Investigation On Microstructure And Mechanical Properties Of Inertia Friction Welded Joints Of TC21 Titanium Alloy

TC21 as a titanium alloy with high tolerance damage properties developed independently by China.Due to its excellent comprehensive performance,it is applicable to the manufacture of key components for space vehicles.Inertial friction welding(IFW),as a welding process to achieve the fabrication of large structural parts,has been widely used in high-tech fields such as aerospace due to the advantages of high quality,low heat input and narrow weld seam.In this research,the IFW joints of TC21 titanium alloy with 100% micro-interface bonding were successfully obtained.Then,Optical Microscopy(OM),Scanning Electron Microscopy(SEM),Electron Back-Scattered Diffraction(EBSD),X-ray Diffraction(XRD),Vickers hardness tester,Nanoindentation tester,and MTS electro-hydraulic servo tester were used to analyze the microstructure evolution of the IFW joints,Static mechanical properties and Low cycle fatigue behavior.The results are as follows:(1)The microstructure evolution of IFW joints of TC21 titanium alloy.The base metal(BM)is a large size "equiaxed grain" with α phase forming the boundary and the α+β lamellae filling the interior.The "equiaxed grain" in the heat affected zone(HAZ)is not deformed,but the secondary α phase is precipitated in the basal-β phase.The "equiaxial grain" in the thermo-mechanically affected zone(TMAZ)elongates toward the welding interface,the intracrystalline transformation into a fine needle α +β microstructure,and the residual lamellar α phase fracture.The large-size "equiaxed grain" in the weld zone(WZ)disappears and is replaced by the small-size "recrystallized equiaxed grain",which consists of a grain boundary α phase forming the profile and the fine needle α + β microstructure filling the interior.The grain/phase boundary from the base metal(BM)to the weld zone(WZ)gradually increases.And the β-phase grain boundaries in the base metal(BM)are all small-angle grain boundaries,and the β-phase stable elements(Mo,Nb,Cr)are aggregated in the large size β-phase.However,there are large angle β grain boundaries in the weld zone(WZ)and the β-phase stable elements are uniformly distributed.As a matter of course,the percentage of β-phase decreases from 14.6% in BM to 3.8% in WZ.(2)Static mechanical properties of IFW joints of TC21 titanium alloy.The hardness of the base metal(BM)is the lowest,about 330 HV;the heat-affected zone(HAZ)has a hardness of about 400 HV;the thermos-mechanically affected zone(TMAZ)has a similar hardness value to the weld zone(WZ),about 430 HV.The equivalent modulus of the weld zone(WZ)is about 134.15 Gpa and the equivalent modulus of the BM is about 130.24 Gpa obtained by nanoindentation.After correction,the Young’s modulus of the weld zone(WZ)was determined to be about 118.2 Gpa.The plasticity of the weld zone(WZ)is reduced compared to the base metal(BM),but the tensile strength increases about 16%.The tensile fracture in the base material zone(BM)is a mixed ductile-brittle fracture mode,while the tensile fracture in the weld zone(WZ)has more dissociation planes and the tensile fracture is closer to brittle fracture mode.The average Young’s modulus of IFW joints is about 114.8 Gpa,the average offset yield strength is about 941.6 Mpa,the average peak stress is about1025.1 Mpa,the average section shrinkage is 14.95%,the average hardening index is about 0.074,and the average strength factor is about 1596.81 Mpa.(3)Low Cycle Fatigue(LCF)behavior of TC21 titanium alloy IFW joints.During the low cycle fatigue(LCF),the mean cycle life of IFW joints increased exponentially from 333 to 14359 as the strain amplitude was reduced from 1% to0.6%.The cyclic hardening index of IFW joints is about 0.055,and the cyclic strength factor is about 1256.14 MPa.There is no cyclic hardening and only slight cyclic softening of IFW joints during the fatigue cycle.At high strain amplitude(0.9%),the fatigue cracks appeared Multi-source extension,and the fatigue crack propagation region is about 1/3 of the specimen diameter,the final transient fracture region is about 2/3 of the specimen diameter.while at low strain amplitude(0.6%),the fatigue cracks appeared Single-source extension,and the fatigue crack propagation region is about 2/3 of the specimen diameter,and the final transient fracture region is about 1/3of the specimen diameter.Based on the total strain equation and Basquin’s formula,the unknown parameters in the low cycle fatigue life prediction model of TC21 titanium alloy IFW joints were fitted and calculated.The final prediction models were obtained within 6% deviation at high strain amplitudes and 13% deviation at low strain amplitudes.