Effect Of Electropulsing And Aging On The Microstructure And Properties Of TC4 Titanium Alloy

Titanium and its alloys are widely applied in aerospace,marine,petrochemical,health care,energy and light industries,and named as the “third metal” because of their excellent advantages such as low density,high specific strength,excellent corrosion resistance and high temperature resistance.Due to its varied microstrcture and phase transition,titanium alloys usually present a large range of mechanical properties and behavior.So many works have been carried out to optimize the microstructrue for achieving the desired performance.As an instantaneous high energy input processing technique,electropulsing treatment(EPT)could signifiantly affect the diffusion of metal atoms,phase transformation and recrystallization.There are many advantages of this method,such as it can improve the mechanical properties of metal materials and promote energy conservation,which cause its large application in many fields.However,little attention is paid to the solid phase transformation of titanium alloys treated by EPT.In this work,solide phase transition process in TC4 titanium alloy with different initial conditions were systematically studied by combining the EPT and traditional heat treatment(HT),aiming at improving mechanical properties.According to the experiment results,the mechanisms of microstructrue refinement and strengthening were discussed,finally,the optimum pulse current processing parameters of TC4 titanium alloy with different initial conditions were obtained.(1)Coupling of the thermal and athermal effects of electropulsing treatment(EPT)can induce the completed recrystallization and phase transition in the TC4 titanium alloy sheet.Therefore,the different initial microstructures subjected by EPT with appropriate parameter were transformed to widmanstatten structure.Compared with air cooling,the smaller prior beta grains were obtained by water cooling after EPT,which is more beneficial to promote the refinement.In contrast with the traditional solution and aging treatment,ultra-high strength of TC4 titanium alloy treated by electropulsing and aging was obtained.(2)With the increase of EPT discharging duration,stable alpha and beta of the annealed samples were gradually transformed into widmanstatten structure.The microstructure was consisted of alpha,metastable beta and hexagonal close packed martensite(??)under the condition of EPT-400 ms,the better mechanical properties of the specimen were obtained.The specimens were subsequently followed by aging treatment at different temperatures and holding times and the results showed that the optimal parameters were 530 ? for 1 h.During the aging treatment,the metastable phases were decomposed into finely dispersed(?+?)equilibrium phase so that best mechanical properties of the specimen were obtained,the ultimate tensile strength was increased from 1006.19 MPa to 1383.66 MPa with 15.76 % fracture elongation and the hardness was 40.04 HRC.(3)Diverse morphologies of equiaxed structure were obtained at different solution treatment temperatures in the ?+? phase field.The sample appeared more metastable phase at 900 ? as the initial structure,the ? ?? ??? transformation occurred when they were treated by EPT-460 ms.Therefore,abundant grain subboundary formed,resulting in the significant refinement of grain.Besides,dense dislocation densities in beta phase were increased and dislocation mobility was restrained resulting in strengthening of the alloy.The main phase transformation contributing to strengthening of TC4 titanium alloy is martensite transformation on EPT quenching and stabilization during aging treatment.Meanwhile,the best mechanical property were obtained due to collective effect of fine-grain strengthening,dislocation strengthening,phase transformation strengthening,and precipitation strengthening.The hardness was 415.36 HV and the ultimate tensile strength was enhanced from 1006.19 MPa to 1459.26 MPa with 14.28% fracture elongation,resulting in better strength-elongation balance.And the specific strength was increased from0.223 N·m/kg to 0.323 N·m/kg.(4)Platelet/laminar ? structure was obtained from solution treatment temperature of 1080 ? in the single ? phase field.The initial platelet ? structure occurred the reverse ?? ?? ?? transformation treated by EPT.The relatively fine ? colonies was contributed to the strengthening of TC4 titanium alloy.After EPT-460 ms and aging teratment,the microstructure presented much finer secondary alpha surrounded by plentiful precipitated phase(aging ?)and decomposition of the metastable ? phase led aging strengthening of the TC4 titanium alloy.Those factors resulted in the obvious increase of the strength and decrease of the ductility.The crack fracture of the samples exhibited cleavage fracture.