The Refinement Mechanism And Control Of Lamellar Microstructure During Hot Deformation For Ti60 High-Temperature Titanium Alloy

With the increase in thrust-to-weight ratio for the new generation high performance aero-engine,the high-temperature titanium alloy compressor disk with excellent high temperature property is one of the important engine components.The high-temperature titanium alloy compressor disk can adapt to the service environment of aero-engine,meet the performance requirements and exert full potential high specific strength of the material.As the core manufacturing method of the compressor disk,the hot forging technology plays a key role in controlling the final performance of the disk.Therefore,the in-depth understanding of the hot deformation behavior and microstructure evolution mechanism of high-temperature Ti-alloys with different initial microstructures is significant for the accurate control of the microstructural morphologies and properties for the final products.In the present work,the hot deformation behavior of a near-αTi60 high-temperature titanium alloy with different initial microstructures was investigated.And a dislocation density-based constitutive model that considered the effects of initial lamellar thickness was proposed to predict the flow behavior of the alloy.The microstructural evolution and conversion mechanisms for α lamellae with different initial thicknesses were analyzed.Based on the microstructure evolution mechanism,the study of grain refinement control for Ti60 alloy with initial lamellar microstructure were developed through extrusion forming process.The main contents are drawn as follows:By conducting a series of isothermal compression tests under various deformation temperatures and strain rates,the difference between the hot deformation behaviors of Ti60 alloy with initial duplex microstructure and dififferent lamellar microstructures were discussed,and found that the initial lamellar thickness has a significant influence on the deformation behaviors.According to the formation characteristics of lamellar microstructure,the phase transformation kinetics and the mathematical relation between lamelalr thickness and cooling rate during the continuous cooling process of Ti60 alloy was established.The effect of initial alpha lamellar thickness on the flow stress and kinetic parameters were analyzed.For a given temperature and strain rate,the alloy with thinner initial alpha lamellae exhibited obviously higher flow stress than that with thick alpha lamellae.And the relationship between peak stress and initial alpha lamellar thickness follows a noticeable Hall-Petch behavior.The initial alpha lamellar thickness has a significant effect on flow softening,temperature sensitivity,strain-rate sensitivity and apparent activation energy under different hot deformation conditions.The values of these parameters decrease with decreases gradually with the increasing of initial alpha platelet thickness.Based on the internal state variable approach,a dislocation density-based constitutive model that considered the effects of initial lamellar thickness was proposed.The evolution equations of the dislocation density,refinement behavior of lamellar microstructure and Hall-Petch behavior were coupled into the constitutive model.The material parameters in the model were determined by a genetic algorithm(GA)-based objective optimization method.The established constitutive model was applied to calculate the dislocation density and flow stress of the studied alloy.The correlation coefficient and average absolute relative error between predicted and experimental flow stresses are 0.9859 and 6.77%,respectively.The quantitative relationship between steady-state flow stress and equiaxed α-phase size was obtained by the comparison of calculated and experimental results.The results validate the high accuracy of the proposed constitutive model for predicting flow behavior of the Ti60 alloy with different initial lamellar microstructures.The processing maps of the Ti60 alloy with different initial lamellar thicknesses were constructed,and the microstructural evolution and refinement mechanisms for αlamellae with different initial thicknesses were analyzed.The results showed that the peak efficiency of power dissipation decreased with increasing initial lamellar thickness.The interaction effects with different extents of refinement,elongating,kinking and phase transformation of lamellar α accounted for the variation in the efficiency power dissipation in the processing maps.The flow instability region appeared to expand more widely for thicker initial lamellar microstructures during high strain rate deformation due to flow localization and local lamellae kinking.The microstructure observations indicated that the extent of lamellar microstructure refinement,morphological distribution,and microstructure homogenization were significantly different for the alloy with various initial lamellar thicknesses under the same deformation condition.The thin initial lamellar microstructure is more easily fragmented into fine α phase while most of the thick lamellae undergo the buckling and elongation.The refinement mechanisms of the lamellar α with various initial thicknesses were different.For the thick initial lamellar microstructure,the fragmentation of lamellar α is caused by continuous dynamic recrystallization mechanism and the penetration of ? phase.While the collaborative mechanism of continuous dynamic recrystallization and discontinuous dynamic recrystallization promoted the rapid refinement for the thin acicular initial microstructure.Based on the deformation mechanism of lamellar microstructure,the experimental verification for lamellar microstructure refinement of Ti60 alloy was conducted through extrudsion forming process.The microstructure distributions and refinement degree for the Ti60 alloy with different initial lamellar microstructure after extrusion were discussed.The results showed that the microstructure in undeformed region remained the original lamellar microstructure and the lamellar α in grain boundary were fragmented.In the deformation regions,a large number of α phase were elongated and distributed along the direction of metal flow.The degree of refinement of lamellar microstructure was obviously improved with the decrease of initial lamellar thickness and the increase of extrusion ratio.To improve the microstructure refinement under small extrusion ratio,the initial lamellar thickness should be controlled in 6μm.For the initial thick lamellar microstructure,the alloy should be extruded at the temperature around 990°C and higher extrusion ratio.