电火花钛合金加工的数值模拟(英文)

建立电火花钛合金加工的三维有限元轴对称热物理模型。为了更好地预测温度场分布和材料去除效率,采用模型分析了基于电流和脉宽变化的等离子体半径、熔化和气化潜热、能量分配系数和高斯分布的热流密度等影响因素,模拟了单脉冲电火花放电加工钛合金的温度场,并研究了沿放电凹坑的半径方向上温度场分布的放电参数。通过实验数据的对比,验证该模型在材料去除效率方面具有很好的预测效果。     

基于FLUENT的高压断路器弧触头喷溅侵蚀流动特性分析

高压断路器动作时,触头间会形成电弧,并伴随蒸发和喷溅现象,造成触头材料侵蚀和接触系统的失效.为研究高压断路器烧蚀故障的喷溅机理特征,建立了弧触头熔池的流动与传热模型,并运用FLUENT软件分析了触头在不同弧根下的熔池温度场、流场和压力分布.计算结果表明:在两种弧根条件下,均形成了半圆形的熔池,弧根较小的情形下不利于能量的散发和电弧的运动,导致流速较大,增大电极的侵蚀.熔池的流体流速随电流的增大而增大,且电流越大,弧根区内流体流速由最小值上升到最大值的速度越快,熔池深度方向的熔化速度也加快.     

Effects of mechanical milling on the carbothermal reduction of oxide of WC/Co hardmetal scrap

The effects of mechanical milling on the carbothermal reduction of oxidized WC/Co hardmetal scrap with solid carbon were examined. Mixed powders were manufactured by milling the WC/Co hard metal scrap oxide and carbon powder in either a tumbler-ball mill or a planetary-ball mill. The milling type affected the carbothermal reduction of the oxide owing to the differing collision energies (mechanical milling energies) in the mills. The hardmetal scrap oxide powder (WO₃, CoWO₄) milled at high energy was more greatly reduced and at a lower temperature than that milled at lower mechanical energy. The formation of WC by the carburization reaction with solid carbon reached completion at a lower temperature after higher-energy milling than after lower-energy milling. The WC/Co composite particles synthesized by the combined oxidationmechanical milling-carbothermal reduction process were smaller when the initial powder was milled at higher mechanical energy.     

Nanoparticles Modeling in Axially Injection Suspension Plasma Spray of Zirconia and Alumina Ceramics

Suspension plasma spray (SPS) is a thermal spray method in which nanoparticles are injected into the plasma jet with the help of suspension droplets to achieve thin and finely structured nanocoatings. The nanoparticles experience three in-flight stages: injection within the suspension droplets, discharge of the nanoparticle agglomerates after the evaporation of the suspension solvent, and tracking of the nanoparticle or agglomerates during the momentum and heat transfer with the plasma jet before coating. A numerical model is proposed in this paper for nanoparticle injection, discharge, acceleration, heating, melting, and evaporation. Initial values of suspension droplet size and agglomerate size are selected according to typical experimental data. Noncontinuum effects on particle acceleration and heating, known as Knudsen effects, are considered, as well as the influence of evaporation on the heat transfer. After a comparison with the experimental data, this nanoparticle model is applied for zirconia and alumina axially injected into the suspension plasma spray. Trajectory, velocity, and temperature of the in-flight nanoparticles are predicted for different initial sizes ranging from 30 nm to 1.5 μm; the distributions of the particle characteristics for multiple particles in the spray are also presented. The effects of powder size and material, power input, plasma gas flow rate, and standoff distance on the nanoparticle characteristics have been investigated and discussed.     

Observation of keyhole-mode laser melting in laser powder-bed fusion additive manufacturing

Laser powder-bed fusion additive manufacturing of metals employs high-power focused laser beams. Typically, the depth of the molten pool is controlled by conduction of heat in the underlying solid material. But, under certain conditions, the mechanism of melting can change from conduction to so-called “keyhole-mode” laser melting. In this mode, the depth of the molten pool is controlled by evaporation of the metal. Keyhole-mode laser melting results in melt pool depths that can be much deeper than observed in conduction mode. In addition, the collapse of the vapor cavity that is formed by the evaporation of the metal can result in a trail of voids in the wake of the laser beam. In this paper, the experimental observation of keyhole-mode laser melting in a laser powder-bed fusion additive manufacturing setting for 316L stainless steel is presented. The conditions required to transition from conduction controlled melting to keyhole-mode melting are identified.     

Plasma Oxidation of Benzene Using DBD Corona Discharges

This study carried out application of dielectric barrier discharges driven by an AC power supply to benzene conversion in air. Only carbon dioxide and carbon monoxide was found as oxidation products. The effect of discharge space volume on benzene conversion, CO₂ selectivity, and energy efficiency were also investigated, resulting in the facts that the benzene conversion and energy efficiency can be improved obviously using a large EC tape width of a large discharge space volume. The energy efficiency is 1.246 mol-C₆H₆/kWh at an energy density of 31 J/L. The mechanism of benzene oxidation was also discussed.     

单脉冲放电过程中熔池及材料蚀除的高速摄像观测研究

尽管电火花加工技术获得了很大发展,但电火花加工的极间微观现象仍存在很多未知。采用一种新的观测方法,利用高速摄像机实现了对单脉冲放电过程中的熔池及材料蚀除过程的观测。结果表明:放电过程中熔池内的熔融金属处于漩涡运动状态,在放电开始后最初的短暂时间内材料蚀除剧烈,材料骤然气化的同时伴随着熔融材料的抛出;在之后的放电持续过程中发现了熔融液滴抛出现象和熔融金属爆破现象;放电结束后熔池中的绝大部分熔融金属没有被蚀除,而是重新凝固形成熔融再凝固层。     

Untersuchungen des Korrosionsverhaltens ausgewählter Wärmeaustauscherwerkstoffe in Natriumacetat‐Trihydrat‐Schmelze für den Anwendungsfall Latentwärmespeichersysteme

Investigations of the corrosion behaviour of special heat exchanging materials in sodium acetate trihydrate melt for the application latent heat storage The development activities in the area of latent heat storage have more and more increased in the last years. The reason is the search for new energy storage systems of higher energy efficiency and the reinforced utilisation of renewable and alternative energies which are often only irregularly available. The special advantage of latent heat storage is the high energy storage density which could be reached at the melting point of selected storage materials and the resulting reduction of required space respectively the nearly constant temperature. An important topic relating to the development of these storage systems was the investigation of the long time stability, especially the corrosion stability. At the same time the heat exchange material has the function to transfer the heat and coldness from a heat transfer medium, e. g. water which has drawn the energy from an external source to the storage material. Among the storage materials the sodium acetate trihydrate (melting point: 58°C) has interesting advantages. In that reason the investigation of the corrosion behaviour and the choose of suitable materials for heat exchangers was an important goal. In the present report important results of the investigation of selected metals at 80°C in sodium acetate trihydrate are described. The corrosion influencing impurities chloride, sulphate and carbonate which could be contained in the much more inexpensive salt hydrate of technical quality were added to the analytically pure sodium acetate trihydrate. Furthermore the addition of graphite (for increasing the heat conductivity) and the contact with air were investigated. Some promising metals of high corrosion resistance are the result of the investigations.     

移动热源作用下电火花线切割加工电极丝三维温度场模拟

往复走丝和“中走丝”电火花线切割机床是中国独创的电加工设备,在模具制造等行业发挥着重要作用.由于其电极丝是重复使用的,因此电极丝损耗是普遍存在的问题.在电极丝与工件之间相对运动的情况下,建立了移动热源条件下的电极丝传热模型,通过有限元分析对单脉冲放电条件下电极丝表面温度场进行了三维仿真,发现当走丝速度增加时电极丝熔融区...     

Influence of discharge current on machined surfaces by thermo-analysis in finish cut of WEDM

In finish machining of wire electrical discharge machining (WEDM), the discharge current has distinct influences on the machined surface. In order to study the differences in surface morphology under various pulse durations, thermo-analysis was carried out to investigate the mechanism of erosion of the workpiece material using the finite element method. Additionally, related single discharge experiments under different pulse energies were performed. Under the same discharge energy, the comparison of analytical and experimental results shows that a discharge current with a short-duration pulse and a high peak value removes the workpiece material mainly by gasifying, while a discharge current with a long-duration pulse and low peak value removes the workpiece material mainly by melting. It was also found that surfaces machined by a discharge current with a short- and long-duration pulses would have similar roughness values when the pulse energies were almost the same and were high enough; however, the surface morphologies would be totally different. A discharge current with a long pulse duration and a low peak value could not produce craters on the workpiece surface when the pulse energy was reduced to a certain value. However, a discharge current with a short pulse duration and a high peak value could produce clear craters on the workpiece surface. This indicates that a discharge current with a short pulse duration and a high peak value can generate better surface roughness, which cannot be achieved with a current with a long pulse duration and a low peak value.     

高速电弧放电加工放电过程研究

为了促进电弧加工技术的进一步发展,详细了解和掌握其加工现象背后的机理变得十分必要。借助于高速摄像机和光谱仪,对高速冲液条件下的电弧放电过程进行了实验研究。计算结果表明:弧柱温度远远超过普通电火花温度,是一种更高效的去除材料热源。高速摄像结果指出:流体动力断弧机制可有效地干扰电弧、防止出现稳态电弧放电,促进大能量电加工的稳定进行。此外,还对电弧放电产生的蚀坑形貌进行了研究,结果解释了工件正极性加工能得到更大材料去除率、而工件负极性加工能得到更好表面质量的原因。     

AgWC30电接触材料的电弧行为研究

对电弧基本行为的研究一直是电接触材料领域的研究重点,本研究以AgWC30触头为研究对象,借助高速摄像的手段,观测电触头对在不同工作条件下,电弧行为的基本特征和规律,并进行了定量测量研究。结果表明,电弧演化阶段经历起弧、稳定燃烧、迅速熄灭3个过程。起弧阶段存在多点起弧特征,并伴有明显的金属熔滴的喷溅行为,稳定燃烧阶段出现电弧的产生、长大和合并行为,最后电弧迅速熄灭。通过扫描电镜对燃弧后触头表面的形貌分析,判断触头表面存在以WC为骨架覆Ag的大量凸丘,凸丘与熔桥的产生和断裂行为有密切关系,触头分断过程中熔桥和电弧可同时存在,并存在一定的协助作用。     

An introduction to physical phenomena in arc welding processes

As is well known, arc discharges have been applied to various processes such as welding, cutting, spray coating, melting and refining. Unlike electrodeless discharge methods such as high frequency discharge of inductive coupling type (eg. RF discharge), arc discharge is a polarized discharge in which the arc is generated between the positive and negative electrodes.¹ Accordingly, when the arc discharge is applied to welding processes, the material becomes one of the electrodes. In TIG welding, the material, that is, the molten pool, generally acts as the anode to the tungsten cathode. As shown in Fig. 1, TIG welding processes are based on the close energy balance between the 'electrode-arc plasma-molten pool'. On the other hand, for the formation of the molten pool, energy transfer from the arc is also important, but energy transfer in the molten pool after that is extremely important, too. In TIG welding of steels, in which energy transfer by convection current becomes dominant rather than thermal conduction,² the penetration at the weld joint has hugely different geometries according to the difference in convective current phenomena at the molten pool. As a driving force of convective current in the molten pool in TIG welding, four forces have been considered,^2-6 as shown in Fig. 1. They are the drag force (friction force) caused by plasma jet (cathode jet) generated by the arc, the buoyancy force induced by the density difference inside the molten pool, the electromagnetic force induced by the current flow inside the molten pool and Marangoni Force induced by the surface tension gradient of the molten pool. These four forces can be said to be also based on the close force balance of the 'electrode-arc plasma-molten pool', as with the energy balance.     

Wire electrical discharge machining applied to high-speed rotating workpieces

In order to evaluate the influence of high-speed rotating workpieces on wire electrical discharge machining (WEDM), single discharge experiments were carried out inside a grinding machine, in a self-designed wire electrical discharge dressing device (WEDD-device). The shape and size of eroded craters, measured on the workpiece/anode, were found to be highly influenced by the applied relative speed. Based on the crater's shape, its radial expansion speed can be calculated and the slip of the plasma arc column can be measured. Additionally, it was found that the volume of eroded craters increases as relative speed is increased, indicating that higher melting efficiencies are achieved for higher relative speeds. Finally, an electro-thermal model is described and simulation results are discussed, which help to better understand the influence of relative speed on erosion.     

Machining performance of silicon carbide ceramic in end electric discharge milling

A new process of machining silicon carbide (SiC) ceramic using end electrical discharge milling is proposed in this paper. The process is able to effectively machine a large surface area on SiC ceramic with good surface quality and low cost. The effects of machining conditions on the material removal rate, electrode wear ratio, and surface roughness have been investigated. The surface microstructures machined by the new process are examined with a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS), and an X-ray diffraction (XRD). The results show that the SiC ceramic is removed by melting, evaporation and thermal spalling, the material from the tool electrode can transfer to the workpiece, and a combination reaction takes place during end electric discharge milling of the SiC ceramic.     

Experimental investigation of spark generation in electrochemical discharge machining of non-conducting materials

Electrochemical discharge machining (ECDM), also known as spark assisted chemical engraving (SACE), is an effective micro-machining process for non-conducting materials. Process modeling of ECDM, including spark generation and material removal, is not fully established however. Empirical estimation for discharge energy results in large prediction error of material removal and is hard to experimentally validate. In this paper, an experiment-based stochastic model for spark energy estimation is presented. Tapered tool electrodes were fabricated by electrochemical machining (ECM) to improve the consistency of spark generation. Energy of sparks was experimentally determined and fit into a two-component mixture log-normal distribution to reveal electrochemical characteristics of tool electrodes. A finite element based model was established to correlate spark energy and the geometry of removed material. Material removal was treated as heat transfer problem because electrical energy released by spark generation transfers into thermal energy on the workpiece, resulting in material removal due to thermal melting and chemical etching. Predictions of material removal by the model demonstrated good consistency with experimental results.     

Improving machining performance of wire electrochemical discharge machining by adding SiC abrasive to electrolyte

The use of wire electrochemical discharge machining (WECDM) to slice hard brittle materials has recently been studied because its effectiveness is independent of the mechanical characteristics of the machined materials. Therefore, materials with high hardness, brittleness, strength and electrical insulation, which are difficult-to-cut, can be machined. In ECDM, the electrochemical reaction produces hydrogen bubbles, which accumulate around the cathode. A thin gas layer forms on the surface of the electrode and isolates the electrode from the electrolyte. When a voltage that exceeds the critical voltage is applied, continuous discharge occurs. The material near the electrode is removed by the discharge erosion and chemical etching. The use of WECDM to cut electrically insulating materials has only recently been investigated. However, the breakdown of the gas in the bubbles and the vibration of the wire in WECDM strongly affect the shape accuracy. This work aims to improve the over cut quality by adding SiC abrasive to the electrolyte. A mechanism that combines discharge, chemical etching and abrasive cutting is studied. The effects on expansion, roughness and material removal rate (MRR) are discussed. The experimental results reveal that adding abrasive reduces the slit expansion because it increases the critical voltage. The particles disrupt the bubble accumulation to form an isolating layer around the wire, increasing the critical voltage and reducing the discharge energy. The surface roughness is improved because the abrasive helps to refine the micro-cracks and melted zone that is formed by discharge heat erosion. Meanwhile, smaller grit produces lower roughness. The quality of the slit can be controlled; its expansion and roughness of the slit are 0.024 mm and 0.84 um Ra, respectively.     

Investigation on cooling efficiency of grinding fluids in deep grinding

The cooling efficiency of grinding fluids in deep grinding, at different material removal rates and grinding speeds, has been investigated. Two ‘inverse’ methods have been proposed to determine the level of convective heat transfer coefficients of grinding fluids, by matching the theoretical and experimental grinding fluid burn-out thresholds or matching the theoretical and measured grinding temperatures. Instead of using a constant chip melting temperature to estimate the energy partition to the grinding chips, the chip temperature and chip energy were calculated using the newly developed approach considering the variation of chip size, deformation and heat transfer at the abrasive/work interface. The variation of grinding heat taken away by the process fluids and grinding chips under different process parameters has been calculated, which shows the importance of cooling effects by the grinding fluids and the transition of thermal characteristics of deep grinding from cooling dominant to ‘dry’ grinding regime, where a large percentage of grinding heat is taken away by the grinding chips.     

相变潜热对铍材激光钎焊熔池内流场和温度场动态行为的影响

为了能够反映固液相变过程中潜热等对熔池动态行为的影响,基于旋转高斯体热源与高斯面热源相结合的混合热源模型,分析了相变潜热对焊接熔池温度场和熔池形状的影响.计算结果表明,在铍材激光钎焊过程的温度场和流场分析中,潜热的影响不是总可以忽略的,其误差取决于材料在物相转变时交界面温差的范围及材料熔解体积的大小.与不考虑相变潜热的情况相比较,考虑相变潜热时显示出较低的最高温度、较小的熔池范围和较窄的热影响区.通过对铍材焊接温度场、焊缝断面形状模拟结果与实际热循环曲线以及焊接接头形状的对比,表明,考虑潜热的计算模型会得出更精确的结果.     

钛合金TC4不同介质放电加工蚀除机理研究

为研究钛合金TCA在蒸馏水和火花油中放电加工时材料蚀除机理的差异,在相同条件下进行钛合金TC4放电加工试验,分别对放电波形、极性效应、表面微观形貌及加工表面主要成分等进行了对比分析。结果表明：蒸馏水的冷却性和流动性较好,可很好地进行极间冷却并进行消电离,放电能量较大时加工依然很稳定,大大提高了加工效率;相对而言,火花油的冷却性和流动性较差,且易析碳,频繁发生拉弧、二次放电和短路等现象,脉冲利用率低,加工效率较低。蒸馏水中工件表面产生的氧化钛熔点较火花油中形成的碳化钛低,更易于蚀除,因此蒸馏水中可获得更高的材料去除率。