Study On Microstructure And Mechanical Properties Of Friction Stir Welding Al-Zn-Mg-Cu Aluminum Alloy Thick Plates

Al-Zn-Mg-Cu aluminum alloy thick plates have important applications in rail transit,aerospace and military industries,but it is difficult to obtain good quality welded joints using traditional fusion welding technology.Friction stir welding(FSW),as a solid-state welding technology,has been successfully applied to the welding of aluminum alloy thin plates(1.2-6 mm).However,there are often problems such as low strength and uneven mechanical properties along the thickness direction for FSW thick plate joints.Aiming to this problem,the paper systematically studies the microstructure and mechanical properties of Single-Side/Double-Side FSW joints to achieve efficient connection of aluminum alloy thick plates.In this paper,thermocouples are used to test the real-time temperature change process along the transverse and thickness directions of the Single-Side/Double-Side FSW joint.Using metallographic microscope,scanning electron microscope,electron backscatter diffraction technology and transmission electron microscope to characterize the grain morphology,grain boundary distribution,orientation distribution and precipitated phase state and distribution of Single-Side/Double-Side FSW joints.The microhardness,the tensile properties of the layered slices along the thickness direction and the low-cycle fatigue properties of the joints were tested and evaluated.The main conclusions obtained are as follows:At 10 mm from the center of the weld,the maximum peak temperature of Single-Side FSW is 430?,and Double-Side FSW is 392?.During the Single-Side/Double-Side FSW process,the welding temperature is distributed along the thickness direction of the plate.The peak temperature on the advancing side(AS)is about 13? higher than that on the retreating side;The grain size of Single-Side FSW joints gradually decreases along the thickness direction.In the center of stir zone(SZ)of Double-Side FSW joint,the grain size is the smallest,and becomes coarser the closer to the surface of plate.The grain size of the heat-affected zone(HAZ)in the Single-Side/Double-Side FSW joint coarsened under the action of thermal cycling;The?`in the base metal(BM)dissolved in the Single-Side/Double-Side FSW process,and the?'phase in SZ precipitated out again with Al<sub>3</sub>Zr as the core.In HAZ,the precipitated phase underwent a phase transition of?`??,and the?phase grows and coarsens;During the Double-Side FSW welding process,the second In the face welding,due to the hot extrusion of the welded joint on the first face,the preferred orientation of the grains in the SZ zone of the joint changes significantly.In the process of Double-Side FSW welding,the preferred orientation of grains in the SZ on the second welding side is obviously changed due to the hot extrusion on the first welding side.The microhardness of each zone of Single-Side/Double-Side FSW joints is lower than BM,and the lowest hardness zone(LHZ)is located in the HAZ.The average value of the LHZ of the AS is lower than that of the RS;The tensile strength of the Single-Side FSW joint is 369 MPa,and the yield strength is 257 MPa.The tensile strength of the joint and layered slices both reach 62%of BM.The tensile strength of the Double-Side FSW joint is 466 MPa,and the yield strength is 343 MPa.The tensile strength of the joint reached 76%of BM,and the tensile strength of each layered slice was basically same.The Single-Side/Double-Side FSW joint fractures all occurred in the LHZ.This is mainly due to the coarsening of grains in the LHZ,the phase transition of?`??and the coarsening of?;With the increase of the total strain amplitude,the cyclic strain hardening behavior of the Double-Side FSW joint gradually increases.The Manson-Coffin-Basquin model,the Three parameter exponential function model and the Halford-Marrow model are used to predict the fatigue life of the joints.The prediction results of the Halford-Marrow model are most consistent with the experimental test results.The fracture position of the low-cycle fatigue specimen is consistent with the tensile test,and both occur in the LHZ.The fracture morphology shows obvious fatigue crack initiation zone,fatigue crack growth zone,and final fracture zone.The cracks originated on the advancing side surface of the first welding surface of the joint.