ZA合金凝固技术的研究进展

ZA铸造合金强度高,耐磨性好,成为近年的研究热点之一.阐述ZA合金的凝固特点,综述了ZA合金凝固技术国内外的研究现状,概述采用压铸、挤压铸造、喷射成形、超高压凝固、过热处理、外场下凝固等凝固技术对ZA合金微观组织的影响规律,分析了各凝固技术的特点,并对ZA合金凝固技术的前景进行了展望.     

Multilayer bender-type PZT-PZN actuator by co-extrusion process

A multilayer flextensional bender-type actuator, which was composed of five piezoelectric layers and one passive conducting layer, was fabricated using a co-extrusion process. A low-temperature sinterable PZN-PZT was used for the piezoelectric layer. The conducting inner electrode layers and passive bottom layer were fabricated by dispersing silver particles in the PZN-PZT matrix. For the co-extrusion process, piezoelectric and conducting feedrods were made separately by mixing them with polymers, which was followed by the formation of the initial feedrods. The initial feedrods were co-extruded through a reduction die to produce a continuous multilayer sheet. The binder was burnt out and, the multilayer green bodies were then sintered at 900 °C. The design was suitable not only for applying a high electric field with inner electrodes, but also for inducing a residual tensile stress on the piezoelectric layer. A multilayer actuator with dimensions of 50 mm × 20 mm × 0.84 mm generated a displacement of approximately 400 μm at 400 V, which is more than twice the displacement of a simple two-layer actuator with the same dimensions.     

Influence of a low-frequency alternating magnetic field on hot tearing susceptibility of EV31 magnesium alloy

The effect of a low-frequency alternating magnetic field (AMF, 0 A 0 Hz, 5 A 10 Hz, 10 A 10 Hz, 15 A 10 Hz) on the hot tearing susceptibility (HTS) of a magnesium alloy (EV31) was systematically studied using a combination of experiment and numerical simulation. By observing the macroscopic hot cracks in hot joints of the “T” samples, the hot tearing tendency of the samples was analyzed. The HTS of the alloy can be predicted via numerical simulation and the crack susceptibility coefficient (CSC). The microstructure and morphology of the hot tearing zone of EV31 were investigated using scanning electron microscopy (SEM). Results show that increasing the magnetic field strength reduces both the alloy solidification temperature range and the dendrite coherency temperature, which increases the feeding time during solidification and decreases the HTS of the alloy. When the magnetic field parameters are 10 Hz 15 A, the EV31 alloy shows the lowest HTS. The main component of the second phase in the microstructure is Mg₁₂Nd. This study also found that the electromagnetic field can effectively refine the grains, purify the melt, and reduce the oxide content in the melt. The obtained simulation results are consistent with the experimental results.

     

Comparative study on local and global mechanical properties of 2024 T351, 2024 T6 and 5251 O friction stir welds

The effect of the microstructure heterogeneity on the global and local tensile properties of friction stir welded joints in 5251 (O temper) and AA2024 (T351 and T6 tempers) aluminium alloys has been investigated. Micro-tensile tests parallel to the welding direction have been carried out in the regions representative of the main microstructural zones. The digital image correlation technique (DIC) has been used during transverse tensile tests for mapping the strain distribution and to determine the local stress–strain curves. A 3-D finite elements model has been developed to predict the weld behaviour from the tensile curves of the individual regions of the weld.The tensile properties of the 5251 O weld are relatively homogeneous leading to high ductility and fracture in the base material. In contrast, the tensile properties of the various regions of the 2024 T351 and 2024 T6 welds are very heterogeneous and essentially controlled by the state of precipitation. The thermo-mechanically affected zone is the weakest region where the strain localises during a transverse tensile test. The 2024 T6 base material is stronger than the 2024 T351 alloy, leading to a more pronounced strain localisation during transverse tensile tests and a lower overall ductility. Local tensile data obtained by strain mapping are in good agreement with the curves obtained by micro-tensile tests, and these results can be safely used in a finite elements model to predict the behaviour of the overall weld assembly.     

Lanthanide modified Pt/CeO2-based catalysts for methane partial oxidation: Relationship between catalytic activity and structure

In most cases, reasonable design and construction of Pt/CeO₂-based catalysts and detailed exploration of relationship between its structural characteristics and the catalytic activity are crucial to improve the catalytic performance and reduce the cost. In this work, a series of CeO₂ doped with lanthanide metal ions (La, Nd, Er and Yb) has been successfully synthesized, and then Pt is introduced through impregnation. The morphology, structure and component analysis are characterized by SEM, TEM (HRTEM), EDS, XRD, ICP-AES, XPS, UV Raman, O₂-TPD, H₂-TPR and CO or O₂-pulse chemisorption, and the corresponding catalytic performances are developed by partial oxidation of methane. On the basis of the analysis of the structural properties of various catalysts, it is found that the Pt/CeLa catalyst shows the best catalytic performance due to its low valence state of Pt, excellent oxygen migration capacity and oxygen storage capacity, T₅₀ is 510 °C and the selectivity is superiority. What's more, the modification of CeO₂ by lanthanide metal ions especially La³⁺ can effectively change the oxygen activity of supports, so that this catalyst can be used in various redox catalytic reactions.     

脲醛树脂石蜡微胶囊的制备及其在保温隔热涂料中的应用

以脲醛树脂为壳材料,质量比为1:1:1的液体石蜡、十八烷和固体石蜡组成的混合物为芯材,用原位聚合的方法制备了脲醛树脂石蜡微胶囊,采用红外光谱分析和扫描电子显微镜对微胶囊进行了表征,使用差示扫描量热仪(DSC)研究了石蜡微胶囊的热性能及相变温度。将该微胶囊制成保温隔热涂料,测定了涂料的保温隔热效果及其机械性能。结果显示,脲醛树脂成功包覆了石蜡,微胶囊表面粗糙,粒径为10μm左右,其相变温度分别为2.1°C、15.1°C和42.9°C,相变焓为50.1J/g。与仅含钛白粉的空白涂料相比,添加了空心玻璃微珠、改性硅藻土和脲醛树脂石蜡微胶囊制备的保温隔热涂料最大温差11.5°C,平均温差9.7°C,涂层的机械性能符合国标要求。     

追加浓硫酸法合成溴代仲丁烷

以溴化钠、仲丁醇为原料,水为溶剂,以追加的方式投入浓硫酸,合成了溴代仲丁烷。考察了不同反应条件对产率的影响,得出适宜的反应条件:溴化钠用量为86g,仲丁醇用量为31g(nNaBr∶nC4H9OH=1∶0.5),浓硫酸用量为196g,水用量为114g(即稀释后硫酸浓度为62%),浓硫酸追加量为26g,回流反应55min,产率可达71%以上。最终产物经气相色谱分析得溴代仲丁烷产物含量为89.3%,并对产物进行了红外表征。     

Design of Fe-based nanocrystalline alloys with superior magnetization and manufacturability

Fe-based alloys with a nanocrystalline-amorphous nanostructure exhibit superior soft-magnetic performances; however they generally suffer from the low magnetization because of their heavy doping for an acceptable manufacturability. In this study, we proposed a revolutionary nanostructure-construction concept based on preforming dense nuclei in the melt-quenching process with a critical cooling-rate and refining the nano-structure via transient metalloid-rich interfaces. A novel alloy composition of Fe_85.5B₁₀Si₂P₂C_0.5 was developed via our multi-metalloid stabilization and critical formability strategies by using a total of only 4.6 wt. % light metalloids. This unique alloy design effort leads to unprecedented magnetic properties with the super-high B_s of 1.87 T and µ_e of 1.0–2.5 × 10⁴, which outperform all commercial counterparts and have a high potential to substitute for commercial bulk Si-steels currently used for soft-magnetic applications. This hetero-structuring and lean-alloying strategy provides a paradigm for the next-generation of magnetic materials.     

实验轧机液压伺服零偏自愈控制

实验轧机由于开机时间短、维护不及时等因素易导致其液压系统固有的伺服零偏现象,这对辊缝定位精度产生恶劣的影响,进而造成轧制变形条件发生不可测变化。为了消除该现象对轧制实验结果准确性的干扰,开发了一种伺服零偏自愈控制方法。仿真研究结果表明,当自愈系数与液压辊缝控制闭环比例系数的比值维持系统惯性时间常数的2倍时,可获得最佳控制效果。实际应用结果表明,在有效轧制力工作区间,动态辊缝定位精度小于1μm。     

基于改进的最小二乘支持向量机的退火炉内带钢延伸量预测

带钢在退火炉内进行长时间加热的过程中,由于炉内温度、张力以及带钢运行速度等变量间具有多重相关性且存在生产数据噪声,从而难以准确预测炉内带钢的延伸量。本文提出了一种基于遗传算法的BP神经网络理论(GA-BP神经网络)与最小二乘支持向量机(LSSVM)理论相结合的算法用于目标函数拟合,该算法兼顾最小二乘支持向量机的全局与局部拟合能力,有效避免算法陷入局部最优的缺点,利用改进的BP神经网络算法优化选择LSSVM 模型的惩罚因子和核函数参数,以避免人为选择参数的盲目性,使LSSVM具有更好的泛化能力和预测精度。将该方法应用到退火炉内带钢的延伸量预测,通过现场生产数据仿真模拟进行非线性函数估计,结果表明：本文所提出的方法具有良好的预测精度,可以应用于实际生产。