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Abstract: Directed Energy Deposition Additive Manufacturing (DED-AM) is transformative for the production of larger, geometrically complex metallic components. However, the mechanical properties of titanium alloy DED-AM components do not always reach their full potential due to microstructural features including porosity and regions of lack of fusion. Using in situ and operando synchrotron X-ray imaging we gain insights into key laser-matter interaction and microstructural feature formation mechanisms during DED-AM of Ti-6242. Analysis of the process conditions reveals that laser power is dominant for build efficiency while higher traverse speed can effectively reduce lack of fusion regions. We also elucidate the mechanisms underlying several physical phenomena occurring during the deposition of titanium alloys, including the formation of a saddle-shaped melt pool and pore pushing. The findings of this work clarify the transient kinetics behind the DED-AM of titanium alloys and can be used as a guide for optimising industrial additive manufacturing processes.

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Abstract: The Directed Energy Deposition Additive Manufacturing (DED-AM) of SS316L was studied using in situ and operando synchrotron X-ray imaging to quantitively understand the effect of processing parameters on the melt-pool morphology and surface quality. It was found that surface roughness of DED-AM builds can result from melt pool surface perturbations caused by changes in the melt flow and build stage motion perturbations. Process maps are developed that quantitatively correlate build quality to process parameters including powder feed rate, laser power and traverse speed. How the AM process parameters control build efficacy is clarified, and the processing conditions required to dampen surface perturbations leading to roughness were determined.

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Abstract: A key technique for controlling solidification microstructures is magneto-hydrodynamics (MHD), resulting from imposing a magnetic field to solidifying metals and alloys. Applications range from bulk stirring to flow control and turbulence damping via the induced Lorentz force. Over the past two decades the Lorentz force caused by the interaction of thermoelectric currents and the magnetic field, a MHD phenomenon known as Thermoelectric Magnetohydrodynamics (TEMHD), was also shown to drive inter-dendritic flow altering microstructural evolution. In this contribution, high-speed synchrotron X-ray tomography and computational simulation are coupled to reveal the evolution, dynamics and mechanisms of solidification within a magnetic field, resolving the complex interplay and competing flow effects arising from Lorentz forces of different origins. The study enabled us to reveal the mechanisms disrupting the traditional columnar dendritic solidification microstructure, ranging from an Archimedes screw-like structure, to one with a highly refined dendritic primary array. We also demonstrate that alloy composition can be tailored to increase or decrease the influence of MHD depending on the Seebeck coefficient and relative density of the primary phase and interdendritic liquid. This work paves the way towards novel computational and experimental methods of exploiting and optimising the application of MHD in solidification processes, together with the calculated design of novel alloys that utilise these forces.

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Abstract: Directed Energy Deposition Additive Manufacturing (DED-AM) is one of the principal AM techniques being explored for both the repair of high value components in the aerospace industry as well as freeform fabrication of large metallic components. However, the lack of fundamental understanding of the underlying process-structure-property relationships hinders the utilisation of DED-AM for the production or repair of safety-critical components. This study uses in situ and operando synchrotron X-ray imaging to provide an improved fundamental understanding of laser-matter interactions and their influence on the melt pool geometry. Coupled with process modelling, these unique observations illustrate how process parameters can influence the DED-AM melt pool geometry. The calibrated simulation can be used for guidance in an industrial additive manufacturing process for microstructure and quality control.

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Abstract: Basaltic volcanism is strongly influenced by magmatic viscosity, which, in turn, is controlled by magma composition, crystallisation, oxygen fugacity and vesiculation. We developed an environmental cell to replicate the pressure and temperature during magma ascent from crustal storage to the surface, while capturing crystallisation using in-situ 4D X-ray computed microtomography. Crystallisation experiments were performed at Diamond Light Source, using monochromatic 53 keV X-rays, a pixel size of 3.2 μm, a sample to detector distance of 2000 mm, 1440 projections per 180 deg, an acquisition time of 0.04 s, and a rotation velocity of 3.125 deg.s-1. The redox conditions were controlled using an oxidised nickel disk for each experiment. Our starting materials were samples made of crystal-free glass cylinders (Ø 3 mm) from the 2001 Etna eruption with 0.9 and 0.8 wt. % water content. In the experiments, samples and crucibles were sealed initially by applying ~10 N loads. All samples were then heated up above glass transition (between 800 °C and 900 °C) in order to allow sample homogenisation while preventing volatiles exsolution. We then pressurised each sample by applying uniaxial loads (between 80 and 380 N), using high-degree alumina pistons, in order to generate enough internal pressure to maintain bubble-free samples when the desired high temperature was reached. Once at the initial high temperature, we began experiments via dropping the temperature to different target isothermal (from 1210 to 1130 °C or 1180 to 1110 °C) and isobaric conditions (8 and 10 MPa, respectively). For the whole duration of the experiments, we were able to observe directly and record pyroxene crystal nucleation and growth. Specifically, we were able to observe pyroxene nucleation on bubbles at small undercooling (∆T) and epitaxial growth of pyroxene at large ∆T. An increase of ∆T (up to 50 °C) can be associated with a decompression of a magma chamber or a decompression during magma ascent in the conduit. As ∆T = 30 - 50 °C can be reached in most of the basaltic volcanic systems on Earth, our results provide a feasible explanation of which mechanisms control nucleation and growth of pyroxene crystals in hydrous basaltic magmas. In addition, epitaxial growth promotes a faster increase of the crystal volume. As a larger crystal content translates into a higher viscosity, our results have important implications for magma rheology, and are extremely important to improve our understanding of magma ascent dynamics during volcanic eruptions, and our capacity to predict eruptions and mitigate volcanic hazards.