非转移弧等离子体炬及其射流的数值模拟

通过求解等离子体弧柱区的质量方程、动量方程、能量方程、电动势方程、磁场方程和组分方程,对非转移弧等离子体炬及其产生的等离子体射流的传热与流动特性进行了数值模拟研究,获得了等离子体炬及其射流区域的温度和速度分布。结果表明:呈现上升的伏安特性曲线;等离子体炬出口处温度和速度呈现抛物线分布;等离子体进气速度增加,等离子体速度增加,温度变化不明显;电流增加,等离子体速度和温度都明显增加。     

低压条件下的等离子射流特性分析

低压等离子喷涂由于具有特殊的射流特性,以及可以沉积组织均匀的特殊结构涂层而备受关注.涂层的形成受到等离子射流特性的影响,比如焓值、温度、速度等.研究中利用热焓探针技术在环境压力为3 kPa的条件下,测量了Ar-H₂等离子体不同轴向位置射流中心的焓值和压力,并且进一步计算了射流的温度和速度,以及表征等离子体对粉体加热能力的努森数.结果表明,等离子射流在距离喷嘴出口12.5 mm处的温度为11 000 K;400 mm处降为7 000 K;等离子射流速度在喷嘴出口处25 mm左右达到最大值,约为2 000 m/s;喷嘴外部等离子射流的努森数处于过渡区,对粉体的加热能力较低.     

发展我国的等离子束加工技术

等离子束实际上是由等离子炬及其中合理组织的流体加以约束的压缩电弧射流,它具有远高于普通焊接电弧的功率密度(50～100kW/cm~2)、温度(10000～30000℃)和束流速度(10~2～10~3m/s),因此,它可以迅速地熔化和去除几乎任何材料。等离子束加工(Plasma Beam Machining,PBM)就     

非转移弧等离子体炬传热与流动的数值模拟

以非转移弧等离子体炬为研究对象,对等离子体电弧和阳极的传热与流动进行了数值模拟,并求解了质量方程、动量方程、能量方程、电势方程和磁场方程,获得了等离子体弧的温度、速度和电流密度分布。计算结果表明,阳极内壁面温度变化不大;等离子体炬出口处径向温度和速度分布类似抛物线形分布;提高进气速度或减小电弧电流,均使得电弧阳极斑点向下游移动。     

预埋铜管铜水套凝固过程模拟仿真

为得到预埋铜管与本体良好熔合的浇注温度和通入冷却气体流速,对预埋铜管铜水套进行铸造工艺设计并建立数学模型,设置初始条件和边界条件后,采用Anycasting软件对其进行模拟仿真。结果表明,浇注温度在1 150¹ 200℃,冷却气体流速在0.31 200℃,冷却气体流速在0.3⁰.7 m/s时,预埋铜管与铸件本体能够良好的熔合。     

特厚钢板射流冲击淬火过程厚向冷速试验研究

利用开发的特厚钢板射流淬火试验装置及多通道温度记录仪,测试了射流速度14.0~23.5 m/s、射流压力0.4~1.0 MPa条件下,84 mm、170 mm厚大断面钢板淬火温降曲线,采用有限元方法建立了三维反传热导热模型和表面换热系数模型,对比分析了射流参数和换热区分布对钢板厚向温降、温度梯度和冷速的影响。结果表明:84 mm厚钢板断面冷速与表面换热系数近似正比关系,射流速度为23.5 m/s时钢板心部冷速达3.7℃/s;170 mm厚钢板表面换热对厚向冷速影响减弱,相应的温度遗传效应和断面厚向温度梯度的影响增强。     

低压直流电弧等离子体传热与流动的数值模拟

根据低压等离子体喷涂所使用的F4-VB喷枪结构,建立了三维稳态湍流模型,采用Fluent软件对104 Pa低压条件下等离子体喷涂过程中等离子体的温度、速度和压强分布进行了模拟。结果表明:阴极尖端附近的等离子体温度达到最高值29 000K,随后温度降低。等离子体速度在喷枪内急速上升,离开喷枪出口约0.01m时达到最大值6 100m/s,随后速度降低。等离子体在距离喷枪出口附近约0.07m以内经历了膨胀和压缩过程,等离子体的膨胀和压缩对速度和温度的变化有显著影响。     

切割用等离子弧的数值分析

以转移型直流电弧等离子体切割炬为研究对象,根据局部热力学平衡假设和磁流体动力学理论构建了电弧数学模型,采用数值模拟方法分析了等离子体炬内等离子体的传热和流动特性,阴极形状、锥角、内缩量和喷嘴形状对切割气流的速度和等离子温度分布的影响.并对五种常用形状阴极和两种不同内缩量的割炬内等离子流体速度场和温度场进行了比较.从数值分析的角度验证了阴极斑点较小的阴极能产生较大的焰流速度和热量,以及内缩量的增加对电弧的压缩具有显著作用.     

薄钢板气体射流控冷结构数值模拟与试验验证

对3种用于薄钢板冷却的气体射流控冷结构进行了数值模拟分析,通过比较气体射流控冷结构内风速流场和钢板温度场,确定单均风板气体射流控冷结构可满足薄钢板淬火要求,且冷却均匀性较好,并进行了试验验证。结果表明:与无均风板相比,设置一个或两个均风板时,工件近壁面风速流场与温度场均匀性得到提高,利用单均风板气体射流控冷结构对钢板进行风冷时,在20 s时工件上存在的最大温度差约为63.8 ℃。在20 s内钢板平均温度从920 ℃冷却至Ms点(412 ℃)以下,最低冷速约为27.2 ℃/s,大于马氏体转变临界冷却速度27 ℃/s,表明单均风板气体射流控冷结构可用于薄钢板气体射流淬火工艺。     

直流电弧热喷涂等离子体偏离局域热力学平衡态分析

在大气压力下,利用直流电弧放电产生热喷涂等离子体,采用发射光谱和热焓探针对热等离子体的射流特性进行诊断。文中通过使用氩原子两条特征谱线的辐射强度,采用双谱线相对辐射强度比值法来计算等离子体的电子温度;同时使用焓探针测量等离子体射流的焓值来计算得到气体温度。研究不同氩气流量和电流强度下,热等离子体的气体温度与射流中电子温度的演变情况,并对两者温度差值出现的原因及变化情况进行分析。结果表明,相同条件下发射光谱法测量的电子温度始终高于焓探针测量的等离子体温度,热喷涂等离子体在一定程度上偏离局域热力学平衡态。     

Effect of plasma spray operating conditions on plasma jet characteristics and coating properties

Using statistical design of experiments, the arc current, total gas flow rate, percent secondary gas (He), and powder feed rate have been varied to assess the torch behavior and establish its correlation to coating properties. The torch response includes arc voltage drop, torch efficiency, and plasma jet geometry. High-speed images of the luminous plasma jet for each operating condition have been acquired with a LaserStrobeℳ videocamera, and image analysis has been used to quantify the jet length and jet fluctuations as additional torch responses. Porosity and unmelted particles, which are determined using image analysis of a micrograph of a NiAl coating cross section, were selected as principal coating characteristics. These findings are expected to be useful for optimization of new spray processes and for evaluation of new torch designs.     

Numerical and experimental determination of plasma temperature during air plasma spraying with a multiple cathodes torch

A numerical model for the calculation of hydro and thermo dynamical gas flow in a three electric arc torch, considering electromagnetic influences on gas flow behaviour was described and developed. The modelling results were compared with experimentally determined temperatures, obtained by computer tomography. A characteristic threefold symmetrical distribution of temperature on the torch nozzle outlet was obtained numerically as well as experimentally. Calculated and tomographic temperature contours and their rotations were compared and showed good agreement. In the second step, a model for the free jet was used for the evolution of thermal conditions into plasma jet outside the torch. The comparison of the calculated temperature distributions and those determined by tomography exhibit good agreement.     

Modelling the Plasma Jet in Multi-Arc Plasma Spraying

Particle in-flight characteristics in atmospheric plasma spraying process are determined by impulse and heat energy transferred between the plasma jet and injected powder particles. One of the important factors for the quality of the plasma-sprayed coatings is thus the distribution of plasma gas temperatures and velocities in plasma jet. Plasma jets generated by conventional single-arc plasma spraying systems and their interaction with powder particles were subject matter of intensive research. However, this does not apply to plasma jets generated by means of multi-arc plasma spraying systems yet. In this study, a numerical model has been developed which is designated to dealing with the flow characteristics of the plasma jet generated by means of a three-cathode spraying system. The upstream flow conditions, which were calculated using a priori conducted plasma generator simulations, have been coupled to the plasma jet simulations. The significances of the relevant numerical assumptions and aspects of the models are analyzed. The focus is placed on to the turbulence and diffusion/demixing modelling. A critical evaluation of the prediction power of the models is conducted by comparing the numerical results to the experimental results determined by means of emission spectroscopic computed tomography. It is evident that the numerical models exhibit a good accuracy for their intended use.     

Three Dimensional Modeling of the Plasma Spray Process

Results of simulations of three-dimensional (3D) temperature and flow fields inside and outside of a DC arc plasma torch in steady state are presented with transverse particle and carrier gas injection into the plasma jet. The results show that an increase of the gas flow rate at constant current moves the anode arc root further downstream leading to higher enthalpy and velocity at the exit of the torch anode, and stronger mixing effects in the jet region. An increase of the arc current with constant gas flow rate shortens the arc, but increases the enthalpy and velocity at the exit of the torch nozzle, and leads to longer jets. 3D features of the plasma jet due to the 3D starting conditions at the torch exit and, in particular, due to the transverse carrier gas and particle injection, as well as 3D trajectories and heating histories of sprayed particles are also discussed.     

Relation Between the Arc-Root Fluctuations, the Cold Boundary Layer Thickness and the Particle Thermal Treatment

In plasma spraying, the arc-root fluctuations, modifying the length and characteristics of the plasma jet, have an important influence on particle thermal treatment. These voltage fluctuations are strongly linked to the thickness of the cold boundary layer (CBL), surrounding the arc column. This thickness depends on the plasma spray parameters (composition and plasma forming gas mass flow rate, arc current, etc.) and the plasma torch design (anode-nozzle internal diameter and shape, etc.). In order to determine the influence of these different spray parameters on the CBL properties and voltage fluctuations, experiments were performed with two different plasma torches from Sulzer Metco. The first one is a PTF4 torch with a cylindrical anode-nozzle, working with Ar-H₂ plasma gas mixtures and the second one is a 3MB torch with either a conical or a cylindrical anode-nozzle, working with N₂-H₂ plasma gas mixtures. Moreover, arc voltage fluctuations influence on particle thermal treatment was studied through the measurements of transient temperature and velocity of particles, issued from an yttria partially stabilized zirconia powder with a size distribution between 5 and 25 μm. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

The deformation and cooling of ceramic particles sprayed with a thermal radio-frequency plasma under atmospheric conditions

Common thermal-spray techniques use the strong acceleration of powder particles to produce dense ceramic coatings with high bond strength. The residence time of the powder particles within the plasma jet is correspondingly low, and only relatively small particles can be molten. In this work, on the contrary, an inductively coupled radio-frequency (RF) inductively coupled plasma (ICP) torch was used to spray large oxide-ceramic powder particles under atmospheric conditions. The slow plasma flow of a RF plasma leads to large residence times of the powder particles, so that the powder size of the feedstock can be 100 μm and more. It was observed that these particles will not be strongly accelerated in the plasma and that their velocity at the moment of impact is in the range of 10 to 20 m/s. Ceramic coatings were ICP sprayed with a low porosity and a high bond strength, similar to direct current (DC) or high-velocity-oxygen-fuel (HVOF) sprayed coatings. The morphology of ICP-sprayed particles on smooth steel surfaces, as a function of the surface temperature, is described and compared with DC plasma-sprayed splats. Furthermore, the degree of deformation was measured and determined by different models, and the pronounced contact zones formed between the pancake and the substrate were investigated. The ICP-sprayed ceramic coatings show some special properties, such as the absence of metastable crystalline phases, which are common in other spray technologies.     

Comparisons Between Two Different Three-Dimensional Arc Plasma Torch Simulations

Several numerical models have been developed to study the characteristics of an arc inside the nontransferred plasma torch. A few of them have considered complete geometry of cathode and anode nozzle (type I) whereas others have considered only anode nozzle with cathode tip (type II). In this work, a three-dimensional model is developed to simulate Ar-N₂ arc in type I and type II geometries. Various combinations of the arc length and arc core radius are predicted for the torch power that corresponds to given gas flow rate and current. Various combinations of the same and minimum entropy production for all cases could not be predicted in type II geometry. The difference between velocities predicted in both geometries is larger than that between temperatures. Three-dimensional effect in the plasma jet thermo-fluid fields demises along the axial direction. Torch efficiencies and arc voltages predicted in both geometries are comparable with measurements.     

Suspension plasma spraying of nanostructured WC-12Co coatings

Nanostructured WC-12% Co coatings were deposited by suspension plasma spraying of submicron feedstock powders, using an internal injection plasma torch. The liquid carrier used in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to thin coatings with a fine surface finish. A polyethylene-imine (PEI) dispersant was used to stabilize the colloidal suspension in an ethanol carrier. In-flight particle states were measured for a number of operating conditions of varying plasma gas flow rates, feed rates, and standoff distances and were related to the resulting microstructure, phase composition (EDS, SEM, XRD), and Vickers hardness. High in-flight particle velocities (>800 m/s) were generated, leading to dense coatings. It was observed that the coating quality was generally compromised by the high temperature and reactivity of the small particles. To compensate for this shortcoming, the suspension feed rate was adjusted, thereby varying the thermal load on the plasma. Results showed that a slightly larger agglomerate size, in conjunction with low particle jet temperatures, could somewhat limit the decomposition of WC into brittle W₂C/W₃C and amorphous cobalt containing binder phases. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Analysis of a high-velocity oxygen-fuel (HVOF) thermal spray torch part 2: Computational results

The fluid and particle dynamics of a high-velocity oxygen-fuel (HVOF) torch are analyzed using computational fluid dynamic (CFD) techniques. The thermal spray device analyzed is similar to a Metco Diamond Jet torch with powder injection. The details of the CFD simulation are given in a companion paper. This paper describes the general gas dynamic features of HVOF spraying and then discusses in detail the computational predictions of the present analysis. The gas velocity, temperature, pressure, and Mach number distributions are presented for various locations inside and outside the torch. The two-dimensional numerical simulations show large variations in gas velocity and temperature both inside and outside the torch due to flow features such as mixing layers, shock waves, and expansion waves. Characteristics of the metal spray particle velocity, temperature, trajectory, and phase state (solid or liquid) are also presented and discussed. Particle velocities and temperatures are shown to be lower for this type of torch than previously believed.