Research On Microstructure And Properties Of Joints Of AlCoCrFeNi High-Entropy Alloy Brazed With Ni-Based Filler Alloy

The high entropy alloy’s unique high entropy,lattice distortion and other effects make it characterized by high strength,high hardness and excellent high temperature stability.It is widely used in automotive,aerospace,nuclear reactor and other fields.As a potential structural material,it has become research hotspots.However,current brazing studies on AlCoCrFeNi high-entropy alloys have not been reported.Therefore,it is important to study the high-entropy alloy brazing for its wide application.In this paper,Ni-based brazing filler metal is used to braze the typical AlCoCrFeNi high-entropy alloy in high-entropy alloy system.The effects of brazing process on the microstructure evolution and mechanical properties of AlCoCrFeNi high-entropy alloy brazed joints were studied systematically.The melting process of brazing filler metal and joining mechanism were discussed.The fracture mode of the brazed joint was analyzed by fracture morphology.The relationship between the microstructure and the fracture mechanism was established,and the high temperature stability of the brazed joint was studied.the result shows:（1）The typical microstructure of the brazed joint is divided into two parts:the fusion zone and the diffusion zone.The diffusion zone is close to the AlCoCrFeNi high-entropy alloy.The microstructure of the diffusion zone of the brazing joint at different brazing temperatures is BCC,FCC and CrB.The microstructure of the fusion zone changes with the increase of brazing temperature.When the brazing temperature is 1150℃,1200℃ and 1250℃,the microstructure is Ni-based solid solution and boride were formed.Further,the compound Al₃Ni₅ was precipitated at1250℃;when the brazing temperature was increased to 1300℃ and 1350℃,the fusion zone structure became BCC,FCC and CrB.（2）As the brazing temperature increases,the width of the fusion zone of the brazing seam does not change significantly,while the width of the diffusion zone increases gradually,and the microstructure is refined and the boride is reduced.In addition to the phase change in the fusion zone,when the brazing temperature is increased from 1300℃ to 1350℃,the partial boride morphology changes from the original black to gray,and the microstructure FCC becomes a thinner strip.The structure of it is similar with AlCoCrFeNi high-entropy alloy.（3）The fracture position of the brazed joint indicates that the fracture occurs mostly at the center of the brazing joint at brazing temperatures of 1150℃,1200℃ and 1250℃.When the brazing temperature is 1300℃ and 1350℃,the fracture occurs in the brazing seam and AlCoCrFeNi high entropy alloy base material.After holding time fixed for15 min,the joint shear strength first increase and then decrease with the increase of brazing temperature,reaching a maximum value of 687.2 MPa at1300℃.With the extension of holding time（0 min,15 min,30 min,45 min）,the joint shear strength also first increase and decrease,it reached the maximum when it was holded on 15 minutes.（4）After heat treatment the brazed joint obtained at 1300℃ for 15 min at 800℃ and 900℃,it was found that the microstructure in the brazing joint was refined,and a large amount of fine CrB was precipitated in the FCC phase.The shear test results of the brazed joints after heat treatment were 664.0 MPa and 593.6 MPa,respectively,indicating that the joints under the brazing conditions have good high temperature stability.In this paper,the high-strength connection of AlCoCrFeNi high-entropy alloy was realized by using Ni-based brazing filler mental for the first time,and the high-temperature stability of the joint was verified.