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Control System Development and Experimental Study on Ultrasonic Vibration Feeding

Chinese Journal of Mechanical Engineering 2011年第6期24卷 1083-1088页

作者： ZENG Xianghui qi lehua HUANG Hua LI Yang YANG Fang School of Mechatronics Northwestern Polytechnical University Xi＇an 710072 China Key Laboratory of Modern Design and Integrated Manufacturing Technology of Ministry of Education Northwestern Polytechnical University Xi ＇an 710072 China

Ultrasonic vibration feeding（UVF） method which can quantitatively feed and precisely deposit fine powder is a potential technique for micro *** excitation sources transmit vibration to capillary though the third medium for most UVF *** vibrator is directly touched with the capillary can transmit mechanical energy on the capillary as much as possible,and the powder feeding can be controlled more ***,there are few reports about it.A direct UVF system which integrates the function of micro feeding,process observing,and powder forming was developed in this *** order to analyze the effect of the system factors on feeding,a group of L9（3^3） orthogonal experiments are selected to confirm the effect of level change of *** three factors are capillary nozzle diameter,amplitude and *** flow rate was stable for each combined factors,and the optimum combination for the minimum flow rate are choosing small capillary,small amplitude,and triangular wave *** whole process of feeding includes start point,middle stage and stop *** of feeding was synchronized to vibration when the amplitude of capillary nozzle is larger than critical ***,the feeding process enters the middle stage,the feeding state is observed by the CCD,and it is very stable in the middle *** of feeding can＇t be eliminated during the stop *** features of the deposited powder lines are analyzed; the overflow can be diminished by choosing small capillary and appropriate ratio of the capillary nozzle diameter to the particle *** characters lattices were deposited to validate the ability of quantitatively feeding and fixed feeding of *** of all powder dots show normal distribution,and more than 60% dots are concentrated from 550 μm to 650 μm,and the average diameter for all the dots is 597 μ*** dots positions are well approached to their scheduled positions,and the maximum deviation is 0.27 *** new direct

关键词： ultrasonic vibration feeding (UVF) capillary orthogonal experiment start/stop control quantitatively feeding fixed feeding

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Fractal characterization of pore microstructure evolution in carbon/carbon composites

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Science China(Technological Sciences) 2009年第4期52卷 871-877页

作者： LI MiaoLing qi lehua LI HeJun XU GuoZhong School of Mechatronic Northwestern Polytechnical UniversityXi’an 710072China Research Institute of Carbon-Carbon Composite Key Laboratory of Thermostructure Composite MaterialNorthwestern Polytechnical UniversityXi’an 710072China

A fractal characterization approach was proposed to research pore microstructure evolution in car- bon/carbon (C/C) composites during the chemical vapor infiltration process. The data obtained from mercury porosimetry determinations were analyzed using the sponge fractal model and the thermo- dynamics relation fractal model, respectively. The fractal dimensions of C/C composites at different densification stages were evaluated. The pore microstructure evolution with densification time was studied by fractal dimension analysis. The results showed that C/C composites belong to porous frac- tal structure. The fractal dimensions increase on the whole with decreasing porosity as the densifica- tion proceeds. The fractal dimensions are influenced by the texture of pyrocarbon and decrease with increasing anisotropy from isotropic pyrocarbon to high textural one. Both the complicacy of pore structure and the textural morphology of pyrocarbon can be represented simultaneously by the fractal dimension. The pore evolution of C/C composites in the densification process can be monitored using fractal dimension.

关键词： carbon/carbon composites pore evolution porous medium fractal mercury intrusion porosimetry (MIP)

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Influence of Interfacial Bonding between Metal Droplets on Tensile Properties of 7075 Aluminum Billets by Additive Manufacturing Technique

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Journal of Materials Science & Technology 2016年第5期32卷 485-488页

作者： Hansong Zuo Hejun Li lehua qi Songyi Zhong State Key Laboratory of Solidification Processing Northwestern Polytechnical UniversityXi'an 710072China School of Mechanical Engineering Northwestern Polytechnical UniversityXi'an 7100721China

7075 aluminum billets were fabricated by micro droplet deposition manufacturing technique, and the influence of interracial bonding between metal droplets on the tensile properties was studied. Three sets of samples were manufactured under different temperature conditions, and their mechanical properties were compared. The results show that the temperature of the metal droplets and substrate significantly affect the tensile strength of the sample. Moreover, with proper temperature setting, the 7075 aluminum billets manufactured by micro metal droplet deposition could achieve very good mechanical properties with a tensile strength of 373 MPa and an elongation of 9.95%, which are very similar to those of an ex truded sample. Moreover, a metallurgical bonding diagram based on numerical calculations of interfacial temperature was established to predict the interfacial bonding state. In addition, the fracture morphologies of these specimens were observed. It is indicated that there was a significant transformation of failure mechanism with the improvement of metallurgical bonding, which agreed well with the numerical results.

关键词： Aluminum alloys Mechanical properties Fracture behavior Interfacial bonding Additive manufacturing

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Simulation of Tensile Behaviors of Bamboo-like Carbon Nanotubes Based on Molecular Structural Mechanics Approach Combining with Finite Element Analysis

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Journal of Wuhan University of Technology(Materials Science) 2019年第1期34卷 11-16页

作者： SHU Yang qi lehua School of Mechatronics Northwestern Polytechnical University School of Materials Science and Engineering Northwestern Polytechnical University Center for Composite Materials and Structures Harbin Institute of Technology

A molecular structural mechanics approach combining with finite element analysis(MSM/FEA) was applied to study the microstructure and tensile behaviors of bamboo-like carbon nanotubes(BCNTs). The mathematical model of tensile behaviors of BCNTs was established based on molecular structural mechanics theory. The deformations of BCNTs, with different diameters and compartments set based on the experimental investigation on BCNT structures synthesized by chemical vapor depositon, under tensile load, were analyzed with ANSYS programmed. Results show that the BCNTs have good tensile properties, and those Young's modulus can reach 0.84 Tpa. Through the analysis, it can be found that the Young's modulus of BCNTs depends on the diameters and the length of compartment, which is in good agreement with our experimental tests for the tensile performances of individual BCNT.

关键词： molecular structural mechanics approach bamboo-like carbon nanotubes Young’s modulus experimental test

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Role of hydrogen in the dissociation of CH4 on different graphene by DFT study

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Chinese Journal of Aeronautics 2022年第6期35卷 407-419页

作者： Kun LI Hejun LI Ni LI qiang SONG lehua qi State Key Laboratory of Solidification Processing Carbon/Carbon Composites Research CenterNorthwestern Polytechnical UniversityXi’an 710072China School of Aeronautics Northwestern Polytechnical UniversityXi’an 710072China School of Mechanical Engineering Northwestern Polytechnical UniversityXi’an 710072China

The effect of hydrogen on the growth mechanism of pyrocarbon has attracted much attention. The influence of hydrogen on the dissociation from CH4to C2H2on pristine graphene,N-doped graphene and vacancy graphene have been investigated by using density functional *** are two kinds of heterogeneous reaction pathways when the hydrogen is involved, i.e., dehydrogenation reactions and H-abstraction reactions. The transition state calculations were performed to acquire the reaction pathways on each substrate after obtaining the most stable adsorption configurations of the reactants and products. The results indicate that the adsorptions of reactants are not affected by hydrogen. The dehydrogenation reactions are more favored on vacancy graphene, and the H-abstraction reactions are more favored on the pristine graphene and N-doped graphene. The dehydrogenation reactions on the vacancy graphene are most favored among all these reactions in favor of the deposition of pyrocarbon. The dehydrogenation reactions on these three substrates are affected by hydrogen.

关键词： Abstraction Dehydrogenation DFT Graphene Hydrogen Methane

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Effect of Extrusion Forming on Compressive Strength of Al_(18)B_4O_(33)w/Mg Composites

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稀有金属材料与工程 2009年第A3期38卷 41-44页

作者： Li Zhengjia Li Hejun qi lehua Oy Haibo Northwestern Polytechnical University Xi＇an 710072 China

The aluminum borate whisker reinforced magnesium matrix composites (Al18B4O33w/Mg) were fabricated by pressure infiltration technique and extrusion-vacuum infiltration technique respectively to study the effect of extrusion forming on their mechanical behaviors. The microstructure of the composites was observed by scanning electron microscopy. The compressive strength was investigated and compared with that of the matrix alloy AZ91D. The results show that the extrusion forming could cause fracture of the whisker, but reduce the holes in the composite during impregnation and improve the interfacial bonding between the whisker and the matrix alloy. Compared with that of the matrix as-cast alloy AZ91D, the compressive strength of Al18B4O33w/Mg by the pressure infiltration technique was increased by 22%, while that of Al18B4O33w/Mg by the extrusion-vacuum infiltration technique was increased by 120 %.

关键词：硼酸铝复合材料电子显微镜抗压强度

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Interfacial failure behavior of PyC-C_f/AZ91D composite fabricated by LSEVI

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Journal of Materials Science & Technology 2018年第9期34卷 1602-1608页

作者： Shaolin Li lehua qi Ting Zhang Jiming Zhou Hejun Li State Key Laboratory of Solidification Processing Carbon/Carbon Composites Research Center Northwestern Polytechnical University School of Mechanical Engineering Northwestern Polytechnical University

Carbon fiber reinforced AZ91 D matrix composites with pyrolytic（PyC） coating deposited on fiber surface（PyC-Cf/AZ91 D composites） have been fabricated by Liquid-solid extrusion following vacuum pressure infiltration technique（LSEVI）. Interfacial microstructure and failure behavior of the composites were investigated. Instead of interfacial reaction products, block-shaped interfacial precipitates Mg（17） Al（12） were detected at the interface, which indicates that interfacial reaction was restrained by LSEVI and PyC coating. Nano-MgO was detected at the interface. Interfacial failure behavior of the PyC-Cf/AZ91 D composites,which was the failure between PyC coating and AZ91 D alloy due to the mismatch of thermal expansion and relatively poor bonding, was proposed. Fracture surface of the PyC-Cf/AZ91 D composites was characterized by fibers pulling-out tests. PyC coating served not only as protection to the fibers, but also an adjustment of the interface of the composites.

关键词： MMCs Interface Carbon fiber Failure

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Microstructure and tensile behavior of 2D-C_f/AZ91D composites fabricated by liquid–solid extrusion and vacuum pressure infiltration

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Journal of Materials Science & Technology 2017年第6期33卷 541-546页

作者： Shaolin Li lehua qi Ting Zhang Jiming Zhou Hejun Li State Key Laboratory of Solidification Processing Carbon/Carbon Composites Research Center Northwestern Polytechnical University School of Mechatronic Engineering Northwestern Polytechnical University

2D carbon fiber reinforced AZ91 D matrix composites（2D-C_f/AZ91 D composites） were fabricated by liquid–solid extrusion and vacuum pressure infiltration technique（LSEVI）. In order to modify the interface between fibers and matrix and protect the fiber, pyrolytic carbon（Py C） coating was deposited on the surface of T700 carbon fiber by chemical vapor deposition（CVD）. Microstructure observation of the composites revealed that the composites were well fabricated by LSEVI. The segregation of aluminum at fiber surface led to the formation of Mg_（17）Al_（12） precipitates at the interface. The aluminum improved the infiltration of the alloy and Py C coating protected the fibers effectively. The ultimate tensile strength of 2D-C_f/AZ91 D composites was about 400 MPa. The fracture process of 2D-C_f/AZ91 D composites was transverse fiber interface cracking–matrix transferring load–longitudinal fibers bearing load–longitudinal fibers breaking.

关键词： Magnesium matrix composites Microstructure Tensile properties Fracture behavior

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Study on stable delivery of charged uniform droplets for freeform fabrication of metal parts

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Science China(Technological Sciences) 2011年第7期54卷 1833-1840页

作者： LUO Jun qi lehua ZHOU JiMing XIAO Yuan YANG Fang School of Mechatronics Northwestern Polytechnical University Xi'an 710072 China Key Laboratory of Contemporary Design and Integrated Manufacturing Technology Ministry of Education Xi'an 710072 China

Electrostatic delivery of charged uniform metal droplets is a novel method to deposit metal parts with short process period and low cost. But the delivery process is unstable and delivery accuracy of charged droplets deteriorates because of actions of mutual interactions of closely charged droplets. A droplet generator with droplet charging and deflecting system has been established to find out the way for stably and accurately deflecting droplets. The parameters for generating uniform droplets were first studied. Then the droplet deflections and metal parts depositions with different charge methods were discussed. The results show that the unbalanced electrostatic repelling forces are the main factor to cause the unstable deflection of droplets. The droplets deflection distance can be predicted and controlled by balancing the repelling forces of charged droplets. The metal parts are stably deposited after combining the board charged pulse and substrate motion, indicating that the work can provide useful theoretical and experimental guide for droplet-based freeform fabrication.

关键词： electrostatic delivery uniform metal droplets metal parts freeform fabrication

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A new kind of resin-based wet friction material:Non-woven fabrics with isotropic fiber networks as preforms

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Friction 2021年第1期9卷 92-103页

作者： Yewei FU Le ZHOU Tao YIN Zhongyao LUO Hejin LI lehua qi State Key Laboratory of Solidification Processing Shaanxi Province Key Laboratory of Fiber Reinforced Light Composite MaterialsNorthwestern Polytechnical UniversityXi’an 710072China

As an alternative to short fibers,non-woven fabrics(NWFs)were made using different types of long fibers to optimize the performance of paper-based friction materials and their *** this investigation,the fillers and resin were impregnated into these NWFs to prepare three kinds of wet friction *** tribological,mechanical,and thermal properties of the new wet friction material were *** results indicate that the dynamic friction coefficient of the new friction material is approximately 0.12 and the static friction coefficient is approximately 0.15;the better wear rate is 0.81334×10^(-14)m^(3)·(N·m)^(-1).In addition,the temperature for 10%mass loss yielded 100°C enhancement and the tensile strength was improved by 200%,compared to previously reported *** importantly,the advantages include a simple preparation flow,low cost,and resource *** is a promising approach for the future development of paper-based friction materials.

关键词： non-woven fabric wet friction material continued friction film green manufacture speed

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