Effect Of Hydrogen Heat Treatment On The Uniformity Of Superplastic Deformation Of Laser Welded Joint Of Titanium Alloy

Lightweight structure has become one of the main research directions in the field of modern aviatio.At present,the combination process of diffusion bonding/superplastic deforming is widely used to reduce the weight of the structure,but there are some problems.LBW/SPF technology is expected to replace the SPF/DB technology to solve the problems.It is found that serious deformation inhomogeneity exists in the titanium alloy welded joint after superplastic deformation,which seriously affects the microstructure and mechanical properties after forming and limits the practical application of LBW/SPF technology.Therefore,the influence of hydrogen heat treatment on the superplastic deformation uniformity of laser welded joint of TC4titanium alloy was investigated through hydrogen heat treatment and high temperature deformation test on the TC4 laser welded joint.The results of the study show that:Hydrogen heat treatment can improve the microstructure uniformity of the joint.The gap between the welded joint and the parent material decreases with the increase of hydrogen content and quenching temperature.All structure changes into acicular structure,and uniformity of joints increases.Moreover,hydrogen heat treatment can reduce the microhardness difference of joints and improve the mechanical uniformity of joints.The microhardness of joints decreases with the increase of hydrogen content and the decrease of quenching temperature.When the hydrogen content is 0.16wt%and the quenching temperature increases from T_P-40?to T_P+40?,the microhardness difference between the parent metal and the weld bead decreases from 22HV to 12HV,which means the microstructure uniformity of the joint increases.Hydrogen heat treatment can improve the superplastic deformation properties of laser welded joints.The peak flow stress increases with the increase of the quenching temperature.the peak flow stress decreases with the increase of the deformation temperature and the decrease of the strain rate.The peak flow stress of the joint varies between 5.38MPa⁵2.24MPa.The peak flow stress reaches the minimum value of5.38MPa when the hydrogen content is 0.16wt%,quenching temperature is 880?,and the deformation process is 920??10^-4s^-1.The elongation of the joint varies between123.25%⁵63.60%.The extension of the joint first increases and then decreases with the increase of hydrogen content.The maximum value is 563.60%when the hydrogen content is 0.05wt%,the quenching temperature is 900?,and the deformation process is880??10^-4s^-1.Hydrogen heat treatment can adjust the structure of the joint and improve the deformation uniformity of the joint.The joints with different hydrogen content deform in the two phase region.The structure of parent material and weld bead changes from acicular structure to lamellar structure,and then transformed into equiaxed structures during deformation.Deformed in the single-phase region of?,the structure the base metal and the weld transforms into large-grain?-phase,in which there are the acicular martensite structure.With the increase of quenching temperature and deformation temperature,the?-phase in the joint grows,the content of acicular tissue increases and the overall structure uniformity of the joint increases.The homogeneity of the joint after deformation is characterized by the ratio of the shrinkage ratio between the weld and the parent metal?K?.With the increase of hydrogen content.the deformation uniformity of the joint first increases and then decreases.the greater the quenching temperature is,the greater the uniformity of the joint is.Therefore,the the deformation uniformity coefficient K reaches maximum value 0.989 under the condition of hydrogen content of 0.16wt%,quenching temperature of 960?and the deformation process of 920??10^-4s^-1.