Visualization Of Interface Cavitation And Removal Of Oxide Film During Ultrasonic-Assisted Brazing Process

Ultrasonic-assisted brazing can achieve reliable joining between filler metal/base metal in the atmosphere which is suitable for dissimilar metals, metal and nonmetal, etc. Its technology is relatively mature. However, researches on regularities of ultrasonic propagation in solid or solid-liquid interface and the effect of different base metal surface morphology on the generation and distribution of ultrasonic cavitation bubbles are still lacking. The mechanism about removal of oxide film is not thorough, limiting further applications and promotion the ultrasonic-assisted brazing of industry. Therefore, the main research goal of this paper is the visualization of interface cavitation and removal of oxide film during ultrasonic-assisted brazing process. The distribution of ultrasonic vibration on base metal surface and the law of ultrasonic transmission at solid-liquid interface was simulated. The characteristics of generation and distribution of ultrasonic cavitation bubbles on base metal surface its impact effect on base metals, as well as spreading behavior caused by ultrasonic was analyzed. In addition, the generation and distribution of cavitation bubbles in the molten solder at the solid-liquid interface under ultrasound was preliminary in situ observed by synchrotron radiation imaging method.The ultrasonound propagation in solid base metal and through solid-liquid interface was carried out by the finite element software COMSOL Multiphsics. The simulating results show that the ultrasound amplitude in the edge of plate is large, while the fixed areas of amplitude enhancement and decay exist in the middle of plate. Ultrasound spread into water via the base metal, the acoustic field has gradually expanded to the whole area of water with the propagation of ultrasonic. The acoustic field in water with a certain acoustic pressure intensity was sporadically distributed throughout the solid-liquid interface, waning from the solid-liquid interface to deep area of water. The acoustic pressure in water was remained in 2-20 MPa. Simultaneously, the acoustic field in water is mutually interfering and superposing. Ultrasound spread into molten solder via the base metal, the acoustic field has gradually expanded to the whole area of molten solder as the propagation of ultrasonic. The acoustic pressure in molten solder was remained in 4-40 MPa. With increasing of brazing temperature, the overall acoustic pressure of acoustic field in molten solder gradually decreases, while the cavitation threshold increases, cavitation is becoming more and more weak.The nucleation and distribution of a large number of cavitation bubbles at base metal surface was studied by using high-speed photography method. There are three nucleation modes of cavitation bubbles in the smooth metal surface, respectively, at plate edge, at vacuoles group and at the pits. Finally, the distribution of cavitation bubbles is a sheet-like cavitation bubbles groups, mixed with a small amount of steady vacuoles group. Surface texturing process on base metal has an effect of promoting, gathering, remaining and enhancing cavitation bubbles. A layer of oxide film was anodized in 1060 Al and TC4 base metal surface, the researchers found that the oxide film increases the attenuation of ultrasonic transmission at the solid-liquid interface and weaken the cavitation effect on base metal surface. After ultrasound treatment, the oxide film on base metal surface generated a few cavitation pits with size of several microns to several tens of micrometers. The broken of oxide film on base metal surface is caused by the micro-jets when the cavitation bubbles collapsing.The spreading behavior of the molten solder on base metal surface under ultrasound was also dynamic observed using high-speed photography. The periodic shock ripples on the molten solder surface and the periodic atomization in the leading edge during molten solder spreading process was found. The mechanism of removal of oxide film during ultrasonic-assisted brazing was analyzed, the removal of the oxide film is caused by a common effect of the micro-cracks produced when heating, the impact effect when cavitation bubbles collapsing and the dissolution between the molten solder and base metal. Subsequently, the observation about the generation and distribution of cavitation bubbles in the molten solder at the solid-liquid interface under ultrasound by synchrotron radiation imaging method was explored preliminary.