Fabrication of Titanium Aluminides via Powder-Cored Wire Arc Additive Manufacturing: Microstructural and Mechanical Characterization

Herein, an innovative powder-cored wire arc additive manufacturing (PC-WAAM) process is proposed to fabricate γ-TiAl thin-walled intermetallic alloy. The metallography, phase composition, and mechanical properties at different thin-wall locations are characterized. The results show that the alternatively distributed layer-like microstructure composed of α₂ (Ti₃Al) and γ (TiAl) phases is obtained along the building direction. The content of α₂ phase exhibits the tendency of decreasing from the bottom to top region. This unique microstructure characteristic is closely related to the typical thermal cycling history during deposition. Moreover, the tensile strength and microhardness of the top region are lower than the middle and bottom region. In general, the current PC-WAAM technique shows promising capability of fabricating γ-TiAl intermetallic alloy with low cost. This work becomes a valuable reference for understanding the evolution mechanism of microstructure and paves the way for the flexible and customized additive manufacturing of γ-TiAl alloy.     

Additive Manufacturing of Hard Magnetic Passive Shims to Increase Field Homogeneity of a Halbach Magnet

Obtaining a highly homogeneous magnetic field is desired for field-controlled applications. For example, the resolution of magnetic analysis methods can be improved by generating a stronger and more homogeneous field over the region of interest (ROI). A set of 3D-printed passive shims is fabricated using additive manufacturing to improve the magnetic field homogeneity of a Halbach magnet assembly. The feedstock is a custom acrylonitrile butadiene styrene (ABS)-hard magnet composite filament filled with 60% wt. isotropic NdFeB. Additionally, a method for investigating the remanence is developed and validated. The result reveals a good agreement between the new method and existing measurement techniques for the remanence of permanent magnets. It is also shown that the additive manufacturing procedure has negligible effects on the magnetic properties. Performing a parametric study over a rectangular ROI, an optimized shim configuration is achieved. In the optimized and 3D-printed configuration, the average norm of the magnetic flux density, B_norm, is increased by 13% and, more importantly, a 43% increase in the magnetic uniformity is obtained. These results highlight the great potential of freeform manufacturing, namely, additive manufacturing, to tailor the properties of magnet structures.     

Using Selective Electron Beam Melting to Enhance the High-Temperature Strength and Creep Resistance of NiAl–28Cr–6Mo In Situ Composites

By increasing the density of interfaces in NiAl–CrMo in situ composites, the mechanical properties can be significantly improved compared to conventionally cast material. The refined microstructure is achieved by manufacturing through electron beam powder bed fusion (PBF-EB). By varying the process parameters, an equiaxed or columnar cell morphology can be obtained, exhibiting a plate-like or an interconnected network of the (Cr,Mo) reinforcement phase which is embedded in a NiAl matrix. The microstructure of the different cell morphologies is investigated in detail using scanning electron microscope, transmission electron microscopy, and atom probe tomography. For both morphologies, the mechanical properties at elevated temperatures are analyzed by compression and creep experiments parallel and perpendicular to the building direction. In comparison to cast NiAl and NiAl–(Cr, Mo), the yield strength of the PBF-EB fabricated specimens is significantly improved at temperatures up to 1,027 °C. While the columnar morphology exhibits the best improved mechanical properties at high temperatures, the equiaxial morphology shows nearly ideal isotropic mechanical behavior, which is a substantial advantage over directionally solidified material.     

Absolute Stability of Nonlinear Systems with Two Additive Time-varying Delay Components

In this paper, we present a new sufficient condition for absolute stability of Lure system with two additive time-varying delay components. This criterion is expressed as a set of linear matrix inequalities (LMIs), which can be readily tested by using standard numerical software. We use this new criterion to stabilize a class of nonlinear time-delay systems. Some numerical examples are given to illustrate the applicability of the results using standard numerical software.     

Synchronisation of complex dynamical networks with additive stochastic time-varying delays

In this letter, a class of complex dynamical networks with additive stochastic time-varying delays is investigated. Two kinds of delays in complex dynamical networks are taken into consideration, one is called the nodes-induced delay in this paper, and the other is the network-induced delay. Both of the delays are assumed to obey different stochastic distributions. By utilising a novel Lyapunov–Krasovskii functional and stochastic analysis techniques, a sufficient criterion is obtained in the form of linear matrix inequalities to ensure the synchronisation of the complex dynamical networks. Finally, a numerical example is given to illustrate the feasibility and effectiveness of the proposed method.     

盲的环境补偿方法

环境鲁棒性是语音识别系统的一个很重要的问题。本文讨论了环境加性噪声和卷积噪声对语音数据的影响,以及提高系统的鲁棒性的盲的环境补偿方法。     

纳米二氧化钛催化氧化废水中硫离子的研究

二氧化钛具有较高的光稳定性和反应活性，可以用于光催化氧化法去除水中的有毒物质，本文以含有S^2-的废水为研究体系，探讨了纳米二氧化钛用量、反应起始浓度对光催化氧化反应的影响结果。     

低温烧结Dy-B-Si-O系玻璃介质掺杂Ba_(0.6)Sr_(0.4)TiO_3陶瓷

采用溶胶-凝胶法制备了Dy-B-Si-O系玻璃介质掺杂Ba0.6Sr0.4TiO3(BST)陶瓷粉体,并烧结成瓷。探讨了玻璃介质对BST陶瓷密度、烧结温度和介电性能的影响,利用扫描电子显微镜(SEM)、自动元件分析仪测试了BST陶瓷的显微结构和介电性能。研究结果表明,添加Dy-B-Si-O系玻璃介质降低了陶瓷烧结温度,提高了陶瓷致密度;随着Dy-B-Si-O系玻璃介质中Dy2O3组分含量的增加,介电常数增大;介电损耗先增大后减小,介电损耗最小值约为0.01,可满足在电压可调电容器上的使用要求。     

双峰纳晶介孔TiO2的制备及在染料敏化太阳电池中的应用

采用水热法,以三嵌段共聚物聚氧化乙烯-聚氧化丙烯-聚氧化乙烯(EO20PO70EO20,P123)为模板剂,钛酸四丁酯为前驱体,制备了双峰纳晶介孔TiO2,分别利用X射线衍射(XRD)、比表面积与孔径分析、场发射扫描电子显微镜(SEM)以及透射电子显微镜(TEM)对制备的介孔TiO2的相结构、比表面积、孔径和形貌进行了表征,探讨了P123用量对纳晶TiO2微结构的影响。以双峰纳晶介孔TiO2作为染料敏化太阳电池的光阳极材料组装的太阳电池的光电转化效率达5.18%,与相同条件下以P25作光阳极材料组装的电池效率(4.02%)相比提高了28.85%,初步显示了双峰纳晶介孔TiO2在太阳电池领域中的应用潜力。