Laminar Plasma Research Articles

Numerical modelling of a direct current non‐transferred thermal plasma torch for optimal performance

AbstractA laminar steady‐state 2D axisymmetric model of a direct current (DC) thermal plasma torch using a magneto‐hydrodynamic approach has been developed. The model takes into account the entire torch system comprising the plasma gas injection, the inner region of the torch, and the jet exiting into the ambient environment. Numerical results are obtained for two different power inputs chosen from published experimental data. The temperature predictions at the torch exit are found in good agreement with experimental results. Velocity analysis of the plasma jet has been presented and the impact of electromagnetic force on jet velocity is analysed. The Lorentz force arising due to the coupling of fluid motion and electromagnetic forces shoots up the jet velocity to significantly high values near the cathode tip. Temperature and velocity profiles are in good agreement with the characteristics of a long laminar plasma jet. An operating value of heat transfer coefficient (h) has been suggested for optimal torch operation, thus ensuring a low anode erosion rate and acceptable thermal efficiency. The argon torch has the maximum temperature and longest jet length among the plasma gases considered.

Modeling of the Effect of Carrier Gas Injection on the Laminarity of the Plasma Jet Generated by a Cascaded Spray Gun

In this work, the plasma generated by the cascaded SinplexProTM plasma spray gun was studied by means of numerical simulation. Special attention is given to the laminarity of the plasma flow. The simulation part is divided into two parts: arcing simulation inside the spray gun and plasma jet simulation outside the spray gun. A laminar as well as a turbulent model is used in each case. The results show that, under the investigated conditions, the internal flow of the plasma torch can be considered as laminar with low turbulence and can, hence, be regarded as quasi-laminar flow. If carrier gas is injected into the plasma jet, the ideal laminar plasma jet is often greatly affected. However, the turbulent plasma jet with low turbulence intensity generated by the cascaded SinplexProTM plasma spray gun is less affected and can remain stable, which is beneficial to the plasma-spraying process.

Flat bubble regime and laminar plasma flow in a plasma wakefield accelerator

A simple 2D model of the bubble formation in a plasma wakefield accelerator is developed and investigated. We show that in the case of a flat driver, the bubble may consist of two parts. The first part corresponds to a laminar flow where plasma electron streams do not cross and the second to the two-stream (turbulent) flow. The laminar flow turns out to be robust to the symmetry breaking. Building off of the developed model, we demonstrate that, in the case of the laminar flow and nonrelativistic plasma electrons, the transverse wakefield is absent inside the bubble even in the case of a transversely nonuniform plasma.

Study on the mechanism of plasma jet surface hardening of rail steels by using numerical method

A multi-physics coupled numerical analysis method that fully considers the hydromechanical properties of the plasma jet was created in this paper. It was used to analyze the evolution of the metallic solid-state phase transformation, and that of the stress state, during the process of laminar plasma selective quenching (LPQ). By this method, the mechanism of LPQ was investigated. It is found that the plasma impact can significantly affect the residual stress distribution, and there is a close relationship between the residual stress distribution and the hardened layer distribution; During LPQ, the material bears tremendous alternating stress within the first 10 s, then the maximum stress begins to gradually decrease; 100 s later, the stress distribution stabilizes. This study has contributed to the understanding of the LPQ mechanism of rail steel.

Theoretical study of laminar plasma quenching of rail steel using the fluid-solid-thermal coupling method

Laminar plasma quenching (LPQ) is an effective surface hardening method for improving the wear resistance of rails. Nonetheless, there has been very little systematic theoretical research on the mechanism of how rail materials are strengthened through LPQ. Therefore, based on the fluid-solid-thermal coupling method, a complete theoretical analysis model for LPQ of a U75V rail was established for this paper. The influence of the laminar plasma jet on the spatial distribution of the hardened layer was investigated, and the residual stress distribution on the rail surface during LPQ was reconstructed. The results show that the distribution of the hardened layer will significantly affect residual stress. As plasma flow rate increases, the width-depth ratio of the hardened layer first decreases and then increases. This research is of great significance for understanding the mechanism of rail plasma quenching, and for the application of fluid-solid- thermal coupled methods.

Numerical study of the effect of coflow argon jet on a laminar argon thermal plasma jet

The effects of the velocity and width in coflow argon jet inlet on the flow characteristics of laminar argon thermal plasma jet flowing into the cold air have been studied by the large eddy simulation methods. The Kelvin–Helmholtz instability between argon thermal plasma jet and coflow argon jet causes the transition from a laminar jet to a turbulent jet in the presence of coflow argon jet. Moreover, increasing the velocity and width in coflow argon jet inlet can enhance turbulent transport and provoke coherent structure in the downstream of thermal plasma jet. And the mixing characteristics between argon thermal plasma, coflow argon and ambient air are strengthened. In addition, the width in coflow argon jet inlet has a significant effect on the distribution of temperature in the upstream of thermal plasma jet. It was also found that the transition occurs in advance with the increase of velocity and width in coflow argon jet inlet.

Effects of surface texturing and laminar plasma jet surface hardening on the tribological behaviors of GCr15 bearing steel

Surface texturing (ST) and laminar plasma jet (LPJ) surface hardening were applied to GCr15 bearing steel to improve its wear resistance. Ball-on-disk reciprocating tests were then conducted under dry and oil-lubricated conditions to investigate the tribological behaviors. Morphology and evolution of the frictional interfaces were analyzed by SEM images to reveal the anti-wear mechanism. Results show that LPJ surface hardening can remarkably reduce the wear of GCr15 bearing steel by increasing surface hardness and thus decreasing the actual contact. Whether ST can improve the wear of GCr15 bearing steel mainly depends on the evolutions of frictional interface and wear mechanisms, namely the competitive relationship of stress concentration with trap effect or hydrodynamic pressure effect under dry friction or oil-lubricated condition.

The rolling-sliding wear behavior and damage mechanism of the rail steel treated by plasma selective quenching

Plasma selective quenching (PSQ) is a promising technique for creating discrete hardened spots (PSQ spots) in the surface layer of the rail to increase its service life. However, cracks occur at one side of the PSQ spots after long-term service, hindering the large-scale application of PSQ treated rails. To reveal the effect of PSQ on the wear behavior of rail steel and damage mechanism of PSQ spots, rail rollers with a thickness of 10 mm were firstly PSQ treated by a laminar plasma jet. Then rolling-sliding contact wear tests were conducted to explore the wear behavior and damage characteristics of the PSQ treated rail rollers. In addition, contact simulation of the contact wear tests was carried out. Results showed that the wear resistance of the PSQ treated rail rollers was improved by 1.26–1.75 times. Such improvement is a result of the increased hardness of the PSQ spots and the reduced plastic deformation of the substrate between two adjacent PSQ spots. Severe cracks and breaks were observed at the entry side of the PSQ spots while no obvious damage appeared at the exit side. The mechanism of such a special pattern of damage was that there was a strain difference at the boundary between the entry side of hardened PSQ spot and the soft substrate while no different strains were produced at the exit side. This paper clarified the damage mechanism of the PSQ spots under the rolling-sliding contact conditions, providing the basis for further improvement on the performance of the PSQ treated rail steel.

In-situ heating effect of laminar plasma jet during Mo coatings deposition

Laminar plasma has the characteristics of a long plasma jet, low velocity, and high temperature, which make it able to cover and heat the substrate during the spraying process. The results show that when the standoff distance is short, the coating is strengthened by in-situ heating of the plasma jet and shows higher erosion resistance at high erosion angles and bulk-like material tendency. When the standoff distance is long, the substrate temperature is low, the coating shows higher erosion resistance at a lower erosion angle.

Non-equilibrium modeling on the plasma–electrode interaction in an argon DC plasma torch

A 2D two-temperature chemical non-equilibrium model considering both cathode and anode space-charge sheath is applied to investigate the plasma–electrode interaction in a laminar argon DC plasma torch working at atmospheric pressure. In order to validate the model, the predicted arc voltage is compared with experimental values under different working conditions, and a reasonable agreement is obtained. The distributions of arc characteristics in the plasma column and electrode regions of a DC arc plasma torch are analyzed in detail. Moreover, the effects of thoriated tungsten, pure tungsten and non-uniform thoriated tungsten cathodes on the plasma characteristics inside the DC plasma torch are investigated. It is found that a constricted arc attachment with the highest current density is obtained at the non-uniform cathode, leading to the largest velocity and electric potential drop in the plasma column. The pure tungsten cathode produces the highest temperatures and heat flux along the cathode surface, which is caused by the largest arc voltage and intense space-charge sheath heating. The temperature and heat flux density on the thoriated tungsten cathode are the lowest, leading to a diffuse distribution of current density. The results indicate that the different cathodes can exert an important influence upon the plasma behavior near the cathode. Furthermore, the torch exit parameters are also slightly different due to their different arc voltages and overall input power.

Thermal Characteristics of Arc Column in Stabilization Zone of an Air-Operating Laminar Plasma Torch

Thermal Characteristics of Arc Column in Stabilization Zone of an Air-Operating Laminar Plasma Torch

Laminar plasma jet surface hardening of P20 mold steel: Analysis on the wear and corrosion behaviors

Insight into the effects of the laminar plasma jet (LPJ) surface hardening on the wear and corrosion behaviors of P20 mold steel is critical for seeking an effective way to improve the service life of the injection mold. In this study, the LPJ treatment was conducted on the surface of commercial P20 mold steel. Numerical simulation for the temperature field of the workpiece surface was employed to analyze the transformation mechanism of the microstructure. According to the numerical simulation results, the formation of a hardened layer with the lath-martensite microstructure was attributed to the ultra-fast cooling rate. Alteration in the wear and corrosion behaviors were investigated in detail by a series of ball-on-disk tribological tests and electrochemical corrosion tests. The results showed that the average coefficient of friction for the treated sample decreased from 0.314 to 0.186, and the wear rate decreased from 0.323 × 10−4 to 0.141 × 10−4 mm3/N·m. In addition, the wear mechanism of the surface of the P20 mold steel changed from abrasive wear combined with adhesive wear to mild oxidative wear after LPJ treatment. Improvement of the wear resistance was mainly attributed to the formation of lath-martensite with high hardness. Besides, it was found that the corrosion rate for the treated sample decreased from 0.2427 to 0.0680 mm/year. This was mainly attributed to the generation of the compact lath-martensite structure and the homogeneously distributed Cr element after LPJ treatment. These findings provide the theoretical feasibility for applying the LPJ surface hardening to the P20 mold steel to elevate the service life of the injection mold.

Numerical investigation of crack initiation on rail surfaces considering laminar plasma quenching technology

To recognize the crack initiation life of a corrugated rail surface after quenching, the explicit finite element model referring to real condition is developed for precise frictional rolling contact solutions considering selective quenching. This takes rail profile evolution and the nonlinear material model used for quenching effect into account. Then the simulation results combined with the fatigue model is integrated for the crack initiation life after quenching, the crack initiation on edge of quenching region is mainly caused by the wheel-rail impact induced by irregularities, which is formed by discontinuous material characteristic between quenching and matrix material. Based on this, the most critically susceptible position to crack and number of cycles are predicted, the simulated results are verified through comparison with quenching rail in service.

Deposition and oxidation behavior of atmospheric laminar plasma sprayed Mo coatings from 200 mm to 400 mm under 20 kW: Numerical and experimental analyses

A stable laminar plasma jet with a jet length of above 600 mm was adopted to deposit molybdenum coatings to investigate the behavior of high-melting point metal, molybdenum, and particles during laminar plasma spraying process. The temperature, velocity, and species distribution of the plasma jet were calculated by numerical simulation. In the experiment, the temperature and velocity distribution of the in-flight particles were measured, and the microstructure and properties of the coatings were analyzed. Combining the results of the simulation and experimental results, the deposition behavior and oxidation mechanism of the coatings prepared at the spray distance from 200 mm to 400 mm were discussed. The results indicate that both the laminar plasma jet and injected particles exhibit high temperature and low velocity, and low temperature and velocity gradients. The microstructure of the coatings shows a high similarity and hardly changes when the spray distance exceeds 250 mm. When the spray distance is 200 mm, post-deposition oxidation occurs due to the heating effect of the long plasma jet on the substrate. In contrast, when the spray distance is longer, the particles undergo severe in-flight oxidation, and the oxide formed is uniformly distributed in the coating improving the hardness from 500 HV0.3 to 700 HV0.3.

Numerical analysis of the plasma-induced self-shadowing effect of impinging particles and phase transformation in a novel long laminar plasma jet

Currently, the self-shadowing effect of impinging particles is recognized as a vital factor to form columnar-like coating in the manufacture of thermal barrier coatings. Most of these quasi-columnar coatings are usually prepared under a very low-pressure condition. This paper investigates a novel quasi-columnar yttria stabilized zirconia (YSZ) coating using an atmospheric plasma spray-physical vapor deposition method. The microstructures of the coating present a quasi-columnar structure that is distributed along the cross-section of the coating within certain intervals with a large number of cluster-like structures on the top surface of the coating. A lower particle velocity that contributes to the generation of a mass of vapor YSZ materials is studied via experimental and numerical analyses and these results are compared with other current plasma spray methods. The mechanism of the self-shadowing effect from impinging particles that leads to the formation of a quasi-columnar feature at the boundary layer of the substrate is demonstrated by a three-dimensional numerical simulation and experimental observation. Furthermore, the hybrid growth model of the vapor and droplet co-deposited coating is clarified in this paper.

Splash involved deposition behavior and erosion mechanism of long laminar plasma sprayed NiCrBSi coatings

The behavior of low-velocity NiCrBSi particles with a high surface temperature of 1800 °C during the long laminar plasma spraying process was studied. During the experiment, the maximum length of the laminar plasma jet exceeded 600 mm, and coatings were obtained using a laminar plasma spray at a wide spray distance ranging from 250 to 500 mm. Interestingly, the coatings obtained at a short spray distance of 250 mm showed special alternate porous and compact lamellar structure with island-like bulges that were evenly distributed on the coating surface. This type of structure was possibly formed by a low-velocity splashes deposition process intensified by the furry-like structure, which can be attributed to vapor phase deposition. The coatings obtained at an extremely long spray distance of 500 mm showed massive cracks. Moreover, the medium-distance sprayed coatings of 350–450 mm showed similar lamellar-like microstructure and mechanical performance because of the similarity in the particle states in the low-energy-gradient laminar plasma jet. Thus, a laminar plasma spray shows great reproducibility and potential for improving the spraying efficiency on an uneven plane.

Laminar plasma jet surface hardening of the U75V rail steel: Insight into the hardening mechanism and control scheme

Insight into the mechanism of laminar plasma jet (LPJ) surface hardening is critical for the controllable preparation of ideal hardened layer to improve the wear and fatigue resistance of U75V rail steel. To reveal the surface hardening mechanism, a series of surface hardening experiments on the U75V rail steel surface were performed under different working parameters using a self-designed laminar plasma generator. Results showed that the geometrical dimension, microstructure and hardness distribution of the hardened layer can be adjusted by changing the arc current, the scanning velocity and the anode nozzle diameter. Further characterization and numerical simulation suggested that the geometrical dimension of the hardened layer is determined by the heat affected zone over the critical austenitizing temperature (730 °C), while its microstructure and hardness distribution depend on the local cooling rate below the critical austenitizing temperature. By changing the working parameters, the heat flux density applied on the workpiece surface and the heating time can be controlled to obtain the desired temperature field within the heat affected zone. Not only a full hardened layer but also a transition layer consisting of martensite, pearlite, ferrite and carbides can be achieved. This configuration of LPJ surface hardening process for U75V rail steel indicates that the LPJ surface hardening with the ability of producing hardness gradient is promising for reducing the risk of crack generation on the rail.

Influence of laminar plasma quenching on rolling contact fatigue behaviour of high-speed railway wheel steel

This study aims at determining the influence of laminar plasma quenching (LPQ) on the rolling contact fatigue (RCF) behaviour under water condition of high-speed railway wheel steel. The wheel and rail test rollers were treated by LPQ with different scanning speeds (700, 900, 1100, 1300 mm/min), which generated different depth of heat affected zones (HAZs) and residual compressive stresses in the surface layer. To obtain an insight of the microstructures and their corresponding thermal treatment applied, the temperature evolution was simulated during the LPQ by Finite Element Modelling and the micro-structures of cross-sections below the roller surfaces were characterized. The results showed martensite, retained austenite and undissolved cementite in the HAZ as a consequence of the ultrafast heating and cooling rates. The produced residual compressive stresses resulted in an increase of ~30% of the RCF life indicating that LPQ could effectively improve the RCF resistance. Further investigations showed that the high density dislocation martensitic structure of the HAZ of reduced the plastic deformation, which could delay the RCF crack initiation and decrease the depth of RCF crack growth. During the RCF test, some retained austenite of the LPQ treated wheel roller transformed into twin martensite under the effect of strain induced martensitic transformation. The plastic deformation in the untreated wheel roller resulted in a refined microstructure showing lots of sub-grains generated in both pearlite and ferrite regions. Furthermore, we have observed that the lattice structures of the martensite in the LPQ treated sample and sub-grains of the plastically deformed untreated sample were similar to the BCC structure of the ferrite at the substrate in the high-speed railway wheel steel. We conclude from the obtained results that the LPQ processing enhances the RCF life and that this improvement is influenced by the obtained microstructure in the HAZ.

Application of Similarity Theory to the Characteristics of Laminar Plasma Torch With Pure Nitrogen

Because of the complex generation process, the jet characteristics of laminar plasma torch were always studied by experiments at present. In order to reduce the experimental costs and the workloads, the similarity theory is a good method to study the characteristics of the laminar plasma jet. Thus, with some experimental results under specified conditions, the relationships between the characteristics and the working parameters have been obtained to predict the characteristics with other specified working parameters. Compared with the experimental results, the theoretical arc voltage characteristics, thermal efficiency characteristics, and jet lengths characteristics coincided with the experimental ones with a difference lower than 2%, 2.5%, and 8%, respectively.

Experimental Study on the Design and Characteristics of a Laminar Plasma Torch With Medium Working Power and its Applications for Surface Hardening

Due to its favorable temperature and velocity distributions, a lamina plasma jet, generated by a laminar plasma torch (LPT), is superior to a turbulent plasma jet on some applications. However, most of the reported LPTs usually work at a relatively low output power with a relatively low arc voltage and thermal efficiency, limiting its capability in the applications such as the surface hardening, synthesis of nanoparticles, remanufacturing of large components, and so on. Therefore, this article aims at designing an LPT with medium level output power and a relatively low arc current but a high thermal efficiency. The characteristics and applications of the proposed LPT have been obtained by experiments. The experimental results show the following characteristics: 1) the mean arc voltage of the proposed LPT is higher than 190 V with a maximum working power larger than 25 kW; 2) its mean thermal efficiency and jet length arrive at 50% and 450 mm, respectively; 3) the experimental data of the proposed LPT were used to fit its theoretical characteristics by using the least squares method. According to these fitted curves, the theoretical characteristics of the proposed LPT at a specified working conditions can be predicted; and 4) in addition, the surface of 45# steel was hardened by using the laminar plasma jet to obtain a hardening surface. The maximum hardness of the treated substrate arrives at about 600 HV, which is three times of the untreated substrate.