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

The connection of C/C composite and TC4 alloy is widely used in aerospace field due to their excellent properties.It can effectively overcome the disadvantages of poor machining performance of C/C composites.Brazing was considered as an effective method to achieve the metallurgical combination of the two.However,due to the difference in the thermal expansion coefficient of the two materials,a large residual stress existed in the brazing joint.As a novel carbon material,carbon nanotubes(CNTs)could be added to the traditional fillerfor alleviating the residual stress in the joint due to its advantages as extremely high elastic modulus,high tensile strength,low density,low CET and high temperature resistance.DSC results demonstrated that the CNTs can reduce the melting point of the composite filler.Ti and CNTs in the composite filler react at 790 oC to 840 oC.When the ratio of Ti to CNTs is relatively high,tubular Ti C compounds will be generated which Ti Cu2 compounds nucleate on.On the contrary,when the ratio of the two is low,Ti C compounds were granularand distributed in the Ag(s.s).Appropriate content of CNTs can refine the Ag Cu eutectic microstructure,while excessive addition of CNTs resultes in agglomeration during the reaction of composite filler.The increasing of Ti results in a large number of Ti-Cu compounds.Wetting result of Ag Cu Ti C composite filler showed that the process of wetting behavior is divided into four stages.In the first stage,Ti C reaction layer formed on the C/C composite.Meanwhile,the rate of the decreasing of contact angle and droplet height are fast.The second stage is the growth of Ti C reaction layer with uniform wetting spreading rate.The third stage,occrued through the diffusion of Ti after the complete reaction of CNTs.Finally the process of wetting is completed.The dynamic model in the wetting process of composite filler is composed of two part.The content of CNTs has a great influence on the wettability of the composite filler metal.The wettabiility of composite filler at 0.3wt.%CNTs is the best.In the intermediate zone,the distribution of compounds is dispersed.The typical interfacial microstructure of TC4/Ag Cu Ti C/C/C composite at 880 ? for 10 min is TC4 / diffusion layer/Ti2Cu/Ti Cu/Ti3Cu4/Ti Cu4/Ti C(s.s)+ Ti Cu2 + Ag Cu /Ti3Cu4/Ti Cu4/Ti C/C/C composites.An appropriate increase in the content of CNTs results in the reduction of Ti3Cu4 and Ti Cu4 compounds in the brazing jointthat could improve the shear strength of the joint.With the increase of CNTs,the shear strength of joints first increased and then decreased.And the maximum strength of the joint was 44.3MPa when the shear strength reached 0.4wt.%.The strengthening mechanism of CNTs for brazing joint is mainly dislocation strengthening and Orowan strengthening.CNTs also can refine the Ag Cu microstructure in the brazing seam.The addition of CNTs can effectively relieve the residual stress in the joint and improve the strength of the joint by adjusting the thermal expansion coefficient matching between two sides of the base material.