Research On Process Of Brazing C/C Composite And TC4 With GNSs Reinforced Composite Active Filler

C/C composite is considered as an ideal high-temperature structural material in aerospace field due to its low density, low thermal expansion coefficient, as well as the excellent fatigue resistance and high temperature properties. In order to make up the deficiency of traditional Ag Cu Ti filler, graphene nanosheets(GNSs) strengthened Ag Cu Ti composite filler(abbreviated as Ag Cu Ti G filler) was prepared and used to braze C/C composite and Ti-6Al-4V alloy. The effects of GNSs addition on the wettability of Ag Cu Ti filler, microstructure and mechanical properties of brazed joint were investigated. Process parameters were optimized to further enhance the mechanical properties of brazed joint. Finally, the mechanism of joint formation and the strengthening mechanism of GNSs were analyzed.Ag Cu Ti G filler was prepared by using a QM-SB planetary ball mill. The components of composite filler were uniformly distributed after milling process which had little effect on the phase of filler. Wetting experiments demonstrated that the addition of GNSs decreased the wettability of Ag Cu Ti filler and refined the microstructure of wetting interface microstructure. A small reduction in wettability was occurred when GNSs content was less than 0.3 wt.%, while the wettability decreased obviously when GNSs content exceeded 0.3 wt.%.Typical interfacial microstructure of TC4/Ag Cu Ti G/C/C composite joint is TC4/diffusion layer + Ti2 Cu + Ti Cu + Ti3Cu4 + Ti Cu4 / Ag(s,s) + Cu(s,s) + GNSs + Ti Cu + Ti C/Ti C/C/C composite. The addition of GNSs refined the interfacial microstructure and improved the shear strength of brazed joint. However, the mechanical properties of brazed joint deteriorated when GNSs content exceeded 0.3 wt.%. Under the condition of 880 °C/10 min, the maximum shear strength of brazed joint can reach 23.3 MPa when GNSs content was 0.3 wt.%. Fracture analysis showed that the fracture positions were related to GNSs content and the direction of carbon fibers in C/C composite. A concentration gradient adjacent to TC4 side was formed by the diffusion of Ti and Cu atoms, which led to the formation of different Ti-Cu layers. GNSs dispersed in brazing seam could retard the diffusion of atoms, therefore, the growth of reaction layers near TC4 and C/C composite side were inhibited. The GNSs can also act as the sites of nucleation and promote the formation of Ti Cu and Ti C phase in brazing seam. With the diffusion of element Ti, a Ti C layer was formed adjacent to C/C composite by the reaction between Ti and C/C composite.The parameters of brazing process had significant effects on the microstructure and mechanical properties of brazed joint. When brazing temperature or holding time was insufficient, the diffusion layer and reaction layers near substrates were relatively thin. Rod-like GNSs were distributed uniformly in brazing seam. With the increase of brazing temperature or holding time(900 °C/10 min), the microstructure in brazing seam was further refined, and the GNSs could hardly be observed. A composite brazing seam reinforced by particles was therefore produced in this condition. The joint brazed at 900 °C for 10 min exhibited the highest shear strength of 30.2 MPa and the fracture was mainly occurred in C/C composite. Further increasing brazing temperature or holding time, the Ti Cu phase in brazing seam excessively developed and almost replaced the Ag(s,s). The excessive growth of Ti Cu phase in brazing seam consumed amount of element Cu. Therefore, the thickness of Ti3Cu4 layer near brazing seam was decreased. Meanwhile, a thick Ti C layer formed near the C/C composite side deteriorated the performance of brazed joint. The reinforcements dispersed in Ag matrix caused the strengthening in brazing seam. The strengthening mechanism mainly involved grain refining strengthening, dislocation strengthening and Orowan strengthening. The formation of composite brazing seam not only improved the properties of itself, but also strengthened the mechanical properties of brazed joint by adjusting the mismatch of thermal expansion coefficient and elasticity modulus between substrates and brazing seam.