Investigation On Microstructure And Properties Of The W-W Joints Brazed By Ni-based And Ti-based Fillers

As a kind of refractory metal,tungsten is an important high temperature structural material with high thermal conductivity,low thermal expansion coefficient,strong resistance to radiation,high temperature strength.And it is widely used in national defense,aerospace,electronics,nuclear industry and other fields.The ductile-brittle transition temperature(DBTT)of tungsten is higher than the common metal,which leads tungsten tend to be brittle at room temperature.Due to the brazing temperature above 1000? is around the recrystallization temperature(RT)of tungsten,grain coarsening phenomenon of tungsten marix is very serious.So the connection of tungsten and tungsten is greatly restricted by the substantial brittleness.In this paper,three kinds of Ni-based fillers(BNi2,BNi5,B?p27)and one Ti-based filler(37.5Ti-37.5Zr-15Cu-10 Ni wt.%)were used to braze W-W at different temperature.Differential thermal analysis(DTA),X ray diffraction(XRD),optical microscopy(OM),scanning electron microscopy(SEM),energy spectrum analysis(EDS),microhardness test,shear test,vacuum brazing experiments and fracture analysis methods were conducted to investigate the microstructure and properties of the fillers and joints.Besides,diffusion of fillers and the base metal in brazing process is analyzed,and the relationship between microstructure and properties of the joint is studied in detail.The work of this paper provides an important data reference for the development of the new Ni-based and Ti-based filler metal to braze tugsten and tungsten.The results showed that the diffusion of tungsten from the base metal to braze is not uniform in the joint brazed with BNi2 filler metal.And the voids can be observed in the central of the braze;the microstructure of the joint brazed with BNi5 filler is symmetrical in the middle line of the braze,and there is some small holes in the central of the braze;the fusion of the B?p27 filler and tungsten is better than BNi2 and BNi5 fillers,and the microstructure is very uniform,no cracks,holes and other defects can be observed in the joint.Melting erosion can be observed obviously in the joint brazed with Ni-based filler.This phenomenon was much more serious with the filler metal of BNi2 while it tended to slight when the filler metal was B?p27.The reaction occurred at the interface of filler metal and matrix to produce the intermetallic NiW when using all of three kinds of filler metal,which promoted the connection of filler and base metal.Matrix structures of the brazed seam were nickel-based solid solution.Besides,the elements in the brazing filler metal reacted with the diffusing tungsten to form the compounds such as NiWSi and Co3 W.The shear test results showed that the shear strength of the joint brazed with B?p27 filler metal was the highest,which was 87.38 MPa.35Ti-37.5Zr-15Cu-10 Ni filler metals were used to vacuum braze W/W at 900?,930?and 950? respectively.The results showed that fusion line of brazing and base metal was smooth.No cracks,holes and other defects can be observed at the brazed seam.With the increase of the brazing temperature,joint thickness decreased.When the brazing temperature was 950?,the phenomenon of melting erosion was obvious,and the width of brazing seam was 18?m(decreased about 50% compared with the temperature of 900? and 930?).When the temperature was 930?,the microstructure of the brazed joint was the most uniform.Microstructure was mainly composed of titanium-tungsten solid solution and titanium-zirconium based spherical eutectic.And there were also NiTi,Ni3 Ti,Cu3Ti,CuZr,Cu0.4W0.6,CuNi2 Ti,TiNi0.8Cu0.2 and other compounds in the brazing seam.The diffusion of tungsten from the base metal into the filler metal in the process brazing with Ti-based filler was weaker than the Ni-based filler.The shear test results showed that when the brazing temperature was 930?,the shear strength of W-W joint was the highest,which was 71.96 MPa.In summary,Ni-based fillers and Ti-based fillers all lead to the tight connection between the base metal and filler metal.When the Ni-based filler metal was used to braze W/W,the filler metal reacted with the matrix at the interface to produce compound and the melting erosion was obvious.When the Ti-based filler metals were used to braze W/W,compound and melting erosion were not observed at the interface of the joint.Ti-W solid solution were formed owing to the inter-diffusion between fillers and matrix.And there were no defects can be observed in the interface and braze.