Study On The Interaction Mechanism Between Flux And Coated Metal Of Zinc-aluminum Flux-cored Filler Metal

Zn-Al flux-cored filler metal includes the cladding Zn-Al alloy and the core flux generally using non-corrosive cesium fluoroaluminate flux,which greatly improves the brazing efficiency due to neither additional flux nor cleaning residual flux after welding.Unfortunately,the aging problem in the long-term natural storage of Zn-Al flux-cored filler metal has seriously affected its brazing performance.Except for the embrittlement of the cladding Zn-Al alloy,the aging of the zinc-aluminum flux-cored filler metal also includes the deterioration of the core CsF-AlF3 flux,and black slag presenting at the interface between the cladding Zn-Al alloy and the core flux.In this paper,the microstructures and properties of the fluxes by temperature and damp-heat treatment and the corresponding brazed joint were investigated by means of tissue testing such as SEM,EDS and XRD,and performance testing methods of spreadability test,wettability test,film-removing ability test,microhardness and tensile strength test.The effects of temperature and damp-heat time on the microstructures and properties of fluxes and the interfacial mechanism between the cladding Zn-Al alloy and the core flux without and with damp-heat treatment were studied.The aging mechanism of zinc-aluminum flux-cored filler metal was further analyzed.The results showed that as the treatment temperature of the flux increased,the microstructure of the flux gradually changed from flocculent to ribbed and massive,and a large number of hollow tubular morphology appeared when the temperature exceeded the melting point of the flux.Moreover,the new phase Cs2AlF5,CS3AIF6 appeared in the flux treated at 465 ? and 500 ?,respectively.In the spreadability test of CsF-AIF3 flux,the flux obtained by treating at 500 ? did not show a wetting ring on the pure aluminum plate while the wetting ring was observed under other conditions,and the spreading area of CsF-AlF3 flux increased with the increase of the pretreatment temperature of the flux.In the wetting test of ZnA115 alloy,the spreading area of the zinc-aluminum alloy decreased as the flux pretreatment temperature raised.In the film-removing test,the content of oxygen and zinc always changed in a way of shift,and the flux treated at 285 ? had the best film removal ability.Moreover,the higher the pretreatment temperature of the flux,the lower microhardness and tensile strength.The flux was hygroscopic under the damp-heat condition.In addition,acicular and prismatic CsAlF4 in the flux increased with the prolonged damp-heat time,wherein the acicular CsAlF4 was a whisker.The presence of whiskers during the short damp-heat time inhibited the initiation of cracks in the zinc-aluminum core solder.The prolonged damp-heat time leaded to an increase in the toughness and strength of the whiskers,which accelerated the generation and expansion of cracks.The wetting ring appeared on the pure aluminum plate in the spreadability test of CsF-AIF3 flux,and the spreading area of the flux and the zinc-aluminum alloy reduced with the prolongation of the damp-heat time of the flux.In the film-removing test,with the prolongation of the damp-heat time of the flux,the film removal ability decreased.The longer the damp-heat time of the flux,the lower microhardness and tensile strength.The microstructure of the zinc-containing reaction products before the damp-heat treatment was not significantly different from the microstructure of the flux treated only at 500 ?,except for the fine dense granular morphology in the reaction product containing 6 wt.%Zn.The microstructure of the zinc-containing reaction products after the damp-heat treatment was not significantly different from that of the flux treated only at 500 ?,and the needle-like and block-like morphology increased.In the microstructure of the aluminum-containing reaction products before the damp-heat treatment,the reaction product containing 2 wt.%of A1 element was not significantly different from the microstructure of the flux treated only at 500 ?.However,a large number of small granular morphology and flower-like morphology appeared in the reaction products containing 4 wt.%A1 and 6 wt.%Al,respectively.The microstructure of the aluminum-containing reaction products after the damp-heat treatment showed a flower cluster and a flower-like morphology compared with the microstructure of the flux treated only at 500 ?.At high temperature,ZnF2 produced by the reaction of flux and zinc had little effect on the aging of zinc-aluminum core solder.Cs2O2 produced by the reaction between flux and aluminum produced CsOH in the hot and humid environment,accelerating the aging and brittleness of the zinc-aluminum flux-cored filler metal.In addition,the quality of the joints brazed with the zinc-containing and aluminum-containing reaction products without and with the damp-heat treatment was very poor.In summary,the heating temperature and the hot and humid environment were not conducive to the application performance of the flux.Moreover,the whiskers generated in the hot and humid environment of the flux,and the interaction between the flux and the cladding metal,all had effects on the aging brittleness of the zinc-aluminum flux-cored filler metal.