Formation Mechanism And Corrosion Behavior Of Linear Friction Welded Ti₂AlNb Joint

The Ti₂AlNb-based titanium intermetallic alloys,as a new class of lightweight high temperature structural materials,have many attractive properties such as high specific strength and stiffness,improved oxidation resistance,good creep resistance,and non-magnetism.They have expansive prospect for potential applications in aero-engines industry because of better processability and fracture toughness than conventional titanium intermetallic alloys such as Ti₃Al-based and TiAl-based alloys.The combact capacity of military aircraft mainly denpend on the development of high thrust-weight ratio engine,and employing blisks is one of the most effective way to increase the thrust-weigh ratio.Linear friction welding,as a key technology for manufacturing blisks,has already been applied aero-engine overseas.Therefore,to investigate linear friction welded Ti₂AlNb-based alloy is an urgent assignment.In this paper,Ti₂AlNb-based alloy is linear friction welded to investigate the bonding mechanism of sound joint,and the post-weld heat treatment is carried out to anlysis the relationship between the microstructure evolution and mechanical properties.In addition,according to its service environment,the high-temperature oxidation,hot corrosion and salt-spray corrosion of the joint are investigated,the conclusions are listed as follow:The bonding rate of linear friction welded Ti₂AlNb-based alloy mainly depends on friction pressure,forging pressure and feed velocity,etc.The weld zone is mainly composed of B2 phase,and dominated by low angles grain boundaries.In thermo-mehanically affected zone,the near-weld zone consists of?₂+O mixtures and the morphology of?₂ and O phase in far-weld zone is similar to base metal.The dissolution of O phase during welding process has weakened the mechanical properties of the joint.The microstructure evolution of the joint solution treated under different phase region are studied.The volume fraction of O phase decreases along with the increase of solution temperature.In O+B2 phase region,lath O precipitates in weld zone,the?₂+O mixtures in near-weld zone have gradually turned into aggregate lath O and the rim O phase has formed around the?₂ grains.?₂ phase will precipitate in weld zone in?₂+O+B2 field.And the microstructure of weld zone,near-weld zone and far-weld zone become similar when the joint are solution treated in?₂+B2 field,all of which mainly consist of B2 phase.The influence of aging treatment on morphology and amount of O precipitates are investigate.The amount of acicular O precipitates during aging treatment increase with the increase of primary solution treatment temperature.After aging treatment,static globularization happens in the aggregate O phase formed in O+B2 field and part of the rim-O phase turned into spherical structure.The morphology of O precipitates is mainly controlled by the aging temperature,the size of O precipitates increases with the increase of aging temperature.The effects of post-weld heat treamment on mechanical properties of joint are studied.It is concluded that the precipitation hardening of O phase and refined crystalline strengthening can obviously enhance the microhardness of the joint.As the solution treatment temperature increases,the volume fraction of O phase decreases and the microhardess of the joint decreases.The microhardness of the joint is mainly controlled by the volume fraction of O phase.For tensile property,the coarse lath O phase formend in solution treatment can enhance the plasticity of the joint remarkably,while the secondary acicular O precipitated during the aging treatment can strengthen the joint but reduece the plasticity.The overhigh solution temperature will lead drastic grain growth of B2 grains,thereby reduce the fracture toughness of the joint obviously.Therefore,to inhibit the grain growth of B2 garins and controll the size and amount of O precipitates simultaneously is the key to obtain sound joint with better combination properties.The oxidation kinetics of base metal and joint at 800 ^oC both obey initial parabolic kinetic with low oxidation rate during the first 50 h,and followed by a linear kinetic from 50h to 100 h.The oxidation rate of joint during the first 50 h is lower than that of base metal,but in the linear stage,difference of the oxidation rate between joint and base metal has gradually declined.The higher amount of Al₂O₃ in oxide scale of the joint caused by the preferential Al oxidation results in better oxidation resistance.And as the base metal consists of more?₂phase which apts to be oxided preferentially,oxidation rate of it is relatively higher in the early stage.AlNbO₄ fromed at the expense of Al₂O₃ for all the three zones from 50 h is the major cause for the occurrence of breakaway oxidation.The results of hot corrosion indicate that,the corrosion weight loss phenomenon occurs in both joint and base metal,and the corrosion weight loss rate of joint is lower than that of base metal during the whole hot corrosion process.The hot corrosion product layer of both joint and base metal contains an outer TiO₂+Nb₂O₅ layer and an inner Al₂O₃+Nb₂O₅+TiO₂layer.It is similar to high-temperature oxidation that,the lower corrosion weight loss rate of joint is caused by the higher amount of Al₂O₃ lower amount of?₂ phase in joint than base metal.In the later stage,NaNbO₃ has fisrt fromed in base metal.Pitting corrosion can be observed in both base metal and TMAZ,and the formation and reoxidation of sulfide lead to the formation and growth of corrosion pit.The gaseous escape and acid dissolution caused by Cl and NaNbO₃ respectively will result in the spalling of corrosion products layer,thereby accerlerate the hot corrosion process.The corrosion behavior of joint and base metal under 5%NaCl aqoeous solution condition is discussed.The corrosion resistance of weld zone is the best,and with further awary from weld zone,the corrosion become more and more serious.Among the three phases,?₂ tends to corrode first,the corrosion of O phase takes second place and almost no corrosion occurs in B2 phase.Due to the electric potential difference between joint and base metal,the galvanic corroiosn happens when the joint and base metal corrode as a whole.As a result,the corrosion of base metal is accelerated as positive pole and the weld zone is protected as negative pole.After post-weld heat treatment,the electric potential difference between joint and base metal decreases owing to the homogenization of the microstructure,so the corrosion of base metal is slighter than that under as-weld condtion.