雾化器往复扫描喷射共沉积过程的数值模拟

喷射沉积过程是一个多参数共同影响的复杂过程。通过对雾化器往复扫描规律以及其它工艺参数的研究，可以对喷射沉积过程进行工艺优化。基于对沉积过程的分析，建立了雾化器往复扫描喷射沉积的数值模型。采用计算机数值模拟技术与实验结合的方法，着重研究了喷射成型过程中雾化器的扫描运动对成型的影响；实验证明，根据分析所得到的雾化器扫描曲线，可以准确预测这类成型产品的形状和尺寸。     

A three-dimensional model of the spray forming method

A three-dimensional model has been formulated to calculate the shape of the general preform, using vector calculus. The shape of a rod, tube, plate, or irregular preform can be calculated at given spray forming conditions. The shape of a spray-formed rod was analyzed at various spray forming conditions using the three-dimensional model. The effects of spray forming parameters, such as spray distribution parameters, angular velocity of rotation, withdrawal velocity, spray angle, and eccentric distance on rod shape, were analyzed. The most important parameters affecting the shape of rods are the spray distribution parameters and the withdrawal velocity. The dynamic evolution of rod shape with a stepwise variation of the withdrawal velocity during spray forming was investigated. The effect of a stepwise change of the withdrawal velocity was the same as that of the scanning atomizer. The calculated surface profiles were compared with those of spray-formed 7075 aluminum alloy rods prepared on a pilot scale. The calculated results for the surface profiles were in agreement with those of the spray-formed rods.     

Modeling of spray deposition: Measurements of particle size,gas velocity,particle velocity,and spray temperature in Gas-Atomized sprays

Spray deposition is a novel manufacturing process which is currently being developed for producing near-net-shape preforms. Spray deposition involves the creation of a spray of droplets by a gas atomizer and the consolidation of these droplets on a substrate to create a preform. In order to maximize the metallurgical benefits of spray deposition, a thorough characterization of momentum and heat transfer in the gas-atomized spray is required. The present paper describes measurements of particle size, gas velocity, particle velocity, and spray temperature in gasatomized Sn-Pb sprays. Measurements were performed on steady-state axisymmetric sprays which were generated using a close-coupled gas atomizer with Sn-5 wt p t Pb and Sn-38 wt p t Pb alloys, atomizer gas flow rates of 2.5 g/s (0.56 MPa) and 3.4 g/s (1.04 MPa), melt flow rates of 35 and 61 g/s, and atomizer-substrate distances of 180 and 360 mm. Gas velocities in the range to 4 m/s were measured using Pitot tube and laser Doppler anemometry (LDA). Droplet velocities in the range 3 to 32 m/s were determined from photographic streak-length measurements and LDA. Oil calorimetry of the spray enthalpy indicated that the spray temperature decreased with increasing axial distance from the gas atomizer, increasing gas flow rate, and decreasing melt flow rate. Formerly Doctoral Student, Department of Metallurgy and Science of Materials, Oxford University     

喷射沉积铝合金管坯工业化生产中的控制系统研究

对多层喷射沉积制备大规格管坯工业化生产特点进行了分析，表明Osprey多程技术更适宜于喷射沉积工业化生产。构建了由系统管理层／控制层／设备层组成的三层递阶控制结构。对关键工艺参数的闭环控制技术进行了研究，提出了漏包连续移液和基于积分分离PID的液位精确控制结构和方法，研究了管坯沉积层厚度的在线检测及喷嘴喷射高度在线反馈控制方法，提出了基于沉积室微正压环境的氧含量控制技术。控制系统及相关控制技术已在国内首条自行研制的喷射沉积制备铝合金管坯工业化生产设备中得到应用，已生产出最大长度1500mm、最大壁厚300mm、不同内径的铝合金产品。     

Modelling deposit growth on tilted rotating cylindrical substrate in centrifugal spray deposition

Abstract

Centrifugal spray deposition (CSD) using a tilted rotating cylindrical substrate may offer considerable technical benefits over the conventional CSD process using a reciprocating substrate in producing ring preforms. A model has been developed to calculate the deposit growth rate as a function of the liquid volume flowrate, the substrate radius, the tilt angle, the position of the atomising disc, and the longitudinal position at the substrate. The deposit band length is determined by the substrate radius and tilt angle. The distribution of the deposit growth rate is symmetrical provided the atomising disc is positioned on the substrate axis or displaced along the direction perpendicular to the tilt plane. For a tilt angle of <50·46°, the deposit thickness uniformity can be improved by increasing the displacement. A tapered deposit with a controlled gradient can be obtained if the atomising disc is positioned offcentre on the tilt plane.     

直流等离子喷枪的研究现状及发展趋势

直流等离子喷枪是等离子喷涂系统中最为核心的部件,决定了粒子的熔融软化沉积效果和涂层性能。总结 了阴极、阳极 ( 喷嘴 ) 和气体分配环等关键部件的结构特征对等离子射流的影响。进一步介绍了直流等离子喷枪 的结构优化发展,重点从等离子体射流稳定性和涂层沉积效率提升等角度进行阐述,介绍了级联等离子喷枪对射 流稳定性的优化,以及送粉方式对涂层沉积效率的提升,并展望了未来的发展趋势。     

A mathematical model of the spray deposition process

Spray deposition is a recently developed atomization process desigted to produce high density, bulk metal shapes directly from the melt. The process consists of two basic steps: first, a molten metal stream is atomized using a gas; the spray thus produced is then collected onto a suitably designed substrate. In this paper a mathematical model for the analysis of heat transfer during SD is described. The model is in two parts: the first part calculates the thermal histories of atomized droplets in flight, whereas the second part computes the transient temperature profiles inside the growing preform. More specifically, the mathematical model estimates droplet size distribution, temperatures, fractional solidification and microstructures of the atomized droplets in the spray, and the temperature field and microstructure of the resulting deposit. In contrast atively low (1 to 10 °C/second). The results also indicate that a small fraction of liquid mixed with solid exists at the top of the growing preform during deposition. The tiny pools of liquid may play a role in the formation of the characteristic equiaxed grain microstructure of as deposited preforms. The results of the calculations are very sensitive to the value of the enthalpy of the impinging spray. Therefore, the production of good quality deposits requires accurate control of the heat fluxes during deposition. E.J. Lavernia formerly, Research Associate, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139     

喷射成形过程中圆锭坯外形生长的数值模拟

通过多次试验后得出雾化喷嘴的雾化特性公式,在此基础上建立了一种模拟喷射成形过程中圆锭坯外形生长的数学模型.该数学模型考虑了喷射成形过程中各种工艺参数,如喷嘴的雾化参数、偏心距离、沉积盘的旋转速度和下拉速度等参数的影响.经过模拟计算,得到了锭坯生长的三维外形尺寸,与实际喷射成形制备的锭坯外形对比,二者吻合很好;采用该模型分析了不同时间下锭坯的轮廓形状、偏心距离以及下拉速度变化后的锭坯轮廓形状.综合分析得出,此数学模型可以预测在不同工艺参数下喷射成形锭坯的外形生长过程.     

Processing of Cu–Al–Ni Alloy by Spray Atomization and Deposition Process

The present work describes a new route for the preparation of Cu–Al–Ni alloy strips via spray atomization and deposition route. The route consists of atomizing liquid Cu–Al–Ni alloy with a jet of argon gas in a closed chamber, at a pressure of 1 MPa. The semi-solid Cu–Al–Ni droplets are subsequently collected on the steel substrate placed vertically below the liquid metal stream in the atomization chamber to form a three-dimensional preform. The deposit produced on the substrate contains ~?5% porosity. The microstructural details of the spray deposited Cu–Al–Ni strips explains particularly the presence of porosity, formation of splats during the flight of spray casting and the associated microstructural evolution in Cu–Al–Ni spray deposit are explained.     

Dry Sliding Wear Behavior of Cold Rolled Spray Cast Al–6Si Alloy

In the present investigation, dry sliding wear behavior of spray cast Al–6Si alloy in both as-sprayed and different cold rolled reductions have been studied as a function of applied load. The spray cast preform was cold rolled to increase its densification. The preform showed equiaxed grain morphology containing some irregular pores and these were almost eliminated by the cold rolling. The improved wear properties of cold rolled spray cast alloy are discussed on the basis of microstructural features at different rolling reductions and the nature of debris particles collected from wear test.     

"旋转型气-液雾化喷嘴"的雾化角与流量分布特性研究

对“旋转型气-液雾化喷嘴”的雾化角的变化规律进行了研究，主要考虑了喷嘴的结构参数，气液比(ALR)，液体粘度等因素对雾化角的影响。通过实验测量与拟合，最后给出了喷嘴的雾化角的表达式。同时对喷嘴的流量分布特性进行了研究，指出其流强分布为马鞍型分布。     

Formulation of rod-forming models and their application in spray forming

In rod spray forming, the preform changes its shape continually from that of a disc to a rod (transient-state rod growth) and then maintains its top surface profile once it has settled down (steady-state rod growth). The rod growth mechanism during spray forming was analyzed using rod-forming models. At a sufficiently high substrate rotation velocity, the calculated results based on the three-dimensional time-dependent model (3-D TDM) and the two-dimensional time-dependent model (2-D TDM) were observed to be identical. The calculated results of the rod’s top shape, obtained by the TDMs, were almost identical to those obtained by the two-dimensional time-independent model (2-D TIM), which means that there exists steady-state rod growth. The effects of spray-forming parameters, such as initial eccentric distance, substrate withdrawal velocity, and spray angle, on the shape-evolution behavior were analyzed in terms of the vertex growth velocity ( ). The optimum spray-forming condition to minimize transient-state rod growth was also presented. Experimental verification was made to confirm the proposed forming models.     

Three-dimensional mathematical model for transport phenomena in horizontal chemical vapor deposition reactors

A three-dimensional (3-D) mathematical model describes the transport phenomena and the resulting rate of deposition in horizontal chemical vapor deposition (CVD) reactors. The model employs a finite difference scheme to solve the governing partial differential equations to predict the velocity field, temperature distribution, and concentration profiles of various gas species. The rate of silicon deposition by the reaction of SiCl₄ and H₂ is predicted. The model shows that the buoyancy-driven flow in such reactors has a marked effect on the uniformity of deposition. In these calculations, the concepts of local equilibrium at the substrate and thermal diffusion of SiCl₂ away from the substrate were considered in evaluating the rate of silicon deposition. Incorporation of these factors has significantly improved the predictive capability of the model. The rates of silicon deposition were calculated for two different thermal boundary conditions which had a pronounced effect on the uniformity of deposition. Substrate tilt contributes to the production of a more uniform silicon deposition. The model was used as a computer-aided design tool for process optimization; a simple addition of two fins to the top wall (without any substrate tilt) of the reactor can significantly reduce the secondary roll cell formation and should lead to more uniform deposition.     

一种提高喷雾通风冷却塔换热效果的方法

冷却塔是干熄焦循环水系统的核心设备,其换热效果直接影响干熄焦发电量和耗水率。马钢干熄焦系统中应用的喷雾通风冷却塔在使用中出现换热效果不佳、壁流严重以及塔体振动等问题。分析了喷雾通风冷却塔的基本原理,建立其物理模型,并使用计算流体力学软件进行数值计算,找出了其换热效果差与壁流有直接的关系。最后提出了一种解决方案,通过对喷嘴缩径提高射流角度,减少了壁流,从而有效地提高了换热效果。     

Metal Spray Forming Process Examined Using Mathematical Model

The metal spray forming process was examined using the mathematical model simulation by Baosteel＇s test and developed facilities.The mathematical model comprised of the probability and statistical analysis of the droplet mass behavior and predicted the shape and temperature distribution of the billet during the spray forming process.     

Microstructure, Tensile Properties and Heat Treatment Process of Spray Formed FGH95 Superalloy

 The potential of spray forming for microstructural refining can be attractive to the production of superalloys. A 200 mm spray formed FGH95 superalloy round billet that was produced using a single atomizer has considerably homogeneous, small, equiaxed grains. The measured oxygen content is 2×10-5, and the measured porosity is only 0. 6%. The achieved yield of deposit is in a good range of about 73. 6%. The microstructure and tensile properties of nickel-based spray formed FGH95 superalloy are analyzed. Also, effects of heat treatment on microstructure were discussed. The results show that the spray formed FGH95 superalloy has higher isotropy in tensile property due to its γ′ phase homogeneous distribution and less defects in the microstructure. Regarding processing procedures, the different heat treatment processes affect the microstructure and γ′ phase homogeneous distribution of spray formed FGH95 superalloy.     

Water atomisation of metal powders: effect of water spray configuration

ABSTRACT

We consider the effect of water spray configuration on the fineness and uniformity of a metal powder produced by water atomization of a melt stream. The effects of water spray travel distance, nozzle design, water pressure, melt superheat, and apex angle on the particle size distribution of a metal powder is studied via a laboratory-scale water atomizer; the main focus is on the first two, which are usually fixed parameters of the atomizer. Correlations are proposed relating the mass median size and standard deviation of the powder to the parameters cited. Similar correlations for water pressure, melt superheat, and apex angle have been reported elsewhere; we present data on these effects to confirm the validity of our results, especially as Bi-42%Sn powder has not been studied before. What is new are results on the effect of water spray travel distance and nozzle design on the mass median size and standard deviation of powder.     

Spray casting of steel strip: Process analysis

Near-net shape manufacturing (NNSM) of thin steel sections by spray casting eliminates casting as a separate step with attendant improved microstructures and properties and significant energy savings. The process involves atomization of a stream of liquid metal and deposition of droplets in the generated spray on a moving substrate at mass flow rates of 0.25 to 2.5 kg/s. In this paper, NNSM of steel strip by the Osprey spray casting process is investigated by combining numerical simulation and experiments. Critical input parameters for the computation are quantified utilizing existing state-of-the-art mathematical models and specific experiments. Numerical computation of the consolidation of the spray at the substrate during manufacture of thin sections is conducted using bothcontinuum anddiscrete event (“splat solidification”) approaches to predict: (1) variation of strip thickness in the transverse dimension and (2) isotherms and cooling rates across the strip thickness. Predicted geometries of the strip simulated by the continuum model are in good agreement with measurements. Predicted isotherms in narrow strip by the continuum approach are in reasonable agreement with thermocouple measurements for intermediate thicknesses (2 to 5 mm), and the observed microstructure is consistent with predicted cooling rates. The discrete event model predicts significantly higher cooling rates than the continuum model in the basal portion of the strip. This is consistent with the observed grain size in thin strip (<l-mm thick) and in the basal portion of thick strip. Beyond a threshold thickness, however, the discrete event model confirms the formation and persistence of a partially liquid layer at the growing surface of the deposit with an attendant decrease in the cooling rate. The influence of critical parameters on “splat solidification” is analyzed and assessed. DIRAN APELIAN, formerly Howmet Professor of Materials Engineering at Drexel University