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.     

Particle behavior in supersonic flow during the cold spray process

The cold spray process is a relatively new process that uses high velocity metallic particles for surface modifications. Metallic powder particles are injected into a converging-diverging nozzle and accelerated to supersonic velocities. In this study two-dimensional temperature and velocitiy distributions of gas along the nozzle axis are calculated and the effects of gas pressure and temperature on particle velocities and temperature inside and outside the nozzle are investigated. It was found that acceleration of the gas velocity takes place in the area of the nozzle throat, and it increases and reaches a maximum value at the nozzle exit. Due to compression shocks, irregular changes of the gas jet properties were found in the area after the nozzle and these resulted in the experience of the maximum particle velocity by the change of the particle size at a given gas pressure and temperature.     

Effect of in-flight particle properties on deposition of air plasma sprayed forsterite

Properties of forsterite coatings deposited by two DC-arc plasma spray guns were studied. The guns generate different types and shapes of plasma jets, resulting in different particle/plasma interactions and different microstructures in the coatings due to the different in-flight particle histories. The particle histories are characterized by cross-sectional maps of the plasma jet showing particle temperature, velocity, and particle size distributions and the number of particles correlated with the coating microstructures.     

On some aspects of gas dynamics of the cold spray process

This paper presents an overview of results of recent studies conducted at the Institute of Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Science in the field of gas dynamics and heat transfer of the supersonic air jet under conditions typically used in the cold spray process. These studies are related to various aspects of the problem including a flow in the nozzle and the outflow of the jet, as well as effects of the interaction of the jet with a flat obstacle. They are conducted with a supersonic nozzle with a rectangular section at the exit with a Mach number M ₀ between 2 and 3.5. The gas flow in the nozzle is theoretically and experimentally studied. It is shown that the boundary layer on the walls of the nozzle affects significantly the flow parameters (for example, Mach number M, pressure p, temperature T, and density ρ of the gas). A method of calculation of the gas parameters in the flow core of the nozzle is suggested, and it is shown that they depend mainly on the ratio of the nozzle width to its length. The results of the investigation of the supersonic air jets with stagnation temperature ranging from 300–600 K flowing in the atmosphere are presented. The corresponding dimensions of the jets, profiles, and axial distributions of the gas parameters are obtained. The interactions of the supersonic jet with the flat obstacle are studied. Self-similarity of the distribution of the pressure and of the Mach number on the obstacle surface is shown for the jets with various values of the Mach number and the angle of impingement. The oscillation regimen of the jet impingement, as well as a compressed layer structure is observed with the aid of a Schliren visualization technique. Some problems of heat exchange of the jets with the obstacle are considered. Distributions of stagnation temperature and heat exchange coefficient in the near-wall jet are obtained. The temperature of the obstacle for the stationary case is calculated, and it is shown that for heat conductive materials the surface temperature is lower than the stagnation temperature due to the redistribution of heat inside of the substrate.     

Analysis and modelling of particle velocities in micro-abrasive air jet

Abrasive jet micromachining (AJM) is a non-traditional technology that can effectively remove hard and brittle materials at high cut quality. A key requisite in modelling the AJM process is to determine the velocities of abrasive particles. In this paper, a theoretical analysis for particle velocities within a micro-abrasive air jet is presented and the associated particle velocity models are developed. The particle velocities at the nozzle exit are determined based on the nozzle length, particle mean diameter, particle density, air density and air flow velocity. The distribution of particle velocities along the jet centerline downstream from the nozzle and the particle velocity profile at a jet cross-section are also modelled considering surrounding air entrainment and air-particle interaction. A numerical solution to the models is developed to determine the particle velocities by dividing the nozzle and the jet flow in air into small segments along the jet axial direction. The developed models are finally verified by comparing the calculated particle velocities with those from a particle image velocimetry (PIV) measurement of the velocity distribution in micro-abrasive air jets. It is shown that the model calculations and the corresponding experimental results are in good agreement with less than 4% average errors.     

In-situ particle temperature, velocity, and size measurements in DC Arc plasma thermal sprays

Particle temperature, velocity, and size measurements in DC arc plasma thermal sprays are reported in this article. Experiments were performed using a conventional DC are argon-hydrogen plasma with two 7 wt % yttria-stabilized zirconia powders injected transversely into the plasma jet. Measurements were performed along the axis of the plasma jet as well as at a number of radial locations at several axial positions. It was found that transverse injection of the powder results in the aerodynamic size classification of the powder with the large particles penetrating further across the plasma jet than the smaller particles, which were more readily swept by the high momentum of the plasma jet. Consequently, the particle temperatures were influenced by their degree of penetration into the core of the plasma jet. Average particle temperatures showed a good degree of uniformity radially and decayed with increasing downstream distance. When nanoclustered particles were injected into the plasma, significant differences in particle velocities and temperatures were observed in comparison to the conventional powder under the same plasma operating and particle injection conditions. These differences were attributed to the penetration characteristics of the powder into the plasma jet and the consequent effects on the particle heat up. Hence, axial injection of powder into plasma jets may provide more uniform and axisymmetric particle property distributions compared to the transverse injection schemes.     

Computational study and experimental comparison of the in-flight particle behavior for an external injection plasma spray process

A three-dimensional computational fluid dynamic (CFD) analysis using Fluent V5.4 was conducted on the in-flight particle behavior during the plasma spraying process with external injection. The spray process was modeled as a steady jet issuing from the torch nozzle via the heating of the are gas by an electric are within the nozzle. The stochastic discrete model was used for the particle distribution. The particle temperature, velocity, and size inside the plasma plume at a specified standoff distance have been investigated. The results show that carrier gas flow rate variation from 2 standard liters per minute (slm) to 4.0 slm can increase the centerline particle mean temperature and mean velocity by 10% and 16%, respectively, at the specified standoff distance. A further increase of the carrier gas flow rate to 6 slm did not change the particle temperature, but the particle velocity was decreased by 20%. It was also found that an increase in the total arc gas flow rate from 52 slm to 61 slm, with all other process parameters unchanged, resulted in a 17% higher particle velocity, but 6% lower particle temperature. Some of these computational findings were experimentally confirmed by Kucuk et al. For a given process parameter setting, the kinetic and thermal energy extracted by the particles reached a maximum for carrier gas flow rate of about 3.5–4.0 slm.     

Investigation of particle in-flight characteristics during atmospheric plasma spraying of yttria stabilized ZrO2: Part 2. modeling

Yttria stabilized ZrO₂ particle in-flight characteristics in an Ar-H₂ atmospheric plasma jet have been studied using analytical and experimental techniques. In the previous article,^[1] the primary gas flow, plasma composition, current, and powder feed rate were systematically varied and particle surface temperatures, velocities, and size distributions measured and statistically analyzed. In this paper, a mathematical model for the plasma flow and particle characteristics is presented. Model predictions are compared with the experimental results in Ref 1 and a reasonable correlation is found. A statistical investigation (composite cubic face (CCF)) is performed on the particle predictions, giving fast and simple relationships between gun parameters and particle in-flight properties. The statistical and theoretical models that are presented here combine to form a powerful and cost-effective tool, which can be used in the evaluation and optimization of spray parameters off-line.     

Controlling particle injection in plasma spraying

This paper reviews experimental and analytical techniques that examine the efficiency of systems for the injection of powders in plasma jets used in spray coating. The types of injectors, the experimental techniques for observing particle trajectories and distributions, and the mathematical models used to investigate the momentum and heat-transfer phenomena between particles, carrier gas, and plasma jet are described. Experimental data are presented from numerous examples from the plasma spraying of ceramic powders.     

Three-dimensional simulation of thermal plasma spraying of partially molten ceramic agglomerates

Thermal plasma spraying of agglomerated nanostructured ceramic particles has been studied using computational fluid dynamics. The plasma jet is modeled as a mixture of Ar-H₂ plasmas issuing into a quiescent atmosphere. The particles, modeled as micron-sized spheres, are introduced into the jet outside the plasma gun exit with radial injection. The existence of a simple target in front of the plasma gun is taken into account. The trajectories and state histories of particles of various sizes during their flight through the jet are presented. Moreover, the solid-liquid interface within the particles is tracked in an attempt to predict the amount of unmelted material retained in these particles at various axial distances from the gun exit. The effects of turbulence in the jet on these particle histories are accounted for. It is shown that, for the range of particle sizes and the plasma gun operating conditions studied, both the deposition location and the retained unmolten fraction are strongly affected by the size of the particles. The predictions are significant in terms of showing general trends, which will be useful in identifying processing windows for producing optimally nanostructured coatings.     

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.     

基于响应曲面法的Ar-N2等离子射流特性研究

目的 为了研究等离子射流特性,方法 本文借助响应曲面法,以粒子速度和温度作为指标来反映射流特性的变化,采用Box-Behnken-Design ( 简称BBD)设计分析了电流、主气以及次级气比例这三个影响因素对于射流特性的影响规律以及参数之间的相互作用关系。结果 研究表明：对于粒子速度的影响因素排序中：QAr > I > C, 对于粒子温度的影响因素排序中：I > QAr > C。该喷嘴下实现最佳加热效应的参数配比为：主气大小80L/min、电流450A,次级气比例为17.5%;实现射流最佳加速效应的离子气及电参数配比方案为：主气大小120L/min、电流450A,次级气比例为12.5%;在射流最佳加速效应对应参数下制备AT40涂层进行验证,发现涂层均匀致密,孔隙少。结论：运用响应曲面法分析和解决等离子射流特性影响问题具有科学性和可操作性,借助射流特性的研究能够有效指导涂层制备。     

新型大气长层流等离子体喷涂方法和研究进展

介绍了一种新型大气等离子喷涂方法,该方法采用特殊内通道结构的直流非转移电弧等离子发生器,可以直接在大气条件下获得长度100～1000 mm之间变化的等离子射流。在大气条件下,等离子射流的流动特性具有"长、直、准"的层流或类层流状态,工作时噪音小于80 dB。在工作参数范围内,等离子射流的长度在固定总气流量条件下可以随输出功率的增加而增长;射流的长度在固定输出功率的条件下随总气流量的增加而减小。当使用在大气等离子喷涂技术中时,会为飞行粉末颗粒带来超长的加热和加速过程。文中详细介绍了大气层流等离子喷涂技术的研究历史和研究现状,以及研究团队利用该新型技术制备的6种涂层的显微结构、颗粒的飞行和加热特点,并对比了目前其他大气等离子喷涂技术的结果。结果表明,文中介绍的方法在最低的输出功率和气流量条件下,为金属和陶瓷颗粒提供了超长的飞行和加热条件,表现为较低的颗粒飞行速度和超高的颗粒表面温度。可以在不同的射流长度或喷涂距离下,获得不同的颗粒熔化状态或涂层结构,并发现可以直接在大气条件下获得大规模气液共沉积的涂层。     

低功率等离子体炬生产钛合金粉末的理论与实验研究

探讨使用低功率等离子体炬生产钛合金金属粉末的可能性.设计一种氩气直流非转移电弧等离子体炬,并对其等离子体射流特性和导线温度进行数值分析.喷嘴附件内的最高射流速度为838～1178 m/s,不同气体流速下顶点处的射流速度为494～645 m/s.等离子体气体流速对有效等离子体射流长度无显著影响.利用等离子体射流的温度和速...     

Three-Dimensional Modeling of Suspension Plasma Spraying with Arc Voltage Fluctuations

In this study, a three-dimensional DC plasma torch is modeled using Joule effect method to simulate the plasma jet and its voltage fluctuations. The plasma gas is a mixture of argon/hydrogen, and the arc voltage fluctuation is used as an input data in the model. Reynolds stress model is used for time-dependent simulation of the oscillating flow of the plasma gas interacting with the ambient air. The results are used to investigate the plasma oscillation effects on the trajectory, temperature, and velocity of suspension droplets. Suspensions are formed of ethanol and yttria-stabilized zirconia submicron particles and modeled as multicomponent droplets. To track the droplets/particles, a two-way coupled Eulerian–Lagrangian method is employed. In addition, in order to simulate the droplet breakup, Kelvin–Helmholtz/Rayleigh–Taylor (KH–RT) breakup model is used. After the completion of suspension breakup and evaporation, the sprayed particles are tracked to obtain the in-flight particle conditions including trajectory, size, velocity, and temperature. The arc voltage fluctuations were found to cause more than two times wider particle trajectories resulting in wider particle temperature, velocity, and size distributions compared with the case of constant voltage.     

Effects of spray conditions on coating formation by the kinetic spray process

The kinetic spray coating process involves impingement of a substrate by particles of various material types at high velocities. In the process, particles are injected into a supersonic gas stream and accelerated to high velocities. A coating forms when the particles become plastically deformed and bond to the substrate and to one another upon collision with the substrate. Coating formation by the kinetic spray process can be affected by a number of process parameters. In the current study, several spray variables were investigated through computational modeling and experiments. The examined variables include the temperature and pressure of the primary gas, the cross-sectional area of the nozzle throat, the nozzle standoff distance from a substrate, and the surface condition of nozzle interior and the powder gas flow. Experimental verification on the effects of these variables was performed primarily using relatively large-size aluminum particles (63–90 μm) as the feedstock material. It was observed that the coating formation is largely controlled by two fundamental variables of the sprayed particles: particle velocity and particle temperature. The effects of different spray conditions on coating formation by the kinetic spray process can be generally interpreted through their influences on particle velocity and/or particle temperature. Though it is limited to accelerate large particles to high velocities using compressed air or nitrogen as carrier gas, increasing particle temperature provides an additional means that can effectively enhance coating formation by the kinetic spray process.     

The characteristics of a low-power-consumption plasma jet and ceramic coatings

The effects of the composition of plasma gases (Ar-N₂, Ar-H₂), arc current, and voltage on the temperature and velocity of a low-power (5 kW) plasma torch in the arc field free region has been investigated using an enthalpy probe. Coatings of Al₂O₃-13TiO₂ were deposited under different conditions. The results show that in the Ar-N₂ plasma, the enthalpy, temperature, and velocity change little with arc current and voltage when regulating the nitrogen proportion in the plasma gas. The hardness of the resulting coatings is 800 to 900 kg/mm² HV.300. For Ar-H₂ plasma, however, increases in the H₂ content in the mixture of the gases remarkably enhanced the velocity and heat transfer ability of the plasma jet, with the result that the coatings showed high hardness up to 1200 HV.     

Effect of plasma fluctuations on in-flight particle parameters

The influence of arc root fluctuations in direct current (DC) plasma spraying on the physical state of the particle jet is investigated by correlating individual in-flight particle temperature and velocity measurements with the instantaneous voltage difference between the electrodes. In-flight diagnostics with the DPV-2000 sensing device involve two-color pyrometry and time-of-flight technique for the determination of temperature and velocity. Synchronization of particle diagnostics with the torch voltage fluctuations are performed using an electronic circuit that generates a pulse when the voltage reaches some specific level; this pulse, which can be shifted by an arbitrary period of time, is used to trigger the acquisition of the pyrometric signals. Contrary to predictions obtained by numerical modeling, time-dependent variations in particle temperature and velocity due to power fluctuations induced by the arc movement can be very large. Periodic variations of the mean particle temperature and velocity, up to ΔT=600 °C and Δv=200 m/s, are recorded in the middle of the particle jet during a voltage cycle. To our knowledge, this is the first time that large time-dependent effects of the arc root fluctuations on the particle state (temperature and velocity) are experimentally demonstrated. Moreover, large fluctuations in the number of detected particles are observed throughout a voltage cycle; very few particles are detected during parts of the cycle. The existence of quiet periods suggests that particles injected at some specific moments in the plasma are not heated sufficiently to be detected.     

Numerical and Experimental Investigation of Cold Spray Gas Dynamic Effects for Polymer Coating

Low melting temperature materials such as polymers are known to be difficult to deposit using traditional cold spray techniques. Computational fluid dynamics (CFD) models were created for various nozzle geometries and flow conditions. A schlieren optical system was used to visualize the density gradients and flow characteristics in the free jet impingement region. Based on the CFD models, it was determined that a diffuser placed into the carrier gas flow near the nozzle exit not only leads to lower particle impact velocity required for polymer deposition, but also provides for appropriate application of compression heating of the particles to produce the conditions necessary at impact for successful coating adhesion of these materials. Experiments subsequently confirmed the successful deposition of polyethylene powder onto a 7075-T6 aluminum substrate. Using air as the carrier gas, polyethylene particles of 53-75???m diameter and 0.94?g/cm³ density_, were cold spray deposited onto the aluminum substrate, with a critical impact velocity of 191?m/s. No apparent melting of the polymer particles was observed. Refinements to these concepts are currently under investigation and a patent disclosure for the idea is pending.