Study On Microstructure And Properties Of The Al/Cu Dissimilar Metals Jointed By Friction Stir Welding

Composite joints of aluminum-copper will contribute to weight and cost reduction.The aluminum-copper composite joints have been widely applied in various industries due to their good electrical conductivity,thermal conductivity,corrosion resistance and formability.Friction stir welding?FSW?is a solid-state connection technique and has the advantages of energy saving,high efficiency,environment friendly compared with traditional welding.In this paper,3 mm thick 6061 aluminum alloy and T2 copper produced by FSW,the effects of different processing parameters such as the placement position of the base metal,different offsets,rotation speed,and traverse speed on the formability of welded joints were compared and analyzed.The influence of different rotation speeds and traverse speeds on microstructure,phase composition,intermetallic compounds?IMCs?layer and mechanical properties of joints were also investigated.The main conclusion was as follows:The stir tool with vortex shaft shoulder and threaded pin would attribute to the stir pin adhesion problem at the butt-welded of dissimilar metals.The platform shoulder shape and none-threaded of the stir pin was the best choice for obtaining a good quality weld aluminum and copper.When aluminum was located at advancing side and the stir pin was inserted towards the aluminum side in the range of 0.2¹.0 mm,a good formability joint was easily obtained.The bestwelding parameters for aluminum-copper welded joints range from 750 to 1180 r/min and 30 to 47.5 mm/min.When the traverse speed was constant,the grain size in the nugget zone?NZ?decrease and then increase with the increase of the rotation speed.When the rotation speed was 1180 r/min,the grain size of the NZ becomes smaller as the traverse speed increases.There exist Al₂Cu,Al Cu and Al₄Cu₉ in the NZ.When the traverse speed was fixed,the heat input of the joint increases accordingly with the increase of the rotation speed,and the content of intermetallic compounds increases.When the rotation speed was fixed,the content of intermetallic compounds gradually decreases with the traverse speed.The IMCs layer were formed at the interface,due to some metallurgical bonding occurred in the FSW of aluminum and copper,and the thickness range of the IMCs layer is 1.5⁴.05?m.The growth mechanism of the IMC layer at the aluminum-copper interface:the Al₄Cu₉ phase was formed at the interface near Cu Al side,and the the Al₂Cu phase was formed at the interface near Al side.The grain refinement and high dislocation density caused by severe plastic deformation resulted in the average hardness of NZ being higher than that of the base metal,and the formation of intermetallic compounds was the main reason for the sudden high hardness value.The highest hardness value?403 HV?appeared on the top of NZ.The residual stress in the base material was generally compressive stress.The aluminum and copper sides of the weld zone are tensile and compressive stresses,respectively,and the maximum stress value appears in the region near the shoulder.The tensile strength of the aluminum-copper dissimilar joints was lower than that of the parent metal.The maximum strength could achieve highly as 203 MPa,and the elongation of joints was generally low,with a maximum of 4.9%.There were four types of fracture location of aluminum-copper dissimilar joints.When the parameters were matched,the joints fracture was located on the heat affected zone side of aluminum or the boundary between NZ and thermo-mechanically affected zone.When the joints fracture occurred at the interface,the cracks propagated mainly along the IMC layer with very low strength.The fracture mode was mainly ductile-brittle mixed fracture.