Microstructure Evolution Of Hot Isostatic Pressing Ti-6A1-4V Alloy By Multi-pass Hot Deformation

Aerospace,nuclear industry and military technology and other high-tech technical fields are closely related to the country’s major security industry,and strong bearing capacity,high reliability and lightweight structure of component equipment are necessary.Titanium alloys have excellent properties of strong corrosion resistance,high specific strength,good heat resistance and low density,which have been widely used and become a research hotspot.At present,the high temperature deformation and heat treatment of titanium alloys have been reported,while no research on the three-state structure with excellent properties after multi-pass deformation of Ti-6Al-4V alloy.Therefore,it is necessary to investigate the research on this problem.In this paper,the microstructure evolution of Ti-6Al-4V Ti alloys,which was fabricated by the process of thermal isostatic press,was characterized and effect of heat treatment parameters（temperature and time）was also studied.The thermal deformation test of thermal isostatic Ti-6Al-4V Titanium alloy was carried out on Gleeble-1500 thermal simulation tester,and the effect of different parameters（deformation temperature,strain rate and deformation pass）on the microstructure evolution law was studied.For the alloy after thermal deformation,the alloys with different strain rates(0.001s^-1,0.1s^-1,5s^-1)of two deformation modes（multi-pass,single-pass）were treated by solution treatment and aging treatment.The microstructure evolution of the alloy after deformation of different deformation parameters was studied.When the solid solution treatment conditions of hot isostatic pressing alloys were the same,With the increase of aging temperature rising from 520℃to 560℃,the lamellarα-phases coarsens and the content of equiaxedα-phases increases with the increasing of grain size when the air-colled solution and 4h aging time attached to the deformation.And when the aging temperature was constant,the equiaxedα-phases disappeard and the lamellar precipitated phases increased with the aging time of 4 hours.The slab secondaryα-phases decreased slightly with the increase of temperature when the aging time was 4 hours,while the equiaxedα-phases appeared when the aging time was 6 hours,and the slabα-phases decreased and the secondary strippedα-phases and equiaxedα-phases occupied a dominant position with the increase of temperature.The results showed that in the 1st-pass hot compression process,due to the spheroidization or dynamic recrystallization of lamellarα,the homogeneous fine equiaxed microstructure could be obtained when the temperature is below 900℃..When the temperature is higher than 950℃,the microstructure composed of uniform equiaxedβgrains and lath martensite could be invesigated,which exhibited typical microstructure characteristics inβ-phase filed after water quenching,and the cross-arranged short lamellarαphases precipitated in the intragranular region.β-phases formed a staggered basket structure and had s specific orientation.With the increase of strain rate,and the volume fraction of dynamic recrystallized grains decreased and lamellarαphases in the grains were refined at950℃.When the strain rate was too high,the microstructure was mainly consisted of elongated unrecrystallized coarseβgrain.In the multi-pass compression process,the grains were mainly composed of needle martensite and strip martensite after deformation and quenching.Because of the low strain in 1st-pass deformation,the polygon shape of coarse grain were full of the investigation,which was due to the dynamic recovery law during the single-phase deformation area.When the strain rate is 0.001s^-1,theβ-phase occurs dynamic recrystallization,which causes the tissue to coarsening to a larger size and aggregate to produce recrystallization grains due to the long deformation time.With the increase of strain rate,the unevenness of grain size increases.In the case of the 2nd-pass deformation,as the cumulative strain increased and strain rate reached 0.001s^-1,the grains were in flattened polygonal shape.With the increase of strain rate to 0.01s^-1 and 0.1s^-1,the volume fraction of dynamic recrystallization increased compared with 1st-pass deformation.The microstructure presented the obvious stripe band when the strain rate reached to 1s^-1 and 5s^-1.And in the 3rd-pass hot compression process,the stripped structure distributed in the grains at different strain rates.After the deformation,the quenched structure is composed ofβgrains and martensite,and there are cross-layerαphases in the grains.Compared with single-pass process,because of the low strain of each pass,the non-uniform dynamic recrystallzaitation structure appeared.As the deforamtion continues,when the strain rate was 0.1s^-1,the three-state structure consisting of equiaxedα-phases,asmall amount of strippedα-phases and a amall amount of secondaryα-phases were obtained by solution treatment after the deforamtion of decreased temperature from 950℃to 850℃.And when the strain rate was 0.001s^-1,the three-state structure consisting of strip-likeα-phases,β-tranforming phases and short rod-likeα-phases were obtained by the solution treatment at the deformation temperature of 1050℃.As the deformation temperature reached to 900℃and keep it constant,the three-state structure consisting of equiaxedα-phases,secondaryα-phases andβ-tranforming phases were obtained by the solution-aging treatment when the strain rate was 0.1s^-1.Finally,the alloy with the strain rate of 0.1s^-1 which was deformed at the decreased temperature from 950℃to 850℃,the three-state structure consisting of equiaxedα-phases,secondary strip-likeα-phases andβ-tranforming phases were obtained.