Study On Superplastic Uniform Deformation Mechanism Of TC4 Titanium Alloy Laser Welded Joint

With the demand for lightweight structure in high-speed train,aerospace and other fields,the joints of titanium alloys attracts more and more attention.However,due to the uneven distribution of temperature field in the welding process,there are regional differences in the microstructure and properties of the joints obtained.And the quality of the joints is difficult to meet the actual needs.LBW/SPF(Laser Beam Welding/Superplastic Formation)has the advantages such as small deformation of laser welding and low cost and good quality of superplastic forming technology.It has great application prospects in improving joint performance,especially in the application field of titanium alloy.At present,the non-uniform superplastic deformation of laser welded titanium alloy joints has a negative impact on the structure and mechanical properties of the formed components,which limits the practical application of LBW/SPF technology.Hydrogen and quenching can adjust the structure of titanium alloy and alleviate the non-uniformity of the structure before and after superplastic deformation.However,the mechanism of superplastic uniform deformation of titanium alloy joints after hydrogenated and quenched is still lack of systematic research.Aiming at the uniform superplastic deformation of TC4 titanium alloy joints after hydrogenated and quenched,the effects of hydrogen,quenching and their combined treatment on the microstructures and properties of TC4 titanium alloy joints were analyzed by TEM and SEM.The following conclusions were drawn: hydrogen,quenching and their combined treatment could significantly improve the microstructures and properties of TC4 laser welded joints.After hydrogen treatment,the main changed of TC4 joint structure were that the original ? phase of parent metal decreased and ?phase increased,the weld structure changed from acicular martensite ?' to lamellar structure.With the increase of hydrogen content,the difference of microhardness between weld and parent metal decreased.When hydrogen content was 0.37wt%,the microhardness difference reached the minimum value of 13 HV,which was 66.7% lower than that without hydrogenated.After quenching treatment,the main changes of TC4 joint structure were that the transformation of ? phase to ? phase in parent metal and the growth of acicular martensite ? in weld metal occurred,when quenching temperature was 1000 C,the structure of weld metal was acicular martensite ?',the grain size of weld was slightly larger than that of parent metal;when quenching temperature was1000 C,the difference of micro-hardness of weld metal was slightly larger than that ofparent metal.The minimum is 12 HV,which is 70% lower than that of quenching temperature 940?.After combined treatment,the microstructures of TC4 joints mainly changed into martensite when hydrogen content was 0.16wt% and quenching temperature was 910?.Because of the existence of hydrogen element,the oblique martensite ? appeared in the joints.When the hydrogen content was 0.16wt% and the quenching temperature was 930?,the micro-hardness difference reached the minimum value of 9HV,and the uniformity of microstructure and properties of TC4 laser welded joint was improved obviously.The microstructure evolution of TC4 laser welded joints under hydrogen,quenching and their combined treatment was studied by high temperature tensile test.The effect of hydrogen on superplastic deformation was analyzed.The main results showed: the ? phase transition point decreased after hydrogenated.The superplastic deformation of TC4 laser welded joint could be divided into two-phase deformation and one-phase deformation.The deformation mechanism of superplasticity was the grain boundary slip under the coordination of dislocation and dynamic recrystallization in the dual-phase zone,while the deformation mechanism of superplasticity was the dislocation movement under the coordination of dynamic recrystallization in the single-phase zone.After quenching,the weld and parent metal of TC4 joint undergo the same transformation process,that is,the acicular ?'phase transformed into lamellar structure.And because of the deformation stress at high temperature,lamellar structure was broken and equiaxed under Gibbs-Thomson effect with the effect of dynamic recrystallization.The combine of heat and hydrogen combined the structure evolution process of both.The weld and parent metal undergo the same structure evolution process at high temperature.The addition of hydrogen promoted recrystallization and atom diffusion,softened the TC4 laser welded joint,maintained the continuous superplastic deformation,achieved the goal of superplastic homogenization of TC4 joint,and the superplastic uniform deformation performance of the obtained joint was better.