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291.
《材料科学技术学报》2019,35(7):1261-1269
High depth-to-width ratio friction stir welding is an attractive method for the joining demands of aluminum profiles, which is sparked with its extremely low heat input and high mechanical performance. In this study, the joint formation mechanism was studied by a numerical model of plastic flow combined with experimental approaches. A fluid-solid-interaction algorithm was proposed to establish the coupling model, and the material to be welded was treated as non-Newtonian fluid. The thread structure and the milling facets on tool pin promoted drastic turbulence of material. The thread structure converged the plasticized material by its inclined plane, and then drove the attached material to refill the welds. The milling facets brought about the periodic dynamic material flow. The thread structure and the milling facets increased the strain rate greatly under the extremely low heat input, which avoided the welding defects. The condition of the peak temperature of 648 K and the strain rate of 151 s−1 attributed to the lowest coarsening degree of precipitate. The tensile strength of the joint reached 265 MPa, equivalent to 86% of base material. The amelioration via the material flow model inhibits the welding defects and optimizes the parameter intervals, providing references to extracting process-structure-property linkages for friction stir welding. 相似文献
292.
采用真空熔覆技术在45钢表面制备Ni +WC复合熔覆层并进行阶段性取样,研究镍基复合涂层的形成机制。结果表明:在45钢表面生成与基体冶金熔合、WC硬质颗粒分布均匀的Ni基复合熔覆层。整个熔覆层由4 mm厚的复合层、1 mm厚的过渡层、20 μm厚的扩散熔合区以及250 μm厚的扩散影响区组成。复合层区由WC和分解形成的富W复相碳化物包围在Ni颗粒周围组成;复合熔覆层的主要组成相有γ-Ni固溶体、Cr7C3、Ni2.9Cr0.7Fe0.36、Cr23C6、Ni3Fe、Ni3Si、Ni3B、W2C以及C等;真空熔覆过程包括:镍基合金颗粒达到熔点(900℃)前升温阶段颗粒间微烧结颈的形成、升温达到熔点(1020℃)开始的镍基合金颗粒熔融以及保温阶段(1060℃)的熔合扩散与WC颗粒微区位置的调整。 相似文献
293.
This paper investigates the joint effects of agent dynamic and network topology on the consensusability of linear discrete‐time multi‐agent systems via relative output feedback. An observer‐based distributed control protocol is proposed. A necessary and sufficient condition for consensusability under this control protocol is given, which explicitly reveals how the intrinsic entropy rate of the agent dynamic and the eigenratio of the undirected communication graph affect consensusability. As a special case, multi‐agent systems with discrete‐time double integrator dynamics are discussed where a simple control protocol directly using two‐step relative position feedback is provided to reach a consensus. Finally, the result is extended to solve the formation and formation‐based tracking problems. The theoretical results are illustrated by simulations. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
294.
295.
First-principles calculations of binary Al compounds: Enthalpies of formation and elastic properties
Jiong Wang Shun-Li ShangYi Wang Zhi-Gang MeiYong-Feng Liang Yong Du Zi-Kui Liu 《Calphad》2011,35(4):562-573
Systematic first-principles calculations of energy vs. volume (E-V) and single crystal elastic stiffness constants (cij’s) have been performed for 50 Al binary compounds in the Al-X (X = Co, Cu, Hf, Mg, Mn, Ni, Sr, V, Ti, Y, and Zr) systems. The E-V equations of state are fitted by a four-parameter Birch-Murnaghan equation, and the cij’s are determined by an efficient strain-stress method. The calculated lattice parameters, enthalpies of formation, and cij’s of these binary compounds are compared with the available experimental data in the literature. In addition, elastic properties of polycrystalline aggregates including bulk modulus (B), shear modulus (G), Young’s modulus (E), B/G (bulk/shear) ratio, and anisotropy ratio are calculated and compared with the experimental and theoretical results available in the literature. The systematic predictions of elastic properties and enthalpies of formation for Al-X compounds provide an insight into the understanding and design of Al-based alloys. 相似文献
296.
Cell formation is the first step in the design of cellular manufacturing systems. In this study, an efficient tabu search algorithm based on a similarity coefficient is proposed to solve the cell formation problem with alternative process routings and machine reliability considerations. In the proposed algorithm, good initial solutions are first generated and later on improved by a tabu search algorithm combining the mutation operator and an effective neighborhood solution searching mechanism. Computational experiences from test problems show that the proposed approach is extremely effective and efficient. When compared with the mathematical programming approach which took three hours to solve problems, the proposed algorithm is able to produce optimal solutions in less than 2 s. 相似文献
297.
298.
We present a solution to the problem of tracking relative rotation in a leader-follower spacecraft formation using feedback from relative attitude only. The controller incorporates an approximate-differentiation filter to account for the unmeasured angular velocity. We show uniform practical asymptotic stability (UPAS) of the closed-loop system. For simplicity, we assume that the leader is controlled and that we know orbital perturbations; however, this assumption can be easily relaxed to boundedness without degrading the stability property. We also assume that angular velocities of spacecraft relative to an inertial frame are bounded. Simulation results of a leader-follower spacecraft formation using the proposed controller structure are also presented. 相似文献
299.
提出一种轨迹跟踪级联机器人编队控制方法. 该方法有效结合距离-角度(??-??) 控制和距离-距离(??-??) 控制方案, 并利用无迹卡尔曼滤波算法对Leader-Follower 级联机器人系统的状态进行估计; 根据状态估计结果设计输入-输出动态反馈控制规律, 使得跟随机器人(Follower) 准确跟踪领航机器人(Leader), 确保编队的稳定性和较快的收敛性, 并达到理想的编队控制效果. 仿真实验验证了所提出方法的可行性.
相似文献300.
在此前的汉语未登录词语义预测中,构词相关的知识一直被当做预测的手段,而没有被视为一种有价值的知识表示方式,该文在“语素概念”基础上,深入考察汉语的语义构词知识,给出未登录词的“多层面”的词义知识表示方案。针对该方案,该文采用贝叶斯网络方法,构建面向汉语未登录词的自动语义构词分析模型,该模型能有效预测未登录词的“多层面”的词义知识。这种词义知识表示简单、直观、易于拓展,实验表明对汉语未登录词的语义预测具有重要的价值,可以满足不同层次的应用需求。 相似文献