Synchrotron X-ray Imaging Technique Research Articles

A counter-gravity casting system for in situ high-speed synchrotron x-ray imaging characterization.

Counter-gravity casting (CGC) aims to eliminate turbulent melt flow and defect formation during filling and subsequent solidification by pushing high-temperature melt into the mold cavity against gravity with regulated pressure. However, limited by the opaqueness of molten metals and the complexity of the CGC apparatus, it is extremely difficult to directly quantify the high-velocity mold filling and pressurized solidification in real-time. Here, we report the design and characterization of a CGC system capable of in situ monitoring of mold filling and subsequent solidification processes in the synchrotron beamlines by deploying a high-energy, high-speed synchrotron x-ray imaging technique. The high-velocity melt flow and dendrite growth during pressurized solidification have been quantified for systematical process parameter analysis by investigating time-resolved x-ray images of an exemplary Al-Cu alloy. The high-speed imaging results demonstrate that the in situ CGC system provides a useful way to better understand the fundamentals of mold filling, pressurized solidification, and experimental inputs for high-fidelity modeling in scientific and industrial applications.

Mapping biochemical and nutritional changes in durum wheat due to spoilage during storage

Synchrotron X-ray imaging and spectroscopy techniques were used for studying changes during post-harvest storage of food grains. Three varieties (AAC Spitfire, CDC Defy, and AAC Stronghold) of the Canada Western Amber Durum (CWAD) wheat class were stored for five weeks at 17 % moisture content (wb). Control (dry) and stored moistened seeds were analyzed for biochemical and nutritional changes using synchrotron bulk X-ray fluorescence spectroscopy (SR-XRF), X-ray fluorescence imaging (SR-XFI), and mid-infrared (mid-IR) spectroscopy at the Canadian Light Source (CLS), Saskatoon, SK. All varieties of durum wheat were spoiled at the end of five week, and AAC Spitfire and CDC Defy varieties were most affected in nutritional composition and their distribution than AAC Stronghold. Variable response to changes in biochemical and nutrition were found in all three spoiled varieties of the same durum wheat class.

Gas bubble coalescence in laser directed energy deposition of irregular HDH titanium alloy powder feedstock

In this study, the synchrotron X-ray imaging technique was used to investigate the coalescence of gas bubbles during laser directed energy deposition (L-DED) of irregular hydride-dehydride (HDH) titanium alloy particles onto a titanium alloy substrate. The objective is to better understand the coalescing mechanism of gas bubbles during the L-DED process of unique feedstock materials. The coalescence frequency of gas bubbles is the percentage of bubbles merging and is dependent on collision frequency and coalescence efficiency. Forces in the melt pool flow such as Marangoni forces, buoyancy force, vaporization pressure, and acoustic waves, along with the number of generated bubbles, were the major factors for increasing the collision frequency between bubbles. In addition, the random forest model determined that the diameter of the smaller bubble during collision was the most significant factor impacting the efficiency of coalescence, which was equal to 48% when the smaller diameter was larger than a threshold of 76 μm and 3.5% and when the diameter was smaller than 76 μm. The results also showed that 6.5% of the formed bubbles in the melt pool led to coalescence. Overall, this work can help mitigate pores, verify simulation models, and promote irregular or recycled powder feedstock as effective replacement to atomized feedstock.

X-ray Imaging of Alloy Solidification: Crystal Formation, Growth, Instability and Defects.

Synchrotron and laboratory-based X-ray imaging techniques have been increasingly used for in situ investigations of alloy solidification and other metal processes. Several reviews have been published in recent years that have focused on the development of in situ X-ray imaging techniques for metal solidification studies. Instead, this work provides a comprehensive review of knowledge provided by in situ X-ray imaging for improved understanding of solidification theories and emerging metal processing technologies. We first review insights related to crystal nucleation and growth mechanisms gained by in situ X-ray imaging, including solute suppressed nucleation theory of -Al and intermetallic compound crystals, dendritic growth of -Al and the twin plane re-entrant growth mechanism of faceted Fe-rich intermetallics. Second, we discuss the contribution of in situ X-ray studies in understanding microstructural instability, including dendrite fragmentation induced by solute-driven, dendrite root re-melting, instability of a planar solid/liquid interface, the cellular-to-dendritic transition and the columnar-to-equiaxed transition. Third, we review investigations of defect formation mechanisms during near-equilibrium solidification, including porosity and hot tear formation, and the associated liquid metal flow. Then, we discuss how X-ray imaging is being applied to the understanding and development of emerging metal processes that operate further from equilibrium, such as additive manufacturing. Finally, the outlook for future research opportunities and challenges is presented.

Ultrasound cavitation induced nucleation in metal solidification: An analytical model and validation by real-time experiments

Microstructural refinement of metallic alloys via ultrasonic melt processing (USMP) is an environmentally friendly and promising method. However, so far there has been no report in open literature on how to predict the solidified microstructures and grain size based on the ultrasound processing parameters.In this paper, an analytical model is developed to calculate the cavitation enhanced undercooling and the USMP refined solidification microstructure and grain size for Al-Cu alloys. Ultrafast synchrotron X-ray imaging and tomography techniques were used to collect the real-time experimental data for validating the model and the calculated results. The comparison between modeling and experiments reveal that there exists an effective ultrasound input power intensity for maximizing the grain refinement effects for the Al-Cu alloys, which is in the range of 20-45 MW/m2. In addition, a monotonous increase in temperature during USMP has negative effect on producing new nuclei, deteriorating the benefit of microstructure refinement due to the application of ultrasound.

Experimental investigation of fuel temperature effects on transient needle motion and injection velocity of solenoid type diesel injector

Real driving emission (RDE) test conducted in sub-zero temperature conditions has been acting as a severe emission regulation over the existing test methods for diesel engines. In order to cope with this tightened regulation, many studies have been conducted to interpret the factors causing the deteriorated diesel engine combustion in cold operation conditions. The deteriorated fuel injection performance has been regarded as one of the key contributing factors but the governing mechanisms are not thoroughly understood so far. This study aims to provide a comprehensive understanding of the effects of fuel temperature including sub-zero conditions on fuel injection performance by measuring the transient needle motion and injection velocity of a diesel injector using synchrotron X-ray imaging techniques. The factors causing the deteriorated injector performance and their contributions are discussed thoroughly based on the results. The results showed that the injection duration was shortened as the fuel temperature decreased as a result of decreased needle opening speed and increased needle closing speed. The increase of fuel viscosity and bulk modulus was discussed as the main factors associated with these results. The total injection mass was reduced by lowering the fuel temperature not only by the shortened injection duration but also by the decreased injection velocity. These factors exerted near equivalent contribution to the reduced injection mass at low fuel temperatures.

In-Situ Characterization of Pore Formation Dynamics in Pulsed Wave Laser Powder Bed Fusion.

Laser powder bed fusion (LPBF) is an additive manufacturing technology with the capability of printing complex metal parts directly from digital models. Between two available emission modes employed in LPBF printing systems, pulsed wave (PW) emission provides more control over the heat input compared to continuous wave (CW) emission, which is highly beneficial for printing parts with intricate features. However, parts printed with pulsed wave LPBF (PW-LPBF) commonly contain pores, which degrade their mechanical properties. In this study, we reveal pore formation mechanisms during PW-LPBF in real time by using an in-situ high-speed synchrotron x-ray imaging technique. We found that vapor depression collapse proceeds when the laser irradiation stops within one pulse, resulting in occasional pore formation during PW-LPBF. We also revealed that the melt ejection and rapid melt pool solidification during pulsed-wave laser melting resulted in cavity formation and subsequent formation of a pore pattern in the melted track. The pore formation dynamics revealed here may provide guidance on developing pore elimination approaches.

Experimental investigation of internal two-phase flow structures and dynamics of quasi-stable sheet cavitation by fast synchrotron x-ray imaging

The quasi-stable sheet cavitation produced in a small Venturi channel is investigated using a fast synchrotron x-ray imaging technique aided with conventional high speed photography. The use of x rays instead of visible light solves cavitation opacity related issues, and x-ray phase contrast-based edge enhancement enables high-definition visualization of the internal two-phase morphology. The simultaneous acquisition of time-resolved velocity and void fraction fields through post-processing of the recorded x-ray images reveals, for the first time, the complex diphasic flow structures inside the sheet cavity, which is essentially divided into six characteristic parts. Distinct from the current mainstream view, the globally steady sheet cavitation is found to be characterized by a weak but constantly existing re-entrant flow that can penetrate the entire cavity. The turbulent velocity fluctuations inside the sheet cavity are also investigated. The turbulence level in the reverse flow region is observed to be as low as in the outer main flow, demonstrating the relatively steady status of the re-entrant flow. Unlike the streamwise and cross-stream fluctuations, the shear stress appears to be weakly correlated with the velocity gradient. The collapse of the vapor phase and the vaporization at the upstream cavity interface are found to be the primary causes of shear stress intensification.

Dynamic behaviors of minor droplets and the role of bubbles in phase-separating Al[sbnd]Bi immiscible alloy

Minor droplets growth and solute segregation have their peculiarities in phase-separating immiscible alloys and can take complex pathways during solidification. The dynamic behaviors of liquid phase separation and surface enrichment in solidifying Al-10 wt% Bi immiscible alloys were revealed by synchrotron X-ray imaging technique. The large-sized Bi droplets would precipitate from Al-matrix melt directly and almost all of the droplets possess different existing states. The distribution of Bi droplets, resulting from the heterogeneous nucleation and repeatedly nucleation, varies with the growth time during cooling. And the growth dynamics of Bi droplets is revealed to be inconsistent with the Gaussian model. We also found that hydrogen bubbles can prevent the occurrence of the liquid phase separation process.

Development of low temperature fuel cell holders for Operando x-ray micro and nano computed tomography to visualize water distribution

Synchrotron x-ray imaging techniques, like x-ray computed tomography (CT) and radiography have proven instrumental in expanding the communities knowledge of complex transport and reaction kinetics in electrochemical devices such as fuel cells and electrolyzers. This work presents the development of novel x-ray CT imaging techniques for operando visualization of water within low temperature fuel cells at spatial resolutions spanning the micro and nano scales. The design of operando sample holders, for both micro x-ray CT and nano CT experiments is described in depth, and prototypes of these sample holders were evaluated across a set of requirements, the most important of which are x-ray transmissibility, electrical conductivity and mechanical stability. Water segmentation from micro x-ray CT data was enabled by an image subtraction method, where the image without water is subtracted from the one with water. Through iterative experimentations, the operando nano CT cell was developed to optimize mechanical compression, electric conductivity and gas flow. While three-dimensional fuel cell reconstructions were shown possible, there remain challenges to overcome at typical lower energies (8 keV) due to beam damage, whereas it is not as significant for higher energies (>17.5 keV).

Observation of Metal Micro-Nanoparticles in an Epoxy Matrix during Thermal Annealing Using Synchrotron X-ray Imaging Techniques

We investigated the thermal annealing behavior of Ag-coated Cu nanoparticles in an epoxy matrix by using synchrotron X-ray transmission microscopy, coherent X-ray diffraction imaging, and scanning transmission X-ray microscopy (STXM). We observed thermal-annealing-induced void formation around the metal particles as well as shape changes in the metal particles because of the volume changes in the epoxy matrix. In addition, the density distribution of the metal particles was changed by thermal annealing. Based on the optical density measurements from the X-ray transmittance analysis, we could distinguish between the Cu-dominant region and the Ag-dominant region from the STXM images.

Pressure-induced velocity softening in natural orthopyroxene at mantle temperature

AbstractIn this study, we have measured the compressional and shear wave velocities of (Mg1.77Fe0.22Ca0.01)Si2O6 natural orthopyroxene up to 13.5 GPa and 873 K using ultrasonic interferometry in conjunction with in situ synchrotron X-ray diffraction and imaging techniques. Previous acoustic experiments on orthoenstatite (OEn) MgSiO3 indicated that both compressional and shear velocities (VP and VS) of OEn undergo continuous velocity softening above 9 GPa at room temperature, which has been attributed to the phase transition from OEn to the metastable, high-pressure clinoenstatite HPCEn2. For the first time, our results suggest that pressure-induced velocity softening can occur in natural orthopyroxene at high-temperature conditions relevant to the Earth's cold subduction zones. Estimates of the impedance and velocity contrasts between orthopyroxene (Opx) and high-pressure clinopyroxene (HPCpx) have been calculated, and the possibility of this phase transformation being a plausible candidate for seismic X-discontinuities at depth around 250–350 km is re-evaluated.

In situ Characterization of Laser Powder Bed Fusion Using High-Speed Synchrotron X-ray Imaging Technique

<i>In situ</i> Characterization of Laser Powder Bed Fusion Using High-Speed Synchrotron X-ray Imaging Technique

In-situ observations of dendritic fragmentation during directional solidification of Sn-10wt.% Bi alloy

The stray crystals seriously affect the properties of metals which fabricated by directional solidification (DS). In order to observe the formation of the stray crystals, the growth behavior of Sn-10wt.%Bi alloy was observed by synchrotron X-ray imaging technique using self-developed directional solidification (DS) apparatus. The experimental results show that Sn-10wt.%Bi alloy is oriented with the form of dendrites to grow into the liquid phase during the DS process, and dendrites are arranged neatly to form columnar crystal microstructures. It is also found that parts of the dendrite arms begin to crack. These cracked dendrites drift upward to grow up in front of the solid-liquid interface and form stray crystals.

Trace doping of multiple elements enables stable battery cycling of LiCoO2 at 4.6 V

LiCoO2 is a dominant cathode material for lithium-ion (Li-ion) batteries due to its high volumetric energy density, which could potentially be further improved by charging to high voltages. However, practical adoption of high-voltage charging is hindered by LiCoO2’s structural instability at the deeply delithiated state and the associated safety concerns. Here, we achieve stable cycling of LiCoO2 at 4.6 V (versus Li/Li+) through trace Ti–Mg–Al co-doping. Using state-of-the-art synchrotron X-ray imaging and spectroscopic techniques, we report the incorporation of Mg and Al into the LiCoO2 lattice, which inhibits the undesired phase transition at voltages above 4.5 V. We also show that, even in trace amounts, Ti segregates significantly at grain boundaries and on the surface, modifying the microstructure of the particles while stabilizing the surface oxygen at high voltages. These dopants contribute through different mechanisms and synergistically promote the cycle stability of LiCoO2 at 4.6 V. LiCoO2 is a widely used cathode material in Li-ion batteries for applications such as portable electronics. Here, the authors report multiple-element doping to enable stable cycling of LiCoO2 at high voltages that are not yet accessible with commercial Li-ion batteries.

Direct Visualization of the Behavior and Shapes of the Nanoscale Menisci of an Evaporating Water Droplet on a Hydrophilic Nanotextured Surface via High-Resolution Synchrotron X-ray Imaging.

Despite considerable research interest due to omnipresent and practical importance of interfacial phenomena (e.g., wetting and dewetting) on nanotextured surfaces in the academic and industrial fields, direct visualization of the behavior and shapes of liquid-vapor interfaces between nanoscale structures remains an arduous task because of the resolution limitations of visualization techniques. In this study, we succeeded in a first-hand visualization of the behavior and shapes of the liquid-vapor interfaces of a water droplet between nanometer-scale pillar during evaporation by introducing a synchrotron X-ray imaging technique with spatially high resolution (40 nm/a pixel). On the basis of the visualization data, we intensively analyzed and discussed the spreading and evaporation phenomena of a liquid droplet on hydrophilic nanotextured surfaces.

Random angle grain boundary formation and evolution dynamics during Si directional solidification

The growth of multicrystalline silicon and the formation of a random angle grain boundary, as well as the dislocation generation and expansion is observed dynamically in situ, by Synchrotron X-ray imaging techniques. The focus is kept on a random angle grain boundary since its behavior is particularly important to better understand the HP mc-Si (High Performance Multi-crystalline Silicon) photovoltaic properties. Due to the process conditions and to the grain competition that occurs during the solidification, a facetted {111}/facetted {111} groove is formed by this random angle grain boundary at the solid/liquid interface. It is shown how the shape of the solid/liquid interface allows the change of the preferential {111} growth facet and affects the grain boundary propagation direction. In one of the groove configurations, the two adjacent {111} facets do not have the same growth velocity and as a consequence the corresponding grain boundary does not follow the bisector of the angle between the two facets. Indeed, the direction of the grain boundary is determined by the growth velocities of the facets which control the grain competition. Moreover, under these experimental conditions a clear relationship is observed between the existence of random angle grain boundaries and the local generation of dislocations as well as their expansion. By comparison, dislocation emission is not observed at the level of Σ3 {111} grain boundaries.

In situ synchrotron X-ray imaging of 4140 steel laser powder bed fusion

An in situ synchrotron X-ray imaging technique was used to examine laser powder bed fusion (LPBF) of a chrome-molybedum, AISI 4140, ferrite–bainite steel, from a moving laser source. Since 4140 has received only minimal attention in the AM literature, focus here was on the effects of melt pool dynamics, vapor cavity depths, build layer height, and the origin of porosity on the 4140 as-built microstructure. Four build parameter sets, which enabled variation of laser power, scan speed, laser spot size, and specific energy density (SED), were applied. Vapor cavity and melt pool depths were measured for each single-track laser scan. Direct imaging of the LPBF process demonstrated that the primary source of porosity in the build originates from incorporation of entrapped gas within the powder irrespective of build parameter set. High resolution SEM and EBSD demonstrated that the as-built microstructure did not vary significantly with build parameter set over a broad range of SED. The fast solidification rates observed (0.06–0.08 m/s) lead to fine martensite packet/block size distributions (1–3 µm) with the potential for increased material strength.

Real time observation of binder jetting printing process using high-speed X-ray imaging

A high-speed synchrotron X-ray imaging technique was used to investigate the binder jetting additive manufacturing (AM) process. A commercial binder jetting printer with droplet-on-demand ink-jet print-head was used to print single lines on powder beds. The printing process was recorded in real time using high-speed X-ray imaging. The ink-jet droplets showed distinct elongated shape with spherical head, long tail, and three to five trailing satellite droplets. Significant drift was observed between the impact points of main droplet and satellite droplets. The impact of the droplet on the powder bed caused movement and ejection of the powder particles. The depth of disturbance in the powder bed from movement and ejection was defined as interaction depth, which is found to be dependent on the size, shape, and material of the powder particles. For smaller powder particles (diameter less than 10 μm), three consecutive binder droplets were observed to coalesce to form large agglomerates. The observations reported here will facilitate the understanding of underlying physics that govern the binder jetting processes, which will then help in improving the quality of parts manufactured using this AM process.

Degradation Mechanism of Ni-Rich Layered Cathode Materials during High Voltage Cycling Revealed By Multi-Model Synchrotron X-Ray Imaging and Spectroscopy Techniques

Ni-rich layered cathode materials are important candidates for lithium-ion batteries used for high energy density applications, such as electric vehicles. However, these materials are suffering severe capacity fade caused by surface reconstruction, unstable cathode-electrolyte interface (CEI) formation and micro crack formation during cycling, especially when a high cut off charge voltage is applied (such as 4.6 and 4.8V). To understand the origin of this capacity fade, state-of-the-art diagnostic tools are applied to understand their structure, chemical and morphological properties of LiNi0.94Co0.06O2 (NC) and LiNi0.92Co0.06Al0.02O2 (NCA) cathode materials during cycling. Electrochemical measurement results show that the NCA cathode delivered higher capacity and stable cyclic performance even cycled at a high cut of voltage of 4.8V. In situ X-ray absorption spectroscopy (XAS) results of NC and NCA electrodes measured during the first and 51st charge/discharge cycling between the voltage range of 2.8-4.4V showed that both Ni and Co contribute to the capacity by going through the Ni3+/Ni4+ and Co3+/Co4+ redox couples. The Ni-K edge XAS spectra of NC cathode charged to 4.8V showed slightly reduction of Ni4+ during the high voltage charging. While, XAS spectra of NCA cathode did not showed reduction of Ni4+ when the cell charged to the higher voltage. More interestingly, C, F, O, Ni and Co soft XAS spectroscopy of NC and NCA electrodes at the different state of charge provided more comprehensive insight into surface and bulk chemical properties. The results showed that much thicker CEI layer formed on the NC surface compared with the NCA cathode, indicated that Al-doping effectively reduced the side reaction of cathode materials with the electrolyte. We have also compared the morphological and chemical evolution of NC and NCA secondary particles using full-filed in-situ transmission X-ray microscopy (TXM) during the initial cycle. Results will be presented at the meeting. Acknowledgement This project was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies through Advanced Battery Material Research (BMR) program (Battery500 consortium) under Contract No. DESC0012704.