Study On Technology And Metal Plastic Flow Of Friction Stir Welding Of 6061 Aluminum Alloy

Pin less friction stir welding(pfsw)is a new welding process based on pin based friction stir welding.It has beautiful weld shape and no keyhole defect at the end of welding.It can effectively avoid some shortcomings of traditional friction stir welding in the welding process of aluminum alloy sheet,prolong the service life of the tool and obtain high-quality joints of ultra-thin plate And other unique advantages.At present,the research on pin less friction stir welding is relatively less,especially the visualization of pfsw plastic flow has not been reported.Therefore,it is of great scientific significance and engineering value to study the welding process and metal plastic flow of friction stir welding(FSW)for improving the quality of pfsw joint and expanding the FSW technology.In this paper,6061 aluminum alloy is taken as the research object,the effect of pin less friction stir welding on surface forming and joint strength of 6061 aluminum alloy under different welding parameters is studied,and the plastic metal flow behavior in the welding process is clarified.In this paper,the pfsw weld is divided into three parts,namely the start end,the middle section and the end.The microstructure and properties of the pfsw joint are systematically analyzed through the basic pfsw test and sample inspection.It is found that the shape of defects can be changed or even eliminated with the increase of rotating speed and holding down.When other conditions remain unchanged,the appropriate reduction of welding speed can increase the welding depth.The results show that the microstructure of pfsw nugget zone is fine equiaxed recrystallization,the grains in the heat affected zone are bent and elongated along the rotation direction of the shoulder of the stirring head under the action of the shoulder of the stirring head,and the heat affected zone is strip structure.In the influence of welding parameters on mechanical properties of joint,the mechanical model of tensile load and each parameter was established by response surface method.The three factors and three levels of response surface test were carried out with residence time,welding speed,stirring head speed and holding down as test factors and tensile load of each stage of full weld as test index.The results show that the single factor of rotational speed and residence time,and the interaction of residence time and downforce have a significant effect on the initial tensile load(P<0.01);the single factor of rotational speed,welding speed and downforce have a significant effect on the intermediate tensile load,and the interaction of rotational speed and welding speed has a significant effect on the intermediate tensile load(P<0.001)The influence order of the three factors at the end on the single index is the influence of pressure>residence time>stirring head speed.However,the interaction between the rotating speed of the mixing head and the down pressure has a significant effect on the end load.The effects of different grades of aluminum alloy on pfsw were studied The results show that in the horizontal direction,the metal plastic material on the forward side migrates to the forward direction due to the shear action between the stir head and the material,and the plastic metal material at the center of the weld is mainly mixed or mixed state,and periodically migrates with the movement of the shoulder of the stir head.After that,the backing material moves to the opposite direction of welding along the rotation direction of the shaft shoulder.