Research On The Static And Dynamic Behavior Of A Reverse-polarity Plasma Torch Used For Plasma Atomization

Plasma atomization is considered as a new and disruptive method that mainly relies on high temperature and high enthalpy thermal plasma jets to produce spherical metal powders with high melting point for 3D printing.As the core component for plasma atomization,reverse-polarity plasma torch(RPT)has been used to generated the plasma jet with high temperature and high speed,but its electro-thermal characteristics determine the final quality of the produced spherical powder.Although the RPT has been industrially applied in the production of plasma atomization,there is still a lack of systematic research on its static characteristics(mainly includes current voltage characteristic and thermal efficiency characteristic)and dynamic characteristics(effect of the dynamic behavior of the arc root on the arc voltage and plasma jet).As a result,there is no basis for determining the optimal atomization processing parameters and the uniform quality and fixed yield of spherical powders are not guaranteed.In view of the above problems,a self-manufactured DC non-transferred arc reverse-polarity plasma torch for atomization studied in this thesis.Experiments on the influence of different working conditions on its arc voltage and thermal efficiency were conducted to determine the best processing parameter and produce spherical powders with high quality.At the same time,researches on the characteristics of voltage fluctuations under different conditions were carried out to reveal the large fluctuation of arc voltage caused by large scale arc shunting of the RPT at different working parameters.The parameters range for the stable operation of the plasma torch was determined,so as to ensure uniform quality the produced powders between different batches.The main research content and conclusions are as follows:(1)The static characteristics(current-voltage characteristics and thermal efficiency characteristics)of the RPT under different operating conditions(arc current,gas flow rate,electrode connection mode,nozzle state)were studied experimentally.The results showed that: 1)the arc voltage slightly increases with the increase of current,and significantly increases with the increase of gas flow rate.The power of plasma torch increases with the increase of current and gas flow rate.2)The thermal efficiency increases with the increase of gas flow rate,but decreases with the increase of arc current.3)Under the same working parameters,the arc voltage and thermal efficiency of reverse polarity connection is higher than that of direct polarity connection.4)Under the same working parameters,the arc voltage of plasma torch with Deval nozzle is higher than without Deval nozzle,and the thermal efficiency of plasma torch with Deval nozzle is slightly higher than without Deval nozzle.In addition,when the plasma torch with Deval nozzle,jet velocity can reach supersonic.(2)An electrothermal characteristic model of the reverse polarity plasma torch has been established based on similarity theory.In order to make reasonable predictions for the voltage and thermal efficiency parameters which have not been studied in the experiment,the theory of dynamic similarity has been used to construct a generalized electrothermal characteristics model of RPT.The relationship between the dimensionless working parameters and the electrothermal characteristics was obtained by using the partial experimental data,which can be used to predict the electrothermal characteristics of plasma torch under other working parameters.The accuracy of the model was verified by comparing the predicted values of voltage and thermal efficiency obtained by modeling with the experimental values.Results showed that 1)The enthalpy number obtained from the energy equation has been found to significantly influence the characteristics of the electric arc.The Reynolds number and the thermal conductivity number affect the arc voltage and the thermal efficiency,respectively.The acceleration number has a relatively small influence on the arc characteristics,but it cannot be ignored.2)The predicted values are in good agreement with the experimental values(relative errors are less than 5% and 3%,respectively),which indicate that the modeling results are reliable,and it is feasible to predict the electrothermal characteristics of the RPT in a certain range.(3)The dynamic characteristics of voltage fluctuation were studied by a series of experiments combined with signal processing.The results show that: 1)the arc voltage fluctuation of the reversed polarity plasma torch increases obviously with the increase of arc current,and the relationship between gas flow rate and voltage fluctuation is not obvious.2)It can work stably at high power(close to 25 k W)under 160 A current and150 SLM gas flow.3)Large-scale shunting is prone to occur in the case of large currents,which leads to violent fluctuations in the arc voltage,and the low-frequency voltage signal with saw-toothed wave is in the range of 0.2 ? 3 Hz.4)The short time Fourier transform(STFT)was used to analyze the arc voltage signal in real-time,which can be used to accurately monitor the generation of large-scale shunting and guide the adjustment of the working parameters of the plasma torch.