Research On Structure?Property And Metal Flow Of 6061-T6 Aluminum Joint By Stationary Shoulder Friction Stir Welding

Stationary Shoulder Friction Stir Welding?SSFSW?is a new type of solid-phase bonding technology based on Friction Stir Welding?FSW?,it produces smooth welds with little flash and thickness reduction and creates homogenous heat input through the thickness direction,it has unique advantages compare to conventional friction stir welding,especially in the welding of unequal-thickness materials,high-melting-point materials and corner joints.At present,there are few studies on the quality,the microstructure and properties of SSFSW welded joints,and the metal flow behavior of SSFSW has been less reported.Therefore,it is very important to study its properties and metal flow to improve the quality of SSFSW joints and popularize the SSFSW method.In this paper,we use the welding tools and clamping tools of stationary shoulder friction stir welding,and 5mm thick of 6061-T6 aluminum alloy has been successfully welded by stationary shoulder friction stir welding.The morphology,microstructure,properties and metal flow of the welded joint were analyzed by changing the welding parameters and the shape of the pin.The results show that there are no fins and fish scales in the weld of SSFSW,the thickness of the weld is in accordance with the thickness of the base material;Central symmetry in the Nugget Zone?NZ?,the grain size in the upper part of NZ is smaller than that in the middle part,the composition of the second phase particles is Mg₂Si,which is distributed uniformly in the grains and grain boundary;The grain in the Thermo-Mechanically Affected Zone?TMAZ?is bent and elongated along the rotation direction of the stirring pin;the grain size of Heat Affected Zone?HAZ?is lath-shaped,larger than that of base metal.The microhardness curve of SSFSW joint is close to“W”type,and the hardness of Base metal zone?BM?is the highest,followed by NZ,about 72%of the base metal,and the hardness of HAZ is the lowest.The tensile strength of the welded joint is about 70%-75%of the base metal,and the fracture location is the heat affected zone on the retreating side.the fracture mode of welded joint is quasi cleavage fracture.In electrochemical corrosion test,it is found that if the welding speed is 120mm/min and the rotational speed is 1000rpm,the welded joint has the best corrosion resistance.Intergranular corrosion test show that there are differences in intergranular corrosion performance in each area of the weld,the BM is the best,HAZ and TMAZ follows,and the NZ is the worst.The plastic material of SSFSW joint of advancing side moves to the forward direction of welding as well as to the opposite direction of welding in the horizontal direction,the retreating side metal moves only to the opposite direction of welding,the metal on the upper surface of the weld is affected by the stationary shoulder,and the circumference migration distance decreases,and the forward migration distance increases.In the direction of thickness,the plastic metal of the advancing side enter the weld,migrating mainly in horizontal,and the amount of migration up and down is less,then the retreating side metal enter the weld with horizontal migration.Distance from upper surface 1mm,the amount of metal upward migration is larger than that of downward migration,Distance from upper surface 3mm,the amount of metal downward migration is larger than that of upward migration.With the increase of distance from the weld upper surface,the amount of metal coming back to the weld metal is reduced.