A Study On Mechanical Behavior Of Mechanical Mismatched Fsw Joints

Friction stir welding has been widely used in aviation,vehicles and other fields due to its particularly adaptability to aerospace aluminum alloys.Unlike conventional welding methods,the FSW involves complex heat flow,material movement,and plastic deformation,resulting in a heterogeneous welding microstructure distribution in the FSW joint.The heterogeneous welding microstructure distribution of the joints results in different mechanical properties in different regions of the joint.This heterogeneous mechanical property makes the mechanical behavior of the welded joint difficult to predict.Therefore,it is important to clarify the relationship between the mechanical properties of each region and the overall mechanical properties of the joints.The full field strain measurement of an FSW AA2A12-T4 joint and an FSW AA2A12-O joint was carried out by digital image correlation(DIC)method.The local and global mechanical responses of joint were analyzed by the method of uniform stress assumption.The results show that the joint exhibits higher hardening rate and lower ductility than the base materials.There is a large strain gradient between NZ and BM during the stretching process.The "constraint effect" leads to the overall mechanical properties of the joint being better than the mechanical properties of the weakest region.The yield strength distribution of the two joints is consistent with the hardness distribution.The "incomplete" stress-strain curve is complemented by fitting the Swift isotropic work-hardening model to the experimental results.In-situ tension method was used to analyze the micro-fracture mechanism of the two joints,and the different tissue regions of the two joints were observed in situ.The results show that the FSW AA2A12-T4 joint fractures at the junction of TMAZ and HAZ,and the FSW AA2A12-O joint breaks at BM.The microcracks are easy to sprout at the junction of various microstructures.The FSW AA2A12-T4 joint forward side the large difference in mechanical properties makes it prone to fracture.The "constrained effect" of the high-strength zone on the low-strength zone causes the FSW AA2A12-O joint to break at the base metal away from the weld zone.