Research On The Characteristics Of Low Power, High Efficiency Non-transferred Arc Plasma Spraying

The Plasma spraying has been developed rapidly during last 30 years. The major researches of plasma spraying are concertrated on increasing of coating's quality, process instability and spray efficiency etc. The distributions and magnitudes of the plume temperature and velocity can be influenced remarkably by the arc characteristics. How to take use of the high energy of the arc to accelerate and heat the in-flight particles is a key problem of increasing the spray efficiency. One major way to achieve this purpose is applying of an axial and inner powder-feed method and controlling the distribution of arc in order to transfer more energy to the particles, however, the research on the arc characteristics inside the torch and the factors that influence its characteristics is rather poor. There also exist many problems in the normal inner-powder-feed plasma spraying that need to be overcome; for example, the powder particles tend to deposit inside the torch. The study on the low-power, high-efficiency plasma spraying has become one of the main contents to promote the technology and to increase coating quality. The research on the plasma characteristics inside torch is important because accelerating and heating of the powder particles depend mainly on the fluid characteristics of this region, but the research on this part is rather difficult because of the limitation of the actual measurement methods.By using numerical simulation method and experimental method, this paper studies systematically on the characteristics of low power, high efficiency none-transferred plasma spraying. The major strategies used in this research are as follow: Set up the mathematic modals of the plasma spraying and study the arc, the fluid and the particles characteristics. Discuss the factors and methods of improving the spray efficiency; Measure the temperatures and velocities of fluid and particles with the enthalpy probe and other measurement methods. Analysis the coating properties with proper coating test methods to insure the spray quality. The brief conclusions are stated as below:1. In this paper, the characteristics of plasma plume and in-flight particles of the low power, high efficiency plasma spray were measured by using the enthalpy probe and digital CCD camera. The coating properties were acquired by using coating testmethods. The results show that the characteristics of plasma plume and in-flight particles are similar to those of normal APS. The good properties of AI2O3 coatings by low power, high efficiency plasma spraying can be obtained.2. A mathematical model is developed to describe the temperature, velocity and other parameters profiles in the low power, high efficiency plasma torch and in the resulting plume. A computer software is programmed which can consider the effects of coupling between conservation equations, the changing properties of high temperature plasma gas, the turbulence in the plasma plume etc. The problems of precision and constringency are well solved. The distributions of the current density, velocity and temperature, the plasma heating efficiency, the kinetics and thermodynamics properties of the in-flight particles, the melting regions of the particles are obtained roundly through numerical simulation. The analytical predictions are in reasonable agreement with experimental results.3. The factors that affect the plasma characteristics and heating efficiency of low power, high efficiency plasma spraying are analyzed based on the numerical simulation. The particle characteristics of acceleration and heating during spray process are also analyzed through simulation. The results show that the magnetic force can remarkably affect the plasma temperature and axial velocity distribution inside the torch. On the same condition, increasing the arc length conduces to larger efficiency of plasma heating. The arc length increases with the increase of gas flow rate, and with the decrease of current. The heating efficiency of the plasma torch can increase with increasing the gas flow rate, content of argon or hydrogen, and with decreasing the arc channel diameter or channel length. For inner-powder-feed plasma spraying, the velocity and the temperature of particles increase more quickly in arc-channel region.4. A new-style double anode, inner-powder-feed plasma torch is developed in this research. The elongated arc can be obtained with this torch. The special plasma velocity and temperature distributions, the plasma acceleration and heating processes are studied in this paper. The results show that the plasma heating efficiency can increase by this new-style plasma torch. With this new-style plasma torch, the particle characteristics ofacceleration and heating are improved. The axial velocity at torch exit center and temperature at field-free region increase obviously. The melting position of particle inside torch extends afterward. This phenomenon is of benefit to avoid the particle depositing on the channel wall.In conclusion, this paper studies systematically on the characteristics of low power, high efficiency none-transferred plasma spraying and on correlative problems by using numerical simulation method and experimental method. The results show that applying axial and inner powder-feed method and controlling the arc distribution are effective methods to improve the spray efficiency. Researching on the low power, high efficiency plasma spraying is an important aspect of developing plasma spraying technology. With further theoretic and experimental researches, the technique will exhibit more attractive perspective in its application.