Microstructure and Mechanical Performance of 7075-O Aluminum Alloy Friction Stir Welded Joints
The age-hardened AA7075 in the friction stir welding(FSW)joints exhibits lower plasticity,posing challenges for further deformation processing.In this study,this paper employed a 3 mm thick AA7075-O plate for stir friction butt welding,investigating the impact of welding speed on the joint microstructure and strength-ductility,aiming to provide solutions for achieving highly ductile FSWed joints.The results reveal that the weld nugget zone(NZ)in the AA7075-O joint consists of fine equiaxed grains.With an increase in welding speed,the average grain size(AGS)in the weld NZ initially decreases and then gradually increases.At a welding speed of 40 mm/min,the smallest AGS in the weld NZ is approximately 1.96 μm.The microhardness distribution across the transverse section of the joint shows a convex shape at different welding speeds,indicating significant hardening in the joint region.The weld NZ exhibits the highest hardness,reaching approximately 145HV.The process window for FSW of AA7075-O is broad,with joint strength exceeding the base material(BM)strength at welding speeds within the range of 20-80 mm/min.The elongation of the welded specimen reaches 82% of the BM.The use of AA7075-O not only overcomes the softening issue in the age-hardened AA7075 FSWed joints but also yields a higher elongation.Bending test results demonstrate that the FSWed joints of AA7075-O can withstand larger bending angles,with a maximum bending angle reaching 105°,indicating superior plastic deformation capabilities.
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