Microstructure improvement of C/SiC-Nb brazing joints reinforced with heat penetration

Abstract

Brazing C/SiC composites and Nb is always trouble with high residual stress owing to high property mismatch, especially in terms of the coefficient of thermal expansion. It weakens the joining strength and reduces the joining quality due to welding defects. The traditional method tends to modify the brazing alloy to overcome the problem of property mismatch by introducing reinforcement or interlayer. However, the application of these methods was limited by the low addition and poor distribution, thus inhibiting the development of brazing composites and alloy, as well as the use of composites. An increase in aircraft speed and size requires the composite-alloy joint to exhibit higher joining strength. To achieve this, heat penetration was employed to modify the composite surface of brazing C/SiC and Nb with the AgCuTi brazing alloy. It aims to provide a new idea for the design of the composite-alloy joint. In this study, the microstructure, residual stress distribution, and reinforced mechanism of the joints were investigated in detail. Ni-Cr-Si alloy was introduced to penetrate due to the good reactive diffusion of Ni with SiC. The Ni2Si+C phase was in situ synthesized by replacing the SiC matrix and constructing the penetration zone using carbon fibers. The high strength and elasticity of the carbon fiber dispersed in the penetration zone increase the mechanical properties and decrease the brittleness. Moreover, the wettability of Ni-Cr-Si alloy on C/SiC composites improved as the Cr content increased. The ideal content of penetrating alloy was confirmed as Ni10Si15Cr (wt%). To reduce the brittleness of the composite-alloy joint, such content needs to be removed, so that bonding between alloys can be achieved. The typical microstructure of penetrated joints was C/SiC/Cf+Ni2Si+Cr3Ni2Si+Cr23C6/Ag(s,s)+(Cu,Ni)/Nb. The Cr elements were slightly diffused into the penetrating zone, thus reinforcing the bonding between carbon phases and Ni-Si compounds. The reaction layer of the compounds on C/SiC composites was replaced with the fiber reinforced penetration zone. The depth of this zone directly influenced the joining strength. To investigate in detail the penetration parameters, the penetration depth was optionally and homogeneously controlled. Accurate regulation of the penetration depth is beneficial for various application conditions. In addition, the reinforcing mechanism of penetration focused on the reduction of residual stress and enhancement of the C/SiC-alloy interface. In the penetration zone, the coefficient of thermal expansion gradually changed from C/SiC to AgCuTi alloy, which reduced the residual stress. The stress concentration of the C/SiC interface in the original joint was also eliminated. The fiber bundles supported the residual stress to prevent the appearance of cracks. Consequently, the residual stress in the brazing seam was reduced. The carbon fibers and dispersed phase enhanced the fracture strength and toughness of the penetrating zone. Fracture occurred in the penetration zone rather than in the reaction layer, with breaking and pulling out of fibers. A longer fracture path requires more energy for crack propagation. All these improvements contributed to the joining quality. The average shear strength increased from 85.5 to 115.2 MPa, with a penetrating depth of 77 μm. The penetration treatment of C/SiC composites provides a new method of brazing composites to alloy. Such method is suitable for different brazing alloys and various brazing requirements. Moreover, it enables the surface modification of composites. Thus, it is important to pay attention to the crucial effect of fiber reinforcement on composites. This research aimed to efficiently expand the application of composites and dissimilar joints.