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Grain refining in weld metal using short-pulsed laser ablation during CW laser welding of 2024-T3 aluminum alloy
摘要： The 2024 aluminum alloy is used extensively in the aircraft and aerospace industries because of its excellent mechanical properties. However, the weldability of 2024 aluminum alloy is generally low because it contains a high number of solutes, such as copper (Cu), magnesium (Mg), and manganese (Mn), causing solidification cracking. If high speed welding of 2024 aluminum alloy without the use of filler is achieved, the applicability of 2024 aluminum alloys will expand. Grain refining is one of the methods used to prevent solidification cracking in weld metal, although it has never been achieved for high-speed laser welding of 2024 aluminum alloy without filler. Here, we propose a short-pulsed, laser-induced, grain-refining method during continuous wave laser welding without filler. Bead-on-plate welding was performed on a 2024-T3 aluminum alloy at a welding speed of 1 m min?1 with a single mode fiber laser at a wavelength of 1070 nm and power of 1 kW. Areas in and around the molten pool were irradiated with nanosecond laser pulses at a wavelength of 1064 nm, pulse width of 10 ns, and pulse energy of 430 mJ. The grain-refinement effect was confirmed when laser pulses were irradiated on the molten pool. The grain-refinement region was formed in a semicircular shape along the solid–liquid interface. Results of the vertical section indicate that the grain-refinement region reached a depth of 1 mm along the solid–liquid interface. The Vickers hardness test results demonstrated that the hardness increased as a result of grain refinement and that the progress of solidification cracking was suppressed in the grain refinement region.
关键词： grain refinement,short pulsed laser,laser welding,laser ablation,2024 aluminum alloy,hot cracking,dendrite fragmentation
更新于2025-09-12 10:27:22
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Defect Prevention in Selective Laser Melting Components: Compositional and Process Effects
摘要： A model to predict the conditions for printability is presented. The model focuses on crack prevention, as well as on avoiding the formation of defects such as keyholes, balls and lack of fusion. Crack prevention is ensured by controlling the solidi?cation temperature range and path, as well as via quantifying its ability to resist thermal stresses upon solidi?cation. Defect formation prevention is ensured by controlling the melt pool geometry and by taking into consideration the melting properties. The model’s core relies on thermodynamics and physical analysis to ensure optimal printability, and in turn offers key information for alloy design and selective laser melting process control. The model is shown to describe accurately defect formation of 316L austenitic stainless steels reported in the literature.
关键词： additive manufacturing,solidi?cation cracking,austenitic stainless steel,porosity
更新于2025-09-11 14:15:04
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Theoretical and experimental study of artificially controlled backscattering fiber using femtosecond laser fabrication
摘要： Thirteen alloys including high- and low-strength nickel-base alloys, austenitic stainless steels, and ferritic alloys were irradiated using 2 MeV protons to a damage level of 2.5 dpa at 360 °C and assessed for their susceptibility to irradiation assisted stress corrosion cracking (IASCC) in both BWR normal water chemistry (NWC) and PWR primary water. Cracking susceptibility was highest for high strength nickel-base alloys, followed by the low strength nickel-base alloys and then the low strength iron-base alloys. Cracking in the nickel-based alloys was worst in normal water chemistry, which was reversed for the iron-based alloys. In general, cracking correlated with the degree of microstructure changes, though no single feature could be linked to cracking. IGSCC occurred in both the unirradiated and irradiated conditions in high strength nickel-base alloys with susceptibility being considerably higher following irradiation. In all cases, slip was planar, and the degree of slip localization correlated with the probability of IG crack initiation. Low strength nickel-base alloys showed the same dependence on environment as high strength alloys but were considerably less susceptible to IASCC initiation. Among the low strength iron-base alloys, alloy 800 was most susceptible to IASCC initiation in both BWR NWC and PWR primary water, which also correlated with grain boundary chromium depletion and silicon segregation. Across all alloys, cracking correlated with both the degree of localized deformation and the hardness in the irradiated condition. The agreement is expected as increased hardening also correlates with localized deformation, which is likely a necessary, though insufficient condition for cracking.
关键词： PWR primary water,Hardening,Irradiation assisted stress corrosion cracking,Radiation,BWR NWC,Structural materials
更新于2025-09-11 14:15:04
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Synthesis of vertical graphene nanowalls by cracking n-dodecane using RF inductively coupled plasma enhanced chemical vapor deposition
摘要： A facile and controllable one-step method to treat liquid hydrocarbons and synthesize vertical graphene nanowalls (VGs) has been developed by using the technique of inductively coupled plasma enhanced chemical vapor deposition (ICP-ECVD) for plasma cracking of n-dodecane. Herein, the morphology and microstructure of solid carbon material and graphene nanowalls are characterized in terms of different operating conditions, i.e., input power, H2/Ar ratio, injection rate, and reaction temperature. The results revel that the optimal operating conditions were 500 W, 5:10, 30 μL/min and 800 oC for the input power, H2/Ar ratio, injection rate and reaction temperature, respectively. In addition, the degree of graphitization and the gaseous product are analyzed by Raman spectroscopy and gas chromatography (GC) detection. It can be calculated from the Raman spectrum that the relative intensity of ID/IG is approximately 1.55, and I2D /IG is approximately 0.48, indicating that the graphene prepared from n-dodecane has a rich defect structure and a high degree of graphitization. By calculating the mass loading and detecting the outlet gas, we find that the cracking rate of n-dodecane is only 6%-7% and that the gaseous products below C2 mainly include CH4, C2H2, C2H4, C2H6 and H2. Among them, the proportion of hydrogen in the outlet gas of n-dodecane cracking ranges from 1.3% to 15.1% under different hydrogen flows. Based on our research, we propose a brand new perspective for both liquid hydrocarbon treatment and other value-added product syntheses.
关键词： n-dodecane cracking,ICPECVD,vertical graphene nanowalls
更新于2025-09-11 14:15:04
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CuZnSn(SxSe1-x)4 Solar Cell Prepared by the Sol-Gel Method Following a Modified Three-Step Selenization Process
摘要： In current work, Cu2ZnSn(S,Se)4 thin films have been prepared by the sol-gel method based on dimethyl sulfoxide solution followed by a modified three-step selenization process. The key process of this method is to divide the Se evaporation and annealing into two different stages: employ a thermal cracking Se source in the Se evaporation stage and an above-atmospheric pressure in the annealing process. The morphological, structural, elemental distributional, and photovoltaic properties of Cu2ZnSn(S,Se)4 thin films prepared with the three-step selenization process were systematically investigated. It was found that through this modified selenization process, the formations of secondary phases (ZnSe, CuSnSe3) and a fine-grain bottom layer, which usually exists in the traditional one-step selenization process, were effectively suppressed. These improvements could further reduce the carrier recombination and improve the solar cell performance. The best solar cell is obtained with a short-circuit current density of 28.16 mA/cm2, open-circuit voltage of 404.91 mV, fill factor of 62.91%, and a power conversion efficiency of 7.17% under air mass 1.5 (100 mW/cm2) illumination.
关键词： selenization,thermal cracking Se,CZTSSe solar cell,secondary phases,sol-gel method,above-atmospheric pressure
更新于2025-09-11 14:15:04
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Effect of hydrogen concentration on graphene synthesis using microwave-driven plasma-mediated methane cracking
摘要： Gas phase graphene forms as an aerosol in a microwave plasma among other carbon forms. Consisting of 2-6 sheets per stack with dimensions between 100 – 500 nm, it is referred to as nanographene (NG). Surprisingly, increasing H/C ratio in the feedstock increases the relative graphitic content of the product. Dependence of the different carbon forms upon H/C ratio of the gas feed mixture is shown across multiple analytical characterizations. Attributes of (a) phase quality (pristine nature of NG) and (b) phase quantity (how much NG forms relative to other carbon sp2 phases) are addressed. Phase identification of the forms is performed via transmission electron microscopy with quantification by thermogravimetric analysis, assessing their respective oxidative reactivity benchmarked to commercially available similar carbon products applied as standards. X-ray diffraction differentiates these forms based on varied extent of graphitic structure. Electron energy loss spectroscopy assesses graphitic content by the ratio of sp2/sp3 bonding. Raman spectroscopy supports the observed shift in relative proportions of the carbon forms towards preferential graphitic content with increasing H/C. Selected area diffraction illustrates this for NG. Fringe analyses of nanostructure quantifies this shift for carbon particles. Infra-red spectroscopy reveals complementary C-H bonding as a measure of graphitic quality.
关键词： electron energy loss spectroscopy,thermogravimetric analysis,hydrogen concentration,methane cracking,graphene synthesis,X-ray diffraction,nanographene,microwave-driven plasma,Raman spectroscopy,infra-red spectroscopy
更新于2025-09-10 09:29:36
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Thermal Shock Performance of DBA/AMB Substrates Plated by Ni and Ni–P Layers for High-Temperature Applications of Power Device Modules
摘要： The thermal cycling life of direct bonded aluminum (DBA) and active metal brazing (AMB) substrates with two types of plating—Ni electroplating and Ni–P electroless plating—was evaluated by thermal shock tests between ?50 and 250 ?C. AMB substrates with Al2O3 and AlN fractured only after 10 cycles, but with Si3N4 ceramic, they retained good thermal stability even beyond 1000 cycles, regardless of the metallization type. The Ni layer on the surviving AMB substrates with Si3N4 was not damaged, while a crack occurred in the Ni–P layer. For DBA substrates, fracture did not occur up to 1000 cycles for all kind of ceramics. On the other hand, the Ni–P layer was roughened and cracked according to the severe deformation of the aluminum layer, while the Ni layer was not damaged after thermal shock tests. In addition, the deformation mechanism of an Al plate on a ceramic substrate was investigated both by microstructural observation and ?nite element method (FEM) simulation, which con?rmed that grain boundary sliding was a key factor in the severe deformation of the Al layer that resulted in the cracking of the Ni–P layer. The fracture suppression in the Ni layer on DBA/AMB substrates can be attributed to its ductility and higher strength compared with those of Ni–P plating.
关键词： DBA,cracking,Ni–P electroless plating,Ni electroplating,reliability,grain boundary sliding,AMB,thermal shock test,roughness,metallization
更新于2025-09-09 09:28:46