Metal Droplet Inner Cavity Forming Method Based On Ultrasonic Composite Field Technology

Femtosecond laser processing is the most mature and widely used internal processing technology for transparent materials,but it cannot be penetrated by laser for non-transparent metals,let alone internal processing.Ultrasonic waves can penetrate non-transparent materials,in addition,acoustic levitation creates a non-contact and pollution-free space environment,as a container-free processing method,it has become an indispensable experimental means in the fields of advanced material preparation in microgravity environment and hydrodynamics research of free suspended droplets.The research focus of this paper is mainly to use the cavitation mechanism of ultrasonic acting on the fluid medium to form the droplet inner cavity,and the acoustic radiation force in the acoustic suspension is used to drive the droplet to float stably at the standing wave node,so as to realize the ultrasonic processing technology of metal droplet inner cavity studied in this paperBased on the acoustic cavitation research of pure water,this paper will be extended to the processing of the spherical cavity of metal droplets(gallium indium alloy).The main research contents of this paper are briefly described as follows:(1)The compound field theoretical basis of the mechanism of acoustic levitation and acoustic cavitation.According to the propagation of sound follows the three basic governing equations in fluid mechanics,the one-dimensional and three-dimensional acoustic wave equations with small amplitudes are derived.At the same time,the propagation form of the sound wave and physical quantity of ultrasonic are introduced.Boundary conditions in the computational domain need to be set when obtaining the simulation.Then,the standing wave theory is introduced to describe the working principle of acoustic levitation,and the droplet shape in the acoustic levitation is analyzed by using the acoustic radiation force represented by Gor' kov.Calculate the different liquid samples of cavitation threshold,the preliminary define different liquid samples to the difficulty of the cavitation,as well as using the simulation software simulation of the compound ultrasonic field acoustic pressure distribution,and preliminary verify the maximum sound pressure value of compound ultrasonic field is greater than the hardest cavitation of the cavitation threshold of indium gallium alloy,prove the feasibility of our build compound ultrasonic field.Finally,The cavity of the droplet can be formed by the cavitation mechanism of the fluid medium produce by ultrasound,and the dynamic control equation of a single bubble in the droplet is established.(2)The dynamic control equation of a single bubble in the droplet and the simulation analysis of the behavior characteristics of the droplet dynamic detaching from the solid reflection end.Firstly,the simulation model of a single bubble in ultrasonic field droplet was established.Through the coupling simulation of the PDE equation and dynamic grid in COMSOL Multiphysics software,the pure water droplet is taken as the basic research object,which is extended to the study of metal droplet gallium-indium alloy.Under the condition that other parameters are constant,the motion of a single bubble in the droplet changes in a single variable,and the parameters the of the obvious cavity in the metal droplet are obtained.Secondly,the calculation domain of the driving droplet is constructed,and the two modules of acoustics and fluid are coupled by adding the nonlinear acoustic radiation force in the sound field to the momentum equation of the fluid.When simulating the ultrasonic emitter at different positions,the behavior characteristics of the droplet dynamically separated from the solid reflector are analyzed,and the best test conditions conducive to driving the droplet to the standing wave node are obtained.(3)Experimental study on ultrasonic compound field machining system.Firstly,the test device,test method,and operation steps are introduced,and the vibration analysis of the customized ultrasonic transmitter is carried out to verify that the vibration amplitude of tip and end face is different,which is conducive to the formation of the inner cavity.Secondly,the relationship between droplet cavity radius and ultrasonic frequency is studied.Then,according to the best test conditions of the suspended droplet,a uniaxial suspension field was set up to drive the droplets attached to the solid reflection end until they are stably suspended at the standing wave node.The three ultrasonic internal processing processes of forming the droplet cavity-driving the droplet away from the solid reflection end-stable suspension were realized to complete the metal droplet internal processing.