Open Access

Schädigungsmechanismen von Materialien unter Belastung (Druck, Zug und Torsion) lassen sich in situ durch Röntgen-Computertomographie (XCT) untersuchen. Mit der in-situ Methode können Schäden durch neue oder bestehende Defekte in inkrementellen Verformungsstufen dreidimensional untersucht werden, ohne die Probe zwischen den XCT-Messungen entlasten zu müssen. Dafür hat die BAM einen für XCT-Untersuchungen kompatiblen 20-kN-Zug-/Druck-/Torsions-Prüfstand mit einer μCT-Anlage kombiniert. Diese verfügt über eine 240kV-Mikrofokusröntgenröhre und zwei Flachdetektoren mit unterschiedlichen Auflösungen. Eine Anwendung dieser Methode erfolgt im Rahmen des DFG-Projekts „Verformungs- und Versagensmechanismen von metallischen additiv gefertigten dreifach periodischen Minimalflächenstrukturen“, welches die Verformung unter Druckbelastung von additiv gefertigten Gyroid-Strukturen (Triply Periodic Minimal Surfaces, TPMS) untersucht. Der Projektpartner Mid-Sweden University stellt die Gyroidstrukturen mit Hilfe der Elektronenstrahlschmelztechnik (EBM) her. Die BAM analysiert die Proben, um den Einfluss der Struktur der Gyroide und ihrer durch den Druckprozess bedingten Ungänzen auf das Verformungs- und Schädigungsverhalten unter mechanischer Belastung zu verstehen. In den XCT-Bilddaten lässt sich die Verformung mit Hilfe digitaler Volumenkorrelation (DVC) nachverfolgen und als dreidimensionales Dehnungsfeld darstellen. Diese Methode verbessert das Verständnis des Verformungsverhaltens komplexer additiv gefertigter Strukturen und trägt somit zur Sicherheit in der industriellen Anwendung additiv gefertigter Bauteile bei.

The actual environmental challenges require a huge effort from all industrial sectors to reduce their emissions of greenhouse gases and pollutants. Aeronautics is one of the most emissions-intensive industrial sectors. In that context, two main lines of research should be tackled: (i) new and more energy-efficient processes, such as additive manufacturing, could be used for topology optimized part production; and (ii) the engine efficiency of airplanes should be significantly improved to reduce gas emissions. The latter can be achieved by increasing the engine thermal efficiency, i.e., increasing the turbine inlet temperature. However, only single-crystalline cast materials are currently available for use in the parts of the gas turbine engine that are subject to the highest thermal loads. And these materials - which rely on a special casting technology - lose these original material performances when additive manufactured. In addition, current materials suitable for metal additive manufacturing have a limited range of temperature applications. Therefore, the focus is on the development of new materials targeting higher in-service operation temperatures and durability. Recently, a new Ni-based superalloy (VDM 780) has been developed to ensure microstructural stability up to 750 °C and to be suitable for additive manufacturing like the classic Inconel 718 alloy (extensively used for aerospace applications but with a maximum in-operation temperature limited at 650°C). However, literature currently lacks characterization of the mechanical properties of VDM 780 processed by additive manufacturing. Hence, the goal of this work is to provide a deeper understanding of the high-temperature fatigue properties of this alloy. For that purpose, a comparison of the mechanical properties, fatigue lives, and fatigue crack initiation mechanisms at room temperature, 300°C, 650°C, and 750°C has been conducted. This approach enabled the determination of the fatigue behavior of VDM 780 at elevated temperature. Fractographic analyses revealed fatigue crack initiation at manufacturing defects. Therefore, Hot Isostatic Pressing (HIP) was applied to reduce the amount of porosity. As expected, early results pointed out a trend towards increasing fatigue lives.

Temperature represents a key characteristic in additive manufacturing (AM) processes for metals. As a physical quantity, temperature provides a direct measure of the actual process state and can be an indicator for the process quality. Consequently, process evaluation based on temperature measurements, rather than solely on the monitoring of process radiation in gray values, is expected to offer increased robustness and explanatory power. However, dependable quantitative in-situ temperature measurements remain highly challenging due to extreme temperature gradients, emissivity changes and, in case of the widely used process laser powder bed fusion of metals (PBF-LB/M), due to the required high spatial and temporal resolution. To nevertheless obtain reliable temperature measurements, besides other factors, a robust thermal calibration of the measurement system is fundamental. The de facto standard for the thermal calibration of thermal camera systems is the use of black-body radiators, as set out in several technical norms and standards (among others: ASTM E1933, IEC 62942, VDI 5585, VDI 3511). However, especially in the visible and near-infrared (VIS-NIR) range, this is not always feasible in practical applications. Apart from the high costs of calibrated blackbody radiators, their calibration is usually not valid in this wavelength range and significant deviation to black body radiation occurs. Furthermore, their physical size usually prohibits use within a PBF-LB/M build chamber, so it is not possible to calibrate the entire optical path with this type of calibration source. This limitation is exacerbated by machine- and system-specific optical interfaces (e.g., protective windows and viewports) that must be traversed in operation and can alter transmission and spectral response. In-situ solutions are therefore recommended, as these enable calibration of the complete on-machine optical path. In this contribution, three different proof-of-concept approaches for the thermal calibration of monitoring systems for the PBF-LB/M process are presented and compared: 1) Calibration using blackbody-like radiation sources - For this purpose, the suitability of the pipetting hole of the graphite tube of a graphite furnace atomic absorption device and a graphite cylinder heated by the PBF-LB/M process laser with a suitable borehole are examined. 2) Calibration using a calibrated halogen light source and an isotopic (Ulbricht) sphere with a known color temperature – as special case for the VIS/NIR range and 3) the single-point calibration at the solidification plateau of molten metal samples – the melting also takes place within the PBF-LB/M process chamber using the process laser. While, as described, some of the approaches presented here are ex-situ, others can be carried out in situ in the PBF-LB/M process chamber, allowing the calibration of the complete optical path. As a ground-truth, additional measurements at a calibrated blackbody radiator were performed. The calibration approaches are tested using a Multispectral Optical Tomography (MS-OT) sensor system. MS-OT operates in the VIS-NIR range and constitutes an approach for determining apparent maximum surface temperatures Tmax during the PBF-LB/M process.

Reliable thermal imaging during metal additive manufacturing (AM) enables early detection of process anomalies and supports non-destructive quality assurance. We present a process-integrated approach using hyperspectral high-speed thermography in the short-wave infrared (SWIR) range for laser powder bed fusion (PBF-LB/M) and multispectral thermography in the mid-wave infrared (MWIR) range for directed energy deposition (DED-LB/M). Temperature and spectral emissivity maps are reconstructed simultaneously via temperature-emissivity separation, while uncertainty is quantified through reconstruction residuals. These thermal metrics correlate with defect formation mechanisms such as lack-of-fusion and overheating, enabling inline anomaly detection. The method is experimentally validated under industrial conditions by correct reconstruction of solidification temperatures. By providing reliable in-process thermal data, this approach lays the foundation for inline NDT strategies and standardization in metal AM.

The talk introduces the BAM as a federal research institute. Main areas of research and career paths are highlighted. Current research projects with a focus on ceramic materials are presented to showcase the bandwith of topics worked on in Division Advanced multi-materials processing.