Effect Of CuSn And Ni Powder On The Brazing Process And Seam Performance Of Flux-cored Silver Brazing Filler Metal

With the rapid development of advanced manufacturing,heterogeneous material connection structures emerge in an endless stream.Brazing has gradually become the internationally recognized mainstream connection method with its unique advantages.However,in some applications requiring high flowability and toughness of filler metal,traditional filler metal is difficult to form due to its component limitation.Based on the concept of flux-cored wire,metal powders(CuSn alloy powder,Ni powder)were added to the flux-cored flux.During the brazing process,the micro-zone reaction occured between the multi-element solder skin and the metal powder core,and then secondary metallurgy to achieve in-situ synthesis of high-performance silver brazing filler metal in this paper.The change trend of structure and properties of BAg30CuZnSn brazing filler metal with different content of CuSn/Ni powder were systematically studied by means of SEM,EDS,XRD,etc.The interaction rule between metal powder and outer skin of brazing filler metal during brazing process was revealed,and the flexible control of brazing seam composition was realized.The conclusions are as follows:(1)During the wetting process of brazing filler metal,the low melting point CuSn alloy powder in the core powder would melt prior to the core sheath.It could reduce solid-liquid interfacial tension,and play a leading role in wetting to promote the wetting spread of brazing filler metal.With the increase of CuSn alloy powder in the core,the wetting area of brazing filler metal on copper plate and steel plate increased;The results showed that the composite flux cored silver filler with CuSn alloy powder could be used for induction brazing of copper/steel,and the microstructure of AgCuZnSn alloy brazing joint with uniform composition could be obtained by in-situ reaction in the process of induction brazing.With the increase of CuSn content in the core,the amount of(Ag)in the braze joint increased to join each other,and finally coarsened to grayish white and distributed at the grain boundary.The size of(Cu)in the brazing seam increased gradually,and the eutectic structure of Ag-Cu-Zn decreased to disappear completely.At 30 wt.%CuSn,the brazing seam of copper/steel j oint was uniform and compact.The microhardness of as cast brazing filler metal and brazing joint increased with the increasing content of CuSn alloy powder.The tensile strength(161 MPa)and shear strength(277 MPa)of the joint reached to the maximum when the content of CuSn alloy powder is 30 wt.%.(2)The Ni powder added in the core could melt with the outer skin of the melted brazing filler metal during the wetting process and synthesize silver-containing brazing filler metal in-situ.The melting temperature of brazing filler metal increased with the addition of Ni powder.With the increase of Ni content in the powder core,the wetting area of composite flux cored silver brazing filler metal on copper plate and steel plate first increased and then decreased.The Ni element in the composite powder core could be transferred to the brazing joint during the induction brazing process,and the brazing joint of AgCuZnSnNi alloy with uniform microstructure could be synthesized in situ by fusing with the outer surface of brazing filler metal.A gray interconnected(Cu,Ni)layer was formed at the interface of steel side after adding Ni powder;With the increase of Ni content in the core,the(Cu,Ni)layer becomed columnar and extended to the center of the brazing seam.On the copper side,the thickness of diffusion layer increased with the increase of Ni powder content,and the columnar grains began to coarsen.The Ag-Cu eutectic structure in the center of the brazing seam decreased gradually to disappear completely.The brazing seam structure of copper/steel joint was more uniform and dense at 10 wt.%Ni powder.The results showed that the microhardness of filler metal and brazing joint decreased with the increase of Ni powder content,while the strength of brazed joint improved significantly.The copper/steel joint broake at the red copper side during tensile test,and the shear strength of steel/steel joint increased at first and then decreased.At 10 wt.%Ni powder,the shear strength reached to the maximum value(335 MPa).