雾化参数对H70黄铜粉粒度分布的影响

从气体动力学和流体力学分析了雾化气体压力、金属熔体温度和导液管内径对H70黄铜雾化粉末粒度分布的影响。结果表明：适当地提高气体压力和金属熔体温度或者减小导液管内径，均能使雾化粉末细粉量增加。当雾化气体压力为1．3MPa，金属熔体温度为1160℃，导液管内径为3．5mm时，所制得粉末的粒度分布最理想。     

液态金属超声雾化喷嘴的气雾化性能影响因素

采用Fluent软件建立超声雾化喷嘴气流场的数值模拟模型,研究入口压力、导液管孔径与伸出长度对超声雾化喷嘴气雾化性能的影响,并采用超声雾化法制备GCr15轴承钢粉末,验证入口压力对粉末粒度的影响。结果表明:导液管伸出长度Δh=1 mm时,入口压力越大,越有利于提高雾化效果;在Δh=2 mm或Δh=3 mm条件下,当入口压力小于2.0 MPa时,增大入口压力可有效提高雾化性能,而当入口压力超过2.0 MPa后,增大入口压力对提高雾化性能效果有限。导液管的孔径对气流场结构影响较小,主要通过影响金属液的质量流率来影响雾化效果。雾化实验结果表明,在Δh=1 mm、导液管孔径d=4.5 mm时,2.8 MPa入口压力下的雾化性能优于2.0 MPa与1.2 MPa下的雾化性能,获得的GCr15轴承钢粉末平均粒径最小,整体球形度最佳,该实验结果与数值模拟研究结果一致。     

导液管负压对Ni基合金雾化效率及粉末性能的影响

研究了导液管负压(即导液管末端压力PT)对真空气雾化过程中的雾化效率(即雾化过程中每分钟雾化的金属液质量RA)以及粉末物理性能的影响。结果表明,在其他工艺参数不变的情况下,降低导液管负压,可以降低雾化效率并有效减少"卫星球"的黏附,提高粉末的松装密度和流动性;但导液管负压过低会使得雾化效率太小,粉末的松装密度和流动性反而变差。因此,在真空气雾化过程中,导液管负压存在一个最佳值。     

雾化制粉用喷嘴的改进

针对雾化过程中易被中断的情况，通过对雾化过程中喷嘴中轴气流场压力分布的测量和分析，对喷嘴系统进行了改进，即将导液管装入喷嘴并延伸人喷嘴中轴气流场负压区。结果表明，雾化过程稳定，雾化制粉效率提高。     

微细CuSnAg合金粉末的制备及粉末粒度控制

CuSnAg合金粉末是一种新型的金刚石工具用钎焊粉末.粒度微细、氧含量低是获得良好钎焊性能的保证.本文以紧耦合气体雾化技术对这一粉末进行研制,分别研究了导液管伸出量、雾化气压对粉末的平均粒度与微细粉末收得率的影响.发现导液管的伸出量对雾化效果影响显著,采用较短的伸出量可以提高微细粉末的收得率;而雾化气压的提高可以进一步细化粉末;同时分析表明采用紧耦合雾化的粉末氧含量为0.02%,粉末的形貌呈球形.本文还从理论上分析了导液管的伸出量和雾化压力对雾化的影响机理.     

雾化器导液管内径对无铅焊锡粉末形貌及粒度分布的影响

本试验利用自行设计的超音速雾化制粉装置,研究了导液管内径对SnAgCu系无铅焊锡粉末有效雾化率、粒度分布、球形度及氧含量的影响.研究结果表明:在过热度为250℃,导液管外径为7mm,喷头突出为4 mm,雾化压力为0.7MPa,雾化介质为氮气条件下,导液管内径为3mm,雾化粉末具有最佳的球形度、表面光滑度及粒度分布,同时具有较高的有效雾化率和较低的氧含量;随着导液管内径减小,粉末明显细化,有效雾化率升高,但粒度分布的离散度变大;随着导液管内径增大,粉末有效雾化率急剧降低,粒度分布变差,且表面更粗糙,形状更不规则.     

金属雾化过程中气体流场动力学行为

利用流体动力学软件计算了喷射成形雾化室内气体的流场状态，分析了不同导液管出口长度对气体流动的影响，结果表明：雾化室内气体流速随轴向和轻向的位置变化而发生变化，距导液管出口处距离越远，气体速度越低，对于本文选用雾化器，当导液管出头长度小于1mm时，导液管内气体逆流，会造成金属反喷，导液管出口长度大于1mm时，导液管内气体向下流，产生对液态金属的抽吸作用，有利于喷射。     

表面组装用Sn-Ag-Cu无铅焊锡粉末的制备

利用自行设计的超音速雾化制粉装置,研究了导液管突出高度对Sn3Ag2.8Cu无铅焊锡粉末有效雾化率和粒度分布的影响,并在此基础上研究了合金过热度对其有效雾化率及粉末特性的影响。结果表明:随着导液管突出高度的减小,有效雾化率并不是呈单调变化趋势,而是在突出高度1mm和5mm处分别出现峰值,导液管突出高度为4mm,雾化粉末具有较高的有效雾化率和最佳的粒度分布,而且导液管的突出高度决定着雾化过程是否能够顺利进行;当导液管突出高度为4mm时,随着合金过热度的升高,粉末粒度分布明显改善,但氧含量升高,当过热度为150℃时,Sn3Ag2.8Cu雾化粉末具有最佳的综合性能。     

多流气雾化工艺参数的研究

采用正交实验方法系统地研究了上层气流压力、下层气流压力、金属熔液温度、导液管突出高度对多流气雾化法工艺所制备粉末的平均粒径及粒度分布的影响，通过直观分析和方差分析，评价了各参数对粉末粒度的显著程度，优化了多流气雾化粉末制备法的丁艺参数。以焊锡63A为原材料的实验结果表明：最佳的工艺参数为上层喷嘴压力0．7MPa，下层喷嘴压力0．4MPa．金属熔液温度350℃．导液管突出高度为4mm；所制备粉末的平均粒径为6．3μm，粒度小于10μm的比例达到77．3％．且粒径分加曲线中的第一波峰值达到500nm左右，比现有报道的粒径分布曲线第一波峰值减小了一个数量级；     

超音速雾化器的气体流场分布研究

利用Pitot管对超音速雾化器出口处的气体流场分布进行了研究。研究结果表明，在出口处的雾化气体呈轴线近似对称的双峰分布，峰值沿着远离导液管出口端面的方向而逐渐减小。通过对气体流场分布的分析，提出倒涡流锥概念，可以很好的解释导液管下端形成负压的原因，并认为理想的雾化过程是在倒涡流锥范围内完成的。     

超音速气雾化喷嘴中抽吸压力变化规律的研究

对超音速气雾化喷嘴中抽吸压力ΔP的变化进行了试验研究,重点考察了雾化气体压力P0、气流夹角α、导液管锥顶角β及突出长度h对抽吸压力的影响。试验结果表明,在(0.5～4.0MPa)雾化压力范围内,抽吸压力皆呈负压状态。但抽吸压力变化分三阶段:随着雾化压力的增加,雾化压力在0.5～1.5MPa范围内,抽吸压力减小;雾化压力在1.5～3.5MPa之间时,抽吸压力逐渐增大,并达到一个最大值;当雾化压力超过3.5MPa后,抽吸压力又逐渐降低。锥顶角越大,抽吸压力越大;而雾化压力升高到或大于3.23MPa时,随着突出长度的增加,抽吸压力逐渐减小。     

Studies on metal build upon atomisers during free fall gas atomisation of liquid metals

Abstract

In free fall gas atomisation of liquid metals, the atomised droplets mainly move downward, while some of them fly in an upward direction. Under certain conditions the upward moving droplets will deposit on the surface of the gas nozzle and metal delivery tube of an atomiser, which may cause a hindrance to the flow of liquid metal and gas. The effect of atomisation parameters, such as gas pressure, focal length, apex angle and diameter of gas nozzle, on the metal buildup on the atomiser have been studied during the free fall gas atomisation of lead, zinc and aluminium. The plenum pressure of gas at which deposition of atomised droplets on the surface of the gas nozzle and metal delivery tube takes place has been termed as limiting plenum pressure, and the corresponding gas velocity at the impingement point as limiting gas velocity. It has been shown that the limiting plenum pressure is different for different metals in the same atomiser. The limiting plenum pressure has been found to increase with free fall distance or specific gravity of liquid metals, and with a decrease in apex angle of atomiser. A correlation is proposed to determine the limiting gas velocity for free fall gas atomisation of metals.     

Deep decarburization of iron-based melts

The decarburization of liquid iron with preliminary saturation and without saturation of the metal with hydrogen followed by vacuum and aspiration treatments has been studied. A decrease in the gas pressure above the liquid metal is experimentally shown to increase the deoxidizing capacity of carbon only to certain limits. When a steel is subjected to hydrogen treatment with preliminary saturation of the melt with hydrogen, the formation of hydrogen bubbles enhance the effect of the mixing of the metal bath and substantially increase the melt-gas interface area. As a result, the rate and degree of decarburization increase considerably. The aspiration treatment also makes it possible to decrease the carbon content in the metal due to the creation of an additional reaction surface in the volume of the melt under vacuum conditions.     

Mathematical modeling of fluid flow and level profile during twin-roll strip casting process

In twin-roll strip casting process,transport phenomena of fluid in the molten pool directly affect the process stability and the quality of products.In order to elucidate the fundamental transport phenomena in twin-roll casting,a commercial software called ProCAST was employed to simulate the transient fluid flow and level profile behaviors during the early stage of the process in this study.The coupled set of governing differential equations for mass,momentum and energy balance were solved with the finite element method and the transient free surface problem was treated with a volume of fluids(VOF) approach.The effect of different delivery systems configuration on flow pattern,level profile in the pool was studied and analyzed in this paper. The new wedge metal delivery systems have been optimized for the twin-roll strip caster.It was shown that new type metal delivery systems had a preferable effect on the uniform distribution of fluid and level fluctuation in the pool.The simulation results also provide a valuable basis for the optimization of delivery system and process parameters during the initial pouring stage.     

Flow separation and liquid rundown in a gas-atomization process

“Rundown≓ filming mode was studied in a confined design gas-atomization nozzle of converging/ diverging construction. In this mode, liquid metal runs down to a certain distance up to ≈3 mm on the outer surface of the metal delivery tube in the form of a thin film. Atomization then takes place through the disintegration of this film by the oncoming gas. The rundown effect was simulated by water, and the supersonic gas flow outside the nozzle was visualized by Schlieren photography. It was found that rundown is caused by the liquid being drawn into a separated flow region on the wall of the delivery tube. This separation comes about as a result of the action on the boundary layer of adverse pressure gradients associated with the shock waves in the gas flow. Geometric conditions required for observing rundown are discussed, and a method is outlined for assessing whether a given nozzle design is likely to produce the effect. Evidence is also presented for the filming of the liquid on the tip of the nozzle by radial spreading.     

Aspects of porosity formation in spray deposited thin aluminium strip

Abstract

A characteristic feature of the spray deposited metal strip is that it contains porosity. The present paper has reported and explained the effect of various process parameters such as atomising pressure, deposition distance, melt superheat and melt delivery tube diameter on porosity of the spray deposited thin aluminium strip. Porosity of the spray deposited aluminium strip greatly depends not only on the liquid fraction present on the substrate, but also on the thermal condition through the substrate. It has been shown that there are several types of porosity present in the strip, namely interstitial, mechanically entrapped gas, intersplat boundary, solidification shrinkage and porosity due to hydrogen evolution. The mechanism of their formation has been discussed. An increase in the initial temperature of the substrate lowers the porosity content in the spray deposited aluminium strip.     

Neuropathology of aging

Jejunal feeding often is preferable to gastric feeding, particularly in the following situations: high risk for aspiration; gastric resection (partial or total); gastric pull-up; gastric outlet obstruction; obstructed or nonfunctioning gastrojejunostomy; and gastric dysmotility. The technique of placing a thin tube through a gastrostomy tube and pulling it endoscopically into the proximal jejunum allows delivery of nutrients into the jejunum. The results of this technique, however, have been poor. The direct percutaneous endoscopic jejunostomy technique allows placement of tubes directly in the jejunum with a success rate of around 85% and a minimal complication rate which is comparable to that of PEGs.     

锌冶炼除铁工艺优化改进实验研究

传统铁矾工艺含锌高、含铁低，其中的金属资源无法利用，采用热酸浸出、还原、加压除铁工艺，锌、铁的总回收率都在95%以上，铁渣可作为铁精矿出售，既综合利用了铁资源又高效分离了锌铁，实现了无铁矾渣湿法炼锌。     

Wetting of ceramic particulates with liquid aluminum alloys: Part II. Study of wettability

Wetting phenomena in ceramic particulate/liquid Al-alloy systems were investigated experimentally using a new pressure infiltration technique developed by the authors. Studies were performed on two different ceramic particulates, SiC and B₄C, with four different liquid aluminum alloy matrices, pure Al, Al-Cu, Al-Si, and Al-Mg. Five major variables tested to study wetting phenomena in ceramic/Al-alloy systems were holding time, melt temperature, alloying element, gas atmosphere, and particulate. Metal: ceramic interfaces were investigated with optical microscopy, SEM, EPMA, and Auger Electron Spectroscopy (AES) in order to understand better the wetting process. The threshold infiltration pressure decreased with, temperature as well as with pressurization time for all the ceramic/metal systems. A strong correlation was found between the alloying effect on the threshold pressure and the free energy of formation of oxide phase of the alloying element. More reactive alloying elements were more effective in improving wettability. In air atmospheres, the threshold pressure usually increased markedly as a result of a thick oxide layer formation on the liquid front. Compacts of B₄C particulates showed lower threshold pressures than those of SiC, particulates. Fracture occurred in a generally brittle manner in infiltrated SiC, specimens. AES element profiles on the fracture surfaces showed fast diffusion of Si, and pile-up of C at the metal∶SiC boundaries which promoted fracture through the carbon-rich layer. The fracture surfaces of infiltrated B₄C specimens indicated plastic deformation, hence a more ductile failure mode.