Research On Non-Transfer Arc Characteristics Of Plasma Spraying

Plasma spraying is a widely used thermal spraying technology in industry,which has been widely applied in many fields such as aviation,aerospace,metallurgy,petrochemicals,automobiles,marine protection,electronics,and biology.Currently,the research on plasma spraying technology mainly focuses on how to improve spray efficiency,increase process stability,and improve coating quality.Plasma arc pulsation can easily cause the powder to oxidize or melt incompletely,resulting in a decline in coating quality.Plasma arc pulsation is mainly caused by power supply ripple and arc root movement.The plasma spraying power supply is the main factor affecting arc pulsation.The development of a new type of low ripple plasma spraying chopper power supply can obtain a relatively stable plasma arc.In the study,the chopper power supply was used as the power supply.Through numerical simulation,arc electrical characteristics,and arc images,the non-transferred arc characteristics of plasma spraying were analyzed in detail.In terms of arc numerical simulation,based on magnetohydrodynamics and local thermodynamic equilibrium assumption,a two-dimensional axisymmetric standard k-εturbulent model was established using Fluent software to simulate the plasma arc flow field characteristics of F4 gun,3MB gun,and F1 gun.By comparing the temperature field and velocity field distribution of different spray gun structures,the distribution law of the arc flow field was obtained.At the same time,based on the energy transfer mode,the coupling effect of the multi-physical field of plasma arc was analyzed,and it was found that the maximum velocity point of plasma arc was lagging behind the maximum temperature point,which was beneficial to guide the structural design of the spray gun.In terms of arc electrical characteristics,a high-power low-ripple plasma spraying chopper power supply was developed.At the same time,a synchronous acquisition system for plasma arc electrical signal and high-speed image was built.Under different working conditions of carrier gas composition and flow rate,the relationship between arc volt-ampere characteristics and plasma arc conductivity and thermal conductivity was investigated.The stability of arc ignition,spraying and extinction was tested for different spray guns under chopper power supply conditions.The FFT algorithm was used to transform the arc current signal from time domain to frequency domain,and the influence of carrier gas composition,flow rate,spray gun type,and other parameters variation on plasma arc pulsation was studied.The cause of Helmholtz resonance(4.1 k Hz)generated by large flow rate carrier gas inside the gun chamber was analyzed.In terms of arc image,a series of digital image processing techniques including grayscale processing,image denoising,image enhancement,morphological processing,and edge extraction were used to process the synchronous collected plasma arc images for different process parameters.The arc length and cross-sectional area of plasma arc were compared and analyzed under different process parameters,and the influence of spraying process parameters on plasma arc characteristics was summarized.The change of arc length was used to verify the low-ripple characteristic of the chopper power supply.