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Research on the “∞”-shaped laser scanning welding process for aluminum alloy
摘要： To solve the problems of poor quality and insu?cient joint strength of aluminum alloy weld, a laser scanning welding test platform for aluminum alloy was built. Taking the butt welding of 5052 aluminum alloy with 5 mm and 3 mm thick specimens as the research objects, the study of a “∞”-shaped laser scanning welding process was conducted, and the similarities and di?erences between the welding process and that of single pass laser welding were compared. The in?uence of “∞”-shaped laser scanning welding parameters on the weld formation was investigated by the factor variable method. The change of the surface morphology and dynamic evolution of the molten pool was observed by a high-speed camera system. The optimal combination of the welding parameters was obtained by an orthogonal test. The research showed that the “∞”-shaped laser scanning welding pool had good stability and a good weld quality. Under suitable welding parameters, the length and width of the molten pool could be increased signi?cantly compared with those of the single pass laser welding. The growth rate of the length and width could reach 110.19% and 57.69%, respectively. The surface of the weld was evenly distributed with dense ?sh scales, and the porosity was less than 1%. The tensile strength of the specimen could reach 205.242 MPa, which was 93% that of the base metal; the topography of fracture was full of dimples, and the metallographic structure of the weld was ?ne and distributed with equiaxed dendrites.
关键词： Weld appearance,Weld performance,Laser technique,“∞”-shaped laser scanning welding,Molten pool characteristics
更新于2025-11-28 14:24:20
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Phase Change Behavior of Sn <sub/>20</sub> Sb <sub/>80</sub> /Si?Nano-Composite Multilayer?Thin?Films
摘要： Nano-composite multilayer Sn20Sb80/Si thin films were studied by thermal, electrical and optical methods. Compared with Sn20Sb80, Sn20Sb80/Si film was proved to be a more promising candidate for phase change memory device applications because of its higher crystallization temperature (246?C) and larger crystallization activation energy (2.6 eV). The bandgap was broadened with the increase of Si layer thickness. The crystallization of Sn20Sb80 was restrained after the adding of more Si film layers confirmed by X-ray diffraction patterns. A smoother surface was obtained with the root-mean-square surface roughness of 0.753 nm for [Sn20Sb80(3nm)/Si(7nm)]5 thin film. The measurement using the picosecond laser technique showed that [Sn20Sb80(3nm)/Si(7nm)]5 thin film could achieve the crystalline-to-amorphous phase transform within 2.8 ns irradiated by the laser pulse.
关键词： bandgap,nano-composite multilayer thin films,picosecond laser technique,crystallization activation energy,Phase change memory,surface roughness,Sn20Sb80/Si,crystallization temperature
更新于2025-09-23 15:21:01
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Efficient generation of nitrogen vacancy centers by laser writing close to the diamond surface with a layer of silicon nanoballs
摘要： We proposed a method to effectively fabricate negatively charged nitrogen vacancy (NV?) centers close to the diamond surface by applying femtosecond laser writing technique. With a thick layer of silicon (Si) nanoballs coated, diamond surface was irradiated by high-fluence femtosecond laser pulses. A large number of NV? centers were created around the laser ablation crater area without thermal annealing. The distribution of the NV? centers was expanded to about 50 μm away from the crater center. To demonstrate the function of Si nanoballs, we performed the exactly same laser illumination process on the bare region of the sample surface. In this case, only a few NV? centers were generated around ablation crater. At distance of 32 μm away from crater centers, the NV? density for the case with nanoballs was up to 15.5 times higher compared to the case without nanoballs. Furthermore, we also investigated the influence of laser fluence and pulse number on the NV? density for the case with Si-nanoball layer. Finally, the formation mechanism of NV? centers and the role of Si nanoballs were explained via Coulomb explosion model. The method is demonstrated to be a promising approach to efficiently and rapidly fabricate NV? centers close to the surface of the diamond, which are significant in quantum sensing. Furthermore, the results provide deep insights into complex light-matter interactions.
关键词： single emitters,nitrogen vacancy centers in diamond,silicon nanoballs,femtosecond laser technique
更新于2025-09-23 15:19:57
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Two-Stage Collapse of PNIPAM Brushes: Viscoelastic Changes Revealed by an Interferometric Laser Technique
摘要： Many temperature-responsive polymers exhibit a single-phase transition at the lower critical solution temperature (LCST). One exception is poly(N-isopropylacryamide) (PNIPAM). PNIPAM brush layers (51 ± 3 nm thick) that are end-grafted onto glass beads collapse in two stages. The viscoelastic changes of a PNIPAM brush layers were investigated with an interferometric laser method at different temperatures. This method is able to measure the two-stage collapse of beads coated with a polymer brush layer. When these beads are situated close to a hydrophilic glass surface, they exhibit Brownian motion. As this Brownian motion changes with temperature, it reveals the collapse of the polymer layer. The characteristic spectrum of the Brownian motion of beads are modelled by a damped harmonic oscillator, where the polymer layer acts as both spring and damping. The change of the Brownian motion spectrum with temperature indicates two transitions of the PNIPAM brush layer, one at 36 °C and one at 46 °C. We attribute the first transition to the LCST volume collapse of PNIPAM. Here, changes of the density and viscosity of the brush dominate. The second transition is dominated by a stiffening of the brush layer.
关键词： PNIPAM,viscoelastic changes,interferometric laser technique,brush layers,temperature-responsive polymers,two-stage collapse
更新于2025-09-19 17:13:59