| 7050-T7451 high strength aluminum alloy has been widely used in transportation,shipping industry and aeronautics and astronautics because of its small density,high strength and good plasticity.The application of aluminum alloy is closely related to its weldability,When aluminum alloys are welded by traditional methods,it is easy to produce defects such as porosity and hot cracks,result in mechanical properties of welded joints are seriously affected,which limits the application of aluminum alloys.As a novel welding technology,friction stir welding is characterized with a small welding deformation,less welding holes and fewer cracks.Moreover,the welding equipment is simple and no need to gas shield.However,high strength aluminum alloy FSW joints easily cause fatigue failure under high load or alternating load conditions.Therefore,it is very necessary to study the microstructure characteristics,surface strengthening technology and its mechanism of FSW joints of high-strength aluminum alloy.In this paper,the test material is 7050-T7451 high strength aluminum alloy plate.A series of weld experiments were performed for FS welded joint including microstructural observation,hardness test,SEM and XRD analysis,tensile and fatigue performance test,laser shocking peening,et al.The microstructure and fracture morphology were observed and analyzed,and the fracture mechanism and the distribution of residual stress were also been discussed.The test results show that after the friction stir welding,the welded joints are mainly divided into four parts:the welding nugget zone(WNZ),thermo-Mechanically affected zone(TMAZ),heat affect zone(HAZ)and the base metal(BM).The WNZ has undergone mechanical stir and high welding heat input,and the dynamic recrystallization takes place to form the equiaxed grain,so this region shows the fine grain character.Under the shear force,the grain in TMAZ was significantly elongated while the grain size in HAZ similar to the base metal.Microhardness test on the surface and cross section of the weld indicates the distribution of surface hardness shows the“W”type.The highest hardness of WNZ is 136HV,which reaches97%of the BM.The hardness value of the two sides HAZ is low.The lowest hardness appears in the HAZ of advancing is 115HV,which is 84%of the hardness of the BM.The distribution of cross section hardness shows the“V”type,the bottom of the weld is far away from the welding heat source and directly contacted with the cushion plate,so its cooling speed is fast which lead to the grain at the bottom is smaller than the upper grain and the hardness is relatively larger.However,the hardness in the middle region is the lowest due to its inhomogeneous grain and loose organization.Tensile tests on welding joints show that the tensile fracture locations are all in the HAZ on both sides.When the fracture is in the advancing side of HAZ,the lowest tensile strength is453MPa,which is 88%of the tensile strength in the BM.This may be due to the greater thermal cycle in advancing side of HAZ during the welding process which result in tissue softening and precipitate phase's grow and segregation.Both the brittle fracture of the quasi cleavage morphology and the ductile fracture of the dimple morphology existed in tensile fracture.Fatigue tests on welding joints show that fracture positions at different stress levels are located in advancing side of HAZ,WNZ and BM,the fatigue life is 10×10~5,8.5×10~5and6.9×10~5.The fracture surface of HAZ is coarse and the distribution of dimple is dense,the fracture mode is ductile fracture.The fracture surface of WNZ is smooth and the dimple is small,the second phase particles with larger diameter can be observed,and the fracture mode is ductile fracture.The fracture surface of BM is the smoothest,which is related to the high level of stress.The tire pattern and fatigue step can be observed from the fracture,and the fracture mode is brittle fracture.After laser shock peening,the grain on the upper surface of the weld is finer.Through the observation of SEM morphology,there are holes in the section of the weld,and the hardness test of the weld section shows that the effective layer depth is 1.5mm and the maximum hardness improvement on the surface is 15HV.The comparison of the fatigue fracture before and after the LSP found that the fracture morphology changed little,but the diameter of the dimple was larger and the dimple distribution is more concentrated,the fracture mode was also ductile fracture.After the LSP,the residual tensile stress in the material is changed into residual compressive stress,which can reduce fatigue crack initiation and inhibit fatigue crack growth rate,thus prolonging the fatigue life of material. |
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