Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Eingeladener Vortrag
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The project ProMoAM is presented. The goal of the project is to evaluate which NDT techniques or combination of techniques is suited for in-situ quality assurance in additive manufacturing of metals. To this end, also 3d-data fusion and visualization techniques are applied. Additional ex-situ NDT-techniques are used as references for defect detection and quantification. Feasability studies for NDT-techniques that are presently not applicable for in-situ use are performed as well.
The presentation gives a brief overview of the whole project and the different involved NDT-techniques.
Despite of the significant advances in AM process optimization there is still a lack of experimental results and understanding regarding the mechanical behavior and microstructural evolution of AMparts, especially in loading conditions typical for safety-relevant applications e.g. in the aerospace or power engineering. Within the scope of the presented investigations, a characterization of the fatigue behavior of additively manufactured Ti-6Al-4V in the low cycle fatigue regime was carried out in the range of 0.3 to 1.0 % strain amplitude at room temperature, 250°C and 400°C. The Ti-6Al-4V specimens are machined out of lean cylindrical rods, which were fabricated using powder laser metal deposition (LMD) with an improved build-up strategy. The improved strategy incorporates variable track overlap ratios to achieve a constant growth in the shell and core area. The low-cycle-fatigue behavior is described based on cyclic deformation curves and strain-based fatigue life curves. The lifetimes are fitted based on the Manson-Coffin-Basquin relationship. A characterization of the microstructure and the Lack-of-Fusion (LoF)-defect-structure in the as-built state is performed using optical light microscopy and high-resolution computed tomography (CT) respectively. The failure mechanism under loading is described in terms of LoF-defects-evolution and crack growth mechanism based on an interrupted LCF test with selected test parameters. After failure, scanning electron microscopy, digital and optical light microscopy and CT are used to describe the failure mechanisms both in the longitudinal direction and in the cross section of the specimens. The fatigue lives obtained are comparable with results from previous related studies and are shorter than those of traditionally manufactured (wrought) Ti-6Al-4V. In this study new experimental data and understanding of the mechanical behavior under application-relevant loading conditions (high temperature, cyclic plasticity) is gained. Furthermore, a better understanding of the role of LoFdefects and AM-typical microstructural features on the failure mechanism of LMD Ti-6Al-4V is achieved.
Despite of the significant advances in additive manufacturing (AM) process optimization there is still a lack of experimental results and understanding regarding the mechanical behavior and its relationship with the microstructural features of AM-parts, especially in loading conditions typical for safety-relevant applications. Within the scope of the presented ongoing investigations, a basic microstructural characterization, tensile tests at room and elevated temperature (400°C) as well as a characterization of the fatigue behavior of additively manufactured Ti-6Al-4V in the low cycle fatigue regime are carried out in the as-built state. After failure, different techniques are used to describe the failure mechanisms of the specimens. The AM-Specimens are provided by the Fraunhofer institute for production systems and design technology and investigated at the BAM following the philosophy of the TF-Project AGIL.
aRTist is an easy-to-use and practical simulation tool to generate realistic radiographic images from CAD objects. A dedicated add-on module makes CT simulation easy (virtual computer tomography). Just choose the number of projections.
Unlike in the practice, simulation can separately image the primary and scatter radiation. This allows studying the scattering artefacts in CT (gray-level variations in regions of homogeneous material).
Virtual CT acquisition and reconstruction of complex and noisy scanning trajectories in aRTist
(2019)
In modern CT imaging, simulation has become an important tool to minimize cost- and time-intensive measurements. It is increasingly used to optimize techniques for complex applications, to support the preparation of written procedures, and for educational purposes. We extended the CT simulation software ‘aRTist’ with a module to set-up arbitrary trajectories including disturbing influences during the scan. Moreover, such geometric deviations can be compensated by the internal reconstruction tool.
3D images such as those produces by X-ray tomography can provide a wealth of information on the internal structure of materials, but quantification of specific geometrical or topological characteristics linked to some bulk physical property is far from being straightforward. This study focuses on methods to quantify the differences in physical properties as a function of direction, i.e. their anisotropy, and how it can be linked to measures of anisotropy of the internal structure of the material. The auto-correlation function gives a similarity measure in the volume as a function of distance and direction. This is a cross-correlation of the image with itself fast to compute and relatively insensitive to noise. It is why we focus on this method to compare with the physical property of our DPF material. Diesel Particulate Filter (DPF) materials are porous ceramics that; a) can be used at very high temperatures; b) have very good thermal shock resistance; c) are inert; d) can be manufactured with tailored porosity. Their usual way of production consists of the extrusion of a slurry into the desired filter shape, with successive ceramming at high temperature. This process causes anisotropy at both microscopic and macroscopic levels.
There have been an increasing number of publications on flow chemistry applications of compact NMR. Despite this, there is so far no comprehensive workflow for the technical design of flow cells. Here, we present an approach that is suitable for the design of an NMR flow cell with an integrated static mixing unit. This design moves the mixing of reactants to the active NMR detection region within the NMR instrument, presenting a feature that analyses chemical reactions faster (5–120 s region) than other common setups. During the design phase, the targeted mixing homogeneity of the components was evaluated for different types of mixing units based on CFD simulation. Subsequently, the flow cell was additively manufactured from ceramic material and metal tubing. Within the targeted working mass flow range, excellent mixing properties as well as narrow line widths were confirmed in validation experiments, comparable to common glass tubes.
Diffraction-based experimental determination of Residual Stress in AM parts: A critical discussion
(2019)
Additive manufacturing (AM) technologies have experienced an exceedingly rapid growth, not always paralleled by the knowledge about the materials properties and performance. In particular, residual stress (RS) has been soon recognized as an issue in AM parts, so that parts are always post-heat-treated. Moreover, much effort has been spent on simulating RS, especially using finite element methods.
The experimental determination of RS has thereby become increasingly important, and even simple data constitute (to date) a piece of knowledge to fill the above-mentioned gap. In particular, diffraction methods, which are basically non-destructive, offer enormous possibilities to gain knowledge on real components, since neutrons and synchrotron radiation can penetrate even heavy metals up to several millimeters (or even centimeters). Indeed, some success has been obtained, and the knowledge about the origins of the RS fields, as well as their variation as a consequence of heat or mechanical treatments, has been greatly expanded.
In this talk, a few success stories will be outlined. It will be shown how the determination of RS in metallic parts (with focus on those produced by laser powder bed fusion) has even allowed showing that process parameters that were considered unimportant play a major role in the onset of stress.
However, while RS is starting to be considered in the part design, deposition strategy (e.g. build plate temperature), and even in the definition of the relevant metric to assess the quality of a part, much is still to be investigated about the hypotheses underlying its experimental determination. Therefore, some aspects to be aware of, or even to date unclear will also be discussed, such as the determination of the reference unstrained samples and of the principal axes of stress. All these aspects will draw the path towards a comprehensive understanding of the process-structure-performance relationships in AM materials and parts.
In this talk, the evolution of the demand for engineering applications of neutrons at the ILL in the last 10 years is presented. It is shown that small angle neutron scattering and diffraction are highly demanded by the materials science community. Further development in the field is foreseen, with the use of in-situ (furnace, tension/compression rig, humidity cells, etc.) techniques to associate to the neutron techniques.
The combination of tomographic, microstructural data with other experimental techniques and with modeling is paramount, if we want to extract the maximum amount of information on material and component properties. In particular, quantitative image analysis, statistical approaches, direct discretization of tomographic reconstructions represent concrete possibilities to extend the power of the tomographic 3D representation to insights into the material and component performance. This logic thread equally holds for industrial and academic research and valorizes expensive experiments such as those carried out at synchrotron sources, which cannot be daily repeated.
I will show a few examples of possible use of X-ray tomographic data for quantitative assessment of damage evolution and microstructural properties, as well as for non-destructive testing. Examples of micro-structured inhomogeneous materials will be given, such as Composites, Ceramics, Concrete, and Additively manufactured parts. I will also show how X-ray refraction computed tomography (CT) can be highly complementary to classic absorption CT, being sensitive to internal interfaces.
Additionally, I will show how Neutron Diffraction, which is extremely well suited to the study of internal stresses, both residual and under external load, can well be coupled to the microstructural framework gained by CT, allowing understanding the microstructure-property relationships in materials.