Effect of wetting behavior on generation of uniform aluminum droplets obtained by pneumatic drop-on-demand technique

Micro metal droplet is the basic building block of three-dimensional metal parts fabricated by micro droplet deposition manufacturing (MDDM) technique. In this paper, the effect of wetting behavior between liquid metal and spray nozzle on the generation of micro aluminum droplets produced by pneumatic drop-on-demand (DOD) technique was investigated by simulation and experiment. A finite element model of liquid–gas flow was established based on the improved level set method (LSM). Then the generation of micro liquid aluminum droplets under different wetting conditions was simulated. A series of spraying experiments were also performed on micro droplet deposition experiment platform. The results show that the generation and flight of micro aluminum droplets are influenced by wetting condition between liquid metal and the nozzle surface significantly. Additionally, the effect of wetting behavior on the droplet size was analyzed to achieve the smallest building block. It was found that the droplet radius decreased with the increase of contact angle exponentially, which agreed with the numerical calculation and experiment results. On this basis, a wettability criterion was proposed for selecting nozzle materials. These works would be helpful for the processing optimization and equipment improvement of MDDM technique.     

Effect of process parameters on copper droplet ejecting by pneumatic drop-on-demand technology

Stable generation of copper droplets is a key issue in fabricating copper parts by drop-on-demand (DOD) technology. The process parameters such as supply pressure and electronic pulse width have significant effect on pressure variation and droplet formation. In the present work, a pressure acquisition system was first set up to measure the pressure variation in crucible. Then the measured pressure data were applied on a 2D axisymmetric model as inlet conditions to study the influence of process parameters on copper droplet formation. The results indicated that the peak pressure in crucible increased linearly with the increase of supply pressure. As supply pressure increased, the jet velocity and the limiting length increased to critical value and droplet could be generated. The peak width increased with the rise of electronic pulse width. By increasing the electronic pulse width, the time of pressure above threshold value increased and the jet limiting length grew to critical value for breakup. However, if supply pressure and electronic pulse width were too large, satellite droplets would be formed. Pure copper droplet generating experiments were conducted to obtain appropriate parameters. Single droplet was generated while supply pressure was between 60 kPa and 100 kPa and pulse width was between 550 μs and 1550 μs. Also the range of pulse width varied as the supply pressure increased for generating single droplet. The statistics of droplet diameter suggested that droplet diameter increased with the increase of supply pressure. Electronic pulse width had influence on the droplets size and the standard deviation increased with the increase of electronic pulse width. So small supply pressure and electronic pulse width should be used for generating uniform droplets.     

金属纤维/聚合物复合材料的制备及其电磁屏蔽效能

填充金属纤维的复合材料由于具有优异的电磁屏蔽效能（EMSE）而广泛应用于电磁干扰领域。本研究采用浸渗和机械搅拌法分别制备了316L纤维/环氧树脂和Cu纤维/环氧树脂两种复合材料,并测试了其电磁屏蔽效能。研究表明,当316L纤维长径比从200增加到1000时,316L纤维/环氧树脂复合材料的电磁屏蔽效能逐渐增大,而当长径比从1000增加到3000时,其复合材料的电磁屏蔽效能迅速下降;当316L纤维的含量从10wt%增加到25wt%时,复合材料的电磁屏蔽效能逐渐增大。对于316L纤维/环氧树脂复合材料而言,316L纤维的最佳参数为：纤维直径为?8μm、含量为25wt%、长径比为1000,其复合材料的电磁屏蔽效能最高可达-78dB。对于Cu纤维/环氧树脂复合材料而言,Cu纤维的最佳参数为：纤维直径为?120μm、含量为2.0wt%。     

Formation of aluminide coatings on Ni and austenitic 316 stainless steel by a low temperature FBCVD process

The experimental formation of aluminide coatings on 316 stainless steel grades and Ni by use of a chemical vapour deposition via the fluidised bed technique at relatively low temperatures (550, 560 and 570 °C for the steel and 550-650 °C for Ni) was studied. A mathematical model that predicts the growth rate of the coating for both substrates is being proposed. The experimental results were evaluated by means of optical microscopy. Uniform coatings even for reduced treatment times of a few minutes were produced. The measured coating thickness was compared to the predicted results from the model. The results indicate a linear correlation of coating thickness and treatment time for short treatment times and a parabolic correlation for longer ones. The low treatment temperature allows for energy saving and the mechanical properties of the treated parts remain unaffected.     

Preparation and characterization of superhydrophobic silica-based surfaces by using polypropylene glycol and tetraethoxysilane precursors

Preparation of superhydrophobic silica-based surfaces via sol-gel process by adding polypropylene glycol (PPG) polymer into the precursor solution has been developed. Surface roughness of the films was obtained by removing the organic polymer at 500 °C and then the hydrophobic groups bonded onto the films were obtained by chemical reaction with hexamethyldisilazane (HMDS). Physical properties of the as-prepared films were analyzed by contact angle measurements, scanning electron microscopy (SEM), UV-VIS scanning spectrophotometer and Fourier transform infrared (FT-IR) spectrophotometer. The experimental parameters were varied by the type of silane species, the weight ratio of PPG solution to precursor solution, the hydrolysis time of the precursor solution, the molecular weight of PPG, the casting temperature and the evaporation temperature. The phase separation of the PPG polymer rich domain occurred on the substrates at a lower temperature. The result showed that the contact angles of the films prepared at 5 °C were greater than 150° when the weight ratio of PPG solution to precursor solution was 5. In addition, the transmittance of the films was greater than 80% simultaneously.     

FEA modeling and simulation of shear localized chip formation in metal cutting

The finite element analysis (FEA) has been applied to model and simulate the chip formation and the shear localization phenomena in the metal cutting process. The updated Lagrangian formulation of plane strain condition is used in this study. A strain-hardening thermal-softening material model is used to simulate shear localized chip formation. Chip formation, shear banding, cutting forces, effects of tool rake angle on both shear angle and cutting forces, maximum shear stress and plastic strain fields, and distribution of effective stress on tool rake face are predicted by the finite element model. The initiation and extension of shear banding due to material's shear instability are also simulated. FEA was also used to predict and compare materials behaviors and chip formations of different workpiece materials in metal cutting. The predictions of the finite element analysis agreed well with the experimental measurements.     

Synthesis and characterization of metal oxide multilayers obtained via MOCVD as protective coatings of graphite against oxidation

Zirconia (ZrO₂) titania (TiO₂) and alumina (Al₂O₃) thin films were deposited on a graphite substrate both in mono- and in multi-layer systems, using the metal organic chemical vapor deposition technique, to test their practical qualities as protective coatings against oxidation at high temperatures. The depositions were performed using a hot wall reactor at reduced pressure (0.6 Torr) in the temperature range 350–500 °C, using, as precursors, (η⁵-C₅H₅)₂Zr(CH₂C(CH₃)₃)₂, Ti(OCH(CH₃)₂)₄, and (CH₃CH₂)₂Al(OCCH₃CHCCH₃O), respectively. Surface and topographical analysis of the deposits using X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy techniques as well as thermogravimetric measurements (TG and DTA) in an oxygen flux of mono- and multi-layer systems are reported and examined.     

A theoretical and experimental study on forming limit diagram for a seamed tube hydroforming

The purpose of this work is to establish the forming limit diagram (FLD) for a seamed tube hydroforming. A new theoretical model is developed to predict the FLD for a seamed tube hydroforming. Based on this theoretical model, the FLD for a seamed tube made of QSTE340 sheet metal is calculated by using the Hosford yield criterion. Some forming limit experiments are performed. A classical free hydroforming tool set is used for obtaining the left hand side forming limit strains, and a novel hydroforming tool set is designed for the right hand side of FLD. The novel device required the simultaneous application of lateral compression force and internal pressure to control the material flow under tension–tension strain states. Furthermore, the suitable loading paths for the left hand side of FLD by theoretical formulas and for the right hand side of FLD by finite element (FE) simulations are calculated. Finally, a comparison between the theoretical results and experimental data is performed. The theoretical predicting results show good agreement with the experimental results.     

一种仓储物流自动导引小车的动力学建模及验证

为缩短仓储物流行业中自动导引小车的开发设计周期,构建自动导引小车的仿真模型.从质量和质心出发,采用拉格朗日法建立差速驱动自动导引小车本体的动力学方程.对电机部分进行分析,建立电压响应方程.通过将二者结合,建立负载与质心可变的差速驱动自动导引小车完整的动力学整车模型.通过MATLAB仿真,与实际系统的速度响应进行了对比分...     

Effects of particle arrangement and particle damage on the mechanical response of metal matrix nanocomposites: A numerical analysis

The spatial distribution of reinforcement particles has a significant effect on the mechanical response and damage evolution of metal matrix composites (MMCs). It is observed that particle clustering leads to higher flow stress, earlier particle damage, as well as lower overall failure strain. In recent years, experimental studies have shown that reducing the size of particles to the nanoscale dramatically increases the mechanical strength of MMCs even at low particle volume fractions. However, the effects of particle distribution and particle damage on the mechanical response of these metal matrix nanocomposites, which may be different from that observed in normal MMCs, has not been widely explored. In this paper, these effects are investigated numerically using plane strain discrete dislocation simulations. The results show that non-clustered random and highly clustered particle arrangements result in the highest and lowest flow stress, respectively. The effect of particle fracture on the overall response of the nanocomposite is also more significant for non-clustered random and mildly clustered particle arrangements, in which particle damage begins earlier and the fraction of damaged particles is higher, compared to regular rectangular and highly clustered arrangements.     

Stiffness analysis and experiment of a novel 5-DoF parallel kinematic machine considering gravitational effects

In order to carry out high-precision machining of aerospace structural components with large size, thin wall and complex surface, this paper proposes a novel parallel kinematic machine (PKM) and formulates its semi-analytical theoretical stiffness model considering gravitational effects that is verified by stiffness experiments. From the viewpoint of topology structure, the novel PKM consists of two substructures in terms of the redundant and overconstrained parallel mechanisms that are connected by two interlinked revolute joints. The theoretical stiffness model of the novel PKM is established based upon the virtual work principle and deformation superposition principle after mapping the stiffness models of substructures from joint space to operated space by Jacobian matrices and considering the deformation contributions of interlinked revolute joints to two substructures. Meanwhile, the component gravities are treated as external payloads exerting on the end reference point of the novel PKM resorting to static equivalence principle. This approach is proved by comparing the theoretical stiffness values with experimental stiffness values in the same configurations, which also indicates equivalent gravity can be employed to describe the actual distributed gravities in an acceptable accuracy manner. Finally, on the basis of the verified theoretical stiffness model, the stiffness distributions of the novel PKM are illustrated and the contributions of component gravities to the stiffness of the novel PKM are discussed.     

Workpiece surface roughness and integrity after WEDM of Ti-6Al-4V and Inconel 718 using minimum damage generator technology

Following a brief review of EDM and its use on advanced aerospace alloys including workpiece integrity constraints, data are presented after machining Ti-6Al-4V and Inconel 718. Roughing and finishing (multiple trim cut) strategies were employed on two high specification machines with pulse generators designed to provide minimum workpiece integrity damage. Results include productivity, 3D topographic maps of workpiece surfaces, microstructural and microhardness depth profile data. Average recast thickness was <11 μm, several trim passes showing no apparent recast or damage. Similarly, no significant change in workpiece microhardness variation was observed with cracking confined to the recast layer.     

Microstructure and wear performance of gradient Ti/TiN metal matrix composite coating synthesized using a gas nitriding technology

Surface modification is an attractive method to enhance the surface hardness and wear resistance of titanium. In this paper, a continuous wave 2 kW Nd:YAG laser was used to synthesize Ti/TiN metal matrix composite coating on the surface of commercial pure titanium. The microstructure and the wear resistance of the synthesized metal matrix composite coating were investigated. The synthesized surface Ti/TiN metal matrix composite coating had a pronounced gradient microstructure through the melt depth. Good metallurgical bonding between the reinforcing phase of the metal matrix composite and the titanium matrix was observed. The hardness and wear resistance under block-on-ring dry sliding wear testing conditions of the synthesized Ti/TiN metal matrix composite coating were markedly enhanced.     

Elastic fields generated by a semi-spherical hydride particle on a free surface of a metal and their effect on its growth

The formation of a metal hydride is associated with a large increase of volume relative to the parent metal and therefore in large strain energies. Effects of elastic energy on the hydriding of metals are revealed in the microstructural evolution and kinetics of hydride growth on free surfaces. In the present work, we study in detail the elastic fields set up by a semi-spherical hydride particle growing at a free surface of metal with cubic symmetry, with and without an oxide layer. These systems combine geometric (structural) and material anisotropies.Three stages along the microstructural evolution on the surface of some hydride forming metals exposed to hydrogen at constant pressure were described experimentally. For these stages along hydride growth, correlations with the elastic fields are suggested as follows. (a) A hydride particle at the free surface generates regions of tensile and compressive hydrostatic stress in the surrounding matrix. This may induce a preferred nucleation of new hydrides and formation of clusters of hydrides precipitates, which is indeed observed experimentally. (b) Clustering, on the other hand, may contribute to the cease of growth due to competition on hydrogen. In addition, as the particle grows, changes in the stress fields may retard further diffusion from the surface and be another contribution to the cease of growth. (c) A growing hydride increases the stress in the oxide layer and may finely break it. Then the elastic energy per unit volume drops to its minimum value and the growth may accelerate. The formation of such “growth center” is favored for that hydride precipitate that grow alone and not in a cluster.     

纳米LiInO2光催化剂的溶胶-凝胶法制备与表征

本文利用溶胶-凝胶法制备了LiInO₂纳米材料,采用X-射线衍射(XRD)、扫描电镜(SEM)和紫外-可见吸收光谱等测试手段,研究了制备条件对LiInO₂微观结构的影响因素,并以亚甲基蓝为目标降解物研究了LiInO₂的光催化性能。研究结果表明：制备的LiInO₂纳米粒子具有LiFeO₂的晶型,颗粒尺寸约50-100纳米,制备样品的焙烧温度对其结构和性能产生了明显地影响,在氙灯(300W)照射90 min条件下,纳米LiInO₂对亚甲基蓝的光催化降解率达92%,活性位点捕获实验表明光生空穴在降解亚甲基蓝的机制中占主导作用。     

Synthesis and characterization of cubic boron nitride thin films using the ME-ARE method

Cubic boron nitride (cBN) films were deposited by a magnetically enhanced activated reactive evaporation (ME-ARE) technique. Pulsed DC instead of r.f. power was used to bias the substrate. The effect of deposition parameters such as substrate bias voltage, plasma discharge current and gas flow ratio on the formation of cBN was investigated. BN films were characterized by FTIR and TEM. CBN films with a high content of cubic phase were successfully synthesized. It was found that formation of cBN film requires the bombardment of ions with both high flux and high energy. TEM observation showed that the cBN film had grain sizes of 15-50 nm and that a non-cubic phase BN, 10-15 nm was initially grown.     

Selective separation of Cu(II), Zn(II), and Cd(II) by solvent extraction

An experimental investigation was presented on the separation of Cu (II), Zn (II), and Cd (II) from a rich sulfate leachate of zinc slag by solvent extraction. The results of orthogonal experiments indicate that LIX 984N is highly selective and very efficient in the extraction of Cu (II), and the analysis of variance indicates that the sequence of parameters according to their influence on the separation efficiency is phase ratio>LIX 984N concentration>pH value>extraction time. The optimal condition for copper extraction is obtained as 25% of LIX 984N concentration, 7 min of extraction time, 3:2 of phase ratio O/A, and pH=1.7. The separation of Zn (II) and Cd (II) was performed after the copper extraction from the raffinate. Comparative analysis of the separation with di-2-ethylhexyl phosphoric acid (D2EHPA), D2EHPA-tributyl-phosophate (TBP) synergistic extracting system, and 2-ethylhexyl phosphonic acid mono 2-ethylhexyl ester (HEHEHP) was made at pH=2.0. It is demonstrated that the extraction efficiency with D2EHPA is improved after being saponified by sodium hydroxide, and D2EHPA-TBP synergistic extracting, as well as HEHEHP, has a superior selectivity to Zn (II) over Cd (II).     

Modelling and simulation of chatter in milling using a predictive force model

A predictive time domain chatter model is presented for the simulation and analysis of chatter in milling processes. The model is developed using a predictive milling force model, which represents the action of milling cutter by the simultaneous operations of a number of single-point cutting tools and predicts the milling forces from the fundamental workpiece material properties, tool geometry and cutting conditions. The instantaneous undeformed chip thickness is modelled to include the dynamic modulations caused by the tool vibrations so that the dynamic regeneration effect is taken into account. Runge–Kutta method is employed to solve the differential equations governing the dynamics of the milling system for accurate solutions. A Windows-based simulation system for chatter in milling is developed using the predictive model, which predicts chatter vibrations represented by the tool-work displacements and cutting force variations against cutter revolution in both numerical and graphic formats, from input of tool and workpiece material properties, cutter parameters, machine tool characteristics and cutting conditions. The system is verified with experimental results and good agreement is shown.