The Characteristics Of "S" Line And Its Effect On The Mechanical Properties Of Friction Stir Welded 6082-t6 Aluminum Alloy Thick Plate

6082-T6 aluminium alloy is widely used in aerospace and rail transportation industry because of its good corrosion resistance and welding performance.At present,the connection of 6082-T6 aluminum alloy for vehicle body components is mainly by fusion welding or friction stir welding.There are many problems such as porosity,welding hot cracking and welding deformation when aluminum alloy is welded by fusion welding.Friction stir welding,as a new solid phase connection technology,has the advantages of high welding efficiency,low welding heat input and welding post-weld deformation.In particular,it has significant advantages in the welding of thick aluminum alloy plates.In recent years,friction stir welding technology is becoming more and more mature in the welding of thick aluminum alloy plates,but there are still some serious problems which are easily ignored in engineering,such as tunnel holes,weak bonding and“S”line.In this paper,the double-sided friction stir welding joint of 42mm 6082-T6 aluminum alloy which is unqualified by ultrasonic flaw detection is taken as the research object.By the means of microstructure observation and mechanical property test to systematically study the characteristics,types and the influence on mechanical properties of weld defects which is indicated by ultrasonic flaw detection.Moreover,5.2mm 6082-T6 aluminum alloy plate is welded by friction stir welding to explore the influence of FSW welding parameters on the formation and characteristics of“S”line,which provides experimental and theoretical basis for optimizes friction stir welding technology of aluminum alloy and promoting engineering application.The microstructure and macrostructure of the 42mm 6082-T6 aluminum alloy joints show that the distribution characteristics of“S”line along the thickness direction of weld is obviously different.“S" line mainly distributes in the form of oxide bands with a width of 380?m-1mm at the shoulder affecteded zone away from the surface 5mm.With the increase of distance from the surface,the oxide bands become narrower,and“S”line distributes discontinuously at the pin affecteded zone.The result of EBSD shows that the matrix grain structure around the“S”line is small and distorted.The characteristics of“S”line at shoulder affected zone and pin affected zone are different which also have different effects on the mechanical properties of the shoulder affected zone and pin affected zone of the 42mm 6082-T6 aluminum alloy joint.The tensile strength of the delaminated tensile specimens(the upper and lower delaminated specimens)at shoulder affected zone are 185MPa and 171MPa,respectively,and the elongations are 2.29%and 1.82%,respectively.And the delaminated specimens broke at is in“S”line.The tensile strength of the delaminated tensile specimens at pin affected zone is 171-202MPa,and the elongation is 7.02%-8.32%.At the fracture position of delaminated specimens is at the softening zone of the joint(where the hardness is lowest).Both side bending specimens of joints and surface bending specimens taken from the surface layer fracture in "S" line.The fatigue strength of the joint center layer specimen is 107 MPa,and the fatigue crack initiation site is in the SZ or the TMAZ.The fatigue strength of the upper and lower surface specimens of the joint are 92 MPa and 60 MPa respectively,and the fatigue crack initiation site are mostly near the "S" line of the SZ.The existence of“S”line severely reduced the fatigue strength of the upper and lower surface specimens of the joint.The above results show that the "S"line of the shoulder-affected zone is the fundamental reason for the deterioration of the mechanical properties of the zone.The 5.2mm 6082-T6 aluminum alloy plates are welded by friction stir welding with the travelling speed of 40-320mm/min and the rotation speed of 600-1500rpm.and the characteristic of“S”line in SZ of the joint is analyzed.The results show that the change of rotating speed has an obvious effect on the distribution characteristic of "S" line at shoulder affect zone.When the rotation speed is 600rpm,the travelling speed are 40mm/min and 80mm/min,the characteristic of“S”line with oxide band is observed at the affected zone of the joint shoulder,and the width of band reaches 110 ?m.When the rotation speed increases to 800rpm(travelling speed from 80mm/min to 320mm/min),the characteristic of "S" line with oxide band disappeare at shoulder affected zone.The change of travelling speed has an obvious effect on the characteristic of "S"line at shoulder affected zone.With the same rotation speed,the welding speed decreases,the times of friction between the stirring pin and the metal on the unit length weld increase and the distance between“S”line in the affected zone of the pin and the center of the weld increases.When the rotation speed is 1500rpm and travelling speed is 40mm/min,the characteristic of "S" line at shoulder affect zone disappeare.