A Solid-State Battery Cathode with a Polymer Composite Electrolyte and Low Tortuosity Microstructure by Directional Freezing and Polymerization

Solid‐state Li metal batteries (SSLMBs) combine improved safety and high specific energy that can surpass current Li ion batteries. However, the Li^{+} ion diffusivity in a composite cathode—a combination of active material and solid‐state electrolyte (SSE)—is at least an order of magnitude lower than that of the SSE alone because of the highly tortuous ion transport pathways in the cathode. This lowers the realizable capacity and mandates relatively thin (30–300 μm) cathodes, and hence low overall energy storage. Here, a thick (600 μm) hybrid cathode comprising vertically aligned LiNi_{0.8}Mn_{0.1}Co_{0.1}O_{2} (NMC811)‐rich channels filled with a [LiTFSI+PEGMA+MePrPyl TFSI] polymer composite electrolyte is fabricated by an innovative directional freezing and polymerization method. X‐ray micro‐computed tomography, ion mobility simulations, and DC depolarization show that the cathode structure improves Li^{+} ion diffusivity in the cathode from 4.4 × 10^{-9} to 1.4 × 10^{-7} cm^{2} s^{−1}. In a SSLMB full cell at 25 oC, the cathode provides gravimetric capacities of 199 and 120 mAh g^{−1}, and ultra‐high areal capacities of 16.7 and 10.1 mAh cm^{−2} at 0.05 and 1 C, respectively. The work demonstrates a scalable approach to realizing composite cathode structures with kinetically favorable ion transport characteristics in SSLMBs

Capturing Marangoni flow via synchrotron imaging of selective laser melting

Marangoni flow has a substantial influence on the quality of components fabricated via laser powder bed fusion (LPBF). However, Marangoni flow in melt pools is rarely quantified due to the opacity of liquid metals and the necessity for in situ evaluation. Here we report the findings of high-temporal-resolution synchrotron x-ray radiography experiments tracking the flow in the melt-pool. Dense, highly attenuating tungsten carbide particles are seeded within an elemental powder blend of aluminium and copper of varying composition. Due to the extremely high temporal resolution of greater than 50 kfps at the 31-ID-B beamline at the Advanced Photon Source, USA, we can track the position of tracer particles from frame to frame. This data provides valuable process guidance for optimising mixing and informs the development and validation of multiphysics models

Synchrotron validation of inline coherent imaging for tracking laser keyhole depth

In situ monitoring is critical to the increasing adoption of laser powder bed fusion (LPBF) and laser welding by industry for manufacture of complex metallic components. Optical coherence tomography (OCT), an interferometric imaging technique adapted from medical applications, is now widely used for operando monitoring of morphology during high-power laser material processing. However, even in stable processing regimes, some OCT depth measurements from the keyhole (vapor cavity formed at laser beam spot) appear too shallow or too deep when compared to ex situ measurements of weld depth. It has remained unclear whether these outliers are due to imaging artifacts, multiple scattering of the imaging beam within the keyhole, or real changes in keyhole depth, making it difficult to accurately extract weld depth and determine error bounds. To provide a definitive explanation, we combine inline coherent imaging (ICI), a type of OCT, with synchrotron X-ray imaging for simultaneous, operando monitoring of the full 2-dimensional keyhole profile at high-speed (280 kHz and 140 kHz, respectively). Even in a highly turbulent pore-generation mode, the depth measured with ICI closely follows the keyhole depth extracted from radiography (>80% within ± 14 µm). Ray-tracing simulations are used to confirm that the outliers in ICI depth measurements (that significantly disagree with radiography) primarily result from multiple reflections of the imaging light (57%). Synchrotron X-ray imaging also enables tracking of bubble and pore formation events. Pores are generated during laser welding when the sidewalls of the keyhole rapidly (>10 m/s) collapse inwards, pinching off a bubble from the keyhole root and resulting in a rapid decrease in keyhole depth. Evidence of bubble formation can be found in ICI depth profiles alone, as rapid depth changes exhibit moderate correlation with bubble formation events (0.26). This work moves closer to accurate, localized defect detection during laser welding and LPBF using ICI

Quantifying the effects of gap on the molten pool and porosity formation in laser butt welding

To obtain a better joint quality in butt welding of aluminum, the gap filling process and the quantification of the gap effects on the molten pool characteristic and the bubble formation were realized by a three-dimensional thermal-mechanistic-fluid coupled model, with the consideration of heat transfer, fluid flow, phase change and recoil pressure. The model was validated by the synchrotron-radiation result. The competition between the solidification and melting at the bottom of the molten pool was uncovered to determine the gap filling process and the molten pool morphology. Gap increased the heat loss, and the molten pool tip was elongated due to gap filling. Four phenomena appeared in sequence in the initial stage of butt welding: I. Gap filling; II. Frozen; III. Remelt; IV. Bubble formation. The result also demonstrated that the gap would disturb the molten pool. In the initial stable growth stage of the molten pool, the larger the gap width, the greater the molten pool depth. The sharp change of keyhole depth was due to the necking formation, while the small fluctuation of keyhole depth with larger gap values resulted from the perturbation by the gap. Bubble formation depends on the degree of the fluid flow and the gap filling due to the unique fluid dropping down phenomenon of butt welding with gap. A continuous melt pool cannot be formed when the gap width beyond 20 μm, which is detrimental to the welding quality. These findings are of great significance for guiding the optimization of butt-welding process, such as reducing the roughness of the butt interface or increasing the clamping force to reduce the butt gap

The effects of powder reuse on the mechanical response of electron beam additively manufactured Ti6Al4V parts

High cost of metal powders has increased the demand for recycling of unmelted powder in electron beam powder bed fusion additive manufacturing process. However, powder characteristics are likely to change during manufacturing, recovery and reuse. It is important to track the evolution of powder characteristics at different stages of recycling to produce components with consistent properties. The present work evaluates the changes in Ti6Al4V powder properties during manufacturing by characterising powder particles at different locations in the powder bed; recovery and reuse, through evaluating the effects of the powder recovery system and sieving for 10 build cycles. Heterogeneous powder degradation occurred during manufacturing with the particles closer to the melt zone showing higher oxygen content and thicker α laths with β phase boundaries. Most of them had a hard-sintered and agglomerated powder morphology in contrast to particles at the edges of the powder bed. Recovery and reuse resulted in a refined particle size distribution, but only marginal change in powder morphology. The increased oxygen caused a slight increase in the yield and tensile strengths of the build. The effect of powder reuse on material elongation, hardness and Charpy impact energy was negligible. The high cycle fatigue performance deteriorated with reuse due to the increased lack-of-fusion defects. This might be attributed to the voids formed in the powder bed due to decrease in the number of fine particles coupled with an increase in the number of high-aspect ratio particles

Effect of preheating on the thermal, microstructural and mechanical properties of selective electron beam melted Ti-6Al-4V components

Two-stage preheating is used in selective electron beam melting (SEBM) to prevent powder spreading during additive manufacturing (AM); however, its effects on part properties have not been widely investigated. Here, we employed three different preheat treatments (energy per unit area, E_{A} to a Ti-6Al-4V powder bed. Each standalone build, we fabricated a large block sample and seven can-shaped samples containing sintered powder. X-ray computed tomography (XCT) was employed to quantify the porosity and build accuracy of the can-shaped samples. The effective thermal conductivity of the sintered powder bed was estimated by XCT image-based modelling. The microstructural and mechanical properties of the block sample were examined by scanning electron microscopy and microhardness testing, respectively. The results demonstrate that increasing E_{A} reduces the anisotropy of tortuosity and increases the thermal conductivity of the sintered powder bed, improving the heat transfer efficiency for subsequent beam-matter interaction. High preheat has a negligible effect on the porosity of large AM components; however, it decreases the microhardness from 330 ± 7 to 315 ± 11 HV0.5 and increases the maximum build error from 330 to 400 μm. Our study shows that a medium E_{A} (411 kJ m^{-2} is sufficient to produce components with a high hardness whilst optimising build accuracy

In situ monitoring the effects of Ti6Al4V powder oxidation during laser powder bed fusion additive manufacturing

Making laser powder bed fusion (L-PBF) additive manufacturing process sustainable requires effective powder recycling. Recycling of Ti6Al4V powder in L-PBF can lead to powder oxidation, however, such impact on laser-matter interactions, process, and defect dynamics during L-PBF are not well understood. This study reveals and quantifies the effects of processing Ti6Al4V powders with low (0.12 wt%) and high (0.40 wt%) oxygen content during multilayer thin-wall L-PBF using in situ high speed synchrotron X-ray imaging. Our results reveal that high oxygen content Ti6Al4V powder can reduce melt ejections, surface roughness, and defect population in the built parts. With increasing oxygen content in the part, there is an increase in microhardness due to solid solution strengthening and no significant change in the microstructure is evident

In situ characterisation of surface roughness and its amplification during multilayer single-track laser powder bed fusion additive manufacturing

Surface roughness controls the mechanical performance and durability (e.g., wear and corrosion resistance) of laser powder bed fusion (LPBF) components. The evolution mechanisms of surface roughness during LPBF are not well understood due to a lack of in situ characterisation methods. Here, we quantified key processes and defect dynamics using synchrotron X-ray imaging and ex situ optical imaging and explained the evolution mechanisms of side-skin and top-skin roughness during multi-layer LPBF of Ti-6Al-4V (where down-skin roughness was out of the project scope). We found that the average surface roughness alone is not an accurate representation of surface topology of an LPBF component and that the surface topology is multimodal (e.g., containing both roughness and waviness) and multiscale (e.g., from 25 µm sintered powder features to 250 µm molten pool wavelength). Both roughness and topology are significantly affected by the formation of pre-layer humping, spatter, and rippling defects. We developed a surface topology matrix that accurately describes surface features by combining 8 different metrics: average roughness, root mean square roughness, maximum profile peak height, maximum profile valley height, mean height, mean width, skewness, and melt pool size ratio. This matrix provides a guide to determine the appropriate linear energy density to achieve the optimum surface finish of Ti-6Al-4V thin-wall builds. This work lays a foundation for surface texture control which is critical for build design, metrology, and performance in LPBF

Blue laser directed energy deposition of aluminum with synchronously enhanced efficiency and quality

Directed energy deposition (DED) of aluminum with infrared lasers faces many processing issues, e.g., poor formability, pore formation, high reflectivity, all lowering the productivity. In this paper, we developed and applied a 2 kW high-power (450 nm) blue laser directed energy deposition (BL-DED) of a nano-TiB2 decorated AlSi10Mg composite. The single-track experiment reveals that the required power density of blue laser to form fully melted track is lower than that of an infrared laser (1060 nm). Under the laser power of 900 W with a scanning speed of 4 mm/s, the width and depth of molten pool is approximately 2500 µm and 350 µm respectively with blue laser, while the powders are not fully melted with infrared laser, owing to aluminum's higher absorption at blue laser wavelengths. The area fraction of equiaxed grains accounts for as high as 63% at 4 mm/s. To the best of our knowledge, this result is the highest area fraction of equiaxed grains in a single-track molten pool of DED process. Such a high fraction is mainly due to the low thermal gradient (8 × 105 K/m) of the flat-top blue laser and the refining effect of nano TiB2 particles. Our work demonstrates that high-power blue laser has enhanced both efficiency and build quality compared to DED of aluminum alloys and composites using an infrared laser, which also promises to help process other high-reflectivity materials like copper alloys

Pore evolution mechanisms during directed energy deposition additive manufacturing.

Porosity in directed energy deposition (DED) deteriorates mechanical performances of components, limiting safety-critical applications. However, how pores arise and evolve in DED remains unclear. Here, we reveal pore evolution mechanisms during DED using in situ X-ray imaging and multi-physics modelling. We quantify five mechanisms contributing to pore formation, migration, pushing, growth, removal and entrapment: (i) bubbles from gas atomised powder enter the melt pool, and then migrate circularly or laterally; (ii) small bubbles can escape from the pool surface, or coalesce into larger bubbles, or be entrapped by solidification fronts; (iii) larger coalesced bubbles can remain in the pool for long periods, pushed by the solid/liquid interface; (iv) Marangoni surface shear flow overcomes buoyancy, keeping larger bubbles from popping out; and (v) once large bubbles reach critical sizes they escape from the pool surface or are trapped in DED tracks. These mechanisms can guide the development of pore minimisation strategies