Dynamics And Effect Of Acoustic Cavitation Bubbles In Liquid Metal Within Narrow Gap

Ultrasonic-assisted brazing can shorten the welding time,reduce the joining temperature,improve the wetting and spreading speed of solder on the surface of parent metals,control the generation and distribution of intermetallic compounds.It joins difficult-to-weld materials in atmospheric environment and it has been widely applied in the joining of ceramics,composite materials,and dissimilar materials.The main mechanism of ultrasonic-assisted brazing is cavitation and its effect.Ultrasonic energy is introduced to solder liquid with small size and then the fluctuation of sound pressure appears.When the sound pressure is above cavitation threshold,cavitaion bubbles grow,collapse,rebound and generate high pressure and shock wave in neighbor fluid,which will produce cavitation effect on the material surface,thus accelerating the welding process.In this work,we first simulated the sound pressure in thin liquid metal and then calculated and observed the dynamic behavior of cavitation bubbles,and finally studied the distribution of cavitation force under different technological parameters.Ultrasonic energy is transferred to the liquid metal through the solid-liquid interface,and the alternating positive and negative sound pressure is produced in the liquid.The curve of sound pressure and time is sinusoidal with amplitude of as high as 107 Pa.And compared with the amplitude,the offset is smaller and it will be neglected.The sound pressure is not uniform.In the liquid metal within narrow gap,the contour planes of sound pressure are paralleled to YZ plane.However,in that with free surface,the contour planes are paralleled to XY plane.Bubbles with a small initial radius in liquid experience growthing,shrinking,collapsing under the excitation of sound pressure.Some cavitation bubbles can rebound after collapse.The growth and decrease of cavitation bubbles are closely related to the negative and positive intervals of sound pressure,respectively.The radius change of cavitation bubble is at a great speed,which can reach tens of meters per second on average.Under the same technological parameters,the decreasing speed is greater than that of growing.The pressure in the surrounding fluid of transient cavitation bubbles changes rapidly and it reduces by above 107 Pa when they collapse.In the process of cavitation bubble growth,the fluid around the bubble wall flows outward,and its effect on the wall surface is similar to water hammer phenomenon.When the velocity of cavitation bubble is greater than the velocity of sound in the fluid,shock waves are generated.Therefore,cavitation bubbles can produce cavitation erosioneffect on the solid surface nearby,and the superposition of cavitation erosion results in cavitation pits with different morphology on the solid surface.