Research On The Evolution Of Microstructure And Interface Structure Of TA19 Titanium Alloy And Its Influence On Mechanical Properties

TA19 alloy is a near alpha dual phase titanium alloy that is widely utilized in key structural components of aero-engines due to its high strength to weight ratio,high temperature resistance and corrosion resistance.The relationship between the microstructure and properties of TA19 titanium alloy flash welded joints is studied,and the temperature field of the welding heat-affected zone is simulated to prepare a particular semi-equiaxed microstructure formed in the welded joints in this paper.Moreover,the effect of different solution and aging heat treatment on the evolution regulation and mechanical properties of this microstructure is investigated.In addition,the crack initiation,propagation behavior and fracture mechanism under deformation of conventional equiaxed structure and semi-equiaxed microstructure are explored.The main research contents are as follows:1.There is a particular interface microstructure(semi-equiaxed microstructure,S-EM)in the flash-butt welded TA19 joints.The microstructure is characterized by the globular?_p/?_trans interface in the S-EM mainly disappeared and was replaced by the thin?lamellae that grew through the equiaxed?_pphase,and the equiaxed?_p phase is locally divided by the thin?lamellae.The formation of the S-EM is due to the inhomogeneous diffusion of elemental Mo in the?matrix into the equiaxed?_p phase when the Welded joints are kept above the T_?temperature for a short time,and in the cooling process,the thin?lamellae grow through the equiaxed?_p phase.During the plastic deformation process,the S-EM effectively inhibits the formation of microcracks,and provides a strong barrier for crack initiation and propagation.2.By simulating the temperature field in the flash welding heat-affected zone,the S-EM of TA19 titanium alloy was obtained by a short holding time above the phase transformation temperature then cooled to room temperature.With the increase of the holding time,the characteristics of the S-EM gradually becomes apparent,and the S-EM eventually transforms into lamellar microstructure.After aging heat treatment,the phenomenon of the thin?lamellae grew through the equiaxed?_p phase disappeared,and becomes the microstructure characteristic of the particles precipitated from the interface to the original equiaxed?_p core.When the holding time is the same,with the increase of the cooling rate,the degree of interspersed growth of the thin?lamella to equiaxed?_p phase in the S-EM is reduced,and the thickness of the?_s lamellae in the?transformed microstructure gradually decreases.3.The tensile strength of the S-EM is improved compared with the original microstructure,but the plasticity is reduced.The increase in holding time and cooling rate have the same effect on the tensile properties,which will improve the strength of the S-EM,and reduce the elongation and reduction of area.4.The tensile deformation and fracture behavior of the conventional equiaxed microstructure(EM)and the S-EM in TA19 titanium alloy are investigated by an in-situ method.In the process of plastic deformation,the incompatible deformation occurs between the equiaxed?_p phase and?transformed microstructure in EM,leading to stress concentration and crack initiation at the?_p/?_transinterface.The thin?lamellae in the S-EM inhibit the formation of dense slip bands in the equiaxed?_p phase,which is not easy to cause stress concentration at the?_p/?_trans interface,reducing the probability of crack initiation.Moreover,the ultimate tensile strength of the S-EM is higher than that of the EM with a slight loss of plasticity.