Process And Mechanism Research On Self-Propagating Reaction Assisted Brazing Between TiAl Alloy And C/C Composites

Joining of metals to composites belongs to a hot issue in the joining fields. In order to meet the requirement of joining between C/C composites(noted as CC) and TiAl alloys, which are both high-temperature structural materials, the self-propagating reaction assisted brazing of these two materials was systematically studied. Experimental and theoretical analyses were carried out to optimize the composition of the interlayer. The microstructure and mechanical properties of the joints were analyzed by means of scanning electron microscope (SEM), energy disperse spectroscopy (EDS), X-ray diffractometer (XRD), strength tests and differential thermal analysis (DSC). The interfacial microstructure and microstructural evolution for this special joining method were systematically analyzed. The effect of the parameters on the joining quality and interfacial reaction was investigated.Based on the results of thermodynamics calculation, the adiabatic temperature of Ti-Al and Ti-Al-C interlayer powder was calculated. The minimum preheating temperature to ensure the self-propagation reaction was determined, and the apparent activation energy was aslo calculated. By means of DSC method, the reaction mechanism of the interlayer was preliminary analyzed. The production of Ti-Al alloys was completed using different interlayer powders and brazing filler metals, and the primary joining to C/C composites was applied in appropriate parameters. The interface structure of joint was analyzed and the composition of each phase was also determined.According to the primary joining of TiAl to CC, the secondry joning of TiAl alloy and CC was realized using the optimized AgCuTi and BNi₂ brazing filler metals. The typical joint microstructure of CC and TiAl joined with AgCuTi filler and Ti-Al interlayer powder was CC/TiC/Ag(s.s)+Cu(s.s)/AlCu₂Ti+Ti(Cu,Al)₂/TiAl₃+Ti₃Al+Ti +TiAl /TiAl₃+TiAl₂/TiAl. Using BNi₂ filler and Ti-Al-C interlayer powder, the typical joint microstructure was CC/Cr7C₃/remanent BNi₂/NiAl+Ni₂Al₃/TiAl₃+TiAl+Ti₃Al +Ti+TiC/TiAl₃+TiAl₂/TiAl.Heating temperature had a strong influence on the microstructure and mechanical properties of the TiAl/CC joints. With the increase of the heating temperature, heat release in the interlayer increased and the reaction layer at the CC side became obvious, which led to the increase of joining strength gradually. At the parameters of 750℃/₂0min/₂0MPa, the maximum shear strength of the joint achieved was 11MPa using AgCuTi filler and Ti-Al-C interlayer powder, which was close to the conventional brazing joint strength .The fracture of the joint propagated in the TiC reaction layer. Using BNi₂ filler and Ti-Al-C interlayer powder, the maximum shear strength of the joint reached 10.6MPa at 800℃/₂0min/₂0MPa conditions, and the fracture took place in the Ni-Al reaction layer. Holding time played a slight effect on the microstructure and mechanical properties of joints.The joining mechanism of self-propagating reaction assisted brazing between TiAl alloy and C/C composites was studied. It was found that the SHS reaction took place after Al melt. The heat release in the interlayer quickly transfered to the side of filler metal,and promoted the molten of brazing filler and reaction between CC, brazing filler and interlayer. On the other side, the TiAl₃ layer was formed adjacent to the TiAl alloy,and the formed TiAl₃ reacted with TiAl substrate to form TiAl₂ layer.