coupling of the laser source is modified regarding the different absorption and reflexion behavior of photons on polymers, e.g., the material is semi-transparent in its liquid state. Additionally, the rheological model for highly viscous and viscoelastic materials from the foam formation is reused.

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In this group a numerical tool for multi-criteria optimization is developed. In particular, it is applied on the alloy development of nickel-based superalloys. The scientific research focuses on probabilistic and deterministic models to find all optimum solutions (pareto front) in the search space. By the development of the CALPHAD method in the recent decades, new opportunities to extend the classical alloy development by means of the prediction of physical properties and microstructure arose. Most property models base on thermodynamic and kinetic calculations, which are coupled by the TC API to the commercial software ThermoCalc and DICTRA.

The research is funded by the collaborative research center SFB Transregio 103 (http://www.sfb-transregio103.de/).

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Until today, metallic foams are not common despite their potential for energy absorption and ultra-light components. The main disadvantage are the inhomogenities of the pore structure, which includes variations in the pore size, geometry and wall size. The aim is to understand the underlying effects during foam formation to improve the process.

The implemented software bases on the lattice Boltzmann method, covers he most important physical effects during foam formation and is able to predict modified process strategies. The implementation comprises the hydrodynamic, diffusive and thermodynamic conservation equations applied on free surfaces. The physical models cover the growth, coarsening, reordering and collapse of foam bubbles as well as effects of the whole pore network like aging and drainage due to capillarity and wetting.

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Foam materials are due to their cellular structure an interesting material class with attractive properties. The software used for metal foams was extended to simulate the viscoelastic effects during foaming of polymers. Therefore, the numerical method was extended by a rheological model for viscoelastic fluids applied on free surfaces. With this software, the influence of different process parameters on the foam formation was studied.

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