Investigation On Microstructure And Properties Of The TZM Alloy Brazed By Mo-based Fillers

TZM alloy is a good thermal energy storage material with high melting point,good corrosion resistance,excellent electrical and thermal conductivity and low coefficient of linear expansion.Therefore,TZM alloy is widely used in aerospace,nuclear industry and electronics industry,etc.important fields.However,the melting point of brazing TZM alloys is generally below 1000°C,while the recrystallization temperature of TZM alloys is above 1400°C.In order to expand the application of TZM alloys,the development of new high-temperature filler metal has always been the focus of brazing field.In this paper,industrial TZM alloys were vacuum brazed with self-made molybdenum-nickel-titanium?Mo-52.3Ni-xTiwt.%?high-temperature filler and molybdenum-cobalt-titanium?Mo-63Co-xTiwt.%?[x=0,1,3,5,7,9,]high-temperature filler.Afterwards,metallographic observation,scanning electron microscopy,energy spectrum analysis,X-ray diffraction analysis,wetting test,microhardness test,shear test,and fracture morphology analysis were used to investigated the microstructures and properties of the said alloys and the corresponding brazing joints.The diffusion behavior of Ti element in the during vacuum brazing,and the relationship between the Ti element distribution and the joint microstructures and properties were preliminary discussed and finally further analyzed the influence mechanism.The results show that the TZM alloy was vacuum brazed with Mo-52.3Ni brazing filler metal,the surface of the brazing seam was smooth and flat.However,there were flat holes in the interior of the brazing seam,and most of these holes passed through the inside of the crystal.Tiny pores and inclusions were existed in the brazing seam while vacuum brazing with Mo-52.3Ni-1Ti and Mo-52.3Ni-1Ti brazing filler metal,and the amount of brazing material infiltrating into the parent metal was very small.When vacuum brazing with Mo-52.3Ni-5Ti brazing filler metal,there was slight eroding at the fusion line of the brazing seam.But the width of the brazing seam was uniform,and there were no inclusions,pores and cracks in the brazing seam.When vacuum brazing with Mo-52.3Ni-7Ti and Mo-52.3Ni-9Ti brazing filler metal,the interface structure of the joint changed obviously,and the brazing material had a strong eroding to the base metal.When the TZM alloy was vacuum brazed with Mo-52.3Ni-base brazing filler metal,the brazing seam mainly contained MoNi phase and Ni₃Mo equivalent compounds.With the addition of Ti element,Ti-based solid solution and Ni₃Ti phase appeared in the brazing seam.When the content of Ti in Mo-52.3Ni was 5%,the shear strength of vacuum brazed TZM alloy joint was the highest,which reached 186.43MPa.When the TZM alloy was vacuum brazed with Mo-63Co filler metal,the base metal and the brazing seam were even and uniform,and the base metal was free from erosive phenomena.Meanwhile,the fusion line of the brazing seam was smooth and there was no defect inside the brazing seam.When vacuum brazing with Mo-63Co-1Ti brazing filler metal,micro cracks were found at the braze joints,and the base metal was free of erosive phenomena.In addition,the fusion line of the brazing seam was smooth.When vacuum brazing with Mo-63Co-3Ti brazing filler metal,there were micro-cracks in the brazing seam.The brazing filler metal melted fully in the brazing seam,and the grains in the brazing seam were fine.When vacuum brazing with Mo-63Co-5Ti,there was no galvanic corrosion and inclusions or pores at the joints of the braze joints,while crack defects existed.When vacuum brazing with Mo-63Co-7Ti,the microstructure in brazing joint was coarse,and there were some defects such as shrinkage,shrinkage and microcracks in the vicinity of the fusion line.When vacuum brazing with Mo-63Co-9Ti,there were large shrinkage cavity and clear intergranular permeation phenomenon in the braze joint.When the TZM alloy was vacuum brazed with Mo-63Co base brazing filler metal,the brazing seam mainly contained the compounds of Co₇Mo₆ phase,Co₃Mo phase and Co₂Mo₃.With the addition of Ti content,Ti-based solid solution appeared in the brazing seam.When the Ti content of Mo-63Co solder was 3%,the shear strength of the TZM alloy joint was as high as 191.6 MPa.In summary,the overall performance of vacuum brazing TZM alloys with Mo-52.3Ni-5Ti and Mo-63Co-3Ti was optimal.