Simulation of Arc Root Fluctuation in a DC Non-Transferred Plasma Torch with Three Dimensional Modeling

It is well known that the coating quality of plasma spraying is strongly influenced by the instability of jets in the plasma spray, which is due to arc root fluctuation. Three dimensional (3D) unsteady-state modeling was employed in this research to analyze the arc root fluctuation in a DC non-transferred plasma torch. Numerical calculations on the distributions of gas temperature and velocity in the plasma torch were carried out using argon as the plasma gas. The electrical current density and potential were also discussed. The results indicate that the fluctuation of arc inside the plasma torch is mainly induced by the movement of the arc root on the anode surface. The arc root moves downstream with the flow of gas, and simultaneously the arc is bent by electromagnetic force. When the arc bends close enough to the anode boundary, a new arc root is formed somewhere upstream of the current attachment. In this paper the nature of the arc root fluctuation is presented, and also it is demonstrated that the voltage-drop calculated is larger than that measured experimentally because the plasma inside the torch has some deviation from the local thermodynamic equilibrium state hypothesis used in the current study.     

Effects of Anode Arc Root Fluctuation on Coating Quality During Plasma Spraying

To obtain a coating of high quality, a new type of plasma torch was designed and constructed to increase the stability of the plasma arc and reduce the air entrainment into the plasma jet. The torch, called bi-anode torch, generates an elongated arc with comparatively high arc voltage and low arc fluctuation. Spraying experiments were carried out to compare the quality of coatings deposited by a conventional torch and a bi-anode torch. Alumina coatings and tungsten carbide coatings were prepared to appraise the heating of the sprayed particles in the plasma jets and the entrainment of the surrounding air into the plasma jets, respectively. The results show that anode arc root fluctuation has only a small effect on the melting rate of alumina particles. On the other hand, reduced air entrainment into the plasma jet of the bi-anode torch will drastically reduce the decarbonization of tungsten carbide coatings.     

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.     

Modeling the restrike mode operation of a DC plasma spray torch

A key aspect of the operation of conventional non-transferred direct current (dc) plasma torches is the random motion of the arc inside the nozzle. Various plasma gun designs have been developed to limit the arc fluctuations without increasing the heat load to the anode wall that results in surface erosion and anode wear. However, construction of these plasma torches is highly complex, while the conventional dc plasma torch consists of a small number of elements and is simple to manufacture and maintain. A better understanding of the behavior of the arc-anode attachment and how it depends on operating conditions may help in the design and operation of conventional plasma torches so that the fluctuation of the time-voltage, and therefore the time-enthalpy variation, is as low as possible with a fluctuation frequency adapted to the time characteristic of the powder particles in the plasma jet. This study deals with a three-dimensional (3D) time-dependent modeling of the arc and plasma generation in such a torch operating under the so-called “restrike” mode. The latter is characterized by rather large voltage fluctuations, corresponding to a broad range of conditions used in the manufacturing of plasma coatings. The mathematical model is based on the simultaneous solution of the conservation equations of mass, momentum, energy, electric current, and electromagnetic equations. These make it possible to predict the effect of operating parameters of the plasma torch on the motion of the anode root attachment over the anode surface and the time-evolution of arc voltage and flow fields in the nozzle. 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.     

Fluid dynamic effects on plasma torch anode erosion

Anode erosion in plasma spray torches results in coating deterioration. The usable life of a torch anode is strongly dependent on the fluid dynamic behavior of the plasma inside the torch, which in turn depends on the geometric design of the anode and the operating parameters. To study the relative importance of these effects, cold flow investigations have been performed with a torch having a glass anode with the same geometric dimensions as a commercial plasma torch. The density differences between the arc and the cold gas were simulated by injecting heated helium from the tip of the cathode into the cold argon gas flow from the regular gas injector. Flow visualization was achieved by seeding the flow with micron-sized particles. A finite-element computational fluid dynamics code was used to simulate the cold flow structure. The results were compared with erosion patterns observed with an actual plasma torch. The results indicate that recirculation eddies inside the torch will force a preferred anode attachment, which is different for different gas injectors. The minimization of such recirculation regions by appropriate fluid dynamic design will result in more random attachment of the arc and prolonged anode life. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

等离子喷枪的注气方式对电弧不稳定性的影响

基于局域热力学平衡假设,建立三维非稳态的磁流体动力学模型,在给定气流量和工作电流的条件下,采用数值模拟方法研究不同注气方式对直流非转移弧等离子喷枪内Ar-H2等离子电弧的波动行为和等离子特性的影响。结果表明,与直流进气相比较,采用旋转方式进气时,喷枪内等离子体流的流线呈现出明显的螺旋状分布,电弧波动信号呈现出更高的平均电压和较高波动频率,电弧附着随时间变化呈现出圆周运动现象;喷枪内等离子体的最大温度出现在阴极尖端附近,采用旋转方式注气时等离子平均最大温度较低,而等离子体的速度沿中心轴方向呈现出先增加后减小的趋势。喷枪出口处等离子体的温度与速度同样显示出波动分布,两种进气方式得到的温度分布基本相似,采用旋转方式注气时等离子平均最大速度分布相对较高。     

旋转电弧加热在环状工件内壁沉积金刚石涂层

为实现在大口径环状工件内壁沉积金刚石涂层,研制一种能够产生旋转电弧的分体式等离子炬。利用活动阳极形成的伞状电弧帽,改变平行于轴且向下吹的工作气体旋转电离的方向,使其垂直吹向环状工件内壁。测试不同阳极直径下电弧的工作参数,并用相机拍摄相应的电弧形貌。进行金刚石涂层沉积试验,在内径为180?mm的硬质合金拉拔模具和内径为100?mm的石墨内表面沉积出高质量的金刚石涂层。利用拉曼光谱仪和扫描电镜对涂层的成分、表面形貌等进行测试分析。已沉积金刚石涂层的硬质合金模具成功应用于超高压电缆铝护套的拉拔设备中。      

铝合金变极性等离子弧穿孔横焊焊缝成形规律分析

以穿孔等离子弧焊接过程中形成的穿孔熔池为研究对象,根据熔池热源形态的特点,采用数值模拟与试验相结合的手段研究横焊位置下的铝合金变极性等离子弧焊缝成形.由于焊接速度波动和工件厚度的影响,体热源作用下的穿孔熔池背面存在最高温度点和最大熔宽截面相背离的现象;因此通过对穿孔熔池背面进行分区和定义,提出温宽偏离度概念,即熔池背面最高温度点和最大熔宽截面的偏离程度,用以描述穿孔熔池状态及焊缝成形;通过调节焊枪角度来改变焊接过程中的温宽偏离度,在其它参数不变的情况下减轻重力在焊接过程中对焊缝成形的影响,实现变极性等离子弧穿孔焊接在横焊位置上的良好成形.     

Comparisons of Two Models for the Simulation of a DC Arc Plasma Torch

The hypothesis of local thermal equilibrium (LTE) in thermal plasma has been widely accepted. Most of the simulation models for the arc plasma torch are based on the hypothesis of LTE and its results indicate good validity to mimic the pattern of plasma flow inside a plasma torch. However, according to the LTE hypothesis, electrical conductivity near electrodes is significantly lower because of the low gas temperature. Consequently, it is difficult for electrical current flows to pass between the anode and cathode. Therefore, the key subject for a model concentrating on the LTE assumption is to deal with the low electrical conductivity near the electrodes. In this study, two models determining the electrical conductivity at the vicinity of the electrodes with two different assumptions were used to calculate the flow patterns inside a non-transferred DC arc plasma torch. Gas temperature, velocity, voltage drop, and heat energy of the plasma arc were compared between the two models. The results indicated that the plasma arc inside the plasma torch fluctuates, as simulated by both models. It seems that the model can obtain comparable accuracy with the experimental results if the plasma gas electrical conductivity is determined by nominal electron temperature.     

Arc instabilities in a plasma spray torch

The control over coating quality in plasma spraying is partly dependent on the arc and jet instabilities of the plasma torch. Different forms of instabilities have been observed with different effects on the coating quality. We report on an investigation of these instabilities based on high-speed end-on observation of the arc. The framing rate of 40,500 frames per second has allowed the visualization of the anode attachment movement and the determination of the thickness of the cold-gas boundary layer surrounding the arc. The images have been synchronized with voltage traces. Data have been obtained for a range of arc currents, and mass flow rates for different gas injectors and for anodes displaying different amounts of wear. The analysis of the data has led to quantitative correlations between the cold-gas boundary layer thickness and the instability mode for the range of operating parameters. The arc instabilities can be seen to enhance the plasma jet instabilities and the cold-gas entrainment. These results are particularly useful for guiding plasma torch design and operation in minimizing the influence of plasma jet instabilities on coating properties.     

聚焦气流对大功率等离子弧焊炬工作稳定性的影响

为了进一步探讨聚焦气流对大功率等离子弧焊炬工作稳定性的影响,设计了两种水冷探头,对YJ-700大功率等离子焊炬(带有环形水冷聚焦罩)和通用型LH-300焊炬,在各种参数、喷嘴孔径时,弧对探头表面的径向压强分布、弧对探头的传导热及探头表面的径向温度分布进行了测定。虽然弧对探头的作用力不同于弧对熔池的作用力,但可以用来定最描述弧的刚性及穿透力。探头表面的温度分布及弧对探头的传导热,虽然并非为弧的真实径向温度分布及弧对阳极工件的传导热,但可以说明弧的热特性及阳极工件的热传导规律。试验证明,聚焦气流的存在,不但可以抑制双弧的产生,而且能使弧特性(包括压力特性及热特性)对离子气流量的变化不敏感;探头表面的温度分布对喷嘴直径的变化也不敏感,有利于焊炬的工作稳定性。     

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.     

强电流直流扩展弧金刚石沉积技术的研究

采用自行研制的Q-500金刚石涂层沉积设备,同时采用两种不同结构的等离子体炬对经酸碱二步法处理后的硬质合金钻杆(YF06)分别进行了金刚石涂层的沉积,并对沉积后的样品表面进行了分析和研究.初步的研究结果表明:等离子体炬的结构对金刚石涂层的沉积影响很大,改进前的等离子体炬所产生的弧柱不稳,导致沉积温度波动幅度大,而改进后的等离子体炬能产生稳定的弧柱,沉积温度变化不大并且沉积的金刚石涂层具有大面积、均匀性好和高质量的特点.     

Research on generation of micro-plasma arc and its power intensity

In order to realize practical application of micro-plasma arc welding, the generation method of submillimetre-sized plasma arc was developed through controlling the DC electrical discharge in the range from glow to arc discharge with use of an experimental torch device and a newly designed high-voltage power source. The electrical discharge was established between the cathode of a tungsten electrode and the anode of stainless steel sheet through a plasma constriction nozzle made from heat-resistant material of boron nitride. It is shown that the micro-plasma arc with use of a submillimetre diameter nozzle is stabilized as the size of the plasma beam corresponds to the nozzle diameter and decrease in the nozzle diameter increases the power density of micro-plasma arc at the anode surface. A submillimetre-sized melt spot can be obtained with the current of 1 A and discharge time of 5 ms.     

Arc voltage fluctuations: Comparison between two plasma torch types

In plasma spray process, the arc root lifetime at the anode wall is limited and new arc roots are continuously created according to restrike, takeover or mixed modes. It results in voltage fluctuations at frequencies in the few thousands hertz range. Such phenomena play a key role in the spray process through the fluctuations of particle trajectories and correspondingly their velocity and temperature time-variations at impact. They have been intensively studied for Ar-H₂ plasmas produced by Sulzer-Metco PTF4 and Praxair plasma torch. However, the arc movement depends on several parameters such as the plasma spray parameters, the plasma gas mixture composition, the electrode geometry and the plasma forming gas injector design. In this paper, two commercial plasma torches, PTF4 and 3MB, running both with argon-hydrogen gas mixtures, have been studied. This research work shows the geometry effect and the cold boundary layer thickness on the arc voltage fluctuations, as well as the particle thermal treatment and the resulting coating properties.     

An optical sensing system for seam tracking and weld pool control in gas metal arc welding of steel pipe

A visual sensing system was developed for automatic gas metal arc welding (GMAW) of the root pass of steel pipe. The system consisted of a vision sensor that consisted of a charge-coupled device (CCD) camera and lenses, a frame grabber, image processing algorithms, and a computer controller. A specially designed five-axis manipulator was used to position the welding torch and to provide the vision sensor with automatic access to view the welding position. During the root pass welding, an image of the weld pool and its vicinity was captured using the camera without interference of the intensive arc light by viewing at the instance of a short-circuit of the welding power. The captured image was then processed to recognize the weld pool shape. For seam tracking, the manipulator was used to adjust the torch position based upon the pool image to the groove center. The measured gap size was used to determine the appropriate welding conditions to obtain sound penetration. The welding speed was chosen using fuzzy logic with the knowledge of a skilled welder and measured gap. The automatic welding equipment demonstrated that both welding conditions and torch position could be appropriately controlled to obtain a sound weldment and a good seam tracking capability.     

双电弧共熔池脉冲GMAW电源的复合外特性

利用自行研制的双电弧软开关脉冲GMAW装备,从弧长控制、熔滴过渡以及焊缝成形等角度对双电弧脉冲GMAW三种复合外特性进行了试验研究.结果表明,主机I-I,从机I-I复合外特性的熔滴过渡均匀,可控性好,但电弧自调节作用较差,适合薄板高速焊接;主机U-I,从机U-I复合外特性的熔滴过渡可控性欠佳,但电弧自调节作用较强,较适合大参数厚板焊接;主机I-I,从机U-I复合外特性中主机的熔滴过渡可控性好,电弧自调节能力有限,从机的电弧自调节作用较强,但熔滴过渡可控性欠佳.     

Characteristics of a Plasma Torch Designed for Very Low Pressure Plasma Spraying

Unlike atmosphere plasma spraying (APS), very low pressure plasma spraying (VLPPS) can only weakly heat the feed materials at the plasma-free region exit of the nozzle. Most current VLPPS methods have adopted a high power plasma gun, which operates at high arc currents up to 2500?A to remedy the lower heating ability, causing a series of problems for both the plasma torch and the associated facility. According to the Kundsen number and pressures distribution inside of the nozzle in a low-pressure environment, a plasma torch was designed with a separated anode and nozzle, and with the powder feed to the plasma jets inside the nozzle intake. In this study, the pressures in the plasma gas intake, in the nozzle intake and outside the plasma torch were measured using an enthalpy probe. For practice, SUS 316 stainless steel coatings were prepared at the plasma currents of 500-600?A, an arc voltage of 50?V and a chamber pressure of 1000?Pa; the results indicated that coatings with an equiaxed microstructure could be deposited?in proper conditions.     

基于微束等离子弧的焊缝跟踪

进行了基于微束等离子弧的焊缝跟踪传感可行性研究。利用微束等离子弧发生器产生非转移等离子弧，测量喷嘴与工件之间的等离子焰流的等效电阻，试验发现，该等效电阻与喷嘴距工件的高度之间有明确的对应关系，配合适当的扫描装置，可实现焊缝位置检测功能。进行厂维弧电流、喷嘴结构、电极直径、离子气流量等参数对该等效电阻的影响试验，为这种传感方式在实际应用中的各参数选择，提供了试验依据。     

Improvement of Plasma Spray Torch Stability by Controlling Pressure and Voltage Dynamic Coupling

The development of coating formation processes involving electric arcs depends on process stability and the capacity to ensure a constant reproducibility of coating properties. This is particularly important when considering suspension plasma spraying or solution precursor plasma spraying. Submicron particles closely follow plasma instabilities and have nonhomogeneous plasma treatment. Recently, it has been shown that arc voltage fluctuations in direct-current (dc) plasma torches, showing dominant fluctuation frequencies between 4 and 6 kHz, are linked to pressure oscillations in the cathode cavity of the plasma torch. In this study, first, a method to isolate the different oscillation modes in arc voltage and pressure signals using signal processing methods is presented. Second, correlations between the different modes of oscillations are analyzed following the plasma torch operating parameters. Lastly, it is shown that the use of an acoustic stub, mounted on the torch body, decreases the amplitude of arc voltage fluctuations and slightly increases the mean voltage.