Gas Atomization of Amorphous Aluminum Powder: Part II. Experimental Investigation

The optimal processing parameters that are required to atomize amorphous Al were established on the basis of numerical simulations in part I of this study. In this part II, the characterization of cooling rate experienced by gas-atomized, Al-based amorphous powders was studied via experiments. An experimental investigation was implemented to validate the numerical predictions reported in part I of this study. The cooling rate experienced by the powders, for example, was experimentally determined on the basis of dendrite arm spacing correlations, and the results were compared with the numerical predictions. The experimental studies were completed using commercial Al 2024 as a baseline material and Al₉₀Gd₇Ni₂Fe₁ metallic glass (MG). The results showed that the cooling rate of droplets increases with decreasing particle size, with an increasing proportion of helium in the atomization gas and with increasing melt superheat. The experimental results reported in this article suggest good agreement between experiments and numerical simulations.     

Heat flow in atomized metal droplets

The solidification of spherical droplets with a discrete melting temperature is analyzed using an enthalpy model. Equations describing the cooling of the initially superheated liquid droplet and a numerical heat flow model for its subsequent solidification are presented. Important parameters like times for initiation and completion of solidification, cooling rates and interface velocities in aluminum, iron, and nickel are related to the process variables governing the rate of heat extraction from the droplets. The analysis is performed for the range of Biot numbers of practical interest where Newtonian cooling models are not considered applicable, 0.01 ≤ Bi ≤ 1.o, and the results are presented in the form of normalized or dimensionless quantities. It is shown that the average cooling rate in the liquid prior to solidification can be computed with the Newtonian cooling expressions. However, significant temperature gradients are noted at the droplet surface even for Biot numbers as low as 0.01. Reducing the droplet diameter reduces the time necessary for the initiation and completion of solidification, increases the interface velocities at equivalent fractions solidified and decreases theG _L /R ratio. Although smaller droplet diameters promote higher cooling rates in the liquid at the beginning and in the solid at the end of solidification, the effect at the intermediate stages is more complex and depends on the initial superheat, the Biot number and the thermophysical properties of the material. Formerly Professor in the same Department.     

Modeling of reactive atomization and deposition processing of Ni3Al

A recently developed processing technique for the synthesis of dispersion strengthened materials, reactive atomization and deposition (RAD), is introduced in the present paper. The unsteady state momentum, heat and mass transfer phenomena, including chemical reaction and phase change during RAD processing of molten Ni₃Al droplets with a N₂O₂ gas mixture, are numerically investigated. The effects of oxygen concentration in the atomization gas on gas characteristics, such as the compressibility of gas and heat-transfer coefficient, and on droplet characteristics, such as velocity, temperature, undercooling, cooling rate, and solidification history are determined. The surface cooling of droplets is simulated using a modified Newton's law of cooling. This improvement results in a good agreement between the calculated results and experimental measurements.     

喷嘴射流特征对连铸二冷区蒸汽膜穿透行为的影响

 连铸二冷区铸坯表面温度通常高于900 ℃,此时喷淋液滴接触高温铸坯时不会湿润铸坯表面,仅在其上形成一层蒸汽膜,阻碍了液滴与铸坯表面接触传热。针对以上问题,以国内某钢厂连铸二冷区的扁平型水喷嘴为原型,建立了喷嘴射流仿真计算模型,并对所建模型进行了理论和实验室验证;采用数值模拟的方法研究了喷嘴自由射流区的流场分布,运用连铸喷嘴冷却检测系统测量获得了射流液滴粒径演变规律;结合数值模拟和实验室测定结果,定量分析了喷嘴在不同水流量下射流液滴冲击铸坯表面蒸汽膜深度的变化规律。结果表明,该喷嘴的最大射流速度在喷嘴出口处,射流在喷嘴出口附近出能维持较大的射流速度,且随着水量的增加,射流保持高射流速度的距离也增长;整体射流的轴向速度占比均在80%以上。当喷淋水量越大时,射流液滴粒径变得越小;随着距喷嘴出口距离的增加,射流中心处的液滴粒径逐渐增大并达到最大值;当水流量为9和12 L/min时,液滴粒径基本相同,这表明当水流量增加到一定值时,冷却水量的增加不影响液滴粒径分布。在不同水流量下,随着喷淋距离的增加,液滴穿透铸坯表面蒸汽膜深度呈先增大后略微减小的变化规律,在喷射距离为100~200 mm范围内时,液滴穿透深度最大,这表明喷射高度在该范围时,喷淋冷却效果最好。     

Size and Velocity Characteristics of Droplets Generated by Thin Steel Slab Continuous Casting Secondary Cooling Air-Mist Nozzles

Direct spray impingement of high temperature surfaces, 1473 K to 973 K (1200 °C to 700 °C), plays a critical role in the secondary cooling of continuously cast thin steel slabs. It is known that the spray parameters affecting the local heat flux are the water impact flux w as well as the droplet velocity and size. However, few works have been done to characterize the last two parameters in the case of dense mists (i.e., mists with w in the range of 2 to 90 L/m²s). This makes it difficult to rationalize how the nozzle type and its operating conditions must be selected to control the cooling process. In the present study, particle/droplet image analysis was used to determine the droplet size and velocity distributions simultaneously at various locations along the major axis of the mist cross section at a distance where the steel strand would stand. The measurements were carried out at room temperature for two standard commercial air-assisted nozzles of fan-discharge type operating over a broad range of conditions of practical interest. To achieve statistically meaningful samples, at least 6000 drops were analyzed at each location. Measuring the droplet size revealed that the number and volume frequency distributions were fitted satisfactorily by the respective log-normal and Nukiyama–Tanasawa distributions. The correlation of the parameters of the distribution functions with the water- and air-nozzle pressures allowed for reasonable estimation of the mean values of the size of the droplets generated. The ensemble of measurements across the mist axis showed that the relationship between the droplet velocity and the diameter exhibited a weak positive correlation. Additionally, increasing the water flow rate at constant air pressure caused a decrease in the proportion of the water volume made of finer droplets, whereas the volume proportion of faster droplets augmented until the water flow reached a certain value, after which it decreased. Diminishing the air-to-water flow rates ratio, particularly below 10, resulted in mists of bigger and slower droplets with low impinging Weber numbers. However, increasing the air pressure maintaining a constant water flow rate caused a greater proportion of finer and faster drops with Weber numbers greater than 80, which suggests an increased probability of wet drop contact with a hot surface that would intensify heat extraction.     

Nucleation of solidification in liquid droplets

Analytical and numerical methods have been developed to analyze the solidification kinetics of a mass of liquid droplets dispersed in a fluid or solid matrix using classical nucleation theory. The resulting analytical expressions and numerical calculations can be compared directly with calorimetric measurements of the droplet solidification exotherms to obtain information about the nucleation mechanism. With increasing contact angle at the solid-liquid-matrix triple point, the solidification onset, peak, and end temperatures and exothermic peak height all decrease sharply and the droplet solidification exotherms become broader. Decreasing either the droplet radius or the number of potential catalytic nucleation sites produces a similar but smaller effect. Distributions in droplet radius, contact angle, and nucleation sites have no effect on the solidification peak temperature, but the droplet solidification exotherms become broader and more symmetric. The solidification onset temperature is independent of cooling rate in the calorimeter, but the solidification peak and end temperatures decrease and the exothermic peak height increases with increasing cooling rate. Predicted droplet solidification exotherms are in excellent agreement with detailed experimental measurements on 10-nm-radius Cd droplets embedded in a solid Al matrix. Analytical predictions give best-fit values of 43 deg and 430 for the contact angle and the number of potential catalytic nucleation sites per droplet, respectively; numerical predictions give best-fit values of 43 deg and 750 for the contact angle and the number of potential catalytic nucleation sites per droplet, respectively.     

辐射对气淬渣滴冷却换热的影响

液态钢渣气淬粒化成功的关键在于粒化后液滴的快速冷却固化,渣滴冷却过程的传热方式主要为对流和辐射在构建的渣滴冷却过程数学模型的基础上,利用4阶Runge-Kutta方法重点研究了辐射对渣滴冷却过程的影响规律。结果表明,渣滴冷却经历液相冷却成核、潜热释放和固相冷却3个阶段;在没有考虑辐射换热情况下,渣滴(/%:29.94FeO、2.05Fe₂O₃、42.18CaO、9.33MgO、15.92SiO₂、2.09MnO、1.25Al₂O₃、2.24P₂O₅)在293 K氮气中由1 723 K降温至1 073 K需要606 ms,比综合考虑对流和辐射时的冷却时间延长了17.2%,说明在计算渣滴温降速率时辐射换热的影响不容忽视。     

高纯纳米无定形硼粉的制备研究

本文采用等离子体技术制备了高纯纳米无定形硼粉, 对不同氢气与 BCl3 气体流量比例、 反应气体总流量 及冷却气流量等工艺参数对纯度、 粒度及晶型的影响进行了探究。 结果表明, 在一定范围内, 硼粉纯度随着 H2 通入量在 BCl3、 H2 通入量比例中的上升而增加, 硼粉粒度随着反应气体（BCl3 和 H2） 总通入量和冷却气流量的 上升而下降, 硼粉晶型随着冷却气流量的上升从晶体硼逐渐转变为无定形硼。 通过上述影响规律, 确定 BCl3、 H2 通入量比例 1:4、 反应气体总通入量 2.25 m3/h、 冷却气流量 0.9 m3/h 为较优工艺参数, 通过确定的较优工艺参数制 备高纯纳米无定形硼粉, 获得了纯度高达 99.97 wt.%、 主粒径 30~100 nm、 平均粒径 51.06 nm 的硼粉。     

Droplet solidification and gas-droplet thermal coupling in the atomization of a Cu-6Sn alloy

The accurate prediction of solidification cooling rates of droplets in gas atomization is critical to the production of quality powders and spray deposits. Despite many sophisticated on-line measuring instruments and numerous mathematical models, an experimentally verified analysis of droplet cooling in gas atomization is still lacking. Impulse atomization offers a unique single-fluid atomization technique with controlled gas-droplet heat transfer. This technique was, therefore, used to generate a reliable cell-spacing vs cooling-rate relationship for Cu-6Sn. Subsequently, the cell spacing was measured for gas-atomized (GA) particles that were collected with quench probes in known parts of the melt spray. Using the calibration work done on impulse atomization and the sampling positions in gas atomization, the effect of particle size, spray parameters, and location in the spray are analyzed. This analysis incorporates other on-line diagnostics in gas atomization to relate atomized droplets to GA processing conditions. A comparison between impulse atomization and gas atomization is also discussed. This work clearly demonstrates that the droplet cooling rate is more strongly affected by the gas temperature than by the droplet-gas relative velocity. In addition, the thermal Stokes number is a useful parameter to assess the amount of two-way thermal coupling in a given spray. Hence, sprays with low thermal Stokes numbers having droplets of the same size will experience lower cooling rates and have correspondingly higher gas temperatures.     

热装高炉高温煤气喷雾降温塔CFD数值分析

高炉炼铁采用原燃料热装工艺能显著提高铁前工序显热资源利用效率。为保证高温高炉煤气的净化回收系统安全,须在干法布袋除尘设施前设置1座喷雾蒸发降温塔。采用Fluent软件中的离散相模型对喷雾降温塔内的流场进行了数值模拟,分析得到塔内气相流场、温度场分布以及雾滴运动规律等信息,对比分析了不同喷嘴角度和不同喷雾方向对塔内喷雾降温效果的影响。分析结果表明：采用喷雾蒸发冷却工艺可迅速降低煤气温度,采用雾滴与气体逆流型式且喷嘴角度为30°时,效果最优,雾滴充满塔体整个截面且可完全蒸发。分析结果可为喷雾降温塔的结构设计和喷雾降温过程的优化提供参考。     

Dynamic compaction of titanium Aluminides by Explosively Generated Shock Waves: Microstructure and Mechanical Properties

A detailed microstructural analysis and evaluation of the mechanical properties of titanium aluminides consolidated by novel shock processes^[131] are presented. Successful consolidation was obtained and was evidenced by strong bonding between individual particles. Additions of Nb and Ti and Al elemental powders resulted in enhanced interparticle bonding through intense plastic deformation of Nb and shock-induced reactions between Ti and Al. Rapid cooling of interparticle molten layers yielded amorphous Ti-Al alloys; this interparticle melting and rapid cooling are a unique feature of shock processing. Embrittlement of individual particles of Ti₃Al-based alloy after exposure to 550 °C and 750 °C was observed. There is evidence of phase transformation after preheating the powder, and this fact can explain the high density of cracks obtained with this alloy after high-temperature shock consolidation. Mechanical properties of the Ti₃Al-based alloy were determined at room temperature and the fracture modes were studied. The microstructural observations are correlated with the mechanical properties.     

Dynamic compaction of titanium aluminides by explosively generated shock waves: Microstructure and mechanical properties

A detailed microstructural analysis and evaluation of the mechanical properties of titanium aluminides consolidated by novel shock processes^[13] are presented. Successful consolidation was obtained and was evidenced by strong bonding between individual particles. Additions of Nb and Ti and Al elemental powders resulted in enhanced interparticle bonding through intense plastic deformation of Nb and shock-induced reactions between Ti and Al. Rapid cooling of interparticle molten layers yielded amorphous Ti-Al alloys; this interparticle melting and rapid cooling are a unique feature of shock processing. Embrittlement of individual particles of Ti₃Al-based alloy after exposure to 550 °C and 750 °C was observed. There is evidence of phase transformation after preheating the powder, and this fact can explain the high density of cracks obtained with this alloy after high-temperature shock consolidation. Mechanical properties of the Ti₃Al-based alloy were determined at room temperature and the fracture modes were studied. The microstructural observations are correlated with the mechanical properties.     

Wear-resistant amorphous and nanocomposite steel coatings

In this article, amorphous and nanocomposite thermally deposited steel coatings have been formed by using both plasma and high-velocity oxy-fuel (HVOF) spraying techniques. This was accomplished by developing a specialized iron-based composition with a low critical cooling rate (≈10⁴ K/s) for metallic glass formation, processing the alloy by inert gas atomization to form micron-sized amorphous spherical powders, and then spraying the classified powder to form coatings. A primarily amorphous structure was formed in the as-sprayed coatings, independent of coating thickness. After a heat treatment above the crystallization temperature (568 °C), the structure of the coatings self-assembled (i.e., devitrified) into a multiphase nanocomposite microstructure with 75 to 125 nm grains containing a distribution of 20 nm second-phase grain-boundary precipitates. Vickers microhardness testing revealed that the amorphous coatings were very hard (10.2 to 10.7 GPa), with further increases in hardness after devitrification (11.4 to 12.8 GPa). The wear characteristics of the amorphous and nanocomposite coatings were determined using both two-body pin-on-disk and three-body rubber wheel wet-slurry sand tests. The results indicate that the amorphous and nanocomposite steel coatings are candidates for a wide variety of wear-resistant applications.     

脱硫废水烟道喷射的数值模拟

以某电厂300 MW燃煤机组为例,对脱硫废水烟道喷射系统进行CFD数值模拟,研究了雾化粒径、烟气温度、烟气流速和喷枪布置方式对液滴蒸发的影响。结果表明:雾化粒径明显影响液滴群蒸发质量百分比和蒸发时间,在较低的烟温下(140℃),需要较小的雾化粒径(≤50μm)才能保证液滴完全蒸发;烟气温度对液滴群蒸发质量百分比和蒸发时间的影响显著,随着烟气温度升高,液滴蒸发速率加快,蒸发时间明显减少;烟气流速对液滴蒸发特性影响不大,随着烟气流速升高,烟气量增大,液滴群蒸发时间会降低,而液滴群蒸发质量百分比变化不大;在烟道同一截面处,不同喷枪布置方式对液滴蒸发影响小。     

A mathematical model for marangoni flow and mass transfer in electrostatically positioned droplets

A mathematical model is developed for the free surface deformation, full three-dimensional (3-D) Marangoni convection and solute transport phenomena in electrostatically levitated droplets under microgravity. The electric field is calculated by the boundary element method and the shape deformation by the weighted residuals method. The numerical model for the transport phenomena is developed based on the Galerkin finite-element solution of the Navier-Stokes equations, the energy balance equation, and the mass transport equation. Numerical simulations are carried out for droplet deformation by electrostatic forces and 3-D Marangoni convection in droplets heated by three different heating source arrangements. Results show that the electric forces deform a droplet into an oval shape under microgravity by pulling the droplet apart at the two poles. A two-beam heating arrangement results in an axisymmetric flow and temperature distribution in the droplet. Complex 3-D Marangoni flow structure occurs when a tetrahedral or octahedral heating arrangement is applied. The thermal transport in the droplet is conduction dominant for the cases studied. In general, the convection is stronger with higher melting point melts. The internal convection has a strong effect on the concentration distribution in the droplet. For melts with high viscosities, a significant reduction in velocity can be achieved with an appropriate laser beam arrangement, thereby permitting a diffusion-controlled condition to be developed.     

The Evolution of Oxygen-Based Inclusions in an Additively Manufactured Super-Duplex Stainless Steel

Super-duplex stainless steel powder feedstocks specified for use in directed energy deposition additive manufacturing processes can have an oxygen composition nearly five times higher than that present in comparable wrought forms. A combination of computational thermodynamic calculations and experimental validation showed that high levels of oxygen promoted the formation of oxygen-rich inclusions during directed energy deposition additive manufacturing. These inclusions play an important role in microstructural evolution during the rapid heating and cooling cycles prevalent in additive manufacturing and impact mechanical and corrosion properties. Inclusions observed across the powder feedstock and additively manufactured and post-processed materials exhibited complex structures with a combination of amorphous, metastable, and stable phases. The powder feedstock, which experiences rapid cooling rates during the gas atomization process, yielded amorphous inclusions that were rich in manganese, chromium, silicon, and oxygen surrounded by small crystalline MnS particles. After additive manufacturing, inclusions transformed to a combination of rhodonite (MnSiO₃) and spinel (MnCr₂O₄) with amorphous regions around the exterior. Post-process hot isostatic pressing treatments, which replicate conditions most similar to equilibrium, resulted in the formation of a stable spinel oxide with MnS particles around the exterior, matching the results predicted by thermodynamic equilibrium calculations.

     

Modelling of droplet dynamic and thermal histories during spray forming—II. Effect of process parameters

A computer model has been developed to describe the in-flight dynamic and thermal histories of gas atomised droplets as a function of distance during spray forming. The model has been used to investigate the effects of the dynamic and thermal behaviour of individual gas atomised droplets and the cooling and solidification behaviour of the overall spray. The most influential parameters for a given alloy system, in order of importance, are: (i) droplet diameter and, therefore, the droplet size distribution within the spray; (ii) initial axial gas velocity at the point of atomisation and the subsequent gas velocity decay profile; (iii) melt mass flow rate; (iv) melt superheat at the point of atomisation; and (v) alloy composition. Experimental measurements of gas velocities and droplet size distributions during spray forming allow the spray solid fraction at deposition to be calculated and used in a subsequent computer model of billet heat flow to predict the billet top surface temperatures and solid fractions.     

FeNiCrSiMoMnC 合金气雾化熔滴的 运动与传热行为模拟分析

气雾化过程中合金液流被高压气流破碎后,要飞行一段距离才开始冷却凝固,这称为熔滴的飞行过程。本文以重载耐磨耐腐蚀激光熔覆涂层用FeNiCrSiMoMnC合金为对象,通过建立熔滴飞行过程的模型,并对该过程进行数值模拟,计算讨论了雾化气体初始速率、熔滴直径对熔滴的速度、传热系数等参数的影响。结果表明,熔滴速度呈现先增大至最大值后逐渐减小的趋势,且随熔滴直径的增大,熔滴速度最大值点逐渐降低;传热系数与速度曲线相反,呈先下降后上升的趋势,极小值为2kg/d,熔滴直径越大,与气体速度差越小,传热系数越小;随气体初始速率的增大,熔滴最大速度点上移,同时传热系数也逐渐增大。     

A numerical study of oxidation behavior during reactive atomization and deposition

The oxidation behavior of droplets during reactive atomization and deposition (RAD) is analyzed on the basis of a numerical framework proposed here. Commercial 5083 Al is chosen as a model material; moreover, in the numerical model, nonspherical droplets are approximated as cylinders with a length/diameter ratio of 3. An equation that represents the growth rate of the oxide phases, together with models that describe the dynamic and thermal behavior of droplets, is implemented in an effort to elucidate the oxidation behavior of individual droplets. The numerical results reveal that the oxidation rate of a droplet is extremely high and that the oxide phase grows very rapidly initially, eventually attaining a steady state of limited oxide growth. The overall volume fraction of oxide phases in the RAD material increases with increasing atomization pressure, superheat temperature, and O₂ concentration, whereas it decreases with increasing melt flow rate. The oxygen concentrations in the RAD powders and deposited materials predicted on the basis of numerical analysis are in good agreement with the results from chemical analysis when O₂ concentration is lower than 16 vol pct.