A method for brazing a copper tube and an aluminum tube without brazing flux comprising the following steps: firstly, expanding one end of either of the copper tube or the aluminum tube into a socket by thermal expansion, manufacturing a solder alloy sheet into a thin solder tube (3) by stamp-molding, and placing the thin solder tube into the socket; secondly, inserting the end of the other tube to be welded into the thin solder tube, and tightly abutting the end of the tube to be welded to the bottom of the socket to form a close fitting; and lastly, assembling the copper tube and the aluminum tube on welding clamps so as to supply partial induction heating to the joint portion, then applying appropriate welding pressure and ultrasonic vibration. The method can be used for brazing thin-wall copper tubes and aluminum tubes without brazing flux, and provides the advantages of long fusion lines, tight weld seams, and no slag intrusion. The method greatly improves the air tightness and the bonding strength of the joint portion, and provides high reliability, short welding time and great value in production and use.
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