Real-time tracking of additive manufacturing with high-energy X-ray techniques

Abstract

Additive manufacturing (AM) of metals offers unique design freedom and the ability to tailor the microstructure and properties of components. However, the complex thermal histories and rapid solidification occurring during AM introduce significant challenges in microstructure control and process optimisation. To address these challenges, this work employs real-time synchrotron techniques to elucidate the rapid phenomena that occur during AM. Synchrotron techniques, including synchrotron X-ray diffraction (XRD) and synchrotron radiography, are powerful tools for investigating various AM-related phenomena such as heat source-matter interaction, melt pool behaviour, solidification, and phase transformations in real time. High resolution temporal and spatial synchrotron data enable the correlation of these phenomena with AM processing parameters, thereby advancing the understanding of the AM process and its underlying mechanisms. These insights can be instrumental in process optimisation, alloy design, and the development of computational models.

The contribution of this work to the field of real-time studies in AM is structured into two parts. First, the design and implementation of an electron beam powder bed fusion (PBF-EB) sample environment for real-time synchrotron studies are detailed in Chapter 3. Second, real-time studies of solidification and phase transformations during AM are presented in Chapter 4.

The first part of this work focuses on the design and implementation of a sample environment for real-time synchrotron studies of the PBF-EB process. The sample environment facilitates the investigation of the previously listed AM phenomena during PBF-EB at high process temperatures and under vacuum. Furthermore, it enables the characterisation of phenomena specific to PBF-EB, such as the smoke phenomenon. The design and capabilities of the device for PBF-EB processing and real-time synchrotron measurements are detailed based on collected data.

In the second part, solidification and phase transformations during AM are studied using real-time synchrotron observations in combination with thermodynamic and kinetic modelling.

The change in solidification mode of a hot work tool steel is investigated under PBF-LB processing conditions. In this study, the change from primary austenite to primary δ-ferrite is observed with increasing cooling rate. The observations are correlated with predictions from a solidification model. Furthermore, the developed PBF-EB sample environment is employed to study the solidification behaviour of the same material under a wide range of PBF-EB conditions with lower cooling rates compared to the PBF-LB conditions. The observed phase transformation behaviour is linked to thermodynamic and kinetic modelling, highlighting the importance of process-induced compositional variations.

In addition, the martensite start temperature (Ms) in iron and iron carbon alloy is investigated under PBF-LB conditions using high-speed XRD at 20 kHz. The observed phase transformations are correlated with thermal simulation results, demonstrating cooling rate and composition dependence of the Ms temperature in real-time. Understanding martensite transformation in low-alloyed compositions during PBF processing can facilitate the development of recycling-friendly materials for AM.

This thesis focuses on real-time studies of metal AM, employing synchrotron techniques and linking the results to modelling. The findings demonstrate that in-situ and operando synchrotron studies, combined with computational models accounting for thermal conditions and compositional variations, are effective tools for process and alloy development for AM.In particular, the versatility of the developed PBF-EB sample environment can facilitate future studies on a variety of AM related phenomena.

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