Research On Microstructure And Properties For Solder And Joint Of Brazing Cu/Al

Cu/Al brazing can alleviate the shortage of copper resources and the high price of copper.Cu/Al brazing has wide application in the manufacture of important equipment.Nevertheless,there are still some problems in Zn-Al solders commonly used in Cu/Al brazing.The soldering performance of flux-cored Zn-Al solders is better than that of solid Zn-Al solders,but the flux-cored Zn-Al solders are easier to"ageing",especially under high temperature environment of summer.In this paper,the effects of annealing temperature on the microstructure and properties of solid Zn-2A1 solders,flux-core Zn-2A1 solders and their Cu/Al brazing joints were studied by means of OM,SEM,EDS,XRD,resistivity test,tensile test,hardness test,comprehensive thermal analysis and wetting test.Meanwhile,the effects of heating temperature on the microstructure and properties of the flux and the interaction between the flux and the coated metal are also investigated.The reasons why the flux-cored solders are easier to "ageing" are revealed.The research results have important directive to develop new solders and improve their properties.The results show that with the increase of annealing temperature,the grain size of the solid Zn-2A1 solders gradually grows up,and a large distorted equiaxed grain is formed eventually,and the amount of a-Al gradually increases.The amount of(?+?1)eutectic decreases gradually and disappears eventually.The microhardness increases first and then decreases.The tensile strength decreases gradually,and the toughness rises first and then reduces.The wetting area is the largest when annealed at 100?.The grain size of ?-Zn and CuAl2 phases in brazed joints annealed at 100? is the smallest,and CuAl2 phases on the side of Cu annealed at 175? or above grow up.The shear strength of joints presents the trend of fluctuation,and the shear strength of Cu/Al joints brazed by solders after annealing at 100? is the highest.The recrystallization texture appears in the flux-core Zn-2A1 solders annealed at 100? or 175?,but disappears after annealing at 250?,and forms equiaxed crystal when annealing temperature reaches 325?.With the increase of annealing temperature,the microhardness depresses first and then increases.When annealing temperature reaches 325 ?,the tensile strength decreases greatly.The wetting area of solders and toughness first uprise and then descend.The mechanical properties of brazed joints vary greatly,especially after annealing at 325?,the brazed joints have no use value.With the increase of the heating temperature,the size of fluxes lump microstructure decreases.Fine needle-like microstructure grows on the surface of lump microstructure after heating at 250 ?,and transforms into rod-like microstructure after heating at 325?.The fluxes untreated and heated at 100? are composed of CS2AlF5·H2O,CsAlF4,CsOH and impurities.The main components of fluxes treated at 175? and above are CsAlF4,CsOH and impurities.The wetting area of the fluxes treated at 100? reaches maximum.The microstructure of joints brazed with fluxes treated at 100? consist of coarse dendrite,lamellar eutectic and a small amount of light gray massive phases.There is little difference between the microstructure and properties of joints brazed using untreated and treated fluxes at or above 175?,respectively.The shear strength of the above joints increases first and then decreases,reaching its maximum brazing with fluxes treated at 100?.The flux-cored solders are conducive to improving the weldability compared with solid core-solders.Oxidation corrosion of the coated metal both internal and external leads to a rapid decline mechanical properties of the flux-cored solders during annealing.Furthermore,the corrosion products of the coated metal react with the fluxes results in the failure of the fluxes,which shows that the flux-cored solders are easier to age than solid solders does.