Applying Of Ultrasonic Vibration In Brazing Of Aluminum To Graphite

Ultrasonic was used in this thesis to active brazing of aluminum and graphite under the condition of the air aiming at improving the wettability of filling material on the surface of graphite. It was also adopted in this article that the academic thought of build ultrasonic activation mechanism of liquid solder and wetting mechanism of base metal to research the basic problems about ultrasonic used in the secondary brazing : the mechanism of ultrasonic to the active filler metals and to the interface reaction in auxiliary active brazing and the effect of ultrasonic in the interface shape theory. The active solder-Sn5Ag8 Ti was used in this article to braze the 6063 aluminum and graphite under the condition of the atmosphere and the mechanism of ultrasonic in the auxiliary brazing of the aluminum and graphite was studied by changing the ultrasonic action time.In the experiment, the Sn5Ag8 Ti solder was heated to the brazing temperature for a period of time. The Sn-Ti phase shape and size has not changed during rapid cooling and the low melting point of Sn-Ag phase disappeared. This further proved that the high-temperature phase was without melting and the method would be well save the tissue of the solder with ultrasonic. Results further showed that with the increase of ultrasonic time and the size of the Sn-Ti phase became smaller in the solder. With the extention time of using ultrasonic, the solder temperature rise.Sn-5Ag solder temperature was 0.3 ℃ which was slightly lower than the theoretical temperature. Considering the effect of heat conduction and the experimental error, the theoretical and experimental values basically. The results of calculating of Sn5Ag8 Ti solder showed that solid-liquid interface heat absorption of ultrasonic waves could make almost all Ti6Sn5 phase melt and were close to the melting point of Ti2 Sn,Ti2Sn3, Ti5Sn3 phase. These melt under the action of ultrasonic waved. Ultrasonic energy was added to make the unmelted Sn-Ti compound of the brazing filler in the brazing temperature to melt, due to the solid-liquid interface of ultrasonic sound encountered increased resistance, mechanical ultrasonic vibration into heat energy,heating the solid-liquid interface, higher the local area solder temperature and overall the liquid solder.With the extension of ultrasonic time, the shear strength of braze welding head first increased and then decreased. At last, it reached a maximum of 16 MPa after 10 s.When the ultrasonic was used for a longer period of time, the heat between solid-liquid interface increased so as to make the interface temperature more higher and increased the joint cooling speed. Under the same process, the welded joint of using ultrasonic for a longer time formed cracks faster and easier. At the same time,the shear strength of the joint was lower.Ultrasonic energy heats and vibration could break the oxide film on the surface of the solder and the parent metal. It could make the solder in direct contact with the parent metal and wet it. After the shearing test of brazing welding, we found that the fracture location appeared in graphite side. With the using of ultrasonic for 2 seconds, the solder could be completely spread out on the surface of aluminum while it could be completely spread out on the surface of graphite for 6 seconds. With the continuous using of ultrasonic, the solder could fully infiltrate the parent metal. Under the condition of vacuum, the solder could wet parent metal smoothly without the complete reaction layer. But the interface generated a straight and thick reaction layer with the using of ultrasonic.After the brazing welding head was observed under the transmission electron microscopy, we found that some grains in the interface had reached to nanoscale size about 40 nm. It made the groups of atoms on graphite surface stripped by the ultrasonic impact and then the solder reacted with the peeling groups of atoms forming the reaction layer in nanoscale size which blocked the active Ti in contact with the graphite. At last the thickness of reaction layer became thinner. This phenomenon was a significant impact of ultrasonic on parent metal. Due to the using of ultrasonic, the solid Sn-Ti in brazing filler metal was melted by ultrasonic energy which also improved the concentration of active Ti and its driving force in the interface reaction. Eventually the thickness of reaction layer increased.