Electron beam powder bed fusion (PBF-EB) is an additive manufacturing (AM) technology that enables the fabrication of metallic parts with arbitrary geometric complexity within a vacuum environment. Due to its ability to process materials at high temperatures (> 1000 °C), processing of crack and oxidation sensitive materials, as well as refractory alloys is possible. However, due to limited fundamental understanding of the intricate dynamics during powder consolidation and melt pool formation, the development of advanced processing strategies has mainly been limited to experimentally time-consuming parameter studies, as numerical models have mostly been unable to accurately predict processing conditions at the part or even layer scale. In this study, we perform high-speed in-situ X-ray imaging during multi-layer single track powder melting experiments on MiniMelt, a recently developed, custom-built PBF-EB machine for in-situ X-ray investigations. Our experiments reveal several key melt pool formation dynamics, some of which are being identified for the first time. They show how melt pool formation involves the coalescence of molten powder particles into larger droplets and how these droplets either fuse with the melt pool or solidify as balling particles. They also elucidate the origins of melt pool oscillations and spatter formation and demonstrate how the superposition of these mechanisms can lead to chaotic and escalating movement within the melt. We expect our results to improve and extend the phenomenological understanding of the powder consolidation mechanisms during PBF-EB and to aid in the development of new scanning strategies as well as the validation of numerical models.

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.

Additiv tillverkning (AM) av metaller erbjuder unik designfrihet och möjlighet att skräddarsy mikrostrukturer och egenskaper hos komponenter. Komplexa termiska förlopp och den snabba stelningen som sker under AM medför dock betydande utmaningar vad gäller mikrostrukturkontroll och processoptimering. I detta arbete används synkrotrontekniker för att studera de snabba fenomen som uppstår under AM i realtid. Synkrotrontekniker, inklusive röntgendiffraktion (XRD) och radiografi, är kraftfulla verktyg för att undersöka AM-relaterade fenomen såsom interaktionen mellan energikälla och material, smältpoolens dynamik samt stelning och fasomvandlingar i realtid. Genom att korrelera datan med processparametrar förbättras förståelsen av AM-processen och av de mekanismer som styr. Resultaten är viktiga för processoptimering, legeringsdesign och utveckling av beräkningsmodeller.

Resultaten av detta arbete kan delas in i två delar.

Den första delen fokuserar på design och implementering av en provmiljö, en elektronstråle-printer (PBF-EB), för realtidsstudier vid synkrotronljusanläggningar (kapitel 3). Provmiljön möjliggör undersökning av ovan nämnda AM-fenomen under PBF-EB vid höga temperaturer och vakuum, samt PBF-EB-specifika effekter såsom det så kallade ”smoke”-fenomenet.Printerns design och dess kapacitet för PBF-EB-printning och synkrotronmätningar i realtid beskrivs med stöd av experimentella data.

I den andra delen studeras stelning och fasomvandlingar i AM genom synkrotronobservationer i realtid (kaptil 4), i kombination med termodynamiska och kinetiska beräkningar. Förändringen av stelningsbeteendet undersöktes i varmarbetsverktygsstål under förhållanden typiska för laserbaserad AM (PBF-LB).I denna studie observerades övergången från primär austenit till primär $\delta$-ferrit vid kylhastigheter i intervallet 2.1 x 10^4 K/s till 1.5 x 10^6 K/s. Observationerna kopplades till en stelningsmodell.Dessutom användes den utvecklade PBF-EB-provmiljön för att studera stelning hos samma verktygsstål vid kylhastigheter från 1.5 x 10^3 K/s till 1.6 x 10^4 K/s. De observerade fasomvandlingarna korrelerades med termodynamisk och kinetisk modellering, och visade på processinducerade sammansättningsvariationer. Martensitstarttemperaturen (Ms) i rent järn och Fe–C-legeringar under PBF-LB-förhållanden undersöktes med high speed-XRD vid 20 kHz. De observerade fasomvandlingarna korrelerades med termiska simuleringar och visade att Ms temperaturen beror på såväl kylhastighet som sammansättning. Förståelse av martensitomvandlingen i låglegerade stål under PBF-förhållanden kan underlätta utvecklingen av material för AM som är lättare att återvinna.

Sammanfattningsvis, den här avhandlingen fokuserar på realtidsstudier av metall-AM med hjälp av synkrotrontekniker och kopplar resultaten till modellering. Resultaten visar att in-situ- och operando-synkrotronstudier, i kombination med beräkningsmodeller som tar hänsyn till termiska förhållanden och sammansättningsvariationer, är effektiva verktyg för process- och legeringsutveckling inom AM. I synnerhet mångsidigheten hos den utvecklade PBF-EB-provmiljön möjliggör framtida studier av en mängd olika PBF- och PBF-EB-relaterade fenomen.

A high-energy white synchrotron X-ray beam enables penetration of relatively thick and highly absorbing samples. At the P61A White Beam Engineering Materials Science Beamline, operated by Helmholtz-Zentrum Hereon at the PETRA III ring of the Deutsches Elektronen-Synchrotron (DESY), a tailored X-ray radiography system has been developed to perform in-situ X-ray imaging experiments at high temporal resolution, taking advantage of the unprecedented X-ray beam flux delivered by ten successive damping wigglers. The imaging system is equipped with an ultrahigh-speed camera (Phantom v2640) enabling acquisition rates up to 25 kHz at maximal resolution and binned mode. The camera is coupled with optical magnification (5x, 10x) and focusing lenses to enable imaging with a pixel size of 1,35 micrometre. The scintillator screens are housed in a special nitrogen gas cooling environment to withstand the heat load induced by the beam, allowing spatial resolution to be optimized down to few micrometres. We present the current state of the system development, implementation and first results of in situ investigations, especially of the electron beam powder bed fusion (PBF-EB) process, where the details of the mechanism of crack and pore formation during processing of different powder materials, e.g. steels and Ni-based alloys, is not yet known.

The development of a sample environment for in situ x-ray characterization during metal Electron Beam Powder Bed Fusion (PBF-EB), called MiniMelt, is presented. The design considerations, the features of the equipment, and its implementation at the synchrotron facility PETRA III at Deutsches Elektronen-Synchrotron, Hamburg, Germany, are described. The equipment is based on the commercially available Freemelt ONE PBF-EB system but has been customized with a unique process chamber to enable real-time synchrotron measurements during the additive manufacturing process. Furthermore, a new unconfined powder bed design to replicate the conditions of the full-scale PBF-EB process is introduced. The first radiography (15 kHz) and diffraction (1 kHz) measurements of PBF-EB with a hot-work tool steel and a Ni-base superalloy, as well as bulk metal melting with the CMSX-4 alloy, using the sample environment are presented. MiniMelt enables time-resolved investigations of the dynamic phenomena taking place during multi-layer PBF-EB, facilitating process understanding and development of advanced process strategies and materials for PBF-EB.<br />

Solidification during fusion-based additive manufacturing (AM) is characterized by high solidification velocities and large thermal gradients, two factors that control the solidification mode of metals and alloys. Using two synchrotron-based, in situ setups, we perform high-speed X-ray diffraction measurements to investigate the impact of the solidification velocities and thermal gradients on the solidification mode of a hot-work tool steel over a wide range of thermal conditions of relevance to AM of metals. The solidification mode of primary delta-ferrite is observed at a cooling rate of 2.12 x 104 K/s, and at a higher cooling rate of 1.5 x 106 K/s, delta-ferrite is sup-pressed, and primary austenite is observed. The experimental thermal conditions are evaluated and linked to a Kurz-Giovanola-Trivedi (KGT) based solidification model. The modelling results show that the predictions from the multicomponent KGT model agree with the experimental observations. This work highlights the role of in situ XRD measurements for a fundamental understanding of the microstructure evolution during AM and for vali-dation of computational thermodynamics and kinetics models, facilitating parameter and alloy development for AM processes.

Additive Manufacturing (AM) is becoming an important technology for manufacturing of metallic materials. Laser-Powder Bed Fusion (L-PBF), Electron beam-Powder Bed Fusion (E-PBF) and Directed Energy Deposition (DED) have attracted significant interest from both the scientific community and the industry since these technologies offer great manufacturing opportunities for niche applications and complex geometries. Understanding the physics behind the complex and dynamic phenomena occurring during these processes is essential for overcoming the barriers that constrain the metal AM development. Insitu synchrotron X-ray characterization is suitable for investigating the microstructure evolution during processing and provides new profound insights. Here, we provide an overview of the research on metal PBF and DED using in-situ synchrotron X-ray imaging, diffraction and small-angle scattering, highlighting the state of the art, the instrumentation, the challenges and the gaps in knowledge that need to be filled. We aim at presenting a scientific roadmap for in-situ synchrotron analysis of metal PBF and DED where future challenges in instrumentation such as the development of experimental stations, sample environments and detectors as well as the need for further application oriented research are included.

Ultrasonic additive manufacturing has been used to fabricate laminated composites of commercially pure aluminum and a nanocrystalline nickel-cobalt (nc-NiCo) alloy. The nc-NiCo alloy would not weld to itself but readily welded to aluminum. Thus, by alternating between foils of nc-NiCo and Al, we achieved multi-material laminates with strong interlayer bonding. Electron microscopy showed that the nanoscale grain structure of the nc-NiCo was preserved during deposition and that the nc-NiCo/Al weld interface was decorated with comminuted surface oxides as well as Al-Ni-Co intermetallics.These findings are considered in light of process models of junction growth, interdiffusion, and grain growth, which together reveal how the different pressure- and temperature dependences of these phenomena give rise to a range of processing conditions that maximize bonding while minimizing coarsening and intermetallic formation. This analysis quantitatively demonstrates that using a soft, low melting point interlayer material decouples junction growth at the weld interface from grain growth in the nc-NiCo, expanding the range of optimal processing conditions.

The formation of martensite through quenching is a fundamental mechanism for achieving high strength in steel. Martensite formation in pure or dilute iron requires high cooling rates that are impractical to realize using conventional processing techniques. However, microstructural studies have shown that martensite can form in iron and low-carbon steel after laser-based additive manufacturing (AM). This martensite exhibits unique high hardness. The high hardness has been related to the activation of martensitic transformation at a lower transformation temperature plateau. In this work, we investigate the activation of the martensitic transformation in-situ employing synchrotron X-ray diffraction (XRD) at laser powder bed fusion (PBF-LB) conditions at 20 kHz. A range of cooling conditions is induced by varying the laser scanning velocities and the austenite to martensite transformation is observed in-situ in iron (< 0.003 w.-% carbon) and an iron-carbon (0.36 w.-% carbon) alloy. The XRD results are correlated with thermal process modelling to study the cooling conditions influence on the transformation temperatures. Using this approach, a decrease in martensite start (Ms) temperatures with increasingly rapid cooling conditions was observed. This work highlights the capability of in-situ XRD to obtain insights into phase transformations during rapid cooling. Understanding of the cooling condition dependency of the transformation behaviour in iron and iron-carbon systems at PBF-LB conditions can be instrumental for future alloy design, particularly for design of recycling-friendly lean alloys.

The solidification behaviour of a tool steel during additive manufacturing (AM) with electron-beam powder bed fusion (PBF-EB) is investigated. Solidification is controlled by thermal conditions and alloy composition. To study the impact of these two factors, we perform high-speed operando X-ray diffraction (XRD) measurements of the PBF-EB process over a wide range of processing conditions inducing cooling rates between 1400 K/s and 16000 K/s. The formation and transformation of the high temperature δ-ferrite phase are observed. The results reveal the dependence of the δ-ferrite presence time on cooling rates, quantifying the impact of processing parameters on the δ-ferrite evolution. Furthermore, multi-component diffusion simulations are linked to the experimental conditions accounting for elemental evaporation during PBF-EB processing. The simulations reveal the effect of compositional variations during the process on the solidification behaviour during PBF-EB processing. This work highlights the importance of understanding the interplay between processing conditions and alloy composition in PBF-EB, as well as how the combination of operando XRD with computational thermodynamics and kinetics tools can facilitate parameter and alloy development for the PBF-EB process.