Concentration mode of the powder stream in coaxial laser cladding

The blown powder laser cladding process has recently been greatly enhanced by the development of a coaxial powder feed system. It provides a new route to generate the metal parts directly from CAD drawings. The performance of the coaxial powder feeder depends on various gas flow streams which significantly affect the distribution mode of the powder stream and the deposition rate in cladding.Two types of optical techniques have been adopted in this study to investigate the powder concentration mode of the coaxial jet streams. The mode of the powder stream is also mathematically modelled and compared to the experimental results of stainless steel powder. The Gaussian distribution mode in the transverse direction of the powder stream was identified by theory and experiment at cold stream conditions.     

拉曼高光谱成像系统中光在奶粉层的穿透深度研究

拉曼高光谱成像技术不仅可以获取样本的空间分布信息,图像上每个像素点还包括了完整的光谱信息,因其信息量丰富的特点已在食品安全检测方面得到了应用。本研究探索拉曼高光谱成像系统中光在奶粉层中的穿透深度,以及采集参数和奶粉类型对穿透深度的影响。实验选取均匀奶粉层样品放置于厚度为5 mm的三聚氰胺样本之上,检测奶粉层厚度为0.8～4.0 mm时的三聚氰胺特征峰强度,以此评估光在奶粉层中的穿透性和信号衰减情况。结果显示当奶粉层厚度一定时,随着激光功率变大,拉曼特征峰值随之增加,此外更长的曝光时间也可以使拉曼信号得到增强。在激光功率不小于2 W且曝光时间不小于500 ms时,光在全脂奶粉层的穿透深度可达4 mm。奶粉层厚度在0.8～4.0 mm范围内,穿透奶粉层的拉曼信号随着奶粉层厚度增加呈指数式衰减。在激光功率为8 W、曝光时间为1 000 ms的条件下,光在全脂、低脂和脱脂奶粉层的穿透深度均达到了4 mm。在相同测量厚度下,通过脱脂奶粉层接收的拉曼信号弱于通过全脂和低脂奶粉层接收的拉曼信号强度。研究结果为拉曼高光谱检测中奶粉样品的前处理提供了有益参考。     

铝粉点火微观机理的光谱研究

 铝粉是一种含能高的材料,它被广泛地添加到含能材料中。利用3台单色谱仪和OMA谱仪等多种谱仪技术,研究了铝粉在几种不同环境下的快速反应微观特性。研究表明：铝粉冲击波点火的临界条件和铝粉的物理状态相关。微米铝粉点火的临界温度为2 100 K,它接近Al₂O₃的熔化温度。它表明,在空气中容易氧化的微米铝粉点火,必须使铝粉表面的氧化层熔化。在气相反应中,微米铝粉和氧的反应是主要的;和水的反应是次要的。减小微米铝粉的颗粒尺寸,可以明显提高快速反应温度。 含铝复合燃料中的液体燃料反应后的铝粉才能参与反应;铝粉添加至气相反应介质后将明显提高反应温度。     

弱酸改性钢渣微粉的光谱学分析

以钢渣微粉作为研究对象,采用磷酸溶液改性钢渣微粉制备弱酸改性钢渣微粉,利用X射线衍射仪、比表面积及孔径测定仪和环境扫描电镜对弱酸改性钢渣微粉进行表征,研究弱酸改性钢渣微粉的组成成分、孔结构、微观结构和元素组成。结果表明,适量磷酸溶液可以有效去除钢渣微粉中f-CaO,提高弱酸改性钢渣微粉的孔结构。过量磷酸溶液与钢渣微粉中Ca(OH)₂发生反应,导致弱酸改性钢渣微粉结构坍塌,整体呈现蓬松状。80 g钢渣微粉,1.6~3.2 mL磷酸溶液时,弱酸改性钢渣微粉具有较低的f-CaO含量与良好的孔结构。为进一步拓展钢渣的利用途径提供一定的技术支持和理论基础。     

送粉角度对激光熔覆铁基复合涂层形状特征的影响

通过分析激光熔覆过程中光束、粉末和熔池间的作用机理,建立了送粉式激光熔覆材料有效利用率的数学模型,在此基础上推导了送粉角度与工艺参数之间的定量关系式,并计算了不同送粉角度下的熔覆材料有效利用率、熔高和横截面积。结果表明,在熔覆工艺参数不变的条件下,理论计算的熔覆材料有效利用率、熔覆层高度和横截面积均随送粉角度的增加而增大,且均高于实验检测值。激光熔覆过程中,由于粉末烧损和机械损失,使熔覆材料有效利用率、熔高和横截面积随送粉角度变化出现最大值,理想送粉角度为60。     

Clad profiles in edge welding using a coaxial powder filler nozzle

The powder catchment and clad profile of the edge welding were investigated by experimental and numerical approaches in this study. The clad profile on the edges joined by a coaxial powder filler nozzle with a CO₂ laser was measured and compared with the powder concentration mode, which was confirmed by powder flow visualization and numerical computations.In the numerical simulation of an impinging jet of gas-powder flow on an edge joint, the powder concentration distributed on a V groove joined by two plates was solved by FLUENT software. Based on the Gaussian mode of the powder distribution in the jet flow, a simplified mode function was proposed to estimate the clad profiles in the edge joint.Cladding experiments were performed for mild steel substrates with thicknesses of 2 and 6 mm under 1 kWCO₂ laser irradiation for 304L stainless steel powder. The results show that the concave clad profiles were generated at large incline angles and the powder catchment efficiency might increase with the joint angle and substrate thickness.Based on the analytical results of the cold powder streams impinging on the edge joint, the similarity between the clad profile and the powder concentration in the edge joint is retained only at small incline angles for thin substrates. Due to the heating effects of laser beam spot and the powder re-distribution inside the edge joint, the deviation of the clad profile between the computation and experiment is increased with the incline angle and substrate thickness.     

铅对粉末药型罩性能的影响

 为了研究铅粉对钨铜粉末药型罩性能的影响,在钨铜药型罩原配方的基础上,添加5%～20%的铅粉进行实验研究。实验结果表明：添加铅粉可以减少粉末药型罩的孔隙度;随着铅粉添加量的增加,射流的侵彻深度和入口孔径均增加,当铅含量为10%～15%时,侵彻深度和入口孔径达到最佳值,侵彻深度可以提高30%,再添加铅粉,侵彻深度和入口孔径均随之下降。从粉末药型罩成型以及射流的形成、拉伸及侵彻靶板等方面,对造成这种现象的原因进行了详细分析,获得了铅对粉末药型罩性能影响的一些机理认识。研究结果为铅粉代替铋粉添加到钨铜粉末药型罩以提高其侵彻性能提供了一定的理论依据。     

Book Reprint Characterization of Powders and Aerosols

Characterizing the shape of powder grains is a very active area of research in powder technology. The growing use of complex powder grains, made by super-critical fluids, to increase the solubility of new low-solubility drugs, and the increasing use of powder delivery systems to the lung, are areas demanding increased knowledge of the shape of powder grains. The chapter presented here, from the forthcoming book “Characterization of Powders and Aerosols”, represents the state-of-the-art review for technologists needing to characterize the shape and structure of powder grains which, it is hoped, will be useful to the powder technology community.     

基于Kahan线性化快速计算梯度磁场下磁流变抛光区域抛光粉沉降特性

针对抛光粉沉降特性数值计算这一超大规模非线性问题,基于Kahan线性化解决了超大规模流固耦合计算问题。研究了以羟基铁粉和硅油为主要成分组合而成的抛光粉多相流在梯度磁场下抛光区域的沉降特性。以质量分数70%、粒径5 μm的羟基铁粉和粘度为0.973 Pa·s的硅油组合而成的抛光粉为研究对象,实现了不同的抛光轮转速、不同嵌入深度以及不同羰基铁粉质量分数情况下的沉降规律分析。结果发现:磁流变抛光区域的抛光粉会随着抛光轮转速的增大而增多;当到达出口时,抛光粉的分布趋于稳定状态;抛光粉会随着嵌入深度的增加而增多并存在饱和区;羟基铁粉的质量分数以非线性的方式影响沉降能力。     

粉末颗粒气力加注特性实验研究

粉末发动机是以粉末颗粒为燃料的新型发动机,具有多次起动和推力调节的功能.粉末加注是粉末发动机实验组织过程中的重要环节.本研究通过搭建粉末供应系统开展粉末气力加注实验,研究对比了集粉箱加注位置、流化气量对粉末气力加注特性的影响.考虑了供粉过程中储箱内粉末堆积密度的动态变化,并建立了相应的计算方法,同时还采用控制系统理论揭示了储箱内粉末堆积密度的变化规律.结果表明:在相同条件下,较大的流化气量有利于加注过程稳定,但集粉箱加注率较低;气力加注方式下集粉箱内的粉末堆积密度大于储箱内初始堆积密度;采用较小的流化气量与集粉箱壁面切向加注方式有利于提高粉末粒径分布均匀性;集粉箱壁面切向加注方式下,流化气量较小时储箱内粉末的堆积密度是先增大后减小,且堆积密度最终值小于初始值,而流化气量较大时,储箱内粉末的堆积密度是先增大后减小再增大后减小,且堆积密度最终值大于初始值;储箱内粉末堆积密度的动态变化过程类似于欠阻尼二阶系统,流化气量较小时系统阻尼系数较小,而流化气量较大时系统阻尼系数较大,且是一个变阻尼过程.     

Studying the Avalanching Behaviour of a Powder in a Rotating Disc

The use of a rotating disc to study the avalanching behaviour of a powder is discussed. It is shown that the strange attractor plotted in discreet time maps summarizes useful information on the rheological behaviour of powders and powder mixtures. In particular it is shown that the avalanching behaviour is related to the particle size distribution of the powder and that one can study the changes in rheological behaviour as another powder is mixed with it. The strange attractor patterns generated are dependent upon the environmental conditions under which the experiments are carried out. For this reason the measurements are referred to as an assessment of the holistic powder rheology. The potential use of the disc to study the holistic rheology of powder systems is outlined.     

Influences of different powders on the characteristics of particle charging and deposition in powder coating processes

This experimental study investigated the influences of two different powder systems (coarse and ultrafine) on particle charging and deposition characteristics during electrostatic powder coating processes. Results disclosed that, despite their differences in particle sizes, the two powders behave similarly in deposition process, commonly resulting in a cone-shaped deposited pattern in the inner portion of the substrate and an increase of deposited particles in the fringe region. However, their different properties lead to the discrepancies in their deposition efficiencies, which account for a higher efficiency with the coarse powder. The study further revealed that the coarse powder is superior to the ultrafine powder in charging in-flight particles, which directly contributes to its higher deposition efficiencies. Furthermore, it was disclosed that the two powders exhibit distinct characteristics in charging deposited particles. Compared to the coarse powder, the ultrafine powder is more uniform in charging deposited particles, mainly owing to its greater particle number and higher specific surface area but less mass. In particular, the charging efficiency of overall deposited particles decreases for the ultrafine powder but increases for the coarse powder with increased charging voltage, closely related to their particle properties. However, both powders decrease in charging efficiency of deposited particles with extended spraying duration due to back corona intensifying with spraying.     

3-D finite element modeling of laser cladding by powder injection: effects of laser pulse shaping on the process

This paper introduces a 3-D transient finite element model of laser cladding by powder injection to investigate the effects of laser pulse shaping on the process. The proposed model can predict the clad geometry as a function of time and process parameters including laser pulse shaping, travel velocity, laser pulse energy, powder jet geometry, and material properties. In the proposed strategy, the interaction between powder and melt pool is assumed to be decoupled and as a result, the melt pool boundary is first obtained in the absence of powder spray. Once the melt pool boundary is obtained, it is assumed that a layer of coating material is deposited on the intersection of the melt pool and powder stream in the absence of the laser beam in which its thickness is calculated based on the powder feedrate and elapsed time. The new melt pool boundary is then calculated by thermal analysis of the deposited powder layer, substrate and laser heat flux. The process is simulated for different laser pulse frequencies and energies. The results are presented and compared with experimental data. The quality of clad bead for different parameter sets is experimentally evaluated and shown as a function of effective powder deposition density and effective energy density. The comparisons show excellent agreement between the modeling and experimental results for cases in which a high quality clad bead is expected.     

近红外光谱技术的乳粉品牌溯源研究

采用近红外光谱漫反射模式,结合简易分类技术(soft independent modeling of class analogy, SIMCA)对不同品牌乳粉进行了分类溯源研究。实验共采集了四种不同品牌乳粉,包括光明乳粉54组,荷兰乳粉43组,雀巢乳粉33组以及伊利乳粉8组共138组样品的近红外光谱,通过对预处理后的训练集全谱段数据变量进行主成分分析,得出前三个主成分的累积方差贡献率为99.07%。利用SIMCA类建模法建立的乳粉主成分回归模型对预测集乳粉进行分类,研究结果表明,光明乳粉、荷兰乳粉、雀巢乳粉的识别率分别为78%, 75%, 100%,拒绝率分别为100%, 87%, 88%。因此,近红外光谱结合SIMCA建立的模型具备较好的乳粉品牌溯源能力,为快速、准确鉴别乳粉品牌提供了新思路。     

蒙药牛胆粉与羊胆粉中胆酸类成分的分析测定

胆酸与硫酸加热产生脱水反应,形成共轭双键,在紫外区产生特征吸收峰。用紫外-分光光度法测定了牛胆粉与羊胆粉中胆酸的含量。测定线性范围为0·0033～0·0167mg·mL-1(r=0·9997),加样回收率及RSD分别为98·48%,1·79%,96·46%,2·50%。5批次样品测定表明:牛胆粉中胆酸的含量在40·85%～43·03%之间,羊胆粉在30·88%～32·64%之间。牛胆粉精密度RSD为2·41%,羊胆粉精密度RSD为2·92%。稳定性试验在8h之内测定牛胆粉RSD为0·55%,羊胆粉RSD为0·59%。重复性试验牛胆粉RSD为2·11%,羊胆粉RSD为2·68%。方法简便、快速、便于推广应用,且可有效地控制牛胆粉与羊胆粉的内在质量。     

Relationships Between Particle Properties and Bulk Behaviour for some industrial powders

A series of industrial powders, most of them organic, were characterized in terms of volumetric mean size, size distribution, density, particle shape, water rentention characteristics (under a series of controlled ambient relative humidities) and loosely packed and densely packed voidages (giving the compressibility). The behaviour of the powders in powder handling apparatus (a precision powder filler) was also determined. The minimum and maximum bulk densities of the powders were related to the properties of the individual particles by means of a semi-empirical equation. The filling capacity of the powder handling apparatus was found to be related to the maximum bulk density of the powder while the filling accuracy turned out to be related to the compressibility of the powder. Knowing these interrelations made it possible to predict the powder behaviour in the handling apparatus directly on basis of the particle properties.     

Nonideal regimes of deflagration and detonation of black powder

The explosive and deflagration properties of black powder differ significantly from those of modern propellants and compositions based on ammonium nitrate or ammonium perchlorate. Possessing a high combustibility, black powder is capable of maintaining stable combustion at high velocities in various shells, be it steel shells or thin-walled plastic tubes, without experiencing deflagration-to-detonation transition. It is extremely difficult to detonate black powder, even using a powerful booster detonator. The results of numerical simulations of a number of key experiments on the convective combustion and shock initiation of black powder described in the literature are presented. The calculations were performed within the framework of a model developed previously for describing the convective combustion of granulated pyroxylin powders, with small modifications being introduced to allow for the specific properties of black powder. The thermophysical properties of the products of combustion and detonation and the parameters of the equation of state of black powder were determined from thermodynamic calculations. The calculation results were found to be in close agreement with the experimental data. The simulation results were used to analyze the regularities of the wave processes in the system and their relation to the properties of black powder and the experimental conditions. It was demonstrated that the effects observed could be explained by a weak dependence of the burning rate of black powder on the pressure.     

DPLS和SVM的掺假花椒粉近红外光谱定性鉴别

花椒是我国的八大调味料之一。目前花椒市场掺假现象较为多见,为实现掺假花椒粉的快速定性鉴别,采用了近红外光谱结合化学计量学方法进行了探讨。将麦麸粉、稻糠粉、玉米粉和松香粉以1 Wt/Wt.%的递增梯度分别掺入红花椒粉和青花椒粉中,制备掺假浓度范围为1~54 Wt/Wt.%的掺假花椒粉样品,以掺假花椒粉和纯花椒粉共462份样品依次采集其800~2 500 nm范围的漫反射近红外光谱。采用主成分分析法(PCA)对光谱数据进行分析,前3个主成分累计贡献率达98.72%,做出的得分图表明PCA法对掺假的花椒粉具有较好的区域划分。347份样本作为校正集,以特征谱区2 000~2 200 nm范围的257个采样点的光谱信号作为输入,采用判别偏最小二乘法(DPLS)和支持向量机(SVM)建立定性鉴别模型,经不同光谱预处理,对115份验证集样本进行预测,总体鉴别正确率在97.39%~100%之间,表明该方法是快速定性鉴别掺假花椒粉的一个有效手段。     

生长温度对纳米氧化锌场发射性能的影响

采用锌粉和银粉为蒸发源,用热蒸发法不同温度下制备了四针状纳米氧化锌,并以单独的锌粉做对照。采用扫描电子显微镜(SEM)观察其形貌,X射线衍射(XRD)谱表征其晶体结构。采用丝网印刷方法将其制备为场致电子发射阴极,将阴极与印刷有荧光粉的阳极板组装成二极结构场致发射显示屏,并进行了场致电子发射特性对比实验。结果表明较高温度制备的氧化锌具有较好的场致发射性能;掺杂银粉的蒸发源制备的样品的发射性能明显优于没有掺杂银粉的样品。实验证明ZnO是一种优良的场致发射冷阴极材料,在真空电子方面具有广阔的应用前景。