Hot Deformation And Application Properties Of Ti-V-Cr Type Burn-resistant Titanium Alloy

The applications of conventional titanium alloys have been extremely limited due to their sensitivity to burn in such elevated temperature and high pressure conditions of aircraft engines.The probability of titanium fire accidents increases in the engine by increasing the usage amount of titanium.The development of the burn-resistant titanium alloy is the most direct and effective way to solve the titanium fire incident,which is also the key material to design and develop the engine with high thrust to weight ratio.The ductile fracture criterion and thermal processing map of Ti40 alloy during high temperature deformation have been widely studied,but the basic research of the hot deformation with cracking mechanism,recrystallization mechanism,microstructure evolution and the applied properties with thermo-physical properties of WSTi3515S alloy have not yet been reported.Currently,there are four main problems around the basic research and applied properties research for burn-resistant titanium alloy during hot deformation.Firstly,the existing studies were not comprehensive,mainly focussed on the Ti40 alloy,and the main object was small ingots or small specimens.Secondly,engineering physical model of grain refinement mechanism of single phase alloy is not established.Thirdly,it is not clear to identify the various precipitates existing in microstructures of burn-resistant titanium alloys.Finally,the applied properties are mainly concentrated in the conventional properties,more fundamental thermo-physical properties are rarely reported.All of these problems have seriously hampered the application and engineering process of domestic burn-resistant titanium alloys.Through VAR,thermal simulation isothermal compression tests,physical simulation upsetting of engineered billets,hot extrusion cogging and wrap protect forging experiments,combined with the test and analysis methods of OM,SEM,EBSD,EDS,XRI,TEM,physical properties and mechanical properties,the cracking mechanism,hot deformation recrystallization behavior,microstructural evolution and engineering manufacturing methods of Ti-V-Cr type burn-resistant titanium alloys were studied.In this paper,the physical and mechanical properties,the cracking mechanism,hot deformation recrystallization behavior,microstructures evolution and engineering manufacturing methods of Ti-V-Cr type burn-resistant titanium alloys are studied.The different characteristics between two typical burn-resistant titanium alloys are comparatively analyzed.A new way of engineered manufacturing of large-size ingots and slabs on Ti-V-Cr type burn-resistant titanium alloys is developed.The physical model of grain refinement and mathematical equation of thermo-physical properties on burn-resistant titanium alloys are established.The cracking mechanism and the recrystallization mechanism of hot deformation of burn-resistant titanium alloys are revealed.The obtained main results and conclusions are listed in the following:?1?Based on isothermal compression test and small ingot upsetting test,the hot deformation behavior of Ti-V-Cr type burn-resistant titanium alloys is investigated.It was found that Ti-V-Cr type burn-resistant titanium alloys are easy to crack during hot deformation.Three typical cracking forms,45°shear cracking,radial pattern cracking and intergranular cracking,are summed up.The results of the cracking behavior show that the crack mechanisms of Ti-V-Cr type burn-resistant titanium alloys are closely related to stress state,high temperature oxidation and precipitates forms.Tensile stress,precipitates and high-temperature oxidation on billet surface promote the formation of cracks and holes,which cause the final outbreak of cracking on the weak interface as well as the extension of cracking along the weak direction.?2?Based on the analysis of microstructural evolution behaviors of Ti40 alloy and WSTi3515S alloy forged large-sized billets,the physical model of grain refinement on Ti-V-Cr type burn-resistant titanium alloys is established.Special grain recrystallization mechanism of Ti-V-Cr type burn-resistant titanium alloy indicates that subgrains would be formed due to the the deformation in the grain boundaries.Moreover,when the misorientation between subgrain and parent grain exceeds a value,the recrystallized grains will be separated from the original grain,and grow up in the subsequent heat process,making the alloy homogenized and refined.The influence of large grains and strong solution strengthening is the most important reason for grain-boundary breaking recrystallization of Ti-V-Cr type burn-resistant titanium alloy,which easily results in the probability of cracking along grain boundaries and phase boundaries.?3?The microstructure evolution behavior during heat treatment indicates that there is no evident microstructures and properties change when WSTi3515S alloy was heated below 900?.The small grains in boundaries grew up quickly when annealing temperature exceeded 900?,and then expanded toward the interior of grains.The four main precipitated phases of WSTi3515S alloy are Ti₂C,?TiV?C,Ti₅Si₃ and?phase.The forged microstructures start to change when the heating temperature is 900-950?.When the temperature increases to 950-1000?,the chicken type?TiV?C starts to dissolve.The point-like particles Ti₅Si₃ start to dissolve in the temperatures of1000-1050?.If the temperature exceeds 1200?,spherical Ti₂C dissolves.Small amount of the secondary phases could decrease the ductility at heat exposure temperature of 570?.?4?Based on the NPSM and LCFA testing methods,XRI test results show that the segregation problem of large-size ingot for Ti-V-Cr type burn-resistant titanium alloy can be solved by means of 4 times VAR.The test results of mechanical properties show that the microstructures and properties uniformity of large-size slab were effectively controlled by using of large tonnage extruder and large tonnage forging press.The room-temperature tensile properites,hardness,impact and 540?high-temperature properties of WSTi3515S alloy are similar with Ti40 alloy.Buthigh-temperature creep properties,high-temperature thermal stability and lasting thermal properties are better than Ti40 alloy at 540?.V and C elements are useful to hot strengthening of Ti-V-Cr type burn-resistant titanium alloy,and C elements can stimulate the grain refinement.?5?The test results of thermal physical properties on Ti-V-Cr type burn-resistant titanium alloys and the mathematical equations fitting by the principle of least squares indicated that thermal physical properties versus temperature curves of WSTi3515S alloy and Ti40 alloy,which are relatively stable in the range from room-temperature to600?.Poisson's ratio did not change significantly by changing various temperatures,Young's modulus E and shear modulus G slowly declined linearly with the increasing temperature,specific heat C_p parabolic increases with the temperature,thermal diffusivity D and thermal conductivity K increases linearly with the increase of temperature,the linear coefficient of thermal expansion?L/L₀ and the average linear thermal expansion coefficient parabolic increases with increasing temperature.