Research On Microstructure Evolution Of TC4 Titanium Alloy Fabricated By Spark Plasma Sintering During Multi-pass Hot Compression Deformation And Heat Treatment

The room temperature strength of the titanium alloy tri-modal structure is higher than that of the equiaxed structure,and the plasticity and thermal stability are close to that of the equiaxed structure,and the high-temperature performance is higher than that of the bi-modal structure.This article takes TC4 titanium alloy as the research object,conducts multi-pass cooling-heating（950℃-850℃-900℃）thermal compression deformation experiment.The thermal deformation can refine the grain and construct the microstructure.Heat treatment can adjust the morphology of the tissue.The structure of the alloy finally obtained the TC4titanium alloy with uniform tri-modal structure distribution.The main research contents and conclusions are as follows:In the multi-pass thermal deformation stage,temperature is the main factor that affects the stress value of each pass.In particular,the closure of the pores during first-pass deformation also helps to reduce the deformation resistance.At low strain rates,the stress-strain curve shows obvious characteristics of dynamic recrystallization curves;at high strain rates,the second and third passes curves show discontinuous yielding.When the deformation temperature is constant,the true stress is more sensitive to the strain rate.The greater the strain rate,the greater the true stress.The microstructure of the sample after first-pass deformation is the widmanstatten structure,which is composed of coarseβgrains;after the second passes deformation,the grain boundaryαphase and the intragranular lamellarαphase are coarsened,and the lamellar structure is twisted.And distortion provides the conditions for the spheroidization of the third passes deformed lamellar structure.The spheroidization of the lamellar alpha phase in the thermal deformation stage is controlled by the grain boundary separation mechanism and the lath shear spheroidization mechanism.The spheroidization of the lamellar alpha phase is greatly affected by the amount of deformation.The third passes 10%deformation sample is mainly lamellar structure,and the 30%deformation sample transforms into an equiaxed structure.The effect of strain rate on the structure is mainly manifested in the change of grain size.At low strain rate,the recrystallization process is sufficient and the grains grow rapidly;at high strain rate(1s^-1),the average grain size of the sample whose deformation reached 30%in third passes is refined to4.1μm.The microhardness of the sample with a strain rate of 0.1s^-1and 30%deformation in third passes reaches the maximum,326.3HV.The microhardness of the TC4 titanium alloy sample is less sensitive to strain rate.The main change of the structure after the solution treatment is the deflection and coarsening of the lamellarαphase.The content of acicular martensite produced by water quenching after the solution treatment is related to the content of the equiaxedαof the initial structure.As the content of equiaxedαin the initial structure increases,the content of acicularα’phase in the sample with 30%deformation in third passes is very small.After the solution treatment,the originalβgrains disappeared and the structure transformed into a mesh basket structure.The equiaxedαphase size of the sample with the equiaxed structure in the initial state became larger,and there is a tendency to merge with each other.The sample with 20%deformation in three passes has the largest hardness value after solution treatment,which is404.4HV.During the aging process,the transformation of the sample whose initial structure is Widmanstatten structure and lamellar structure is mainly the coarsening of strip-shapedαphase,and the growth of the equiaxedαphase is not obvious.For the samples with a large amount of equiaxedα-phase in the initial structure,the main transformation after aging treatment is the migration and merging of the strip-shapedα-phase,accompanied by the spheroidization of the primary strip-shapedα-phase.The spheroidization mechanism in the heat treatment stage is controlled by the terminal migration mechanism,which is greatly affected by temperature and time.With the increase of the aging temperature,the spheroidization ratio of the strip-shapedαphase increases,and the content of the equiaxedαphase increases.The increase of the aging time will promote the size of the equiaxedαphase to increase.In this process,it is also accompanied by the spheroidization of the secondaryαphase.The first-pass deformed sample is solid-solved at 950°C for 1h,aging at 530°C for 6h,and then approximates the tri-modal structure;the second passes deformed sample is solid-solved at 950°C for 1h,550°C,6h aging treatment,and gradually transforms into a tri-modal structure,and the hardness value reaches a maximum of 494.5HV.This means that the formation of tri-modal structure needs to control the content of equiaxedαphase in the thermal deformation stage,and the deformation amount should not exceed 50%.Then in the solution aging heat treatment stage can the content of the equiaxedαphase and secondary stripαphase of the structure be guaranteed.