| Power ultrasonic honing is a kind of ultrasonic assisted machining method,which depends on the movement of whetstones to remove the material and change the micro topography of the surface.In the process of machining,cavitation will occur in the grinding fluid due to incoming ultrasonic and a large number of cavitation bubbles appear and collapse.Some complex secondary effects produced by bubble collapse,such as micro-jet,shock wave,sonoluminescence and so on,will have great influence on the material surface,especially the impact effect of the cavitation bubble collapse micro-jet.Scholars at home and abroad have done a lot of work on cavitation observation,cavitation dynamics analysis and cavitation erosion test analysis so far,and the relevant theoretical system has been initially established.The secondary effects produced by cavitation,however,need to be studied deeply.Furthermore,cavitation research in ultrasonic assisted machining such as power ultrasonic honing is obviously insufficient.Therefore,this paper presents the research on impact characteristics of bubble collapse micro-jet in ultrasonic honing.Firstly,the micro machining mechanism of ultrasonic honing and the cavitation mechanism in grinding zone are clarified.Then through the basic theories of ultrasonic cavitation,hydrodynamics,impact dynamics,material mechanics and ultrasonics,focusing on the impact effect of bubble collapse micro-jet on materials and the behavior changes in liquid and solid zones in this process.Finally,some relevant test analysis and demonstration are conducted.In this paper,the main content includes the following aspects:(1)The micro grinding mechanism of ultrasonic honing is studied.The scale effect of material is considered in micro machining process of ultrasonic honing and the strain gradient plasticity theory based on the dislocation mechanism is used,then a single whetstone grinding force model and a specific grinding energy model are established.It is found that the grinding force increases obviously when the scale effect is considered,and the honing depth has the greatest influence on the grinding force.When the honing depth reduces to a certain threshold(about 1.4?m),with the honing depth continuing to decrease,the grinding force shows a trend of nonlinear increase.When the honing depth is less than 4.4?m and then decreases,the specific grinding energy also increases nonlinearly.(2)The changes of single cavitation bubble characteristics in the grinding area and influences of honing parameters on bubble characteristics are investigated under the consideration of thermal effect.A dynamic model of single cavitation bubble in the ultrasonic honing grinding area is established.The model is based on the bubble dynamics,and the condensation and evaporation of kerosene steam,ultrasonic honing pressure and honing head speed are considered.The change rules of bubble radius,temperature,pressure and number of kerosene steam molecules inside the bubble are numerically simulated in the process of bubble moving by Matlab.The amplitude of bubble radius is greatly suppressed in the ultrasonic honing environment.However,the rate of movement of the bubble is faster.Meanwhile,the minimum values of pressure and temperature are larger,and the number of kerosene steam molecules is less.By studying the effect of ultrasonic honing factors on the movement of the cavitation bubble,it is found that honing pressure has a greater influence on bubble evolution characteristics,while rotation speed of honing head has a minor effect and the reciprocating speed of honing head has little impacts.(3)A three-dimensional fluid-solid coupling model of bubble collapse micro-jet impact on a wall is established.For the physical phenomenon that bubble collapse micro-jet impinge on a wall,simple mathematical calculations of hammer pressure and stagnation pressure are carried out,respectively,and the results show that sound pressure and honing pressure have little effect on the process of micro-jet impingement on a wall and can be neglected.The material strain rate can be as high as 10~4-10~6 s~-11 due to the micro-jet impact,and the Johnson-Cook rate correlation material constitutive model is employed.Then based on fluid mechanics,solid mechanics and impact dynamics,one-dimensional and three-dimensional fluid-solid coupling models of bubble collapse micro-jet impact on a wall are established,respectively.(4)The behavior changes of solid and liquid regions during micro-jet impact are discussed.By Coupled Eulerian Lagrangian method,the changes of wall behavior are analyzed under different conditions of micro-jet impact.When the bubble collapse micro-jet impacts on a wall vertically,Micro pits appear on the material surface.The peak pressure,maximum deformation depth and maximum equivalent strain on the wall all appear at the edge of the jet impingement.Plastic deformation on the wall occurs mainly at the early stage of the impact,and the equivalent strain is circular distribution.Obviously,the greater the micro-jet velocity is,the greater the wall pressure is.Pit depth is determined by both micro-jet velocity and micro-jet diameter,and increases with their increase.Pit diameter is mainly related to the micro-jet diameter and d_p/d_j?0.95-1.2,while pit's diameter-to-depth ratio is mainly negatively correlated with the micro-jet velocity and its value is about 15-80.When the micro-jet impacts with different angles,The lateral jet velocity on the back side of the micro-jet is lower than that on the front side,and the pressure,pit depth and equivalent strain of the back side are all higher than that of the front side.With the increase of the impact angle,wall pressure,pit depth and equivalent strain are all first slightly increase and then decrease,especially the shock effect of micro-jet is the strongest at 15o.In addition,the behavior of liquid region is analyzed by coupled Smoothed Particle Hydrodynamics with Finite Element Method during the micro-jet impact.The speed of lateral jet formed in the process of micro-jet impact is higher than the impact velocity of micro-jet and can be up to 1.6 times in this paper,and the central particles of micro-jet are reverse movement and upward bulge.(5)The cavitation observation test in ultrasonic honing,the bubble collapse micro-jet impact test and inversion analysis,and the ultrasonic honing cavitation orthogonal test are carried out,respectively.It is observed that obvious cavitation occurs in the liquid around the whetstones.Based on the theory of spherical indentation test,the inversion analysis of micro pits on the surface of material under micro-jet impact is carried out,and the results indicate that equivalent stress and equivalent strain of the pit,and impact strength and velocity of the micro-jet are closely related with pit's diameter-to-depth ratio.When the ratio is in the range of 16-68,micro-jet impact strength is about 420-500 MPa and the corresponding micro-jet velocity is about 310-370 m/s.Through the ultrasonic honing cavitation orthogonal test,the effects of distance,ultrasonic amplitude and test time on pits'maximum diameter,surface erosion rate and surface roughness are studied.The results show that the main factors influencing the pits'maximum diameter are distance and ultrasonic amplitude in turn,meanwhile the shorter distance and the greater amplitude result in the larger pits'maximum diameter.The influence of test time on the surface erosion rate is the most significant,followed by distance.The longer time and the shorter distance,the greater degree of surface erosion and the stronger the whole cavitation intensity.Amplitude has the greatest influence on the surface roughness.The sample surface roughness reduces and surface quality improves in the condition of distance of 5 mm,ultrasonic amplitude of 65%and test time of 1/3 min.Therefore,cavitation effect can help to enhance the workpiece surface quality under certain conditions. |
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