Simulation of particle-shock interaction in a high velocity oxygen fuel process

High velocity oxygen fuel process (HVOF) involves supersonic two-phase flow of gas-solid particles. Two kinds of shocks are formed in a typical high velocity oxygen fuel process. Adjustment of the overexpanded flow to the atmospheric pressure at the exit of the nozzle results in formation of shock diamonds while high speed flow impingement on a substrate creates bow shock. The latter is found to be responsible for deviation of the injected particles from their trajectories near the substrate, which significantly reduces the chance of some particles landing on the substrate. An attempt is made to study the behavior of particle trajectory as it interacts with the bow shock formed near the substrate. The strength and location of bow shock was found to vary for different substrate geometries and standoff distances. In this work, various particle sizes impinging on substrates with various configurations (flat, concave, and convex) are simulated and the effect of shock diamonds and bow shock on particle trajectory is studied. 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.     

Effect of Nozzle Material on Downstream Lateral Injection Cold Spray Performance

In cold gas dynamic spraying, the gas nature, process stagnation pressure and temperature, and the standoff distance are known to be important parameters that affect the deposition efficiency and coating quality. This investigation attempts to elucidate the effect of nozzle material on coatings produced using a downstream lateral injection cold spray system. Through experimentation, it is shown that the nozzle material has a substantial effect on deposition efficiency and particle velocity. It is proposed that the effects are related to complex interaction between the particles and the internal nozzle walls. The results obtained lead to the conclusion that during the particle/nozzle wall contact, a nozzle with higher thermal diffusivity transfers more heat to the particles. This heat transfer results in lower critical velocities and therefore higher deposition efficiencies, despite a noticeable reduction of particle velocities which is also attributed to particle-nozzle interactions.     

Numerical Study on the Effect of Substrate Size on the Supersonic Jet Flow and Temperature Distribution Within the Substrate in Cold Spraying

This study investigates the effect of substrate size on the supersonic jet flow regime, particle acceleration and temperature distribution within the substrate in cold spraying. A computational fluid dynamics approach is employed in the present work to achieve this objective. The simulated results show that substrate diameter has some effect on the gas flow regime and the consequent particle motion. When the substrate diameter is smaller than the nozzle exit diameter, the thickness of the bow shock formed in front of the substrate is rather small, which contributes to increase the particle impact velocity. With increasing the substrate diameter, the bow shock thickness increases gradually up to the point at which the substrate diameter is beyond the nozzle exit diameter. A further increase beyond this has almost no effect on the flow regime and the bow shock thickness. Besides, the current numerical work also reveals that the temperature distribution within the substrate can be significantly influenced by substrate thickness. With an increase in substrate thickness, the substrate temperature presents a downward trend, which means that higher inlet temperature may be required for thick substrate to achieve the same preheating effect.     

微磨料空气射流加工玻璃微流道结构研究

采用微磨料气射流加工技术(MAJM),对普通硅酸盐玻璃进行微流道槽加工.对通过掩膜加工与非掩膜加工效果相比较,得出在掩膜加工过程中,由于磨料的二次反弹冲蚀作用,所加工出的槽,槽壁较陡;在掩膜加工过程中,改变喷射距离、喷射压力、喷嘴移动速度和磨料流量四个加工参数,对加工槽的轮廓、加工过程中的材料去除率、磨料的流量效应进行研究,得出当喷射距离为4 mm,喷射压力为0.5 MPa,喷嘴移动速度为1.0 mm/s,磨料流量为0.117 g/s时,加工效果最好.通过倾斜喷射与掩膜加工相结合,研究了磨料磨粒的二次反弹冲蚀现象,为复杂三维微结构的加工,提供了一种新方法.     

Numerical Studies on the Effects of Stagnation Pressure and Temperature on Supersonic Flow Characteristics in Cold Spray Applications

Low-temperature particle coating requires supersonic flow. The characteristics of this supersonic flow are investigated using a nonlinear turbulence model. The low-temperature, supersonic particle deposition technique is valuable because its rapid and dense coating minimizes thermal damage to both particles and substrate. It has excellent potential for industrial production of low-cost thin films. Stagnation pressures and temperatures of the supersonic nozzle range from 4 < P _o < 45 bar and 300 < T _o < 1500 K, respectively. The exit Mach number, M _e, and velocity, V _e, range from 0.6 to 3.5 and 200 to 1400 m/s, respectively. The effects of stagnation pressure (P _o) and stagnation temperature (T _o) on supersonic flow impinging upon a substrate are described. In other words, the energy loss through shockwaves and shear interactions between the streaming jet and surrounding gas are quantified as functions of P _o and T _o. P _o is decreased because of friction (loss ranges from 40 to 60%) while T _o is nearly conserved. To realize the nozzle exit condition of P _e = P _amb, we demonstrate that P _o should be adjusted rather than T _o, as T _o has little effect on exit pressures. On the other hand, T _o is more influential than P _o for varying the exit velocity. Various nozzle geometries yielding different flow characteristics, and hence, different operating conditions and coating performances are investigated. The corresponding supersonic flows for three types of nozzles (under-, correctly , and over-expanded) are simulated, and their correctly expanded (or shock-free) operating conditions are identified. Diamond shock structures induced by the pressure imbalance between the exiting gas and the surrounding atmosphere are captured.     

Investigation on in-flight particle velocity in supersonic plasma spraying

0Introduction Asakindofsurfaceengineeringtechnology,thermal sprayingcanprovideprotectiveorfunctionalcoatings whicharewidelyusedinmanyindustrieslikechemistryin dustry,papermaking,electricengineering,powerplant, aviation,automobileproducing,steelmill,glass…     

Influence of Flow Swirling and Exit Shape of Barrel Nozzle on Cold Spraying

Traditionally, in cold spray two-phase supersonic jet formed with the help of converging-diverging nozzle are used. In this study an alternative design of cold spray nozzle is proposed in which a high velocity two-phase flow is created using an intense flow swirling in a constant section barrel (cylinder) with double-edged bevel exit. As a result, a high velocity gas-powder mixture jet is produced presenting a fan-shaped jet spreading at a large angle in one plane and approximately of equal size along the normal to this plane. This results in greater angles of particle deposition and, hence, in larger deposition widths, with the maximum width of deposition spot reaching 25 barrel diameters. The performed experimental study proves the new nozzle design to be appropriate for deposition of cold-sprayed coatings.     

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.     

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.     

后混合水射流喷丸工艺对18CrNiMo7-6渗碳钢表面性能的影响

目的探究不同后混合水射流喷丸工艺对18Cr Ni Mo7-6渗碳钢表面性能的影响。方法运用超景深三维显微系统、三维表面形貌测量系统、X射线残余应力分析仪及HV-1000显微硬度计等,对后混合水射流喷丸前后试样的表面形貌、表面粗糙度、残余应力及显微硬度随层深的变化情况进行分析。结果后混合水射流喷丸时,弹丸和水会对试样表层产生一定的冲蚀、磨损、剪切作用,使试样表面产生新的凹坑。表面粗糙度Ra值随着喷射压力P及喷射靶距H的增加而增大,随着喷嘴移动速度v的增加而减小。试样显微硬度最大值都出现在表面,且随层深的增加,硬度值逐渐减小,喷射压力P=300 MPa时,表面硬度值达到62.8HRC,比试样初始表面硬度值增加了7.35%。试样材料所能引入的残余压应力具有固有最大值σmirs,当引入的残余压应力未达到σmirs时,所产生的最大残余压应力值σmcrs随喷射压力P的增加而增大,但随喷射靶距H和喷嘴移动速度v的改变变化不大。当引入的残余压应力达到σmirs时,所产生的最大残余压应力值σmcrs即为σmirs,不再改变,但是最大残余压应力距表面距离值zm仍会随着喷射压力P的增加而增大。结论后混合水射流喷丸后,试样表面粗糙度变化较大,表层显微硬度有一定提高。残余应力的分布主要与喷射压力P有关,而与喷射靶距H和喷嘴移动速度v关系不大。     

Abrasive waterjet micro-machining of channels in metals: Comparison between machining in air and submerged in water

Abrasive water jet technology can be used for micro-milling using recently developed miniaturized nozzles. Abrasive water jet (AWJ) machining is often used with both the nozzle tip and workpiece submerged in water to reduce noise and contain debris. This paper compares the performance of submerged and unsubmerged abrasive water jet micro-milling of channels in 316L stainless steel and 6061-T6 aluminum at various nozzle angles and standoff distances. The effect of submergence on the diameter and effective footprint of AWJ erosion footprints was measured and compared. It was found that the centerline erosion rate decreased with channel depth due to the spreading of the jet as the effective standoff distance increased, and because of the growing effect of stagnation as the channel became deeper. The erosive jet spread over a larger effective footprint in air than in water, since particles on the jet periphery were slowed much more quickly in water due to increased drag. As a result, the width of a channel machined in air was wider than that in water. Moreover, it was observed that the instantaneous erosion rate decreased with channel depth, and that this decrease was a function only of the channel cross-sectional geometry, being independent of the type of metal, the jet angle, the standoff distance, and regardless of whether the jet was submerged or in air, in either the forward or backward directions. It is shown that submerged AWJM results in narrower features than those produced while machining in air, without a decrease in centerline etch rate.     

磨料射流铣削工艺参数优化

目的对表面粗糙度和材料去除率作为输出参数的磨料水射流铣削45#钢过程进行研究,旨在寻找最优加工参数。方法对射流去除材料机理进行了分析,设计并进行了以磨料粒度、射流压力、横向进给距离、靶距为加工工艺参数的田氏正交实验。采用Minitab对不同实验参数组合下磨料水射流加工45#钢的表面粗糙度、材料去除效率进行了数据分析,并从材料去除机理方面,对4种加工工艺参数对于铣削表面质量和材料去除效率的影响程度和影响趋势,以及各因素之间的交互作用进行了分析。结果对射流铣削面表面粗糙度影响较显著的因素是横向进给距离,射流压力次之;对于材料去除效率,磨料粒径的影响最显著,横向进给距离次之。结论综合材料去除效率和表面粗糙度值,选出最优加工参数:磨料粒径2000目,射流压力120~160 MPa,喷嘴横移距离1.0~1.5 mm,靶距约30 mm。     

送粉位置对等离子喷涂YSZ粒子熔化状态的影响

研究了等离子喷枪采用3个送粉位置时距离喷嘴90mm处YSZ粒子的熔化状态(包括粒子温度及速度、粒子粒度分布、粒子撞击基体后的扁平形貌),其中送粉位置1和2为枪外送粉,送粉孔轴线距离喷嘴端面分别为9mm和3.5mm,送粉位置3为枪内送粉,送粉孔轴线距离喷嘴端面5mm。结果表明,送粉位置沿等离子射流逆流方向移动时(分别对应送粉位置1、2和3),粒子熔化效果明显增强,粒子扁平化程度提高;涂层中未熔颗粒和孔隙百分比由33.25%减小至10.13%,涂层结合强度由16.25MPa增加至37.25MPa,涂层表面剥落20%时经历的热震次数由35次增加至130次;采用送粉位置3时,喷涂功率33.60kW,低于其他送粉位置的电弧功率(46.75kW),实现了低功率制备高性能YSZ涂层。     

Numerical Modeling of Suspension HVOF Spray

A three-dimensional two-way coupled Eulerian-Lagrangian scheme is used to simulate suspension high-velocity oxy-fuel spraying process. The mass, momentum, energy, and species equations are solved together with the realizable k-ε turbulence model to simulate the gas phase. Suspension is assumed to be a mixture of solid particles [mullite powder (3Al₂O₃·2SiO₂)], ethanol, and ethylene glycol. The process involves premixed combustion of oxygen-propylene, and non-premixed combustion of oxygen-ethanol and oxygen-ethylene glycol. One-step global reaction is used for each mentioned reaction together with eddy dissipation model to compute the reaction rate. To simulate the droplet breakup, Taylor Analogy Breakup model is applied. After the completion of droplet breakup, and solvent evaporation/combustion, the solid suspended particles are tracked through the domain to determine the characteristics of the coating particles. Numerical simulations are validated against the experimental results in the literature for the same operating conditions. Seven or possibly eight shock diamonds are captured outside the nozzle. In addition, a good agreement between the predicted particle temperature, velocity, and diameter, and the experiment is obtained. It is shown that as the standoff distance increases, the particle temperature and velocity reduce. Furthermore, a correlation is proposed to determine the spray cross-sectional diameter and estimate the particle trajectories as a function of standoff distance.     

沉积时间对钼薄膜结构和热疲劳性能的影响

采用直流磁控溅射在不同沉积时间条件下制备钼薄膜,并用电子束热负荷装置对薄膜进行热疲劳性能试验,利用X射线衍射仪(XRD)对其结构和残余应力状态进行测试分析,用扫描电镜(SEM)对钼薄膜热疲劳前后形貌进行表征。结果表明:薄膜沿(110)取向择优生长,呈柱状晶结构,薄膜内存在张应力,残余应力随沉积时间增加而逐渐减小。热循环试验后均未出现薄膜脱落现象,但均产生表面裂纹。随着沉积时间由4h增加至8h,疲劳裂纹由穿晶断裂的直线裂纹转变为沿晶开裂的曲折裂纹,同时成膜过程中的退火效应使薄膜的晶粒长大、晶粒结构更加趋于完整、残余应力减小,从而使薄膜疲劳裂纹减小。     

Supersonic Nozzle Flow Simulations for Particle Coating Applications: Effects of Shockwaves,Nozzle Geometry,Ambient Pressure,and Substrate Location upon Flow Characteristics

Characteristics of supersonic flow are examined with specific regard to nano-particle thin-film coating. Effects of shockwaves, nozzle geometry, chamber pressure, and substrate location were studied computationally. Shockwaves are minimized to reduce fluctuations in flow properties at the discontinuities across diamond shock structures. Nozzle geometry was adjusted to ensure optimal expansion (i.e., P _exit = P _ambient), where shock formation was significantly reduced and flow kinetic energy maximized. When the ambient pressure was reduced from 1 to 0.01316 bar, the nozzle’s diverging angle must be increased to yield the optimum condition of minimized adversed effects. Beyond some critical distance, substrate location did not seem to be a sensitive parameter on flow characteristics when P _amb = 0.01316 bar; however, overly close proximity to the nozzle exit caused flow disturbances inside the nozzle, thereby adversely affecting coating gas flow.     

基于响应曲面的Ar气氛超音速等离子喷涂钛涂层

采用超音速等离子喷涂可低成本、高效率制备钛涂层。采用响应曲面法（RSM）中的Box-Behnken（BBD）设计分析了Ar流量、功率、喷涂距离3个因素与超音速等离子射流中钛粒子飞行速度和温度的交互性,利用SEM和显微硬度计研究了钛涂层的微观结构和显微硬度。结果表明：建立的线性模型可靠,喷涂距离对粒子飞行速度和温度影响最大,且随喷涂距离增加粒子飞行速度减小温度增加,而Ar流量和功率对粒子飞行速度和温度的影响与喷涂距离相反。超音速等离子喷涂制备出的钛涂层硬度较低,且呈多孔结构,随粒子飞行速度增加孔隙率降低。     

Parameters of the Gas-Powder Supersonic Jet in Cold Spraying Using a Mask

Behavior of an axisymmetric cold spray supersonic gas-particle jet passing through an axisymmetric aperture having a diameter smaller than the jet dimensions is considered. The particular case studied in the paper is the interaction between a supersonic jet delivered by a nozzle with a 7.8 mm exit diameter and a solid plate with a 3 mm aperture. The gas and aluminum particle parameters after passing through the aperture are experimentally and numerically studied. Under conditions used in this study, the decelerating influence of the aperture on the aluminum particle velocity is found negligible for spray distances less than 40 mm.     

Atmospheric plasma spraying of yttria-stabilized zirconia coatings with specific porosity

The interdependence between plasma spray process parameters and porosity of YSZ coating microstructures was investigated with simultaneous consideration of the deposition efficiency. Based on a factorial experimental plan, the argon plasma gas flow, the current, the interaction of argon flow and current, and the spray distance for the Triplex II plasma gun were found to yield the main contributions to porosity as well as to deposition efficiency.Each of these three process parameters has a significant individual effect on the in-flight particle velocities and temperatures. The contribution to the effects on porosity arises almost exclusively from the particle temperature. Regarding the deposition efficiency, the larger contribution originates from the particle velocity.To achieve a targeted high porosity at reasonable deposition efficiency a simple linear regression model was applied yielding an argon flow of 50 slpm and a current of 470 A at a spray distance of 200 mm as the optimum parameter set. The average particle temperature estimated for this optimum is just above the melting temperature. At this setting, a porosity of 17.7% and a deposition efficiency of 32.5% may be expected.At a greater spray distance and lower power density (lower current and/or higher argon plasma gas flow) the deposition efficiency was observed to drop considerably. The cooling of the particles here becomes critical, i.e. the particles are only partly molten. This was verified by an analysis of the density distributions of measured in-flight particle temperatures.     

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.