Effect Of Electropulsing Treatment And Aging On Microstructure And Properties Of TC4 Titanium Alloys In Different Initial States

Titanium and titanium alloys have excellent characteristics such as high specific strength,strong corrosion resistance and good mechanical properties.They are widely used in high technology fields such as aerospace.However,the research on titanium alloys for aviation in China started relatively late,relying mainly on imports for a long time to maintain the production demand of the aviation industry.With the rapid development of domestic titanium alloy industrial production,achieving toughening of titanium alloys is a major problem to be solved at present.The traditional heat treatment has the problems of excessively high temperature and too long holding time,which lead to grain coarsening,so the ultra-high strength and toughness of titanium alloy cannot be achieved.The high density electropulsing treatment technology has the advantages of fast heating speed,short pulse processing time and energy saving,and has achieved good effective in many aspects such as hindering crack growth,promoting recrystallization and phase transition.In this paper,TC4titanium alloys in different initial states were used as the research object,the combination of electropulsing treatment and traditional heat treatment were used to improve the tensile strength and plasticity of the alloy.The effects of electropulsing treatment and aging on the microstructure and properties of TC4 titanium alloys in different initial states were analyzed.A new heat treatment process method to obtain high strength TC4 titanium alloy was explored.The main research results of this paper were as follows:(1)The equiaxed structure and bimorphic structure were obtained by subjecting the rolled and annealed TC4 titanium alloy to solid solution treatment at different temperatures.With the increase of solid solution temperature,the proportion of primary?phase transformed to?phase gradually increased,the volume fraction of primary?phase decreasesd.(2)With the increase of solution temperature,the tensile strength and elongation of TC4titanium alloy increased.The tensile strength of TC4 alloy subjected to solid solution treatment at 850 ~oC,900 ~oC and 950 ~oC were 1024 MPa,1085 MPa and 1128 MPa,respectively,the corresponding elongations were 13.3%,18.4%and 21.5%,respectively.(3)Electropulsing treatment effectively reduced the phase transition thermodynamic barrier of the hexagonal martensite to the?phase.The solute content of the hexagonal martensite was high and the transition to the?phase was preferential.A large number of?phase nucleation sites were formed in a short period of time and?stable element rich region was formed.More?phase was retained at room temperature and the grains were significantly refined.The average grain size of the 900 ~oC solid solution TC4 alloy after optimization of the electropulsing parameters was refined from 5.0?m to 2.8?m.After the electropulsing treatment,The best tensile strengths of TC4 titanium alloy in solid solution treatment at 850 ~oC,900 ~oC and 950 ~oC subjected to electropulsing treatment were 1219 MPa,1205 MPa and 1215 MPa,and the corresponding elongations were:14%,17.2%and 16.8%.Compared with the solid solution state TC4 titanium alloy without electropulsing treatment,the tensile strength was increased by 19.0%,11.3%and 7.7%,respectively.(4)Different initial states TC4 titanium alloy subjected to electropulsing treatment followed by aging treatment,the metastable hexagonal martensite in the microstructure was severely decomposed during aging,and the dispersed(?+?)phase was precipitated.The secondary?phase was precipitated at the original?grain boundary,which had effective improved the tensile strength of the alloy.850 ~oC solid solution state,900 ~oC solid solution state and 950 ~oC solid solution state TC4 titanium alloy subjected to electropulsing treatment followed by aging treatment,after experimental parameters optimized,the optimal tensile strength corresponding to TC4 titanium alloy in different initial states were:1348 MPa,1455 MPa and 1315 MPa,and the corresponding elongations were:12.5%,14.2%and14.6%.Compared with the initial solid solution TC4 titanium alloy at the corresponding temperature,the tensile strengths were increased by 31.6%,34.1%,and 16.6%,respectively.The tensile strength of TC4 titanium alloy corresponding to the traditional solid solution+aging treatment increased by 20.2%,22.0%,and 5.4%in turn.(5)The 900°C solid solution TC4 titanium alloy was subjected to electropulsing local multipoint discrete strengthening treatment.As the electropulsing time increased,the microstructure of the alloy matrix area did not change significantly,and the content of the?phase in the middle axis of the strengthened unit structure increased first after the reduction,the?-transformation tissue content increased,the oxygen element content increased first and then decreased,and the hydrogen element content increased.After the aging treatment,the content of the secondary?phase increased,and the hexagonal martensite was severely decomposed,the dispersed(?+?)phase was precipitated.The oxygen content increased and the hydrogen content decreased.After the optimization of the experimental parameters,the optimal tensile strength of the TC4 titanium alloy with local single row multipoint discrete strengthening treatment was 1331 MPa and the elongation was 12.9%when the electropulsing was applied for 80 ms.