Study On Local And Global Mechanical Properties Of Friction Stir Welded AA2024 Alloy Joints

Friction stir welding?FSW?as a promising solid-state process has a prominent advantage in the fabrication of high strength aluminum complex components.However,there are still many issues to be resolved,such as the non-uniform deformation behavior induced by heterogeneous microstructure across the joint and its effect on the joint properties,and the mechanical response of FSW joints under complex loading.These would severely limit the widespread use of FSW in aerospace and railways industry in China,and also seriously hamper the establishment and improvement of joint design criteria and performance evaluation standard.In this research,the local and global properties in the FSW joints of 3.175 mm thick 2024-T3aluminum alloy were therefore systemically investigated.The effect of backplate diffusivity on the temperature,microstructure and mechanical properties of FSW joints was investigated by numerical simulation and experiments.The non-uniform deformation behavior induced by heterogeneous microstructure across the joint was revealed in detail.Moreover,the size effect and anisotropic response of joint properties were also studied.Results in this study could provide theoretical guidance and reference data for FSW structural design and weld quality control.The main conclusions are as follows:Simulated results show that backplate diffusivity has a significant influence on not only the peak temperature but the final temperature distribution.With increasing the backplate diffusivity,the time spent above 195oC presents a nearly linear decrease and the average cooling rate increases sharply,but the time for reaching the peak temperature is nearly invariable.Moreover,using a backplate with higher or lower thermal conductivity is not appropriate in FSW because it cause joint defects,and thus resultant mechanical properties is very poor.Specially designed composite backplate effectively prevents the dissolution of GPB zones and the coarsening of Al₂CuMg phase in the heat affected zone?HAZ?,and promotes the reprecipitation of GPB zones in the stired zone?SZ?.The width of softened zone in the joint is decreased by using composite backplate.The joints produced under composite backplate exhibit ultrafine grains of less than 1.2?m,and a tensile strength of 442 MPa?as much as 93.5%of base metal?and an elongation of as much as 16.2%?as much as 72.7%of base metal?.These values are extremely high compared to tensile strength of 423 MPa and elongation of 10.6%under a monolithic steel backplate typically used in FSW.Base on tensile tests and the digital image correlation?DIC?method,nonuniform deformation characteristics of FSW joint were studied in detail,and the correlation of local and global mechanical respone was also developed.The data obtained by DIC clearly indicate that the plastic deformation focus on the 10 mm wide softened zones in the weld centre,where the average elongation reaches 13%,obviously higher than global elongation of 8.2%.But the yield stress in that zone is only 291 MPa,lower than global yield stress of 305 MPa.In addation,the local yield stress across the joint is distinctly different,which is within the range of 270 MPa-325 MPa.The theoretical and experimental studies revael the obvious size effects of tensile properties of joints and its inducement factors.The ultimate tensile strength is independent on the sample size,but yield stress and elongation are significantly impacted by the sample size.The elongation drops sharply while the yield stress gradually increases with increasing the gage length from 12.5 to 25 mm,but with further increase in gage length,the elongation and yield stress are almost not changed.The size effect of the tensile behavior is mainly caused by various ratios of weld width to gauge length of samples.In order to avoid sample size effect on the tensile properties of FSW joints,the critical gauge length of sample should be no less than 25mm,which roughly equals to about 2.5 times of tool shoulder diameter.Uniaxial tensile tests at various loading angles with respect to the welding direction show the strong fracture and yield anisotropies of the FSW joints.With increasing the loading angle,the yield stress,tensile strength and elongation of the joints first decrease gradually and then increase.The yield stress is invariably the lowest at 45°loading angle.The tensile strength and elongation are always the lowest at loading angles of 60°or 75°,rather than the commonly used 90°.Furthermore,the Yld2000-2d yield function can accurately describe the in-plane anisotropy of the joints compared to the Hill48 and Yld89 yield functions.Rotation speed has a distinct effect on the mechanical properties of the FSW joints,resulting in evident changes in the shapes and magnitudes of the initial yield surfaces.The magnitude of the positive y-intercept monotonically decreases as the increase of the rotation speed.