The Study on Brazing Process and Mechanism of C/C Composites and Ti6Al4V

Author:Song Xin Zuo

Supervisor:zeng zuo zuo

Database:Doctor

Degree Year:2018

Download:7

Pages:125

Size:8075K

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Carbon/carbon(C/C)composites was widely applied in aviation and aerospace areas due to the further studied in this materials.However,there are still some problems in the fabrication of large size and complex C/C parts due to the restrictions of weave technology of perform,densification process and machine,which can restrain the development and application of C/C composites.To solve those problems,reliable joining technique of C/C composites is very important.Ti6Al4 V as the most widely used titanium alloy,its joining componets with C/C composites,to further expand the applied engineering area of this two kinds of materials.Since the huge difference in coefficient of thermal expansion(CTE)and elastic modulus between C/C composites and Ti6Al4 V.In order to decrease the thermal stresses caused by the huge CTE mismatch between C/C and Ti6Al4 V and improve the ambient and high temperature mechanical properties of joints,several different reinforcements were introduced into the brazing alloy to join C/C and Ti6Al4 V.Effect of the different reinforcement on the interfacial microstructures and mechanical properties were invested.According to SEM、EDS and XRD testing methods,the joints formation mechanism and fracture mechanism were studied.The microstructures of joints using TiCuZrNi brazing alloy were different when the brazing temperature and the soaking time were changed.When the brazing temperature decreased or the soaking time shorterned,the TiC reaction layer adjacent to C/C composites was thin and the quantity of Ti(s.s)in the braze was low.When the brazing temperature increased or the soaking time prolonged,the TiC reaction layer adjacent to C/C composites was thick and the quantity of Ti(s.s)in the braze increased.When the brazing temperature was 940℃ and the soaking time was 10 min,a maximum average shear strength at room temperature was 22.3±3.5MPa.The fracture section was mainly occurred at C/C composites side and inside the TiC reaction layer.The effect of fraction of CNTs in brazing alloy on microstructures and mechanical properties of brazed joints was studying.Combined with the finite element simulation,the effect of the content of CNTs in TiCuZrNi brazing alloy on distribution of residual stress was discussed.When the brazing temperature was 940℃ and the soaking time was 10 min,a maximum average shear strength of joints with 1wt.% CNTs at room temperature was 38.2±2.3MPa.The heat resistance of joints with 1wt.% CNTs between room temperature and 350℃ was tested.After 10 cycles,the shear strength retention was 95%;after 30 cycles,the shear strength was 20.0± 1.3 MPa and the shear strength retention was 52.6%.The effect of fraction and size of graphite in brazing alloy on microstructures and mechanical properties of brazed joints was studying.When the size of graphite was large,the fracture was observed on the C/C side.Because the large size and layerd structure of graphite in the joints could not effectively alleviate the residual stress.The maximum average shear strength of joints with 0.5 wt.% graphite(particles size is about 50μm)at room temperature was 25.3±1.6 MPa,which was little higher than that of joints without graphite.The effect of fraction of GNPs in brazing alloy on microstructures and mechanical properties of brazed joints was studying.In the same brazing parameters,the formed TiC and GNPs were uneven distributed in the braze when 3 wt.% GNPs were introduced.Meanwhile,the TiC layer adjacent to the C/C composites was discrete since the high quantity of GNPs existed could consume most of the Ti element from the brazing alloy.The average shear strength of joints at room temperature firstly increased and then decreased when the amounts of GNPs increased.The maximum average shear strength of joints with 1 wt.% GNPs at room temperature was 42.2±3.5 MPa.All the shear strength of joints at high temperature was higher than those obtained at room temperature because the joints were equivalent to conduct stress relief annealing during the high temperature test.The effect of brazing alloy form and Cu foil thickness on microstructures and mechanical properties of CrC modified C/C composites and Ti6Al4 V brazed joints was studying.The results showed that the shear strength of joints using brazing alloy foil was much lower than that of joints with brazing alloy powder.This is because that the TiC layer formed at the interface with the chromium carbide was dense and coherent and limited the formation of TiC particles inside the brazing alloy.The shear strength of joints with 40μm Cu foil was 39±8.5 MPa and the fracture mainly occurred at the C/C side.