The Effect Of Solid-liquid Interaction On Microstructure And Corrosion Property Of Brazing Sheet During Brazing

The strong liquid-solid interaction of multi-layer aluminum sheets during brazing leads to dissolution,corrosion,isothermal solidification and liquid film migration（LFM）.This not only reduces the brazing performance of multi-layer aluminum sheets,but also reduces the uniformity of as-brazed microstructure distribution,destroys the potential gradient design of multi-layer aluminum sheets and seriously damages the corrosion performance of multi-layer aluminum sheets.In this paper,the interaction between molten clad layer and solid core layer during brazing and its effect on as-brazed microstructure were studied.On this basis,the composition and structural design of multi-layer aluminium sheets are optimized to avoid the negetive effects caused by liquid-solid interaction,aming to provide theoretical support for the development of multi-layer aluminium sheet with better performance.The results show that strain induced liquid film migration（SILFM）usually occurs in the micro-deformed multi-layer aluminum sheets.Due to the small strain,the core alloy only recovers and does not recrystallize before the clad material melts.When the core material is in contact with the molten clad material,the residual storage energy provides the driving force for SILFM.The transport process of liquid film migration is tracked by Ni element labeling.The results show that the molten clad continuously supplies the liquid film through the liquid film channel formed on the grain boundary.After brazing,a large number of Si,α-Al（Fe,Mn）Si and Al₂Cu particles are accumulated on the surface of AA4343/AA3003/AA4343 multi-layer aluminum sheets.The first principle calculation results show that Si,α-Al（Fe,Mn）Si and Al₂Cu particles are cathode phase,and the intrinsic potential differences between them and Al matrix are 500-1350 m V,140-210 m V and120-470 m V,respectively.These phases acts as cathode phases to continuously accelerate the corrosion of Al matrix.After the sea water acidified accelerated test（SWAAT）for 25 days,the intergranular corrosion in AA4343/AA3003/AA4343 multi-layer aluminum sheets has basically penetrated the whole sheet thickness,and the sheet completely fails.After the post-brazing heat treatment at 325°C,a large number ofα-Al（Fe,Mn）Si particles are precipitated in the diffusion zone of aluminum sheets.Due to the precipitation ofα-Al（Fe,Mn）Si particles,the content of solid solution[Mn]and[Si]of Al matrix in the diffusion zone decreases,which reduces the corrosion potential of the diffusion zone.The optimized potential gradient of the clad layer-diffusion zone-core layer can greatly improve the corrosion resistance of the alloy.The sufficiently large potential difference in AA7072/AA3003/AA4343 multi-layer aluminum sheets and the uniformity of AA7072 microstructure can provide good protection for core alloy.After the SWAAT for 20 days,AA7072/AA3003/AA4343 multi-layer aluminum sheets did not penetrate.Due to the good corrosion potential gradient design,AA4343/3Z19/3Z45/AA4343 multi-layer aluminum sheet still has no obvious penetration corrosion after the SWAAT for 25 days.The supersaturated solid solution Mg and Si in the core material precipitate during the natural aging process to form GP zone,resulting in significant aging strengthening effect.The aging strengthening basically reached the peak after the natural aging for 21d.The yield strength of four-layer aluminum sheet increased sharply from 59 MPa to 91 MPa.The tensile strength increased sharply from 175 MPa to 218 MPa.