颗粒在超音速等离子喷涂中的流动特性及熔化细化历程

在等离子喷涂多物理场耦合的基础上考虑了颗粒所受的热泳力和压力梯度力,对超音速等离子喷涂过程中颗粒运动的特性进行研究,利用SprayWatch-2i对飞行颗粒进行在线监测并与计算值进行对比。结果显示:距喷嘴出口90～100 mm时,颗粒的温度和速度最高,是超音速等离子喷涂的最佳喷涂距离。在0.4 ms时,颗粒中心最低温度达到3000 K,已高于8YSZ的熔点,在这个时候颗粒已经完全熔化。颗粒在等离子射流中发生细化,在距离喷嘴出口100 mm处直径小于5μm的颗粒所占比例超过了50%,颗粒主要发生的是振动破碎。实验验证了超音速等离子喷涂数值模拟的正确性,小直径颗粒的计算值更接近实验值。     

[例15]高速电弧喷涂技术在机件再制造中的应用

正高速电弧喷涂是在普通电弧喷涂枪的基础上,通过特殊设计的高压气体喷嘴产生的超音速气流将熔化的金属材料雾化成微滴,并高速沉积到工件表面形成致密涂层的表面工程技术。喷涂铁基熔滴速度在喷枪出口附近由传统喷涂的100m/s,提高到250m/s,结合强度平均提高约10MPa。该技术     

超音速电弧喷涂粒子速度的测定

在电弧喷涂中,喷涂粒子的速度是影响涂层质量的重要因素,超音速电弧喷涂系统利用超音速气体雾化,加速喷涂粒子,增加喷涂粒子的速度,从而改善了涂层质量,采用Ｋｏｄａｋ１０１２型高速运动分析仪对喷涂粒子的速度进行了测定,结果表明,喷涂φ２．２ｍｍＴ８钢丝时,喷涂粒子的平均速度可达到２９１ｍ／ｓ。     

超音速等离子喷涂过程颗粒运动特性（英文）

在对等离子喷涂多物理场特性分析的基础上,考虑拖曳力、热泳力、压力梯度力等受力,对颗粒的运动特性进行研究。利用SprayWatch-2i对飞行颗粒进行在线监测并与计算值进行对比。结果表明:距喷嘴出口90~100 mm是超音速等离子喷涂的最佳喷涂距离。超音速等离子喷涂颗粒所受气流的拖拽力要大于普通等离子喷涂。当距离喷嘴出口90 mm处时,颗粒的最低温度达到3800 K,已高于Al_2O_3的熔点,此时颗粒已完全熔化。颗粒在等离子射流中发生细化,在距离喷嘴出口100 mm处直径小于5μm的颗粒所占比例超过了50%,颗粒主要发生的是振动破碎。通过实验验证了超音速等离子喷涂数值模拟的正确性。     

多功能超音速火焰喷涂喷枪拉伐尔喷嘴的设计

粒子的速度和温度对多功能超音速火焰喷涂层的性能有决定性作用,它取决于经拉伐尔喷嘴加速后的焰流速度和温度。文中计算了拉伐尔喷嘴入口、喉部与出口三个特征截面的燃气速度和温度,并对喷嘴特征尺寸和型面进行了优化设计。保证在氧气和空气助燃状态下都能达到超音速,实现了焰流速度与温度在大范围内的连续可调。并在喷嘴的出口形成负压以实现送粉的通畅,焰流速度调节范围约为1400～2150m／s．温度调节范围约为1400～2600℃,可满足金属、合金和陶瓷材料的喷涂要求。     

高速电弧喷涂熔滴速度的数值模拟及试验

熔滴速度是电弧喷涂涂层性能的主要影响因素之一。本基于空气动力学和二相流流体力学理论建立了高速电弧喷涂雾化气流化和熔滴速度的数学模型，并进行了数值模拟；同时用试验方法测试了气流速度及Al,3Cr13熵滴在不同喷涂距离处的平均速度；数值计算结果与试验数据基本吻合。结果表明，雾化气流的速度和距喷嘴一定距离内将保持初始速度（约650m/s)，然后随喷涂距离的增大而衰减，这与超音速气流通过Laval喷管后所产生的膨胀波和压缩波相互作用有关；熔滴在雾化飞行过程中经历了先加速后减速的过程，小熔滴能在较短的距离内被加速到最大速度；达到最大速度之后，小熔滴由于惯性力较小而迅速减速，而大熔滴则因较大的惯性力而减速不明显；熔滴速度的变化是由熔滴的Reynolds数决定的。Al和3Cr13熵滴的最大速度在0．3m喷涂距离之内均超过音速。     

双丝电弧喷涂Ni-Al过程金属熔滴行为

对双丝电弧喷涂Ni-Al过程进行了研究。利用VOF双相流模型和标准k-ε模型,分析了不同直径熔滴在不同雾化气体压力下的变形和破碎过程,以及Weber数对熔滴破碎过程的影响。同时对不同速度的熔滴撞击基体的变形和凝固温度场进行了详细分析,揭示了双丝电弧喷涂粒子速度的飞行动力学规律。计算结果显示:双丝电弧喷涂熔滴破碎形式为一次破碎和二次破碎形式及爆炸式破碎形式。Weber数与雾化气体压力呈近似线性关系。提高雾化气体压力可以提高熔滴飞行速度,改善熔滴雾化效果。双丝电弧喷涂粒子在雾化气体中的初始加速较快,然后趋于平缓和稳定。当喷涂粒子加速飞行距离为200 mm,且粒子的粒度范围为5~50μm时,喷涂粒子的飞行速度仅能达到初始气流速度的15%~45%。熔滴的形态、凝固层变化和温度场变化一致,并获得了熔滴冷却速度范围为3.1×107~7.6×107 k/s。     

喷嘴出口直径对冷喷涂射流流场及基板最佳位置影响的数值分析

冷喷涂过程中,喷嘴出口后射流流场的波系结构对粒子冲击基板时的速度有很大影响.利用CFD软件Fluent对不同出口直径的喷嘴后单相自由射流和两相冲击射流流场进行了模拟计算.结果表明,喷嘴出口直径对冷喷涂射流流场及粒子冲击速度会产生一定的影响;在喷嘴出口与基板之间存在一个最佳距离使得粒子能够获得较大的冲击速度,该最佳距离会随喷嘴出口直径的增加而线性增加.     

冷喷涂工艺中射流过程的数值模拟研究

在冷喷涂材料表面改性技术研究中，喷嘴出口的超音速气流射流流场直接影响喷涂的效果和喷涂质量，故喷嘴出口后射流的流场数值计算分析非常重要。通过对数学模型的工程简化，完成气相射流流场的数值模拟，给出了可压缩气体压力、温度和速度分布，根据计算结果，完成对实验的优化设计。     

喷嘴下游形状对冷喷涂粒子加速行为影响的数值模拟研究

目的 冷喷涂过程中喷嘴内流道关键尺寸是影响粒子加速的重要因素。虽然喷嘴下游长度与扩张比是喷嘴的2个最重要参量,但目前对喷嘴优化设计的研究仍有深入空间,比如喷嘴下游形状。方法 本文利用计算流体动力学的软件ANSYS/Fluent进行数值模拟研究,在与传统锥形下游相比较后,设计了钟形下游与喇叭形下游喷嘴。同时研究了喷嘴下游形状对气流与粒子加速行为的影响。结果 对于钟形下游喷嘴,其气流速度在过喉部后迅速增加到较高数值,随后变化缓慢;对于喇叭形下游喷嘴,其气流速度在过喉部后先增加缓慢,直至截面积开始快速增加时,气流速度迅速增加;喷嘴下游形状对粒子撞击基板时的速度有一定影响,且随着喷嘴下游长度和粉末粒度的变化而改变。对于Cu粉,当为下游100 mm短喷嘴时,锥形下游喷嘴对10~20 μm的粒子加速效果最好。当粉末在20 μm以上时,喇叭形下游喷嘴的加速效果最好。对于下游220 mm长喷嘴,钟形下游喷嘴对10~30 μm的粒子加速效果最好。当粉末在30 μm以上时,锥形下游喷嘴的加速效果最好。对于Al粉,当为下游220 mm长喷嘴时,钟形下游喷嘴对10~50 μm的粒子加速效果最好,喇叭形下游喷嘴加速效果最差。结论 冷喷涂喷嘴下游形状对气体与粒子加速有显著的影响。     

A Study of Wire Breakup and In-Flight Particle Behavior During Wire Flame Spraying of Aluminum

Although wire flame spraying has been used for many years, there has been relatively little attention given to understanding the process dynamics. In this work, imaging of the molten wire tip, particle imaging using the Oseir SprayWatch system and particle capture (wipe tests) have all been employed to quantify plume behavior. Aluminum wire feedstock is melted and then breaks up close to the exit of the spray nozzle in a non-axisymmetric manor. The mean velocity and diameter of the particles detected by the SprayWatch system change little with standoff distance with values of approximately 280 m/s and 70 µm, respectively, for the spray parameters employed. The particle diagnostic system could not detect particles ?45 µm in diameter, and it is estimated that these account for no more than 53% of the sprayed material. Overall, wire flame spraying generates a surprisingly stable particle stream.     

低温爆炸喷涂WC涂层的理论与试验研究

为解决爆炸喷涂过程中WC颗粒由于高温导致的脱碳问题,本文设计了一种带有拉瓦尔喷管的爆炸喷涂装置,利用该装置进行了爆炸喷涂WC涂层的理论和试验研究。基于等熵流的数值计算结果表明,喷管可以有效降低载气的温度,并可将WC颗粒加速至1000m/s以上。利用压力传感器测试了喷管入口和出口处的压力,计算了气流马赫数。在喷管出口处采用激光遮挡法测试了粒子的速度,测试结果与理论计算结果具有较好的一致性。采用扫描电镜、金相显微镜和XRD对碳化钨颗粒、涂层进行了表征,结果表明喷管的使用可以有效避免脱碳现象。该研究为解决爆炸喷涂碳化钨过程中的脱碳问题提供了一种新思路。     

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.     

高速电弧喷涂枪结构优化的试验

试验优化了一种高速电弧喷涂枪的结构.用SprayWatch-2i热喷涂监测系统测试了喷枪的雾化熔滴速度,并研究了Al涂层和3Cr13涂层的组织和性能.结果表明,丝材和导电嘴的存在,严重扰动了雾化气流流态,两丝夹角和丝交点离喷管出口距离也显著影响了喷管气流场的分布特征;改进设计喷枪的雾化熔滴速度显著提高,其3Cr13雾化熔滴的最高速度达到210 m/s;涂层组织和力学性能也得到改善,喷涂Al涂层和3Cr13涂层时,结合强度分别提高了55%和39%,硬度值也分别增加了26%和9%,涂层的显微组织更均匀致密,孔隙率更低.     

Spray parameters and particle behavior relationships during plasma spraying

Using laser anemometry, laser fluxmetry, and statistical two-color pyrometry, the velocity, number flux, and surface temperature distributions of alumina and zirconia particles in dc plasma jets have been determined inflight for various spraying parameters. The flux measurements emphasized the importance of the carrier gas flow rate, which must be adjusted to the plasma jet momentum depending on the arc current, nozzle diameter, gas flow rate, and gas nature. It has also been shown that the particle trajectories depend both on the particle size and injection velocity distributions and that the position and tilting of the injector plays a great role. The particle size drastically influences its surface temperature and velocity, and for the refractory materials studied, only the particles below 45 μm in diameter are fully molten in Ar-H₂ (30 vol%) plasma jets at 40 kW. The morphology of the particles is also a critical parameter. The agglomerated particles partially explode upon penetration into the jet, and the heat propagation phenomenon is seriously enhanced, particularly for particles larger than 40 μm. The effects of the arc current and gas flow rate have been studied, and the results obtained in an air atmosphere cannot be understood without considering the enhanced pumping of air when the plasma velocity is increased. The Ar-He (60 vol%) and Ar-H₂ (30 vol%) plasma jets, when conditions are found where both plasma jets have about the same dimensions, do not result in the same treatment for the particles. The particles are not as well heated in the Ar-He jet compared to the Ar-H₂ jet. Where the surrounding atmosphere is pure argon instead of air (in a controlled atmosphere chamber), he radial velocity and temperature distributions are broadened, and if the velocities are about the same, the temperatures are higher. The use of nozzle shields delays the air pumping and increases both the velocity and surface temperature of the particles. However, the velocity increase in this case does not seem to be an advantage for coating properties.     

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.     

Effect of Powder Injection Location on Ceramic Coatings Properties When Using Plasma Spray

The effect of powder injecting location of the plasma spraying on spraying properties was studied. Three different powder-injecting methods were applied in the experiment. In the first method, the particles were axially injected into the plasma flow from the cathode tip. In the second method, the particles were radially injected into the plasma flow just downstream of the anode arc root inside the anode nozzle. In the third method, the particles were radially injected into the plasma jet at the nozzle exit. The alumina particles with a mean diameter of 20 μm were used to deposit coatings. Spraying properties, such as the deposition efficiency, the melting rate of the powder particles, and the coating quality were investigated. The results show that the spraying with axial particle injecting can heat and melt the powder particles more effectively, produce coatings with better quality, and have higher deposition efficiency. 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.     

Numerical analysis of the effect of arc spray gun configuration parameters on the external gas flow

The objective of present study is to investigate the effect of different gun configuration parameters on the external gas flow by using a CFD model, and to optimize the design of an arc spray gun based on the modeling results. A converging nozzle and a converging/diverging nozzle for arc spraying were compared by calculating their flow distributions using a two-dimensional (2D) axially symmetrical model. Other parameters such as location of the intersection position and angle between the two wires were also analyzed in a three-dimensional (3D) model. The results show that the converging/diverging nozzle is more favorable for droplet atomization, moving the intersection position from the nozzle outside to the nozzle exit and using a moderate intersection angle are better for improving the flow dynamics properties. Consequently, an innovative design to upgrade the original HAS-01 type gun was put forward, and the modified design was experimentally compared with the original one by measuring the in-flight droplet size and velocity.     

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.