| Silver-based brazing filler metals are widely used in important fields such as aerospace,automotive engines and electronics benefited from its moderate melting point,good wettability,higher mechanical properties(strength,plasticity,toughness),excellent corrosion-resistance,and high strength of brazing joints.The appearance of flux-cored filler metals solves the problems such as low brazing efficiency,unstable quality of brazing joints,and environmental pollution caused by flux loss,and these problems are resulted from the separate use of brazing filler metal and flux during the conventional brazing process.In this paper,the effects of heating temperature on microstructures and properties of silver-based filler-metal/flux were investigated,which have guiding significance for the improvement of production process of flux-cored silver-based brazing filler metal.In this paper,characterizations and tests such as OM,SEM/EDS,XRD,DSC,wettability test,spreadability test,hardness test,and tensile test were used to investigate the effects of heating temperature on microstructures and properties of silver-based solid brazing filler metal BAg30CuZnSn,flux and their brazing joints.In addition,the actions of flux to brazing filler metal in brazing filler metal/flux composites treated in thermal fields with various temperatures were investigated and the influencing mechanism evolution of flux was further revealed.The results showed that as the annealing temperature increased with the same holding time,there were almost no new phases were produced in separately annealed brazing filler metal.The phases composition of annealed filler metal were mainly Cu-based solid-solution,Ag-based solid-solution,(Cu+Ag)eutectic,the compounds of CuZn and(Cu Sn).At the annealing temperature of 300～450 0C,the morphology and quantity of flaky Cu-based solid-solution and(Cu+Ag)eutectic changed obviously.As the annealing temperature increased,the hardness and tensile strength of brazing filler metal exhibited the following evolutions:increasing firstly,then decreasing,and finally increasing again,the change tendency of elongation after fracture was just the opposite to that of tensile strength.Moreover,the hardness,tensile strength reached to a minimum and elongation after fracture reached to a maximum,respectively,at the annealing temperature of 400 ℃.In addition,with the annealing temperature of 400 ℃ and 450 ℃,both solidus and liquidus temperature grew slightly,but the wetting spreading area and the tensile strength of brazing joint were both reduced.The main phase composition of the flux is KBF4,H3BO3,HBO2,B2O3,KHF2,and K2B4O7.In addition,the new compounds of KBF(OH)3,K2B3O3F4OH were produced after the separate heating of flux.With increasing heating temperatures,the flow area of flux raised continuously,and the microstructures,morphologies of flux showed various evidently.Nevertheless,the flux treated by different heat temperatures did not affected the wettability of sliver-based brazing filler metal and the tensile strength of brazing joints.Silver-based brazing filler metal/flux composites were treated in thermal fields with different temperatures.At the thermal fields temperature of 150～300 ℃,the microstructure change of the silver-based brazing filler metal is the same as that of the filler metal separately annealed under the same temperature.At the temperature of 350～450 ℃,a region composed of flaky Cu-based solid-solution and Ag-based solid-solution was formed at the boundary of the silver-based brazing filler metal adjacent to the flux,which is widened with ascending temperature.Moreover,small corrosion pits begins to appear on the outer surface of the filler metal when the temperature is higher than 300 ℃.In addition,as the thermal fields temperature increased,the tensile strength of silver-based brazing filler metal lifed firstly and then fell continuously,showing a lower level within 350～450 ℃ than that of filler metal annealed separately;the solidus-liquidus temperature and the wetting spreading area of filler metal did not show evidently change.In summary,the silver-based solid brazing filler metal BAg30CuZnSn was most suitable for plastic processing at an annealing temperature of about 400 ℃.The heating temperature lower than 450 ℃ of flux did not affect the tensile strength of brazing joints.Nevertheless,the microstructure and tensile strength of brazing filler metal will be affected by flux when the filler metal/flux composites were treated in thermal fields with 350 ℃. |
