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Numerical and experimental study on keyhole and melt flow dynamics during laser welding of aluminium alloys under subatmospheric pressures
摘要： Porosity defects was highly related to the keyhole and melt flow dynamic during laser welding process. In this paper, a novel 3D numerical model was developed to describe the keyhole dynamic and melt flow behaviors during laser welding of 5A06 aluminium alloy under subatmospheric pressures. The effect of ambient pressure on laser welding process was taken into consideration by optimizing the boiling point of aluminium alloy and recoil pressure of evaporated metallic vapor jets based on vapor–liquid equilibria calculation and Wilson equation. A moving hybrid heat source model was employed to describe the laser energy distribution under subatmospheric pressures. Numerical results indicated that a wider and deeper keyhole with less humps was produced under subatmospheric pressure comparing with that of atmospheric pressure. The vortices in the rear keyhole wall became unapparent or even disappeared with the decrease of ambient pressures. The melt flow velocity on the keyhole wall was larger under a lower pressure. A smaller difference between boiling point and melting point was produced and this led to the formation of a thinner keyhole wall and improved the stability of molten pool. Larger recoil pressure produced under subatmospheric pressure was responsible for the weakened vortices and enhanced melt flow velocity. Bigger keyhole opening size, larger melt flow velocity, thinner keyhole and the weakened vortices all resulted into the reduction of porosity defects during laser welding of aluminium alloys. Based on the simulation results, the plasma distribution, weld formation and porosity defects had been demonstrated. The compared results showed that the simulation results exhibited good agreements with the experimental ones.
关键词： Porosity defects,Keyhole stability,Numerical simulation,Subatmospheric pressure,Melt flow dynamic,Laser welding
更新于2025-11-28 14:24:20
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Melt Electrowriting of Electroactive Poly(vinylidene difluoride) Fibers
摘要： Poly(vinylidene difluoride) (PVDF) has piezoelectric properties suitable for numerous applications such as flexible electronics, sensing and biomedical materials. In this study, individual fibers with diameters ranging from 17-55 μm were processed using melt electrowriting (MEW). Electroactive PVDF fibers can be fabricated via MEW, while the polymer could remain molten for up to ten hours without noticeable changes in the resulting fiber diameter. MEW processing parameters for PVDF were investigated, including applied voltage, pressure and temperature. A rapid fiber characterization methodology for MEW that automatically determines the fiber diameters from camera images taken of microscope slides was developed and validated. The outputs from this approach followed previous MEW processing trends already identified with different polymers, although overestimation of the < 25 micron fiber diameters was observed. The transformation of the PVDF crystalline phase to the electroactive β-phase – without poling – was confirmed using piezo-force microscopy and revealed that the PVDF fibers possess piezoelectric responses showing d33~19 pm/V.
关键词： melt electrospinning writing,piezoelectric,electrohydrodynamic,piezoresponse force microscopy,3D printing
更新于2025-09-23 15:23:52
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Thermal analysis and crystal growth of doped Nb2O5
摘要： The systems Nb2O5–Ta2O5 and Nb2O5–V2O5 were investigated using thermal analysis, X-ray powder diffraction and thermodynamic simulations. Solid solution formation is possible for both systems; furthermore, both contain one intermediate compound, VNb9O25 or Ta2Nb4O15, respectively. Phase relationships for pure niobium(V)-oxide and tantalum(V)-oxide were studied under ambient pressure. It was found that both compounds can occur in two stable solid modifications. For niobium(V)-oxide this are the monoclinic high-temperature modification (H-Nb2O5) and an orthorhombic low-temperature modification (T-Nb2O5) and for tantalum(V)-oxide a tetragonal high-temperature form (α-Ta2O5) and an orthorhombic low-temperature form (β-Ta2O5). Based on these results, crystal growth experiments with various compositions from both systems were carried out using the optical floating zone (OZF) technique.
关键词： B2. dielectric materials,A2. growth from melt,A1. doping,A1. phase diagrams,B1. niobates
更新于2025-09-23 15:23:52
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The influence of Marangoni effect on the growth quality of multi-crystalline silicon during the vacuum directional solidification process
摘要： A multi-field coupling model of heat-flow-thermal stress was established to investigate the influence of Marangoni effect on the growth quality of multi-crystalline silicon (mc-Si) during the vacuum directional solidification (VDS) process. The simulation results showed that the Marangoni effect has a significant effect on the distribution of temperature and thermal stress as well during the VDS process due to the increased velocity of melts. The enhanced flow incurred more homogeneous temperature distribution of silicon melts and reduced radial temperature gradient, which then leads to the flatter solid/liquid (s/l) interface. However, as temperature gradient of the crystal increased, thermal stress of the silicon ingot was strengthened as a result. The growth quality of crystal was more desirable when adopting a pulling-down rate of 10 μm/s, in which case the (111) surface was advantageous throughout the entire crystal growth process by XRD detection. Finally, the reliability of the numerical simulation result was verified by the experiment.
关键词： Multi-crystalline,Melt flow,Vacuum directional solidification,Numerical simulation,Marangoni effect
更新于2025-09-23 15:22:29
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Tailoring Grain Boundary Chemistry of Polymeric Carbon Nitride for Enhanced Solar H2 Production and CO2 Reduction
摘要： Photocatalytic water splitting is a promising and clean way to mimic the plant photosynthesis in a sustainable manner. Improvements of the quantum efficiency and optical absorption in the relevant range are certainly the further steps to approach practicality. Here, we reported that these issues can be readily addressed when 5-aminotetrazole, a monomer with still higher nitrogen content, is used for the synthesis of carbon nitride. The molten salt mixture NaCl/KCl is used as a high temperature solvent to tailor the grain boundary structure and chemistry. Visible light quantum efficiency for H2 production of 0.65 could be obtained in the presence of K2HPO4 as double layer modifier. This value is very high, considering that this number is a convolute of light to charge couple conversion, charge localization, as well as a successful oxidation and reduction reaction.
关键词： H2 production,Grain boundary,Polymeric carbon nitride,Photocatalysis,Salt melt
更新于2025-09-23 15:22:29
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Tailoring Grain Boundary Chemistry of Polymeric Carbon Nitride for Enhanced Solar H2 Production and CO2 Reduction
摘要： Photocatalytic water splitting is a promising and clean way to mimic the plant photosynthesis in a sustainable manner. Improvements of the quantum efficiency and optical absorption in the relevant range are certainly the further steps to approach practicality. Here, we reported that these issues can be readily addressed when 5-aminotetrazole, a monomer with still higher nitrogen content, is used for the synthesis of carbon nitride. The molten salt mixture NaCl/KCl is used as a high temperature solvent to tailor the grain boundary structure and chemistry. Visible light quantum efficiency for H2 production of 0.65 could be obtained in the presence of K2HPO4 as double layer modifier. This value is very high, considering that this number is a convolute of light to charge couple conversion, charge localization, as well as a successful oxidation and reduction reaction.
关键词： Grain boundary,Salt melt,Photocatalysis,H2 production,Polymeric carbon nitride
更新于2025-09-23 15:22:29
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High-Performance Biobased Unsaturated Polyester Nanocomposites with Very Low Loadings of Graphene
摘要： Graphene-reinforced tung oil (TO)-based unsaturated polyester nanocomposites were prepared via in situ melt polycondensation intergrated with Diels–Alder addition. Functionalized graphene sheets derived from graphene oxide (GO) were then extracted from the obtained nanocomposites and carefully characterized. Furthermore, dispersion state of the graphene nanosheets in the cured polymer composites and ultimate properties of the resultant biobased nanocomposites were investigated. Mechanical and thermal properties of the TO-based unsaturated polyester resin (UPR) were greatly improved by the incorporation of GO. For example, at the optimal GO content (only 0.10 wt %), the obtained biobased nanocomposite showed tensile strength and modulus of 43.2 MPa and 2.62 GPa, and Tg of 105.2 ?C, which were 159%, 191%, and 49.4% higher than those of the unreinforced UPR/TO resin, respectively. Compared to neat UPR, the biobased UPR nanocomposite with 0.1 wt % of GO even demonstrated superior comprehensive properties (comparable stiffness and Tg, while better toughness and thermal stability). Therefore, the developed biobased UPR nanocomposites are very promising to be applied in structural plastics.
关键词： unsaturated polyester resins,graphene,in situ melt polycondensation,tung oil,biobased polymer nanocomposites
更新于2025-09-23 15:21:21
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Growth of high purity zone-refined Boron Carbide single crystals by Laser Diode Floating Zone method
摘要： We report the growth of 4 mm diameter x 50 mm long Boron Carbide (B4C) with large single crystal regions using a Laser Diode Floating Zone (LDFZ) method at varying growth rates of 5-20 mm/hr. These materials were grown using polycrystalline B4C as a seed. Microstructural characterization shows the presence of a significant number of twinning-boundaries along the growth direction ([001]h) oriented in the (1210)h plane. At faster growth rates >10 mm/hr, the crystal orientation was reproducible, suggesting a twin-plane mediated growth mechanism. On the contrary, at slower growth rates <10 mm/hr the crystal orientation was not reproducible, suggesting a critical rate for twin-plane mediated growth to dominate. Zone refinement of these crystals led to a significant reduction of trace impurities to better than 99.999 wt % purity, at the expense of increased twinning. Powder x-ray diffraction confirms that the bulk is rhombohedral B4C, consistent with the microstructural analysis. The X-ray reciprocal space maps reveal the growth direction to be close to the [001]h direction, and the corresponding ω-rocking curve width is ~530arcsec. The rocking curve consisted of 3 distinct peaks, indicating in-plane mosaicism, consistent with the twinning observed. Berkovich nano-indentation of the key (001)h plane showed 41 ± 1 GPa hardness, with a Young’s modulus of 520 ± 14 GPa, comparable to literature reports.
关键词： A2. Single crystal growth,A1. Defects,A1. X-ray diffraction,A1. Characterization,A2. Growth from melt
更新于2025-09-23 15:21:01
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Brazing, Laser, and Electron-Beam Welding of Additively Manufactured GRCop-84 Copper for Phased Array Lower Hybrid Launchers
摘要： Recent advances in selective laser melting 3-D printing technology allow additive manufacturing of lower hybrid current drive (LHCD) RF launchers from a new material, Glenn Research Copper 84 (GRCop-84), a Cr2Nb (8 at. % Cr, 4 at. % Nb) precipitation hardened alloy, in configurations unachievable by conventional machining. Cr2Nb crystals pin grain boundaries within the copper matrix resulting in high tensile strength and resistance to annealing at elevated temperatures. Brazing, laser, and electron-beam welding (EBW) techniques are explored for joining a thin-walled GRCop-84 waveguide with zero porosity and minimal internal surface roughness. GRCop-84 wets well with the silver solder, CuSil, and Cusil-ABA brazes, once the durable surface oxide is mechanically removed. GRCop-84 melt pool size and flow during EBW is reduced compared to oxygen-free copper (OFC). Pulsed laser and e-beam welding maintains the Cr2Nb precipitate size; precipitate coarsening occurs in conduction mode e-beam welding.
关键词： joining processes,fusion reactor design,magnetic confinement,Annealing,tokamaks,copper alloys,melt processing,materials processing,phased arrays,waveguide
更新于2025-09-23 15:21:01
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Influence of the Wire Feeding on the Wetting Process during Laser Brazing of Aluminum Alloys with Aluminum-Based Braze Material
摘要： The wetting behavior in laser brazing can be designated as inconstant, caused largely by external process discontinuities such as the wire feeding. To reveal periodic melt pool propagation effects that occur during laser brazing of aluminum and for a better understanding of those effects in laser brazing in general, this paper analyzes high-speed recordings of the brazing process with aluminum alloy. It is demonstrated that two main effects of periodic melt pool behavior in different frequency scales occur during the process, related directly to the wire feeding.
关键词： laser brazing,aluminum,process frequencies,melt pool behavior,process dynamics
更新于2025-09-23 15:21:01