Effect Of Shot Peening On Microstructure And Fatigue Crack Growth Of 7050 Aluminum Alloy FSW Joint

In this paper,the friction stir welding(FSW)test of 7050 aluminum alloy is carried out first,and the microstructure and mechanical properties of the joints under different rotation speeds are compared.Aiming at the problem of uneven microstructure and poor fatigue performance of the joints after friction stir welding,select the best performance joint for shot peening.A comparative study of microstructure,mechanical properties,residual stress,fatigue crack growth rate and other tests were carried out to explore the effect of shot peening on the microstructure and fatigue properties of FSW joints.(1)With the increase of the rotation speed,the grain size of nugget zone gradually increases,the grain deformation size of the thermo mechanical affected zone increases,and the grain size change of the heat affected zone is smaller.The microhardness distribution of the joint shows that the hardness of nugget zone is the highest,followed by the thermo-mechanical affected zone,and the heat affected zone is the lowest.The change of hardness is that the microhardness of nugget zone decreases with the increase of the rotation speed,and the hardness of the heat affected zone decreases with the increase of the rotation speed.When the speed is low,the fracture mode of the joint is ductile-brittle mixed fracture.When the speed increases,the fracture mode changes to ductile fracture.The increase of the speed causes the residual stress in nugget zone to increase first and then decrease in the transverse direction,and the stress in the depth direction first decreases and then increases.The residual stress along the transverse direction of the heat affected zone decreases with the increase of the speed,and the stress along the depth direction increases with the increase of the speed.(2)Select the joint with the best overall performance for shot peening,and the surface roughness of the joint will increase with the increase of shot peening time.Under different shot peening time treatments,the surface of the joint shows different layered strengthening gradient structures.The thickness of the surface strengthening layer reaches the maximum 150 ?m at25 minutes.The grain size of the surface strengthening layer is nanometer,and the grain size of the surface layer continues to decrease with the increa se of shot peening time.Compared with the welded state,the surface microhardness of the joint has a greater increase under different shot peening times,and the surface hardness of the joint is increased from148.8HV in the welded state to 213 HV in the shot-peened state.Different shot peening time treatments have little effect on the tensile strength of the joints.Compared with the welded state,the tensile strength is slightly improved.The increase of the shot peening time makes the residual compressive stress introduced into the joint gradually increase,the maximum value position of residual stress continues to deepen.(3)Select shot peened joints with better comprehensive performance and conduct fatigue crack growth test.After shot peening,the fa tigue life of nugget zone of the joint is the shortest,followed by the thermomechanical affected zone,and the heat affected zone has the longest life.The fatigue life of all areas of the shot peened joint is increased by more than 25% compared with the welded joint.The fatigue crack growth rate of the shot-peened joint the nugget zone is the fastest,and the heat affected zone is the slowest.The fatigue crack growth rate in different areas of shot-peened joints is lower than that of welded joints.At the initial stage of fatigue crack growth,fatigue lines appear on the fatigue fractures in all areas of the shot-peened joints,and fatigue lines and tire pattern indentations appear in the fatigue fractures in the thermo-mechanical affected zone.During the stable period of fatigue crack growth,fatigue lines and secondary cracks appeared in nugget zone and the thermo-mechanical affected zone.Compared with the welded state,the shot peened state has more fatigue marks on the fatigue fracture,more fatigue steps,and more secondary cracks and holes.The fatigue crack growth mode of nugget zone is transgranular and intergranular,and the fatigue crack growth mode of the heat affected zone is mainly transgranular.