| Ultra-thin wall materials are widely used in aerospace,medical devices,integrated circuit and other important industries and key fields,has an irreplaceable role.Due to the thin material,traditional forming technology can not meet the forming needs of ultra-thin wall materials.It has the forming difficulty,easy to deform,burst and other problems.Therefore,laser shock microforming technology is developed for the manufacture of ultra-thin wall materials.It has attracted widespread attention because of its high processing efficiency,low cost,good flexibility and so on.However,laser shock microforming uses plasma detonation wave to make the material deformed.So it exist ablation and adiabatic shear failure phenomenon.In addition,as the thickness of the workpiece drops to a certain extent,it is easy to break and wrinkle.In this paper,a new laser cavitation impact microforming technique is proposed which uses a pulsed laser to breakdown the liquid medium to induce a single bubble,and then applies a compound loading source such as plasma shock wave,bubble collapse shock wave and micro-jet to form the material with thickness of 10 ?m and below.The experimental results show that the laser cavitation impact microforming workpiece has clear edge contour,good surface quality,uniform cross-sectional thickness distribution and good symmetry.This paper carries out a series of basic research on the mechanism and characteristics of laser cavitation impact microforming,and the main work is as follows:(1)Theoretical study of the laser cavitation impact forming mechanism.Firstly,the feasibility of laser cavitation impact forming was investigated by analyzing the mechanism of laser-induced bubble formation and bubble dynamics theory.Secondly,we studied the relationship of bubble size changes with time based on the Keller-Miksis equation.Analyzing the formation and development process of plasma shock wave and shock wave pressure characteristics in water,and the shock wave pressures of bubble collapse during isothermal and adiabatic processes was analyzed.The asymmetric collapse of spherical bubble and formation mechanism of micro-jet were revealed.Finally,the pulsation law of the bubble near a single-wall was studied,and the cavitation impact microforming mechanism of single-wall was revealed.In addition,the multi-wall laser cavitation forming technique was proposed,and the double-wall cavitation impact forming mechanism was revealed by studying the pulsation law of the bubble between the double-wall surfaces.(2)Numerical study the impact characteristics of laser cavitation.Based on the VOF multiphase flow model,combining with the bubble dynamics formula,and taking into account the liquid viscosity,surface tension,compressibility and other factors,the bubble shape,pressure field and velocity field distributions were analyzed.In addition,the micro-jet impinging velocity and water hammer pressure were obtained under different standoff distances.On this basis,the bubble dynamics and shock wave pressure characteristics between the double walls were simulated.The bubble shape,pressure field and velocity field distributions under different wall distances were obtained,as well as the micro-jet velocity and water hammer pressure.(3)Experimental study of the shock wave pressure characteristics and its influencing factors.The experimental study of bubble oscillates near a single wall was carried out to reveal the pulsation law and shock wave pressure characteristics.The influence of standoff distance and laser energy on bubble dynamic characteristics and bubble center displacement were studied,and the bubble pulsation law under different dimensionless parameters was obtained.The influence of wall distance on bubble oscillation period was studied,and the variation trend of bubble oscillation prolongation factor with dimensionless parameters was obtained.In addition,the acoustic pulse signals of the bubble under different wall distances and laser energies were studied to reveal the shock wave pressure characteristics.On this basis,the double walls experimental studies of bubble were carried out to reveal the pulsation law and shock wave pressure characteristics.The effect of wall distance on bubble dynamic characteristics was studied,and the bubble center displacement under different dimensionless parameters was obtained.In addition,the effect of wall distance on bubble oscillation period was studied,and the variation trend of bubble oscillation period with bubble radius under different wall distances was obtained.In addition,the acoustic pulse signals of the bubble under different wall distances were studied to reveal the effect of wall distance on shock wave pressure characteristics.(4)A single-wall cavitation impact forming system was constructed to study the laser cavitation forming characteristics of ultra-thin wall materials.The effects of the wall distance and impact times on the surface topography,forming depth and roughness of the workpiece were investigated.The variation trend of Oxygen element content with impact times was obtained by line scanning analysis of micro-jet impact area through EDS.The thickness distribution of workpiece cross-section was studied and the variation trend of workpiece cross-sectional thickness with laser energy was obtained.The influence of cavitation impact microforming on the nano-hardness and elastic modulus of the workpiece was analyzed.At the same time,we constructed a double-wall cavitation forming system to study the influence of wall distance on the morphologies,forming depth and roughness of the workpiece.The cross-sectional thickness of the workpiece as a function of laser energy was studied to obtain the ideal forming effect.The results show that the laser cavitation impact microforming technology is particularly suitable for the processing of ultra-thin wall materials with good fabrication effect,no surface ablation,large forming depths,clear edge contours,and good symmetry.It can meet the fabrication and requirements of micro-nano devices. |
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