Study On Filler And Brazing Technology For CuAgZr Alloy

Due to high efficiency thermal conduction,heat exchangers are key components in variety of fields,such as energy,power system and gas separation system.Accompanying with the rapid development of aerospace,navigation,hydrogen energy,electronic power and other fields in recent years,the development of heat exchangers gradually turns to higher heat efficiency,higher strength,enhanced corrosion resistance and miniaturization directions.The materials used for heat exchangers are developed from traditional aluminum alloys and stainless steels to copper alloys which have a high strength and simultaneously a high conductivity.In particular,the CuAgZr alloys are usually utilized for fuel separation heat exchanger in the field of hydrogen catalytic combustion.The brazing connection between the fin and the baffle in the heat exchanger processing will significantly affects some key properties,such as the connection strength,heat transfer efficiency,and anti-shocking property and hence the service reliability.This pape studyed the connection of CuAgZr alloys which is vacuum brazed by different brazing materials and corresponding brazing processes.By using X-ray diffractometer,differential scanning calorimetry(DSC),Scanning Electron Microscope(SEM),hydraulic universal testing machines,and etc,the microstructure and shear strength of brazed joints produced by different brazing materials at a wide temperature range of 500-900? were investigated.The brazing materials include:AgCuZnMn filler for vaccum brazing at 800-900?;AgCuGaInSn filler for vacuum brazing at 600-700?;and SnAgCu(Si)filler for brazing-aging intergration at 500?.The main results of present paper are list below:(1)The brazed joints of CuAgZr alloy by using AgCuZnMn filler mainly consist of the regions of substrate,diffusion zone,interface zone and filler metal zone.The interface zone and filler metal zone are composed of Cu-rich phase,Ag-rich phase,AgCu eutectic structure and coarse CuZr phase.The microstructure and shear strength of the brazed joint are highly dependent on brazing temperature,holding time,and weld width.Especially,the microstructural morphology of interface zone and filler metal zone,the size of CuZr grain,and the content of Mn solute in the filler metal zone are greatly sensitive to the technological parameters that affect the dissolution and isothermal solidification behavior of substrate to the filler metal zone.The CuZr phase reduces the shear strength of the joints.The high is the temperature is,and the larger is the CuZr grain,and therefore the lower is the shear strength.However,the shear strength of joints is enhanced with the Mn solute content,which is remarkably varied with brazing temperature and weld width.The lower is the brazing temperature,the larger is the weld width,the higher is the Mn content,resulting in an improved shear strength.In comparison,the holding time has slight effect on the shear strength of the joint.In order to ensure the service reliability of the joint,the CuZr phase size should be as small as possible,while the Mn element content should be as great as possible.In these considerations,the brazing process with large weld width and low brazing temperature for a long time is favored for the connection of CuAgZr alloy.The optimal brazing process of AgCuZnMn filler for brazing CuAgZr alloy was finally determined to be 200?m weld width at 840? for 20min,which yielded to strong brazed joint with a shear strength of about 259 MPa.(2)A series of AgCuGaInSn strip fillers containing different Ga,In,and Sn elements(GaInSn,in total content of 5wt%-15wt%)were prepared by traditional smelting and processing technology.These fillers are mainly composed of Cu-rich phase with solutionized Ga solute and Ag-rich phase with solutionized In.With the increase of the total content of GaInSn elements,the ductility of the filler decreases and the processing difficulty increases.Based on the machinability and DSC test results,the Ag61.2Cu23.8Ga3In10.5Sn1.5(in wt%)filler metal with a liquid point of 675? was selected as the preferred filler metal for brazing CuAgZr alloy.By using this filler,various brazing joints were produced in different weld width and different brazing temperature to investigate the evolutions in microstructure,shear strength,and fracture behaviors.It was found that the joints were mainly composed of Ag-rich and Cu-rich phases,where some tiny need-shaped Cu phase existed with in the Ag-rich phase while Sn segregation was detected within the middle region of the joint with a thicker weld width.The shear strength of joints is dependent on the brazing temperature and weld width that affect the Cu solubility in the substrate.Generally,the shear strength increased with raising brazing temperature and widening the weld width.By comparing with the brazed joint derived from AgCuZnMn filler,the joints derived from the AgCuGaInSn filler displayed the comparable shear strength but enhanced plastic deformation.In addition,the AgCuGaInSn filler metal has lower vapor pressure and brazing temperature,which makes it more suitable for vacuum brazing the CuAgZr alloy.(3)Sn-based filler was developed for brazing-aging integration at 500?.By optimizing the composition of SnAgCu(Si)filler as well as the weld width,the brazed joint consisting of Cu-rich phase and Ag-rich phase.highlighted as an all-metal solid solution phase,was produced by using(39.8-39.062)Sn-43.2Ag-16.8Cu-(0.20-0.938)Si(wt%)filler in a weld with of 7-10?m.This ensures a high brazing strength of more than 200MPa and simultaneously improves the plastic deformation of the brazed joint as charazterized by a fracture mode of microvoid aggregation.