High Rotational Speed Friction Stir Welding Of Dissimilar Aluminum Alloy Thin Plates Process Research And Residual Stress

In recent years,with the continuous development and promotion of industrial manufacturing intelligence,aerospace,household appliances,automotive industry and other manufacturing fields have shown a trend of development of precision,miniaturization and lightweight.The application demand for thin-walled parts is increasing day by day,and energy saving,emission reduction and sustainable development have become the theme of social development.Replacing steel with aluminum alloy with low density is one of the important ways to achieve lightweight design.Aiming at 1mm thick 6061-T6 and 5083 dissimilar aluminum alloy friction stir welding,we independently designed a large shaft shoulder type stirring head suitable for thin plate high speed FSW,and studied the influence of the relative position of the base metal on the flow behavior of dissimilar metal and mechanical properties of the welding joint.The effects of welding process parameters on welding surface defects,joint microstructure and mechanical properties were analyzed.The size and distribution of the residual stress on the upper surface of the thin plate under different welding parameters were tested by XRD.Through this experiment,the following conclusions are drawn:The relative position of FSW base metal of dissimilar aluminum alloy has an important effect on the weld metal flow behavior and joint performance.When 6061-T6 aluminum alloy is placed on the forward side,the plastic metal fluidity in the weld zone is better,and the zigzag interface can be formed.Compared with 5083 placed on the forward side,the mechanical properties of the welded joint are better,the average tensile strength is increased by 8%,and the fracture morphology changes from parabolic dimple to equiaxial dimple.Joint regional characteristic of observations available,welding nuclear area of thermal cycle peak temperature highest occurrence of dynamic recrystallization grain to fine equiaxed crystal,grain of heat engine affected zone due to thermal cycling and mechanical force is a combination of a general and plastic material flow in the same direction of grain orientation,heat affected zone group as far away from the weld center,by welding thermal cycle function weaker than welding area,dynamic recrystallization process that occurs not only respond to reaction.The overall hardness distribution curve of the welded joint presents a"W"shape with unequal height on both sides.On the one hand,the nugget undergoes fine grain strengthening,on the other hand,the strengthening phase Mg₂Si dissolves,and the final hardness can reach more than 80HV.The heat affected zone on the side of 5083 aluminum alloy is the lowest hardness area of the whole welding joint.This is consistent with the tensile test results of the joint,and the tensile fracture location is mostly in this region.When d=6mm,v=300mm and?=10000,the tensile strength of the welded joint reaches 236MPa and the elongation is 6.6%after fracture,showing the best mechanical properties.The distribution law of the longitudinal residual stress on the upper surface of the weldment perpendicular to the direction of the weld is a typical bimodal characteristic.It is shown as tensile stress within about 10mm on both sides of the weld center,and the range of variation is-20MPa?120MPa.The peak tensile stress appears at the edge of the shaft shoulder.However,the transverse residual stress value is obviously smaller than the longitudinal residual stress value,and the variation range is smaller,ranging from-20MPa to 40MPa.The surface longitudinal residual stress of the forward side is obviously higher than that of the backward side.There is a certain degree of stress release at the starting and ending positions of welding,and the residual stress value is small.