| Titanium alloys have good special strength,stiffness,corrosion resistance,high temperature behavior and biocompatibility,they are widely used in the fields of Aeronautics,astronautics,chemical and biological medicine fields.With the larger and more integrated requirements of the aircraft,the titanium alloy structures need good fracture toughness and fatigue properties in addition to the requirements of high strength.Therefore,it is of great significance to study the hot deformation characteristics of high strength and high toughness titanium alloys and the effect of hot processing on the microstructure and properties of them,it is important for alloys' application on aircraft.Ti-6554(Ti-6Cr-5Mo-5V-4Al)alloy is a new type of titanium alloy with high strength and fracture toughness.It has broad application prospects in civil and military aircraft.In this paper,the characteristics of thermal deformation and heat processing diagram of Ti-6554 titanium alloy are systematically studied.The effect of thermal deformation processing and heat treatment on the microstructure and properties of the alloy is studied.The optimum processing range and the best matching of strength-ductility and fracture toughness are determined.In this paper,the characteristics of thermal deformation and heat processing map of Ti-6554 titanium alloy are systematically studied.The effect of thermal deformation processing and heat treatment on the microstructure and properties of the alloy is studied.The optimum processing range and the best matching of strength-ductility and fracture toughness are determined.The thermal compression test of Ti-6554 alloy was carried out at nearby the phase transition point.The thermal deformation characteristics of the alloy were obtained.The constitutive equation of the alloy was calculated and the processing map was drawn.The results show that the flow stress of alloy is very sensitive to temperature and strain rate during high temperature deformation.Under the same strain rate,the flow stress decreases with the increase of temperature.At the same temperature,the strain rate increases and the flow stress increases.According to the processing map,the alloy has a high dissipation coefficient in the 0.01-0.03s-1,800-890? and the deformation mechanism of the alloy is dynamic recovery and dynamic recrystallization;When alloy deformed two-phase region,the high dissipation coefficient in 740-770?,0.01-0.03s-1,the microscopic deformation mechanism is mainly dynamic recovery.They are good processing ranges.The instability zone is mainly in the region of 740-900?,0.05-1 s-1.Deformation in unstable region will cause local plastic flow.The effect of heat treatment on the microstructure and properties of the alloy bar was studied in single phase region deformation processing and two phase heat treatment.The results showed that the main composition phases of the alloy after heat treatment in single-phase region were primary strip a phases and needle like secondaryaphases.The strengthening of the alloy mainly comes from the precipitation strengthening of the secondary a phase.The higher the solid solution temperature,the lower the aging temperature,the more the secondary a precipitates,and the higher the strength is.The plasticity is related to the content of the primary a phase.The more the primary a phase in the alloy is,the better the plasticity of the alloy is under the same strength.Under the typical heat treatment system,the strength of the alloy can reach 1300MPa to 1600MPa and the elongation is 2%to 10%.The fracture toughness of the alloy is related to the crack propagation path in the alloy structure.The more twists the path,the higher the fracture toughness.After 760?/lh,AC+560?/2h.AC,the fracture toughness of the alloy is the highest,reaching 54.4 MPa·m1/2,at this time the strength is 1341MPa,the elongation is 10%.The alloy bar is deformation processed in two phase zone and heated in single phase zone.The relationship between the grain size and the fracture toughness was studied,and the best solution system was determined.The effect of aging heat treatment on the microstructure and properties was studied.The results show that the grain size increases with the increase of solution temperature and the increase of solution time.The fracture toughness of the alloy with a grain size of 125?m is the highest,and the solution regime is 8900?/1h.The strength of the alloy from 1200MPa to 1500MPa under different aging regimes,and the elongation is between 3%and 9%.The strengthening mechanism is similar to the heat treatment in the two phase zone of single-phase zone processing bar,which is determined by the number and size of secondary alpha phase,and the plasticity is negatively correlated with the strength.The fracture toughness of the alloy bar processed in two phase region is generally better than that of the alloy bar processed in the single-phase area,which is more than 55MPa·m1/2.This is due to the solid solution in the single-phase region makes the microstructure construction more uniform.When the alloy strength is 1350MPa,the elongation is 6%and the fracture toughness is 65MPa·m1/2.Through the performance test and optimization,the best strength plastic toughness matching is obtained. |
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