Study On The Method Of Ultrasonic-assisted Brazing Of Copper And Graphite

At present,the connection of metal and graphite is widely used in electrical industry,the connection of copper and graphite is more necessary in the commutator for the core parts of electrical equipment.The connection between graphite and copper is usually carried out under vacuum or inert gas protection,but it is limited by the size of furnace,it is difficult to achieve mass production in industry.With the advent of ultrasonic assisted brazing,it is possible to connect copper with graphite under air condition.Ultrasonic assisted reactive brazing technology is adopted in this thesis,two kinds of active solders,Ag-Cu-Zn-Ti filler at high temperature and Sn-Ag-Cu-Ti filler at low temperature,were selected and developed.The function of ultrasonic vibration in brazing process was analyzed,and the influence of brazing process and brazing parameters on mechanical properties of joint was studied,the connection mechanism was also discussed.The results show that: using Ag-Cu-Zn-Ti filler metal,the technology of ultrasonic assisted reactive brazing can realize the connection of copper and graphite under atmospheric condition.In the process of ultrasonic vibration from 2 s to 20 s,the existence of oxide film was not found in the brazing seam.When the brazing joint was analyzed,no metallurgical bonding between the filler metal and graphite was found razing temperature is an important factor affecting the quality of joints.At higher temperatures,the metal base material and the active filler metal are oxidized seriously at the time of welding,which increases the difficulty of removing the oxide film and makes brazing difficult.Microstructure analysis of brazed joints obtained by applying 2 s ultrasonic vibration shows that a large amount of broken oxide film exists on the graphite side of the brazing seam,with the prolonging of the ultrasonic time,the oxide film in the brazing seam decreases gradually,when the ultrasonic vibration time is 10 s,the oxide film in the brazing seam disappears completely,and the two kinds of base materials form a good joint.The microstructure of brazed joint from the copper side followed by copper/Cu-Zn intermetallic compound/Ag based solid solution+ Cu based solid solution/Ti C reaction layer/graphite,the connection between filler metal and graphite side is mainly reaction wetting bonding.Increasing the ultrasonic vibration time and increasing brazing temperature will cause the growth of interfacial reaction layer of Ti C,but it can not continue to increase,and the effect of applying ultrasonic time the shear properties of brazed joint,when the ultrasonic wave is not more than 10 s,the ultrasonic wave can improve the strength of the joint,if the ultrasonic vibration is applied continuously,the shear strength of the joint will decrease.The optimum technological condition is brazing temperature of 820 degrees,ultrasonic vibration time 10 s,10 min holding time,under this brazing condition,the maximum strength of brazed joints is 18 MPa.With Sn-Ag-Cu-Ti active filler metal,the connection of copper and graphite can be realized by ultrasonic wave under atmospheric condition.In the process of 2 s to20 s,no oxide film was found in the seam.When the brazing joint was analyzed,it was found that the metallurgical bonding between the filler metal and graphite was not found.There was no Ti C reaction layer formed on the graphite side when the 20 s was treated by ultrasound.The combination of the two kinds of parent material is mainly driven by the ultrasonic wave,the flow of the solder to the inner hole of the parent material graphite the mechanical connection formed by cooling the joint to normal temperature.The interface microstructure of brazed joints was Cu/Cu3 Sn intermetallic compound/Cu6Sn5 intermetallic alloy/Sn-Ti alloy,Sn-Ag alloy and a small amount of Cu-Ti alloy/graphite.When the brazing temperature is 450 degrees and 12 s ultrasonic is applied,the maximum joint strength is 16 MPa.The brazing joint of copper and graphite obtained by this kind of low temperature active brazing filler metal has a shearing strength greater than that of the base metal graphite itself.