Microstructures,Properties,and Joining Mechanisms Of Tungsten Heavy Alloy/superalloy Joints Based On Amorphous Filler Design

Tungsten heavy alloys possess high density,high strength,high thermal conductivity,low thermal expansion coefficient,and excellent corrosion resistance,oxidation resistance,and impact toughness,which are considered as ideal materials for high temperature components in the aerospace,nuclear,military,and machinery manufacturing industries.Tungsten heavy alloys are manufactured by powder metallurgy precision forming process,large size and complex structural parts are difficult to form at one time due to the limitation of shape and dimension.It is necessary to join tungsten heavy alloy with other materials to make full use of the structural and functional advantages of composite components.As the physical,chemical,and mechanical properties of tungsten heavy alloys are special,the joining of tungsten heavy alloy is relatively difficult.With the application of tungsten heavy alloys and components in high temperature fields such as nuclear fusion reactors,the service conditions of joints are harsh,which puts forward higher requirements for the reliability and efficiency of tungsten heavy alloy joining technology,and the joining problem has become increasingly prominent.In this work,based on the joining feature of tungsten heavy alloy/superalloy dissimilar materials,the diffusion bonding and brazing processes were explored.The Cu-Ti based multi-component amorphous fillers with high composition uniformity and low melting temperature were designed and fabricated.The interfacial microstructures-mechanical properties-joining mechanism of tungsten heavy alloy/superalloy joining system were studied.The effects of brazing parameters on the interfacial microstructures and mechanical properties of joints were studied.The high temperature shear strengths,corrosion behaviors,thermal fatigue properties and underlying mechanisms were analyzed in detail.Tungsten heavy alloy/superalloy dissimilar materials were joined by diffusion bonding and brazing processes.The Cu-Ni-V-Ti interlayer alloy system was designed,and the effects of bonding temperature and holding time on the interfacial microstructures,mechanical properties,and defects of diffusion bonding joints were studied.The microstructure of the interlayer was（Cu,Ni）ss,a small amount of Ni₂V,Cu₄Ti₃ phases and Ti,V particles.Under the conditions of diffusion bonding temperature of 1100°C,holding time of 40 min and bonding pressure of 5 MPa,the shear strengths of the joints at room temperature and 400°C were 171.6 MPa and 113.8 MPa,respectively.The typical joint defects included voids,cracks,and brittle intermetallic compounds.Tungsten heavy alloy and superalloy were brazed with Cu-Ag based fillers.The wettability of Cu-Ag based fillers and the microstructures and properties of brazed joints were studied.Under the condition of brazing temperature of 1050°C and holding time of 20 min,the wetting area of Cu Ag₃₀V_0.4（wt.%）filler on the surface of tungsten heavy alloy was 132.4 mm²,the equilibrium contact angle was 15.94°.The shear strengths of the brazed joint at room temperature and 400°C were223.8 MPa and 93.1 MPa,respectively,and the fracture occurred at the interface of tungsten heavy alloy side and the center region of the brazing seam.The insufficient wettability of the brazing filler,low interfacial bonding capacity of the tungsten heavy alloy side,and less high melting point alloy elements at the brazing seam accounted for the poor high temperature performance of the brazed joints.Aiming at improving the technological performance and processability of brazing fillers,the Cu₅₀Ti₅₀,Cu_44.5Ti_44.5Ni_5.5Zr_5.5 and Cu_44.5Ti₃₉Ni_5.5Zr_5.5V_5.5（at.%）amorphous fillers were designed.The phase structures,melting characteristics,and wettability of crystalline/amorphous fillers with the same compositions were analyzed.The matrix phases Cu Ti,Cu Ti₂ and brittle phases Ti₂Ni,Ni Zr and Ni V₃,etc.were detected in XRD patterns of crystalline fillers,while uniform microstructures without crystalline phases and segregation were observed in amorphous fillers.The solidus temperature T_m and liquidus temperature T_lof crystalline Cu₅₀Ti₅₀ filler were 865.9°C and 970.9°C,respectively,while the solidus temperature T_m and liquidus temperature T_l of amorphous Cu₅₀Ti₅₀ filler are 861.7°C and936.0°C,respectively.Under the same composition,the solidus temperature T_m,liquidus temperature T_l,and the melting range of amorphous fillers decreased,and the addition of alloying elements Ni,Zr and V reduced the solid/liquid temperatures of the fillers.A good wettability was obtained in amorphous filler,and the quasi-equilibrium contact angle of the amorphous Cu_44.5Ti₃₉Ni_5.5Zr_5.5V_5.5 filler/tungsten heavy alloy wetting system was 1.74°after wetting at 1000°C for 300 s.During the brazing process,the atom diffusion activation energy was reduced by releasing of the latent heat of crystallization of amorphous filler,promoting the adsorption and diffusion of active elements to the triple line,and the solidified microstructures of brazing seam were relatively refined.The effects of brazing parameters on the interface formation and mechanical properties of tungsten heavy alloy joints brazed with amorphous fillers were studied.The erosion of the liquid filler to the binder phase and the interfacial metallurgical reaction were effectively suppressed through controlling the brazing temperature and holding time,and thus the separation of W particles was reduced.The separation of W particles was caused by the infiltration of Ti and Cu elements,which reacted with the binder phase to form（Fe,Ni）Ti and Cu_0.8Fe_0.2Ti phases.The dihedral angle between adjacent W grains decreased,and the W grain boundary was partially dissolved.During the solidification process,the W particles gradually separated from the matrix with the movement of the solid-liquid interface.Under the condition of brazing temperature of 925°C and the holding time of 20 min,the alloying reaction of the brazing seam and the metallurgical bonding of the brazed interface were enhanced,and only a small amount of W particles with a dimension less than 20μm was observed at the interface.The shear strengths and high temperature performance of the joints were superior,and the fracture occurred at the central region of the brazing seam.The interfacial microstructures of tungsten heavy alloy and superalloy dissimilar joints brazed with amorphous fillers were studied,and the corrosion behaviors and thermal fatigue properties were analyzed.The amorphous Cu_44.5Ti₃₉Ni_5.5Zr_5.5V_5.5 joint showed the highest Ecorr(-0.911 V_SCE),the lowest Icorr(2.858 m A·cm^-2),the highest charge transfer resistance(1610.5Ω·cm^-2),indicating the excellent corrosion resistance.The corrosion resistance mechanism was revealed as follows:with the addition of Ni,Zr and V alloy elements,more competitive phases at the brazing seam were produced and the primary batteries increased,and the corrosion dissolution of the superalloy substrate was inhibited;a dense surface passivation film（Fe₂O₃-Ni O-Co O-VO）was formed to prevent the penetration of corrosive ions Cl^-and O^2-,and thus the expansion of micro-galvanic corrosion was effectively inhibited.The typical fatigue defects included W particle migration,continuous cracks in the diffusion layer,voids in the brazing seam,and uneven and coarsened microstructures.The mechanical properties of brazed joints after thermal fatigue tests were quantitatively analyzed,and the correlation between interfacial microstructures and mechanical properties was established.The shear strength of joint brazed with amorphous Cu_44.5Ti₃₉Ni_5.5Zr_5.5V_5.5 filler was 508 MPa.After 500 thermal cycles,the shear strength of the joint decreased to 368 MPa,and the strength retention rate was 72.4%.Combined with TEM analysis of the crack tip and residual stress distributions,the crack propagation behavior under cyclic thermal fatigue load was analyzed,and the underlying mechanism of thermal fatigue defects was revealed.