Research On Diffusion Bonding Process And Mechanism Of Ti-Al Alloy

As new lightweight high temperature Ti-Al alloys,high Nb containing TiAl alloy?TAN?and Ti₂ AlNb alloy are ideal materials for aeronautic and aerospace application.In the present research,TAN alloy itself as well as TAN alloy and Ti₂ AlNb alloy were successfully bonded by solid diffusion bonding directly.The effects of diffusion bonding process on the interfacial microstructure and mechanical properties were studied in details.In particular,the best diffusion bonding process were summarized.In addition,fracture analyses were carried out to investigate the relationship between joining properties and interfacial microstructure.Furthermore,the formation and evolution of interfacial microstructure were summarized comprehensively.The characteristic of the interface was dominated by recrystallization change during direct diffusion bonding TAN alloy,the process of which was divided into six stages with increasing bonding temperature or prolonging holding time.The pores at the interface disappeared with atoms diffusing sufficiently,the process of which was named pores closure stage.In the prophase of recrystallization,the boundary of less-distorted grain protrudes to the serious-distorted grain with the piling up of dislocations at the interface.In the later stage of recrystallization,the boundary extended to the side of the smaller distortion area with the further accumulation of dislocations,and the recrystallization nucleation was completed.In recrystallization growth stage,recrystallization grains grew and dislocations decreased constantly with the increase of bonding temperature or holding time.In the occurence stage of continuous dynamic recrystallization and the phase transformation,continuous dynamic recrystallization occurred at the interface when a certain temperature was reached,and the phase transformation occurred in the parent metal and joints which resulting in the increase of ?2 phase that degenerated the properties of the joints when bonding temperature reached 1125°C.In the complete formation stage of joints,large and more recrystallized grains were found at bonding interface,when the maximum shear strength reached 154.3MPa.The effects of holding time and bonding pressure on the microstructure and properties of the joints had some similarities with bonding temperature.However,excessive bonding pressure in turn resulted in the decreasing of recrystallized grain size.The fracture modes of joints were dominated by the brittle failure under different diffusion bonding process.In the process of direct diffusion bonding TAN alloy and Ti₂ AlNb alloy,the change of interface was dominated by phase transition,which can be divided into four stages,with the increase of bonding temperature or holding time.The voids at the interface disappeared because that atoms from both parent metals sufficiently diffused,which was named pores closure stage.Al?Ti,Nb?₂ phase and ?2-Ti₃ Al phase were generated at the interface due to the accumulation of Nb atoms and the diffusion of Al atoms,which the structure was Ti₂ AlNb alloy/Al?Ti,Nb?₂+ Ti₃Al/Ti₃Al/TAN alloy.And,this process was named the formation stage of?Al?Ti,Nb?₂+ Ti₃Al?mixing layer and Ti₃ Al layer.In the stage of O phase formation,Al atoms passed into the mixing layer into Ti₂ AlNb parent metal,resulting in the formation of O phase layer.At this time,the interface structure was Ti₂ AlNb alloy/O/Al?Ti,Nb?₂+Ti₃Al/Ti₃Al/TAN alloy.With the further diffusion of atoms,Al?Ti,Nb?₂ phase gradually disappeared,and O phase layer became discontinuous,and the Ti₃ Al transion layer was formed at the interface,the process of which was named the formation stage of Ti₃ Al transition.At this time,an appropriate joint microstructure of Ti₂ AlNb alloy/O/Ti₃Al/TAN alloy was achieved,when the maximum shear strength reached 66.12 MPa.The fracture mode was brittle fracture under different diffusion bonding process.