Study On Movements And Material Removal Mechanism Of Nanoparticles In Fluid Jet Polishing

With the development of modern science and technology,the demand for super-smooth surface is increasing.Fluid Jet Polishing(FJP)is an advanced surface processing technology that can be used for complex surfaces,which was proposed in the 1990 s.It can be used to obtain a surface with nanoscale roughness,no damage and complete lattice.There are still many unsolved problems with FJP technology,which limit the further improvement of FJP processing capacity.To solve this problem,it is necessary to have a deep understanding of the material removal mechanism in FJP.However,the existing researches mainly focus on the optimization of processing parameters,improvement of removal function and design of processing path,and less on the mechanism of material removal.The results show that the impact of nanoparticles on the surface of the workpiece is the main cause of material removal.Therefore,it is a key to reveal the material removal mechanism in FJP to study the movement law of nanoparticles in the jet and the material removal effect of the collision between nanoparticles and the workpiece surface.In the view of the above research purposes,the thesis carries out the following aspects of research:Firstly,a set of experimental equipment for jet polishing was set up,which was composed of jet circulation system,motion control device and temperature control stirring system.The jet speed can be set between 20m/s and 80m/s,and has the functions of circulation,stirring and heating.The nozzle and the working surface can realize the relative movement of four degrees of freedom,which can meet the requirements of basic jet experiment.In this paper,further experiments verify the speed stability and processing ability of the equipment.Meanwhile,the multi-phase flow field model in the process of jet polishing was established by using the hydrodynamics simulation software,and the discrete phase model(DPM)was used to track the trajectory of particles in the jet.The movement law of particles in the flow field and the collision characteristics between particles and workpiece surface were obtained.By comparing the experimental observation with the simulation calculation,it is found that the morphology of material removal in jet polishing is quite consistent with the collision density distribution of particles on the workpiece surface,which indicates that the collision between particles and workpiece surface is the main reason for material removal.In order to further explore the material removal characteristics,the effects of jet velocity,particle size and particle density on the particle workpiece surface collision probability are further studied.It is found that when the particle size is micron,the collision probability and particle size increase in a power function.The results of experiments show that the particles diameter has a significant effect on the morphology of the material removal.With the increase of particle size,the removal profile of the material changes from "W" to "U" shape.Further analysis of the material removal characteristics of single particle collision shows that with the increase of particle size,the material removal mode of particle collision is changed from chemical adsorption to bombardment removal.When nanoparticles collide with the workpiece surface,the removal method is chemical adsorption removal,and the surface of the workpiece is atomic level removal,and the nano-scale processing surface can be obtained.When the particles collide with the micron,the removal method is bombardment removal.At this time,more surface atoms can be removed by single collision,but the surface processing quality is poor.Moreover,by analyzing the relationship between material removal morphology and particle collision velocity distribution,it is found that the normal velocity is the key to material removal.Compared with the experimental observation and numerical calculation results,it is found that the normal velocity of particle collision should be greater than a certain critical velocity when material removal is realized.Furthermore,the material removal morphology and particle collision characteristics under oblique incidence are studied,and the existence of critical normal velocity of material removal is confirmed.Finally,a device for measuring the surface potential of workpiece in jet polishing is built,and it is proposed to study the particle workpiece surface collision law and the micro material removal characteristics of collision by measuring the charge transfer in jet polishing.By using the device,the surface potential of workpiece during fixed-point polishing was measured,and the effect of particle concentration on charge transfer in the process of jet was studied.The device provides a new experimental method for studying the mechanism of FJP material removal.