Study Of The Interfacial Behavior And Corrosion Resistance Of The Cu/Al Joints By Frequency Induction Brazing

Due to the lack of copper resources, the price of copper has been very high.“Aluminum instead of copper” has become the hotspot of the materialsubstitution in the air-conditioning refrigeration industry. Cu/Al joints are moreand more widely applied in the air-conditioning refrigeration industry with thematuring of “aluminum instead of copper” technology, but the deficiencies ofCu/Al joints in mechanical properties and corrosion resistance still exist. Thispaper selected three kinds of Zn-Al flux-cored wire (FC502, E05NC andSUPER5A) and achieved the connection of copper and aluminum by highfrequency induction brazing, and this paper systematically studied themicrostructure, mechanical properties and corrosion resistance of the joints.The research results showed that the microstructure of the joint after FC502brazing is mainly composed of α-(Al) solid solution, Zn-Al eutectic, Cu-Znintermetallic compound and Zn-Al-Cu intermetallic compound. There areobvious differences between Cu3.2Al4.2Zn0.7brittle phase and braze metal in termsof strength and plastic, under the action of external force, easy to become thesource of crack. The shear strength of the joint is32.2MPa. The fracture is flushlight, which belongs to brittle fracture. As the same time, the joint is prone topitting and intergranular corrosion, corrosion resistance performance is poor. Themicrostructure of the joint after E05NC brazing is similar to the joint’s afterFC502brazing. But because of adding a certain amount of Cu elements and traceelements of Ag, the IMC thickness becomes small. There are a few small sizes ofCuAl2phase in the brazing seam. The shear strength of the joint is34.4MPa.Corrosion resistance of the joint is better than the joint after FC502brazing. TheMicrostructure of the joint after SUPER5A brazing is mainly composed Al basedsolid solution with Zn based solid solution mixture and grey blocky CuAl2phase. Because of adding a certain content of Ag in the brazing filler metal, the brazingfiller metal has good wettability on the surface of Cu/Al. The joint has fewerdefects after brazing. The shear strength reached39.3MPa. There are many largeand small uniformly distributed dimples in the fracture, which belongs to ductilefracture. Its corrosion resistance is the best of the three kinds of the Cu/Al joints.Polarization curves results showed that corrosion potertial of the copperbase material is the highest, corrosion current density is the smallest, corrosionresistance is the best. But aluminum base material is on the contrary. Thecorrosion current density of the joint after FC502brazing is the largest of thethree kinds of Cu/Al joints, so its corrosion resistance is the worst. The corrosioncurrent density of the joint after SUPER5A brazing is the least, its corrosionresistance is the best, followed by the joint after E05NC brazing. The results ofpolarization curves and the results of Cu/Al joints after corrosion are consistent.At present, using high viscosity SAC305solder paste brazing brassoscillator, but the solder has a large number of pores. On the one hand, theexistence of pores has hider trouble on product performance. On the other hand,SAC305belongs to high Ag (≥1%) solder with the higher cost. Thus, this papertakes Sn-58Bi, Sn-17.0Bi-0.5Cu (SBC1705) and SAC305as the research object,solderability of three kinds of high viscosity pastes on the brass surface and theinternal porosity of brazing joints were compared, in order to select a suitablesolder paste with low price for the induction brazing process of brass oscillctor.The research results showed that, in the aspect of weldability, SBC1705paste is the best, SAC305paste is relatively poor. In the solder joint porosity, theformer is less than1%, the latter is5-7%, both in the less than25%qualifiedrange. PIM test results of oscillator after two kinds of solder pastes soldering arein line with the standards of communication industry. But the former is morestable. For Sn-58Bi solder paste, the solderability on the brass surface isrelatively good, but its resistivity is relatively large. Comprehensiveconsideration, SBC1705solder paste is more suitable for induction brazingprocess of brass oscillator.