The Welding Mechanism And Performance Adjustment Of Friction-stir-welded Joints Of High-strength Aluminum Alloy

2024-T351aluminum alloys were chosen as the model materials to study the metalflow behaviors under high welding parameters. Moreover, the mechanisms ofstrengthening and toughening of the friction stir welded joints treated by the deepcryogenic treatment accompanied with low temperature aging treatment were investigated.Besides, the cryogenic tensile properties and fracture behaviors under different weldingparameters were evaluated.The thread of the pin can mix the plastic metal up and down during the friction stirwelding. The grain size in the shoulder driven zone is larger than other zones due to thehigh heat generation, particularly on the retreating side, the recovery phenomenon afterwelding being found.Low-temperature aging treatments with and without deep cryogenic pretreatments,were conducted for friction-stir-welded joints of2024-T351aluminum alloys. Themicrostructures and mechanical properties of the welded joints after the above treatmentswere investigated. Two obvious soft zones were found to be located at the retreating andadvancing sides of the heat-affected zone (HAZ) in the as-welded joint. After a singlelow-temperature aging treatment (LTA), the soft regions close to the base metal in theHAZ almost vanished, while the soft regions close to the thermal-mechanical-affectedzone were not significantly affected. Moreover, tensile fracture occurred along thesegregation bands of the precipitates in the nugget zone (NZ). This resulted in a decreasein the elongation of the as-welded joint from74%to38%that of the base metal. Afterlow-temperature aging with a deep cryogenic pretreatment at-196℃, the features of thesoft zones were similar to those in the case of the single LTA, but the tensile fracture at thesegregation bands in the NZ was inhibited. Consequently, the elongation of the jointsimproved along with an increase in tensile strength. The possible mechanisms related tothe improvement of tensile properties of the joints were discussed.The friction stir welded joints under different welding parameters of Al-Cu-Mg alloyexhibited excellent tensile performance at deep cryogenic temperature (DCT). High heatinput improved the uniform elongation of joints and endowed them with favorable tensile strength and higher plasticity at DCT, although the obvious softening regions still exist inthe joints. Moreover, the crack path of fractured joints showed zigzag-like patterns. Theimproved tensile performance of the joints can be attributed to enhancements in workhardening capability and coordination deformation of the joints at DCT. In addition, theeffects of welding heat input on the tensile deformation and fracture behaviors of the jointsat DCT were discussed.