Study on the Process and Mechanism of Constrained Electrical Explosion Spraying

Author:Han Feng

Supervisor:zhu liang


Degree Year:2019





Electrical explosive spraying(EES)is a new type of surface modification,maintenance and manufacture technology based on the study of electrical explosive mechanism.It utilizes the high-density current generated by discharge of pulse capacitor bank to heat spraying material,which will make spraying material experience liquifaction,expansion and eventually explosion.The high-temperature and high-speed particles generated by such explosion process will be sprayed to the surface of the substrate to form coating.The output energy during the spraying process can be easily controled,making the EES process available for conductive spraying material with different melting point.Such coating process is of great engineering application potential.However,in the course of current researches,coating quality and spraying efficiency can not meet the actual application requirements,so the EES process was still in the laboratory research stage.For this reason,constrained electrical explosive spraying(CEES)process was proposed in this paper,and multi-functional CEES devices which can efficiently spray different forms of spraying materials(wire,powder and foil)were also developed.By using this device,the main scientific issues involved in the process of CEES were systematically studied.Firstly,the focus issue in the CEES which is the constraint and regulation mechanism of spraying particles was revealed.The slotted confinement chambers were used for constraining the spraying particles.A series of spraying experiments were carried out by changing the confinement parameters(confinement structure,confinement depth and width)of the confinement chamber and the energy density(57-152J/mm3)of the spraying material.It was found that the high quality coating can be obtained only in the vertical confinement chamber;when the confinement depth was 100 times the diameter of the spraying material,the uniformity and compactness coating can also be guaranteed;when the confinement width was reduced from 6 mm to 2 mm,the uniformity of the coating can be improved and the thickness of the coating can also be increased gradually.It was concluded that the thermal expansion effect caused by ohmic heating of spraying materials combined with the pressure effect created by surface breakdown arc in confinement chamber dominate constraint and regulation mechanism of spraying particles,influencing the following deposition behavior of spraying particles and formation of the coating.Secondly,the energy deposition characteristics of spraying materials which affect the formation of spraying particles were analyzed and discussed.The energy deposition characteristics and modes of wire and powder conductor were studied respectively.For wire conductor,when the wire diameter was same,the higher the initial charging voltage,the more energy the material deposits.When the initial charging voltage was same,the energy deposits decrease as the wire diameter exceeds a certain limit value.Whether the wire conductor was the refractory Mo wire or the fusible Al wire,the energy deposition mode had better to be adjusted to slow mode by changing the initial charging voltage and wire diameter,the resulting coating is the most uniform and compact;For powder conductor,two energy deposition modes were found.One was smooth oscillation mode,the other was catastrophe mode.The high quality coating can not be formed in smooth oscillation mode,whereas in the catastrophe mode,the coating was the most uniform and compact.By means of increasing initial charging voltage and reducing particle size of raw powder,the formation of bypass current on the spraying material will be restrained,leading to the energy deposition mode of powder can be changed from smooth oscillation to catastrophe mode,which will improve the coating quality.Thirdly,by using the self-designed spraying particles collection device and the particles flight characteristics test system,the flight characteristics(size distribution,temperature,velocity,spatial distribution and impact angle)and deposition behavior of spraying particles after explosion were tested and analyzed.The results show that the CEES is a kind of high temperature and high speed spraying process.When the initial charging voltage was 10 kV,the instantaneous velocity of the captured spraying particle as it impact on the substrate was more than 2000 m/s,the external surface temperature of which was exceed 700℃;The temperature and velocity of the spraying particle increases when the initial charging voltage was raised from 8kV to 11kV;The resulting interface of the coating and substrate was metallurgical bonding.It was also found that the size distribution of sprayed particles have a significant influence on their flight trajectory,the uniform size distribution and unified flight trajectory were the key factors to obtain an ideal coating;When the spraying distance was 10 mm and the initial voltage was 10 kV,the minimum impact angle of the spraying particles which can form ideal coating was 19°,and the deposition efficiency of the resultant coating can exceed 50%.Based on the above understanding of the formation and development of CEES particles,the morphology,phase structure,deposition efficiency,thickness and adhersive strength of refractory Ta10W and fusible Ni60A coatings were characterized and analyzed respectively.The results show that the Ta10W coating consists of three crystalline phases:Ta,Ta2N and FeTaO4,the average grain size of which were 90 nm;There are both crystalline and amorphous phases in Ni60A coating,it was made up of FeNi and SiO2 phase,the average grain size of which were 86 nm.The spraying particles will destroy and cut the substrate material to a certain extent,making the substrate elements appear on the coating surface,the fully metallurgical bonding interface can be created.Increasing the energy density from 22.4 J/mm3 to 151.6J/mm3,the deposition efficiency of both coatings under 10 times continuous electrical explosions show the similar trend of increase first and then decrease.The maximum deposition efficiency of Ta10W was 53%,while that of the Ni60A was 47%.It can be inferred that the coatings sprayed by the CEES have the potential to become structural coatings.Finally,in order to promote the CEES process to practical application,based on the research and understanding of processing mechanism of the CEES,a prototype of the equipment of the CEES for spraying the inner wall of the pipe was designed and developed.The tube with length of about 1 m and diameter of 50 mm can be efficiently and stably sprayed.