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The overview of the activity of Federal Institute for Material Research and Testing (BAM, Belin, Germany) in the field of additively manufacturing material characterization will be presented. The research of our group is focused on the 3D imaging of AM materials by means of X-ray Computed Tomography at the lab and at synchrotron, and the residual stress characterization by diffraction (nondestructive technique). Also, two successful research project in collaboration with CAM2, Sweden are presented.
X-ray refraction is analogous to visible light deflection by matter; it occurs at boundaries between different media. The main difference between visible light and X-rays is that in the latter case deflection angles are very small, from a few seconds to a few minutes of arc (i.e., the refraction index n is near to 1). Importantly, deflection of X-rays is also sensitive to the orientation of the object boundaries. These features make X-ray refraction techniques extremely suitable to a) detect defects such as pores and microcracks and quantify their densities in bulk (not too heavy) materials, and b) evaluate porosity and particle properties such as orientation, size, and spatial distribution (by mapping). While X-ray refraction techniques cannot in general image single defects, they can detect objects with size above a few wavelengths of the radiation.
Such techniques, especially at the Synchrotron BESSY II, Berlin, Germany, can be used in-situ, i.e. when the specimen is subjected to temperatures or external loads.
The use of X-ray refraction analysis yields quantitative information, which can be directly input in kinetics, mechanical and damage models.
We hereby show the application of non-destructive X-ray refraction radiography (SXRR, 2D mapping also called topography) to problems in additive manufacturing:
1) Porosity analysis in PBF-LM-Ti64. Through the use of SXRR, we could not only map the (very sparse) porosity distribution between the layers and quantify it, but also classify, and thereby separate, the filled porosity (unmolten powder) from the keyhole and gas pores (Figure 1).
2) In-situ heat treatment of laser powder bed fusion PBF-LM-AlSi10Mg to monitor microstructure and porosity evolution as a function of temperature (Figure 2). By means of SXRR we indirectly observed the initial eutectic Si network break down into larger particles as a function of increasing temperature. We also could detect the thermally induced porosity (TIP). Such changes in the Si-phase morphology upon heating is currently only possible using scanning electron microscopy, but with a much smaller field-of-view. SXRR also allows observing the growth of some individual pores, usually studied via X-ray computed tomography, but again on much smaller fields-of-view.
Our results show the great potential of in-situ SXRR as a tool to gain in-depth knowledge of the defect distribution and the susceptibility of any material to thermally induced damage and/or microstructure evolution over statistically relevant volumes.
For how trivial or provocative it can sound, the best neutron spectrometer in the world does not produce science and technology by itself. By definition of “Materials Science”, neutron scattering data on engineering materials must be used as a tool to understand, and even tailor, materials performance. In order for this to happen, neutron data need to be
1. Acquired under the most relevant condition possible
2. Coupled to other experimental techniques
3. Capitalized by means of proper simulations and data analysis
Point 1- calls for an intense use and the development of top-notch of in-situ techniques; Point 2- means that the sole use of neutron data will not lead to any solution of a global problem; All points above hint to the fact that access to neutron sources is not routine, and therefore it is imperative to search ways to make neutron data rentable and sustainable for the material science and industrial research community.
In this presentation, and based on two examples, we will show a couple of strategies to combine neutron data with other experiments, and with theoretical models to raise the validity of experiments to the level of problem-solving. As one might imagine, these are only a few among the almost infinite combinations possible to help improving material properties, performance, and safety, i.e., ripe for everyday use.
X-ray refraction is analogous to visible light deflection by matter; it occurs at boundaries between different media. The main difference between visible light and X-rays is that in the latter case deflection angles are very small, from a few seconds to a few minutes of arc (i.e., the refraction index n is near to 1). Trivially but importantly, deflection of X-rays is also sensitive to the orientation of the object boundaries. These features make X-ray refraction techniques extremely suitable to a) detect defects such as pores and microcracks and quantify their densities in bulk (not too heavy) materials, and b) evaluate porosity and particle properties such as orientation, size, and spatial distribution (by mapping). While X-ray refraction techniques cannot in general image single defects, their detectability is simply limited by the wavelength of the radiation.
We thereby show the application of X-ray refraction 2D mapping (topography) and tomography to different sorts of problems in materials science and technology: 1) Sintering of SiC green bodies; 2) Porosity analysis in additively manufactured alloys; 3) Fiber de-bonding in metal and polymer matrix composites.
Such techniques, especially at the Synchrotron BESSY II, Berlin, Germany, can be used in-situ, i.e. when the specimen is subjected to temperatures or external loads. Applications of in-situ X-ray refraction radiography on aluminum alloys and composites are also shown.
The use of X-ray refraction analysis yields quantitative information, which can be directly input in kinetics, mechanical and damage models.
The residual stresses and load transfer in multiphase metal alloys and their composites (with both random planar-oriented short fibers and particles) will be shown, as studied by neutron diffraction, by X-ray computed tomography, and by a model based on the reformulation of classic Maxwell’s homogenization method.
Contrary to common understanding and state-of-the-art models, we experimentally observe that randomly oriented phases possess non-hydrostatic residual stress. Moreover, we disclose that the unreinforced matrix alloy stays under hydrostatic compression even under external uniaxial compression.
The recently developed modeling approach allows calculating the residual stress in all phases of the composites. It rationalizes the presence of deviatoric stresses accounting for the interaction of random oriented phases with fibers having preferential orientation. It also allows the explanation of the unconventional in-situ behavior of the unreinforced alloy and the prediction of the micromechanical behavior of other similar alloys.
Additiv gefertigte (AM) dreifach periodische metallische minimale Oberflächenstrukturen (TPMSS, aus dem Englischen Triply Periodic Minimum Surface Structures) erfüllen mehrere Anforderungen sowohl im biomedizinischen als auch im technischen Bereich: Abstimmbare mechanische Eigenschaften, geringe Empfindlichkeit gegenüber Herstellungsfehlern, mechanische Stabilität und hohe Energieabsorption. Allerdings stellen sie auch einige Herausforderungen in Bezug auf die Qualitätskontrolle dar, die ihre erfolgreiche Anwendung verhindern können. Tatsächlich ist die Optimierung des AM-Prozesses ohne die Berücksichtigung struktureller Merkmale wie Fertigungsgenauigkeit, interne Defekte sowie Oberflächentopographie und -rauheit unmöglich. In dieser Studie wurde die quantitative zerstörungsfreie Analyse von Ti-6Al-4V-Legierung TPMSS mit Hilfe der Röntgen-Computertomographie (XCT) durchgeführt. Es werden mehrere neue Bildanalyse-Workflows vorgestellt, um die Auswirkungen der Aufbaurichtung auf die Wanddickenverteilung, die Wanddegradation und die Verringerung der Oberflächenrauheit aufgrund des chemischen Ätzens von TPMSS zu bewerten. Es wird gezeigt, dass die Herstellungsgenauigkeit für die Strukturelemente, die parallel und orthogonal zu den hergestellten Schichten gedruckt werden, unterschiedlich ist. Verschiedene Strategien für das chemische Ätzen zeigten unterschiedliche Pulverabtragsfähigkeiten und damit ein Gradient der Wanddicke. Dies wirkte sich auf die mechanische Leistung unter Druck durch die Verringerung der Streckspannung aus. Eine positive Auswirkung des chemischen Ätzens ist die Verringerung der Oberflächenrauhigkeit, die möglicherweise die Ermüdungseigenschaften der Bauteile verbessern kann. Schließlich wurde XCT eingesetzt, um die Menge des zurückgehaltenen Pulvers mit der Porengröße des TPMSS zu korrelieren, wodurch der Herstellungsprozess weiter verbessert werden kann.
Additively manufactured (AM) metallic sheet-based Triply Periodic Minimal Surface Structures (TPMSS) meet several requirements in both bio-medical and engineering fields: Tunable mechanical properties, low sensitivity to manufacturing defects, mechanical stability, and high energy absorption. However, they also present some challenges related to quality control. In fact, the optimization of both the AM process and the properties of TPMSS is impossible without considering structural characteristics as manufacturing accuracy, internal defects, and as well as surface topography and roughness. In this study, the quantitative non-destructive analysis of TPMSS manufactured from Ti-6Al-4V alloy by electron beam melting was performed by means of laboratory X-ray computed tomography (XCT).
Additively manufactured (AM) triply periodic metallic minimum surface structures (TPMSS, from the English Triply Periodic Minimum Surface Structures) fulfill several requirements in both biomedical and engineering fields: tunable mechanical properties, low sensitivity to manufacturing defects, mechanical stability, and high energy absorption. However, they also present some quality control challenges that may prevent their successful application. In fact, optimization of the AM process is impossible without considering structural features such as manufacturing accuracy, internal defects, and surface topography and roughness. In this study, quantitative nondestructive analysis of Ti-6Al-4V alloy TPMSS was performed using X-ray computed tomography (XCT). Several new image analysis workflows are presented to evaluate the effects of buildup direction on wall thickness distribution, wall degradation, and surface roughness reduction due to chemical etching of TPMSS. It is shown that the fabrication accuracy is different for the structural elements printed parallel and orthogonal to the fabricated layers. Different strategies for chemical etching showed different powder removal capabilities and thus a gradient in wall thickness. This affected the mechanical performance under compression by reducing the yield stress. A positive effect of chemical etching is the reduction of surface roughness, which can potentially improve the fatigue properties of the components. Finally, XCT was used to correlate the amount of powder retained with the pore size of the TPMSS, which can further improve the manufacturing process.