Microstructure And Properties Of Vacuum Brazed Joints For Mo-Cu Alloy And GH4169 Superalloy

Mo-Cu alloy is made of two immiscible metals including Mo and Cu. Mo-Cu alloy has been concerned in recent years due to its excellent physical properties including high-temperature strength, high hardness, high melting point, high electrical and thermal conductivity, and relatively low thermal expansion coefficient. Consequently, Mo-Cu alloy has been widely applied in the fields of aerospace, nuclear and electronic industries. Therefore, Mo-Cu alloy is considered to be an attractive structural material. However, the low temperature embrittlement and less resistance to elevated oxidation of Mo-Cu alloy have limited its practical applications seriously.In this work, the brazing of Mo-Cu alloy with GH4169 superalloy was conducted with Ni-Cr-Si-B filler metal and Ni-Cr-Si-B amorphous filler metal in a vacuum of 1×10-5 Torr. Good combination between Mo-Cu alloy with GH4169 superalloy was prepared at 1100-1150℃ for 20min. The microstructure and fracture morphology of Mo-Cu/GH4169 brazed joints were characterized by means of optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS). Shear strength and microhardness were evaluated by electromechanical universal testing machine and microhardness tester. It is hoped that the results of the present work would provide a theoretical foundation and experimental basis for the brazing of Mo-Cu alloy with GH4169 superalloy, as well as broaden the engineering application scope of Mo-Cu alloy.The results indicated that Ni-based filler metal (Ni-Cr-Si-B and Ni-Cr-Si-B amorphous filler metal) had a good wetting with Mo-Cu alloy and GH4169 superalloy, and there were good bonding interface without crack or micro-hole observed in the brazed joint. According to the solidification characterization and microstructure, Mo-Cu/GH4169 brazed joint was divided into three zones:isothermal solidification zone (ISZ), athermal solidification zone (ASZ) and diffusion affected zone (DAZ).When Mo-Cu alloy and GH4169 superalloy was brazed with Ni-Cr-Si-B filler metal, the diffusion of boron and silicon from brazing filler metal into base metal at the brazing temperature is the main controlling factor pertaining to the microstructural evolution of the joint interface. When the brazing temperature was 1100～1120℃, the brazed seam was mainly made up of γ-Ni solid solution, Ni3Si, Ni3B, CrB. And the Ni3Si particles dispersed in γ-Ni solid solution. When the brazing temperature was 1150℃, the brazed region was only composed of γ-Ni solid solution.The diffusion of boron was the key controlling factor of solidification. The solidification process of brazed seam zone can be divided into the isothermal stage and athermal solidification stage. The athermal solidification stage was concluded as following:(1) the residual liquid solidified into γ-Ni solid solution and the residual liquid L1 was rich in B; (2) Ni-B binary eutectic reaction:Liâ†'γ-Ni+N3B and the residual liquid L2 was rich in Cr; (3) Ni-Cr-B ternary eutectic reaction:L2->γ-Ni+ Ni3B+CrB and the residual liquid L3was rich in Si; (4) Ni-Si-B ternary eutectic reaction:L3->γ-Ni+Ni3B+Ni6Si2B.The microhardness of brazed joint was higher than base metal. The maximum microhardness appeared in the DAZ of Mo-Cu alloy.When the brazing temperature was 1120℃, the fracture originated from the interface beside Mo-Cu alloy. The shear strength was 223MPa and shear fracture showed brittle transgranular fracture with tearing edge and secondary cracks. When the brazing temperature increased to 1150℃ shear strength reached 256 MPa and shear fracture mainly showed brittle fracture, but plastic fracture characteristics appeared in some areas.