Studying Characteristics Of Plasma Spraying With High Arc Voltage And Low Arc Current

Over the past decades,plasma spraying,as a well-established technology in modern surface engineering,has been applied widely to industry practice in depositing coatings with various functions such as wear-,corrosion,high temperature-resistance. However,there exist some shortcomings that prohibit the plasma spraying to obtain the coatings of high quality.The arc shunting of the anode arc root increases the oxygen content in the plasma jet,which results in the oxidation and phase change of some materials such as metals and carbide.Due to the subsonic velocity of the plasma flow, the powder particles cannot be effectively accelerate,which leads to low density of the coatings and low bond strength between coating and substrate.In recent years,some researchers paid more attentions on better melting the powder particles and increasing its deposition efficiency.It is necessary to develop new-style plasma torch that can be adapted to various requirements.The plasma spraying is developing in the direction of high energy,high velocity,controlled atmosphere and axial particle feeding.Recently,Prof.Yang Gao in Dalian Maritime University has developed a novel bi-anode plasma torch,which consists of two insulated anode nozzles that are in different distance from the cathode tip.The first anode is used to ignite plasma arc,and the second one is used to lengthen the arc column.This new-style torch can remarkably enhance the arc voltage and reduce the.arc shunting.The paper experimentally studied the thermal and electric characteristics of the plasma arc generated by the bi-anode torch, including the temperature and velocity of the plasma jet,the torch efficiency,the U-I characteristics in different gas composition and the influence of the gas flow on the arc voltage.The paper also compared the coatings deposited by the bi-anode torch and by a normal one,and made numeric simulation to optimize the spray parameters.Beside,the effects of the particle injecting position on the coating qualities were also studied.The main conclusion can be stated as below;1.Under an equal power input,the temperature and the velocity of the plasma jet in anodeâ…¡mode was significantly higher than that in anodeâ… mode.The reason is that the torch operated in anodeâ…¡mode had a higher thermal efficiency because the self-magnetic constriction concentrating the arc energy along the center of the nozzle and reducing the thermal diffusion to the cold wall.2.In anodeâ… mode,the arc always shows rising voltage-current characteristic, which is consistent with a traditional one.In anodeâ…¡mode,the voltage-current characteristic of the arc relates to the thermal conductivity of the plasma gas,pure argon arc of low thermal conductivity shows a rising voltage-current characteristic while Ar-H₂ arc of high thermal conductivity shows a dropping voltage-current characteristic.3.In Anodeâ…¡mode,higher gas flow always increased the arc voltage,while in Anodeâ… mode,it may cause the voltage drop because a laminar flow will change to a turbulent one when increasing the gas flow,which decreases the breakdown voltage for the short-circuit and reduces the arc length.4.Under the same input power and hydrogen content,the plasma spraying applying axial particle injecting can better melt the powder particles,produce coatings with better quality,and get higher deposition efficiency.It proved the axial particle injecting is superior to internal radical particle injecting and external injecting.5.For a traditional torch,the fluctuation of the anode arc root enhances the air entrainment into the plasma jet,which will increase the decarbonization of the tungsten carbide coatings.There exists large content of W₂C in WC-Co coating deposited by the traditional torch.The bi-anode torch can effectively prohibit the decarbonization of WC-Co coatings because it can restrict the movement of the anode arc root and reduce the oxygen content in the plasma jet.6.A numeric simulation has been made to study the behaviors of the powder particles in the plasma flow.The accelerating and heating of the particles with different size and material were calculated under different injecting velocity.The paper gives an optimized injecting velocity for alumina particles with different size.