Deformation Behavior And Mechanism Of The Laser-welded TA15 Titanium Alloy Tube For Hot Gas Bulging

TA15 is a near-α titanium alloy with a chemical composition of Ti–6.5Al–2Zr–1Mo-1V(in wt.%),which is widely applied in the aerospace industry,marine,chemical industry,energy and transportation technology due to its outstanding properties including superior strength-to-weight ratio,excellent corrosion resistance moderate elevated temperature strength and good weld ability and machinability.Recently,in order to further improve the mobility performance of the aircrafts,rockets and ships,more and more attentions were paid to Ti-alloy tubular components in the fuel pipe system.However,most of tubular components were made of pure titanium or low strength Ti-alloy,and it is still difficult to form medium or high strength Ti-alloy parts,especially the forming of welded tube due to the limited understanding of the effect of weld seam on the tube formability.Therefore,in order to guide the forming of medium or high strength welded Ti-alloy tubular parts,the deformation behavior and mechanism of the laser-welded TA15 Ti-alloy tube for hot gas bulging were investigated systematically.To investigate the formability and deformation mechanism of the Ti-alloy,hot tensile tests were carried out in the temperature ranging from 650 ℃ to 800 ℃ and initial strain rate ranging from 0.001 S-1~0.1 S-1 for both the initial and annealed materials.The microstructure evolution was systematically studied.The results show that when the annealed material was deformed under the effect of dynamic recovery(DRV),the stress kept almost constant after the peak stress.The microstructure characterics included grains rotation,grains deformation and dislocation interaction.When the annealed material was deformed under the effect of dynamic recrystallization(DRX),the stress decreased quickly after the peak stress and trended to be constant gradually with the increasing strain.The main DRX mechanism was discontinuous dynamic recrystallization(DDRX).The main deformation mechanism of the initial material deformed at 800 ℃、0.001 S-1 was grain boundary sliding(GBS)facilitated by DRX,which is beneficial for the formability.To investigate the deformation behavior and mechanism of the laser-welded TA15 joint,hot tensile test,microstructure characterization and heat treatment were carried out.The results show that the joint had high strength than the parent material.When the transverse sample was subject to deformation,the whole sample could endure large deformation with the joint complete,but the joint deformed less than the parent material due to the higher strength.The laser-welded near α TA15 titanium alloy joint tends to have a good formability at high temperature and small strain rate.When the initial strain rate was 0.001 S-1,the elongation of the joint exceeded 100% at temperatures ranging from 800 ℃ to 900 ℃.The hot deformation mechanism of the laser-welded TA15 titanium alloy joint was continuous globularization,which was mainly achieved by DRX and strain-induced phase transformation of α→β.At the early stage of globularization,substructures like LAGBs and subgrains formed in the α lamella under the effect of DRV;at the middle stage of globularization,subgrains turned into independent equiaxed grain with HAGBs gradually by progressive rotation and continuous dynamic recrystallization(CDRX)was the main mechanism;at the mid to late stage of globularization,more and more equiaxed grains appeared in the materials,leading to more DDRX and less CDRX.After the double annealing process,the secondary α precipitated in the matrix improving the strength of the parent material,so the gap between the weld joint and parent material was reduced.Based on the results of the tensile tests,the hot gas free bulging experiments were carried out.The results show that the laser-welded TA15 titanium alloy tube has a good formability and a maximum bulging ratio of 73.9% was achieved at 800 ℃ and 10 MPa constant gas pressure.The weld seam hinders the deformation of the tube a bit during the hot gas free bulging due to its high strength,but a relatively uniform deformation was obtained before the plastic instability happened.Materials in the vicinity of weld seam and in the area opposite the weld seam have a smaller thickness than materials in other areas.The tube did not fail at weld seam but at the parent materials near or opposite the weld seam randomly.The bulging temperature,loading path and the initial microstructure of the tube all had a great influence on the formability of the laser-welded TA15 Ti-alloy tube.The tube tended to have a good formability at high temperature and small pressure.At the same forming condition,the initial tube had a better formability than the annealed tube.The high stored energy in the initial materials facilitated the occurrence of DRX at the early stage of the bulging,which refined the grains and improved the formability of the welded tube.Therefore,the main deformation mechanism of the initial tube under hot gas bulging was also GBS facilitated by DRX The tube after the DRX annealing process had the worst formability.For the tube after the DRX annealing,both of the strain rate and dislocation density increased with the increasing bulging height,which would reduce the uniform deformation ability of the material and local necking and bursting would happen easily.For the tube after the double annealing,the weld seam was partially globalized;in the parent material,secondary α deformed firstly and the DRX happened during the bulging process which leaded to the microstructure refining after the bulging.The deformation uniformity was improved after the double annealing process.The deformation compatibility between the weld seam and parent material was improved after the double annealing process.When the bulging height of the weld seam was 8 mm,the bulging height deviation of the weld seam and parent material was reduced by 24.6% for the tube after the double annealing compared with that of the initial tube with the same bulging height in the weld seam.