Modeling On The Powder Production Process And Powder Quality Control In Plasma Atomization

Metal additive manufacturing is a novel manufacturing technique that uses spherical titanium powder to create non-standard titanium alloy products with complicated shapes.This technique is widely used in the aerospace,automotive,and medical industries.However,the preparation of high-quality spherical titanium powder remains a bottleneck problem in the industrial application of additive manufacturing technology in China.Plasma atomization(PA),one of the important methods for powder preparation,can produce high-quality spherical titanium powder with high sphericity,high purity,small average size,and narrow size distribution by using a high-temperature(4000 K)and high-speed(1000 m/s)plasma jet to impinge and melt a metal wire,and then atomize the molten metal.The powder production process of PA includes wire feeding,atomization of the metal wire,and powder collection.Among them,the process of wire feeding and powder collection is simple and easy to understand,while the atomization of the metal wire(referred to as the plasma atomization process)involves complex coupled heat and mass transfer behaviors between the plasma jet,metal wire and molten metal,leading to a lack of theoretical research on the heat and mass transfer behaviors during the plasma atomization process and their effect on powder quality.Consequently,the prediction of powder quality and the optimization of the process parameters lack theoretical guidance,which hampers the further development of PA.Thus,this dissertation aims to investigate the heat and mass transfer behaviors during the PA process and their specific effects on the powder quality by experimental observation,numerical simulation and theoretical derivation.On this basis,the mapping relationship between the atomization conditions and the powder quality can be established,and the preparation of powders can be more controllable.The main research contents and conclusions of this dissertation are presented below:1.The five stages of the PA process were defined,and the phenomenological models of heat and mass transfer used for describing the atomization process were established to achieve decoupling research on PA.The main contributions of this study are as follows:(1)The five stages include metal melting,molten metal transfer,generation of the pre-breaking molten droplet(Pb MD),breakup of Pb MD,and spheroidization of small droplets.The first three stages were further identified as the plasma primary atomization(PPA),while the fourth stage was identified as the plasma secondary atomization(PSA);(2)Based on the heat and mass transfer characteristics of each stage,phenomenological models were established to describe the generation of the molten pool,the transfer of the molten metal,and the generation and breakup of Pb MD.Such models achieved the decoupling description on the complex PA process.2.In response to the inadequacy of research on the heat and mass transfer behavior during the PPA process,this study conducted an in-depth analysis on the heat and mass transfer behavior of PPA and established its theoretical models,so that the effects of the PPA process on the powder quality can be revealed,and a theoretical foundation for calculating the size of PPA can be prepared.The main contributions of this study are as follows:(1)Theoretical models of the heat and mass transfer behavior during the PPA process were constructed,including the heat transfer model between the plasma jet and the metal wire,the energy utilization efficiency model of PPA,the gas-solid-liquid coupling model,and the transfer model of molten metal.Based on these models,schemes were made for increasing powder yield,and phenomenological models of the PPA process were established;(2)The heat and mass transfer behavior during the PPA process was revealed.The study found that under the impingement of the plasma jet,the surface of the wire alternately exhibits two behaviors,i.e.,the expansion of the molten pool and the transfer of the molten metal.This phenomenon affects the average size and yield of the powder;(3)A quantitative model of the PPA process was established.The heat and mass transfer behaviors during the PPA process were quantified into two key influencing parameters(the phase transition time and the peeling time),and calculation models were developed based on their physical meanings.By controlling the values of these two parameters,the heat and mass transfer behavior during PPA process can be regulated.3.In response to the difficulty in determining the size of Pb MD in PA,a calculation model on this size was established in this dissertation,and experimental investigations were conducted to reveal the variation of the size of Pb MD with the arc current and gas flow rate.On this basis,the size and size distribution of Pb MD can be predicted,and a theoretical foundation for the study of the PSA process can be prepared.The main contributions of this study are as follows:(1)Based on the heat and mass transfer behavior of the PPA process,a phenomenological model for the generation of Pb MD was established,and a calculation model for the size of Pb MD was further developed.It was found that the variation of the calculated results was consistent with that of the experimental results;(2)Experimental observation revealed that the average size and size distribution of Pb MD increase with the increase of arc current and decrease with the increase of gas flow rate,which also drives the relevant parameters of the powder to change in the same way.Moreover,the size of Pb MD in PA is found to be 10 times smaller than that of gas atomization,providing compelling evidence for the superiority of PA in preparing spherical fine powders.4.In response to the problems of the unclear mechanism of the PSA process and an unknown average size of small molten droplets after the Pb MD being broken,the breaking mechanism of Pb MD was revealed,and a calculation model on the average size of small molten droplets was established,so that the average size of the produced powders could be predicted.The main contributions of this study are as follows:(1)Through the use of the theory of the Weber number and the experimental observations,it can be declared that the breaking form of Pb MD is mainly catastrophic breakup;(2)A numerical simulation of the PSA process was conducted,and a phenomenological model of PSA was established.It was found that the Pb MD will undergo plastic deformation when impinged by the plasma jet,and then the unstable oscillations will appear on the surface of Pb MD.When the oscillation amplitude reaches its maximum,the Pb MD will be broken and form multiple small droplets;(3)A calculation model on the average size of small droplets was established.By combining the theory of oscillation fragmentation and the equation for calculating the maximum wavelength of oscillation waves while considering the conditions of PA,the calculation model on the average size of small droplets can be established,enabling the quantitative prediction of the average size of the produced powders.5.The theoretical model proposed in this dissertation was verified by producing spherical titanium powders using industrial-scale equipment of PA.The main contributions of this study are as follows:(1)A simulation analysis of the flow field distribution around the metal wire was conducted under the industrial-scale conditions of PA to predict the powder quality based on the theoretical model.The results showed that the temperature and velocity around the wire exhibited a “triangular symmetry”distribution,with the lowest values in the gap region between adjacent plasma generators and the plasma jet in this region tending to move away from the metal wire;(2)Based on the flow field distribution,methods were proposed for predicting the average size,the variation of the size distribution,and the spheroidization situation of the powders;(3)Experiments were conducted to verify the reliability of the theoretical models.By optimizing the arc current and gas flow rate,the average power of the plasma generators was reduced to below 27.7 k W,and the average size of the produced powder was reduced to below 189 μm,of which the smallest average size was only50.9 μm,better than the relevant records of the similar equipment abroad.The experimental results for average size,size distribution variation,and spheroidization of the produced powders were consistent with the theoretical prediction.The variation of the calculated average size was consistent with that of the experimental results,and the errors between the calculated and experimental results were between 10% and 18%.The proportion of irregular powders in the prepared powder was 0,and the proportion of close-to-spherical powders approached 100%,which was consistent with the prediction results of the theoretical model.These results further confirm the reliability of the theoretical model proposed in this dissertation.6.The schemes for controlling the powder quality based on the industrial-scale PA equipment were proposed,including:(1)The scheme for controlling the average size of powders.The average size of powders can be predicted using the calculation model after changing the process parameters;(2)The scheme for controlling the size distribution of powders.Increasing the gas flow rate of the plasma generator or decreasing its arc current can effectively reduce the size distribution of powders;(3)The scheme for controlling the spheroidization of powders.Through the calculation model of the average size of droplets and the spheroidization judgment inequality,while considering the conditions of PA,the spheroidization situation of the produced powders can be predicted after changing the process parameters.