Study On High Rotation Speed Friction Stir Welding Of 6061-T6 Aluminum Alloy Sheet

6061-T6 aluminum alloy has been widely used in the construction,transportation,aerospace,weapons and other industries.It is a medium strength material with good toughness,good plasticity and corrosion resistance.However,due to the problem of the welding property of the aluminum alloy itself,the conventional welding method may cause defects such as pores,inclusions,and cracks.Especially in aluminum alloy sheet,the heat input is difficult to control and the post-weld deformation is severe.In order to solve this problem,the high rotation speed friction stir welding test of 1mm thick 6061-T6 aluminum alloy sheet has been carried out with different morphological FSW tools and different welding processes.The surface of the welded joint is shaped,the microstructure and degree of deformation of the weld joint are measured.The mechanical properties of the joints and the force conditions of the FSW tool are analyzed.The effect of the shape of FSW tools on the welding quality shows that when the shape of the stirring pin is a prism table,the rotation speed is 11,000 rpm,and the welding speed is 300 mm/min,the ideal mechanical properties are obtained,and the tensile strength reaches 87.2% of the base metal.The hardness value of each zone of the weld is high and evenly distributed.The section of the weld seam presents a "flat T" shape.The heat affected zone and the thermo-mechanical affected zone are narrow.The crystal structure of the weld zone is dense,and no obvious "S curve" is found.The result of the influence of pin angle on the welding quality indicates that the angular table-type stirring head with an angle of 110° between the side of the stirring pin and the shaft shoulder has a moderate volume,a strong stirring effect,and the "S curve" of the joint welding zone is not easy to appear.The welding process window is wide and the mechanical properties are optimal.When the angle between the side of the stirring pin and the shaft shoulder is 100°,the FSW tool has the largest volume and the strongest stirring effect.The "S curve" does not appear in the weld zone of the joint but the weld joint is not dense enough and the mechanical properties are worst.When the angle between the side of the stirring pin and the shaft shoulder is 120° and 130°,the volume of the FSW tool is small and the stirring effect is weak.The obtained weld surface is better but the "S curve" exists in the weld zone.Welds are slightly less dense and have lower overall hardness and tensile strength.The detection results of the forward resistance test show that when the spindle rotation speed is 11,000 rpm and other welding parameters are constant,the forward resistance of the FSW tool is gradually increased with the increase of the welding speed.In particular,when the welding speed reaches 500 mm/min,the welding joints show a groove defect,and the peak value of the forward resistance of the FSW tool reaches 100 N.The oscillation amplitude is large and the oscillation frequency is high.When the welding speed is 300mm/min and other welding parameters are unchanged,the forward resistance of the FSW tool decreases slightly with the increase of the spindle rotation speed.When the spindle rotation speed reaches 13,000 rpm,the welded joints show flash or even groove defects.The forward resistance of the FSW tool is larger and the oscillation frequency is higher.The results of EBSD analysis of weld microstructure show that the deformation of the weld zone varies greatly in high rotation speed conditions.The heat affected zone is mainly composed of deformed microstructures.The proportion of grain boundaries at low angles is 50%,and the average grain size is 16.9μm.Subgrain structure and recrystallized structure exist in thermo-mechanical affected zone and the grain size and morphology are not uniform.There are equiaxed and elongated ribbons.The proportion of grain boundaries at low angles is 23.3%.The average size of grains in this region is 10.2μm.Metal in the nugget zone undergoes severe plastic deformation and flow under high rotation speed stirring.The grain structure is mainly composed of fine equiaxed grains.The average grain size is 9.3μm,and equiaxed grains with high angle grain boundaries exist.The proportion of grain boundaries at low angles is significantly reduced,accounting for only 20.1%,lower than the proportion of conventional friction stir welding.