Study Of Thermomechanical Process And Electron Beam Welding Properties Of Titanium Alloy Ti-6246

The Ti-6246 is a high-temperature titanium alloy with high strength.It was designed for long-term load-carrying application at temperature up to 400?and short-term load-carrying application at temperature up to 540?.At this temperature range its mechanical properties are very impressive and the alloy is particularly suitable for compressor disk and fan for intermediate temperature application.The effect of hot working,heat treatment and welding properties on microstructure and mechanical properties of Ti-6246 was studied systematically in this thesis by using the techniques of optical microscopy,scanning electron microscopy and transmission electron microscopy,which was aimed to supply a beneficial support for application of Ti-6246.The hot deformation behavior of Ti-6246 alloy was investigated by the hot compression.The true stress-strain curves were affected by the work hardening and dynamic softening.The flow stress decreased with the increasing deformation temperature and increased with the increasing strain rate.The Zener-Hollomon parametic method and sinusoidal function were used to study the thermal activation energy(?+?phase region,Q=429.9 kJ·mol-1.?phase region,Q=245.6 kJ·mol-1)and their constitutive equation.The processing maps of Ti-6246 have been established by Dynamic Material Model(DMM)to evaluate the optimum hot processing conditions and reveal the instability regions.The investigated alloy has the optimum hot-working condition at 895?and 0.5 s-1,since the material undergoes dynamic recrystallization to produce a fine-grained microstructure.At 860?and 10-3 s-1,the alloy exhibits dynamic recovery.The regions with low temperature and high strain rate were instable.The manifestations of the instabilities have been observed in the form of localized deformation.The effect of heat treatment temperature and cooling rate on the microstructure,phase and tensile deformation behavior of Ti-6246 alloy were investigated.In the temperature range of the experiment,the composition of the phase were mainly affected by the cooling rate.The?" martensitic was observed in prior?phase after solution heat treatment and water quenching.The?" phase was transformed to fine a plates after aging process at 595?for 8 h.While air cooling produces a fine transformed?microstructure.Meanwhile,both the size and volume fraction of the secondary a grain were increase with increasing solution heat temperature during air cooling process.Double yielding phenomenon was observed in engineering stress-strain curves in samples after water quenching.Meanwhile,the first yield strength increase with increasing solid solution temperature.After aging process,both in water quenching and air cooling samples,the strength of the alloy increase but the plasticity decrease when compared with the material before aging process.In this work an optimal mechanic property in strength and ductility was achieved in samples after heat treated at both 900 and 920?followed by aging at 595?.The EBW weldment of Ti-6246 alloy is composed of fusion zone(FZ),heat affected zone(HAZ)and base material(BM),and only?and?phase in the as-welded FZ was observed through X-ray diffraction and selected area electron diffraction.The microhardness profiles of the Ti-6246 EBW joints exhibited an uneven distribution.All tested tensile weldments fractured at base material region,the fracture surface showed small dimples and exhibited ductile fracture mode.Post weld heat treatment(PWHT)temperature would have effects on the Ti-6246 alloy weldment when the size and shape of a phase in the BM,HAZ and FZ changed.The decreasing of tensile strength of weldment was ascribed to the coarsened a laths in BM,the increasing of elongation of weldment was thought to due to the ductility improvement in the FZ,HAZ and BM after PWHT.Microstructure at the regime of welded joint for Ti-624x alloy changes as function of Mo content.The futher the content of Mo,the larger the size of the beta columnar grains in the weld fusion zone of Ti-624x alloy.The microstructure at the heat affected zone is composed of a large number of equal axial grains,and the grain size decreases with the increase of the distance from the fusion zone.Inside the columnargrains at the weld fusion zone,there is lamellar microstructure,and with the increasing content of Mo alloy,the size of the fusion zone of alpha phaseinside columnar grains at the fusion zone gradually decreased.The microstructure of the heat affected zone is the transition organization between the matrixand the fusion zone,and with the decrease of distancefrom molten pool line gradually turninto the lamellar microstructure from duplex microstructure.The distribution of secondary phase in different zones of the welded joint is different,which leads to the inhomogeneous distribution of the micro hardness of the welded joint.And with the increase of Mo content,the hardness of the welded joint was gradually increased,and the hardness go through a pronounce change when the content of Mo element at 4%?5%.With the increase of Mo content,the strength of the fusion zone gradually increased,but plasticity decreased graduallyat room temperature.