Research On Brazing Process And Mechanism Of SiC_f/SiC Composite Material And GH3039 High Temperature Superalloy Using Cu-based Filler

SiC fiber strengthened SiC composite ceramic（SiC_f/SiC）with great resistance to high temperature,oxidation,and corrosion was widely used in the parts of aeroengines,for the high strength,small specific gravity,and certain functions.However,because of its poor processing,SiC_f/SiC is often welded with other materials for complex construction.Ni-based high temperature alloy GH3039 is one of best choices on account of its good inoxidizability,thermal fatigue resistance,weldability and cold workability.Nevertheless,due to the mismatch of Coefficient of Thermal Expansion（CTE）between the 2 materials and overreaction at the interface during brazing process,the reliability of the joints are not satisfactory.This study aimed to obtain high stability brazing joints of SiC_f/SiC and GH3039 with great mechanism properties of various surving temperature conditions,and made designs of several Cu-based active brazing filler,relieving the residual stress of joints by soft welding seam and restraining the overreaction of ceramic through constructing in-situ VC protective layer on the surface of SiC_f/SiC.And finally,the evaluation of the service reliability was measured through thermal shock and thermal oxidation tests.A gradient interconnection joint of GH3039 and SiC_f/SiC was fabricated through a CuAuV filler and the interfacial reaction mechanism of filler and base materials was also demonstrated.This novel CuAuV filler was designed for releasing the residual stress through the gradient structure of joint and soft character of filler.When the connection temperature was 1025 ^oC,the interface consisted of Cr₃C₂,Ni₂Si,（Ni,Cu）₅Cr₃Si₂,and CuAu_ss.When the connection temperature was 1050 ^oC,the joint began to form gradient structure,and was composed of Cr₃C₂,Ni₂Si,CuAu_ss,Ni₃₁Si₁₂,C,and Ni_ss;the filler would penetrate inside the SiC_f/SiC,and Ni₂Si inside the joint transformed to Ni₃₁Si₁₂ completely.Cr₃C₂ would continue to be generated with the dissociative C and Cr at 1100 ^oC,and Ni₃₁Si₁₂ began to gather.The gradient structure of joint strengthened the mechanical properties,the maximum shear strength was 44.3±5.8 MPa at 1075 ^oC for 10 min,which was 3.65 times that of joints without gradient structure.The high temperature performance（at 750 ^oC）of the joint was also good,and ensured by the heat resistance and the high temperature skeleton of Cr₃C₂and Ni₃₁Si₁₂dispersed in the matrix.The high temperature performance of the joint was at least84%that of the room temperature strength in same processing parameter,the maximum high temperature shear strength was 38.7±4.5 MPa at 1075 ^oC/10 min.In the following stage,in order to further promote the mechanical properties of GH3039-SiC_f/SiC heterogenerous joint,an innovative CuNiV filler was designedwhich possessed higher melting point and higher strength.The interface reaction and wetting behaviour between CuNiV4 and SiC_f/SiC were studied firstly.The interface consisted of（Cu,Ni）_ss,Ni Si V,Ni₂Si V₃,Ni₂Si,VC at 1125 ^oC for 10min.The amount of Ni-Si-V ternary phase in the joint was reduced and transformed to Ni₃V or Ni₃Si with the increase of V and brazing temperature.Then,the heterogeneous joints of SiC_f/SiC and GH3039 with CuNiV were further investigated,the joints consisted of（Cu,Ni）_ss,dispersed Ni Si（V,Cr）,and compounds distributed on the surface of SiC_f/SiC such as Ni₂Si,VC,and Cr₃C₂.The reaction process inside the heterogenous joint was that,with the increase of brazing temperature and holding time,Ni Si（V,Cr）was gradually formed and transformed into Ni₂Si（V,Cr）₃,eventually transformed in to Ni₂Si（V,Cr）₃completely,and distributed throughout the joint.What’s more,a compact VC layer was observed at the surface of SiC_f/SiC,where the element diffusion was obvious hindered.This layer was defined as a protective layer for SiC_f/SiC,and prevented the overreaction which made the heterogenerous joints unstable.The heterogeneous joints possessed the best mechanical property by CuNiV4at 1125 ^oC/10 min,of whom the room temperature and 750 ^oC strengths were103.9 MPa and 78.2 MPa,separately.Such data was great improved comparing to CuAuV,owing to the enhancement of fine strengthening phase inside the filler metal and the protective effect of the in-situ VC layer at the surface of SiC_f/SiC.In the solidification process,the produced fine phase Ni Si（V,Cr）and Ni₂Si（V,Cr）₃dispersed in the（Cu,Ni）_ss matrix,adding the grain nucleation sites and refining the grains of（Cu,Ni）_ss matrix.What’s more,the hard renforced phases also increased the resistance to dislocation motion while loading.The fine-grain strengthening and second-phase strengthening effect realized the promotion of strength of filling region of the heterogenerous joint.Moreover,the fracture of CuNiV4 joint showed obvious fiber pull-out characteristics which CuAuV joint did not possess,demonstrating the importance of VC protective layer.The VC layer prevented the SiC_f/SiC from overreaction and the structure of SiC_f/SiC reserved in the ceramic reaction zone,causing the great mechanical properties promotion.The heterogeneous joints also underwent thermal shock and corrosion testing at 750 ^oC,expressing good performance.The joint maintained 86%strength after 6h high temperature oxidation,and exhibited certain mechical properties after thermal shock.The results showed the brazing joints good thermal shock properties and thermal corrosion properties.