Effect Of Thermal-Mechanical Treatment On The Microstructure And Properties Of 2195-T4 Aluminum-Lithium Alloy Friction Stir Welded Joints

With its high specific strength and high weldability,2195 aluminum-lithium alloy has become the primary choice for launch vehicle structural materials.Friction stir welding can effectively avoid welding defects and improve the mechanical properties of joints.It is widely used in aluminum-lithium alloy welding.In order to improve the hardness of whole welded joints and increase its tensile properties,this paper uses 2195-T4 aluminum-lithium alloy as the welded material to carry out friction stir welding experiments to study the changes in mechanical properties of the welded joints during the artificial aging process and the joints before and after aging.At the same time,this article explores the changes in the mechanical properties of the joints and the evolution of the microstructure caused by the amount of deformation through the rolling deformation treatment of the welded joints.On this basis,the pre-deformation+artificial aging process is used to process the welded joints to achieve a uniform distribution of the microhardness of the joints and a significant increase in the welding coefficient,providing a theoretical basis and process reference for the formulation of the actual production process.The main conclusions are as follows:The use of 2195-T4 for friction stir welding can improve the hardness distribution of the joint and achieve the purpose of reducing the fluctuation range of the hardness in different areas of the joint.At this time,the tensile strength and elongation of the joint are 440.2MPa and 11.2%,respectively,and the welding coefficient is 91.8%.The artificial aging treatment of the joint after 2195-T4 friction stir welding can significantly increase the joint hardness,but the joint hardness still shows a"W"-shaped distribution during the aging process,and there are obvious weak hardness areas.The best artificial aging process is 160?artificial aging for 64hours.At this time,the tensile strength of the joint is 489.5 MPa and the elongation is 4.5%.The HAADF-STEM results show that after the T4 welding joint,there is a very small amount of coarsening T₁phase precipitation in the HAZ near the BM,and only a small amount of granular?'phase precipitation in BM,NZ and TMAZ.After64 hours of artificial aging treatment,it was found that obvious T₁ phase precipitation appeared in each area of the joint.According to statistics,it was found that the T₁ phase density in the HAZ area was much lower than other areas.Research on the post-weld aging process shows that the aging treatment can promote the precipitation of T₁ phase in the joints after 2195-T4 friction stir welding and increase the strength of the joint.However,the different characteristic areas of the joint have a big difference in the precipitation pow er of the T1 phase.The precipitation power in the HAZ area is significantly lower than other areas.Therefore,the number of T1 phases in the HAZ area after peak aging is much less than other areas,and the strengthening effect is not obvious enough,so t he joint The hardness still presents a"W"-shaped distribution,and it breaks along the junction of the HAZ/TMAZ zone when it is stretched at room temperature.The strength of the 2195-T4 joint after rolling deformation increases with the increase of the deformation,and the elongation change law is opposite to the strength.When the deformation is 15%,the tensile strength of the joint is the highest,reaching 482.2MPa,and the elongation is 7.0%.The microstructure characterization results found that rolling deformation can reduce the grain size of each area of the joint,increase the dislocation density within the alloy,and then increase the strength of the joint.The optimal process for artificial aging of 2195-T4 joints after pre-deformation is 15%pre-deformation+64h artificial aging.At this time,the hardness distribution of the joint is relatively uniform,and the difference in hardness value between different areas is small.The tensile strength of the joint is 543.4Mpa,Its welding coefficient can reach 91.1%,but the elongation is only 3.3%.he HAADF-STEM bright field characterization results of the weld nugget area of the 2195-T4 joint after 15%pre-deformation+artificial aging treatment show that the increase in deformation will significantly increase the dislocation density inside the alloy,which can promote the T1 phase at the dislocation site during artificial aging Nucleation,thereby improving the precipitation efficiency of T1 phase.Therefore,the overall hardness of the joints artificially aged for 32h after 15%deformation is relatively high,the degree of fluctuation is not large,and the tensile strength is significantly improved.