Brazing of carbon fiber reinforced SiC composite and TC4 using Ag–Cu–Ti active brazing alloy

Abstract

Carbon fiber reinforced SiC (C_f/SiC) was successfully joined to TC4 with Ag–Cu–Ti alloy powder by brazing. Microstructures of the brazed joints were investigated by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). The mechanical properties of the brazed joints were measured by mechanical testing machine. The results show that the brazed joint mainly consist of TiC, Ti₃SiC₂, Ti₅Si₃, Ag, TiCu, Ti₃Cu₄ and Ti₂Cu reaction products. TiC + Ti₃SiC₂/Ti₅Si₃ + Ti₂Cu reaction layers are formed near C_f/SiC composite side while Ti₃Cu₄/TiCu/Ti₂Cu/Ti₂Cu + Ti reaction layers are formed near TC4 side. The thickness of reaction layers of the brazed joint increases with the brazing temperature or holding time increasing. The maximum shear strengths of the brazed joints at room temperature and 500 °C with brazing temperature 900 °C and holding time 5 min are 102 and 51 MPa, respectively.

Introduction

Carbon fiber reinforced SiC (C_f/SiC) ceramic matrix composites are lightweight, hard, and wear resistant and stable in oxidizing environment up to a high temperature. Owing to the embedded carbon fibers, they have an excellent combination of mechanical properties. Therefore, C_f/SiC composites are promising new structural materials for a variety of high-temperature burner environments, including in hypersonic aircraft thermal structures, advanced rocket propulsion thrust chambers, cooled panels for nozzle ramps, turbo pump blisks/shaft attachments, and brake disks [1]. TC4 is a type of titanium alloy, which is one of the most promising alternative lightweight heat resistant alloys. Furthermore, TC4 can be readily welded, forged and machined [2], [3].

Like most ceramics, however, due to C_f/SiC composite is brittle and difficult to manufacture into work pieces with large dimensions and complex shapes, which substantially increases preparation cost. Therefore, the development of joining techniques is very important for joining of C_f/SiC composite to itself or to metals, especially Ti alloy.

Fabrication of complex large-scale structural components requires robust integration technologies capable of assembling smaller and geometrically simple parts. Bolting and riveting are not advisable to join C_f/SiC composite due to its brittle nature and machine holes of C_f/SiC composite. Meanwhile, the high service temperature of the joint impedes the use of bonding adhesives [4]. So far, diffusion bonding [5], [6] and brazing [7], [8], [9], [10], [11] have been reported for joining C_f/SiC composite. However, the high bonding temperature (up to 1200 °C) limits the application of the method of diffusion bonding. Brazing, due to its simplicity, lower cost investment and potential as a mass production process, is used extensively. Thus, brazing is becoming an effective method to join C_f/SiC composite to itself or metals.

The eutectic composition of Ag and Cu in weight percent is 72Ag–28Cu, and it exhibits a low melting point (780 °C) as well as excellent fluidity upon melting. Eutectic Ag–Cu alloy with a few percent of Ti is the most frequently considered active brazing filler. Ag–Cu–Ti active brazing filler metal possesses not only good wettability on ceramics but also an appropriate brazing temperature for TC4, which does not degrade the TC4. Decreasing the brazing temperature and time are always recommended with the advantages of decreased interfacial reactions, decreased erosion of substrates and minimum loss of base-metal properties. It is reported that the higher strength is obtained with Ag–Cu–Ti filler metal than Ni-based or Ti-based filler metal [12]. However, the brazing mechanism and the possibly related interfacial reactions between C_f/SiC composite and Ag–Cu–Ti active filler have been less often reported. The microstructure evolution and strength evaluation of the brazed joints using Ag–Cu–Ti filler metal need to be further studied.

In the present study, the microstructures of C_f/SiC composite and TC4 brazed joints are investigated, the interface evolution mechanism of the C_f/SiC composite/Ag–Cu–Ti/TC4 joint is analyzed and the mechanical properties of the joints are also discussed with the Ag–Cu–Ti alloy brazing filler metal.