Aluminum Fluoride Acid Cesium - Potassium Aluminum Brazing Flux On Aluminum Fluoride Synthesis

As a non-corrosive brazing flux, the Nocolok flux(KF-AlF₃) is widely used in brazing aluminum and its alloys. The melt point of this flux is 558℃and the biggest disadvantage in brazing is the operation temperature (600℃) is too high for most of the aluminum alloys whose collapsed temperature are lower than this , so they cannot braze with it. Therefore, it has becoming an important aspect for developing a flux which has the features of non-corrosive, insoluble and low melt point.The system CsF-AlF₃ is a reliable application prospect flux. From the phase diagram we can see that, the eutectic point E₂ contains x(AlF₃)42.0 mol% and x(CsF)58.0 mol% and the melt point is 471℃. But the price of CsF is expensive, therefore the emphasis of the research for fluxes is decreasing the cost and improving the activity. So it has great sense to study the ternary system CsF-AlF₃-KF for adjusting the melt points improving the performance and decreasing the cost. Most of the papers are conjecturing the feasibility of the ternary system CsF-AlF₃-KF from the theory, which has guide meaning but did not do research in application. There are little reports about the ternary system CsF-AlF₃-KF as a flux, especial in the brazing aluminum and its alloys.Based on the phase diagram of ternary system CsF-AlF₃-KF, this paper prepared a series of samples from cesium carbonate, potassium carbonate, aluminum hydroxide and hydrofluoric acid by chemical method, the samples were investigated by melting and spread ability test, then improved it from the samples that can melt clearly, has larger spread area and low content of CsF which is FM-2(AlF₃:CsF:KF=43 mol%:18 mol%:39 mol%). Tested the melt point with naichi STA409PC Differential Scanning Calorimetry ; analyzed the structure and pattern of the fluxes with D/MAX2200VPC power X ray diffraction instrument and S-3400N scanning electronic microscope; tested the spread area according to the GB11364-89 and shear strength by CMT4204 microcomputer controlled electron universal tension test machine with brazing filler metal Zn-Al30, lap joint, high frequency and industrial pure aluminum. The results showed that: the melt point of the fluxes FM-2-4 (AlF₃:CsF:KF=A4:C2:K4)and FM-2-5(AlF₃:CsF:KF=A5:C2:K5) is about 520℃, the content of CsF is low , so the cost is low too ; the main phase of the fluxes is Cs₂KAl₃F₁₂ and the rest are K₂AlF₅·H₂O or (and) KAlF₄; the pattern is simle and regulur; the spread area is large and larger in 560℃than that in 540℃, the shear strength of the brazing joint is 55 MPa, close to the strength of the base metal, meet the requirement of actual brazing.This paper also studied the relationship between the brazing joint and mechanical properties ,tested the microstructure of the brazing joint with metallographic microscope and the hardness with MH-5D micro hardness instrument; observed the fracture pattern with JCM-5700 SEM. The results show that: there are no gas hole, slag patch or crack in the brazing joint with the fluxes FM-2-4(AlF₃:CsF:KF=A4:C2:K4) and FM-2-5(AlF₃ :CsF:KF=A5:C2:K5); the center of the joint has the highest hardness, larger than the place the joint bonding with the base metal and the base metal is the lowest.; the fracture pattern is gliding with the fluxes FM-2-4 and FM-2-5 so the shear strength is high . There is intimate relationship between the shear strength, metallographic and fracture pattern of the brazing joint.