Microstructure And Mechanical Properties Of 7050 Aluminum Alloy FSW Joint After Rolling Treatment

The structure and mechanical properties of different areas of friction stir welded joints are quite different,which makes the welded joints easy to become weak points of the welded structure.In this paper,7050 aluminum alloy is selected for friction stir welding.After optimizing the process parameters,the welded joints obtained under the best process parameters are selected for rolling treatment.By studying the microstructure,grain orientation,microhardness,tensile properties and fatigue crack growth resistance of the welded joint before and after the rolling treatment,the influence of the rolling treatment on the joint is explored.The research conclusions are as follows:(1)Rotation speed and welding speed have a great influence on the heat input of welding.When the heat input is too high,the weld is easy to form defects such as flash;when the heat input is too low,the weld is easy to form defects such as surface cracking.The grains in the nugget zone are fine equiaxed grains.As the heat input increases,the average grain size increases,and the recrystallization ratio increases.The main textures in the upper part are {011}<211>Brass deformation texture,{236}<385>Brass R and{001}<110> rotating cube shear texture.The hardness of the heat-affected zone on the advancing side of the joint is the lowest,the hardness of the upper part of the nugget zone is the highest,and the hardness gradient on the advancing side is larger.When the rotation speed is 900r/min and the welding speed is 80mm/min,the overall mechanical properties of the joint are the best,with the tensile strength and elongation reaching77% and 87% of the base material respectively.(2)The rolling treatment of 1,2 and 3 passes effectively reduces the surface roughness of the joint,and the plastic deformation with large strain,high strain gradient and high strain rate is introduced on the joint surface to realize the nanometerization and homogenization of the joint surface structure.According to the changes of strain and displacement,the organization from the subsurface layer to the surface layer of the joint is divided into microplastic deformation layer,transition layer and deformation layer.The results show that the grain morphology of the microplastic deformation layer is basically unchanged;The grain boundary of the transition layer grains gradually deflects to the direction parallel to the surface of joint,and its aspect ratio increase;The grains of the layer have a nano-layered structure,and the average layer thickness reaches 68 nm at 20 ?m from the surface.The texture of the welded joint changes from(001)[94?0] recrystallized texture to(001)[11?0] rotating cubic shear texture,and finally to <111>?RD fiber texture.The Schmid factor of the slip system of the main texture becomes smaller and smaller when stretching along RD direction.(3)After rolling treatment,the hardness of the surface layer in each area of the joint tends to be uniform,and the average hardness of the nugget zone increases to244 HV.After three passes of rolling treatment of the joint,the tensile strength increased by 21.2%,and the elongation decreased by 17.3%,which improved the tensile strength of the joint while ensuring the plasticity.Rolling treatments with different passes all increase the fatigue crack growth life of the joint and reduce the fatigue crack growth rate of the joint.With the increase of rolling passes,the joint fatigue crack growth resistance continues to increase.Fatigue striation can be found at the fracture surface in the early and middle stage of stable fatigue crack growth,and their spacing decreases with the increase of rolling passes.