Research On Hot Deformation Property Of Laser-welded Joint And Hot Gas Bulging Property Of Laser-welded Tube Of Ti-22Al-25Nb Alloy

Owing to its high specific strength,better heat-resistance,corrosion and creep resistance,tubular parts of the Ti-22Al-25 Nb alloy have a bright application prospects in aviation.Hot metal gas forming technology can be used to fabricate high strength tubular parts with complex cross-sections.The common forming processes of such tubular parts are to form the tube blanks first and then to weld for the finished parts.However,the tubular parts obtained by this method are poor in terms of shape and dimension precision.Therefore,sheet rolling combining laser beam welding techniques for tube blanks and hot metal gas forming technique for finished parts are proposed to make Ti-22Al-25 Nb alloy tubular parts.Deformability of the weld joint at elevated temperature attaches great importance for the bulging properties of the tube.Therefore,deformation behavior and microstructure evolution during hot working processes of the laser-welded joint of Ti-22Al-25 Nb alloy should be investigated systematically and deeply.Deformability of the laser-welded Ti-22Al-25 Nb alloy sheet and the bulging property of the welded tube at elevated temperature are investigated.Besides,both the microstructure evolution and deformation mechanism of the weld joint are studied.Tensile tests of samples with a transverse joint of Ti-22Al-25 Nb alloy are carried out under the deformation temperatures of 850⁹30?and a strain rate of 0.001s^-1.Results show that 875?is a critical temperature which determines the fracture mode of the welded samples as well as the sealing temperature of the welded tubes during bulging process.The reasonable deformation temperature of the welded blank is930⁹90?.Tensile tests of samples with a longitudinal joint of Ti-22Al-25 Nb alloy are carried out under the deformation temperatures of 930⁹90?and strain rates of0.125⁰.0002s^-1.The constitutive equation of the weld joint at elevated temperature is established and the microscopic deformation mechanism is revealed:lamellar phase of the weld bended and spheroidized under deformation,dynamic recrystallization of ?/B2 phase contributes to the improved plasticity of the weld joint.Additionally,the volume fraction of the chain-shaped?₂ phase along the ?/B2 boundaries decreases with increasing temperature,promoting boundary migration and hot deformability of ?/B2 grains.Hot metal gas bulging tests of the laser-welded Ti-22Al-25 Nb alloy tube are carried out based on the hot tensile deformation behavior and the corresponding deformation mechanism.Results show that the weld seam is thinned under the biaxial tensile process which improves the uniform deformation ability between the weld and the base metal.Unreasonable matching of bulging temperature and pressure is bad for the reduction of the joint and decreases the maximum expansion ratio of the tube.The best bulging temperature of the welded tube of Ti-22Al-25 Nb alloy is 970⁹90?.The result shows that the maximum expansion ratio of the laser-welded tube is about80.6%.The weld joint has better deformation compatibility when the strain rate is around 0.001s^-1.Besides,microstructural evolution of the uniform deformed weld during bulging process shows that dynamic recrystallization and dynamic recovery are the deformation mechanism of the fusion zone,and that dynamic recovery is the main mechanism.Laser-welded Ti-22Al-25 Nb alloy experiences complex microstructure evolution during high temperature deformation.As an important part of microstructure evolution,texture evolution of ?/B2 phase in the fusion zone during the laser beam welding,uniaxial tensile and bulging processes is investigated in this article.Results show that there is strong ?/B2 phase texture?111?[??????2]in the as-welded joint.The intensity of textures?111?[0???1],?001?[1???0]decrease and that of the texture?111?[1???0]increases with increasing tensile temperature.In the bulging process,grain fragmentation and recrystallization in the joint break the texture{001}<100>formed during the welding process.The thickness deformation is larger than the width deformation,which results in the rolling texture?111?[1???0].