Microstructure And Mechanical Properties Of Friction Stir Welding Joints For 7B56–T4 Aluminum Alloy Thick Plates

In this paper,a wide range of welding parameters were selected to study the friction stir welding（FSW）and dual rotation friction stir welding（DR–FSW）of 7B56–T4 high-strength aluminum alloy sheet with a thickness of 11 mm and 21 mm,respectively.OM,SEM,TEM,EBSD,Vickers hardness（HV）and tensile test machine were used to study the origin of the welding defects and the relationship between the welding parameters,microstructure characters and mechanical properties.And further improvement of the strength of FSW joints were conducted by post-weld heat treatment.DR–FSW was preliminarily explored.The influence of welding process parameters on the microstructure and mechanical properties of the DR–FSW joints was analyzed.Friction stir welding test shows that the welding defects can be eliminated by optimizing the welding process parameters.Defect-free FSW 7B56–T4 joints with thickness of 11 mm could be obtained under low rotation rate of 500 r/min and welding speeds of 100 mm/min and 250 mm/min.Compared with the BM,NZ was characterized by fine and equiaxed grains due to dynamic recrystallization.And the proportion of high angle grain boundaries in NZ is more than 80%.With the increase of welding speed,the grain size in the NZ become smaller.Small amount of long stripηphase were observed in the NZ.And Al₃（Zr,Sc）dispersoids often act as effective heterogeneous nucleation sites forηphase.In the HAZ,there were a large number of coarseningη′/ηprecipitated phases.The distribution of microhardness of the FSW joints shows“W”pattern,with two low hardness zone were observed on retreating side（RS）and advancing side（AS）,respectively.The lowest hardness zone（LHZ）was located in the HAZ.With welding speed,the hardness of NZ and LHZ increase gradually.The tensile strength of FSW joint increases with the increase of welding speed.When the rotation speed is 500 r/min and the welding speed is 250 mm/min,the maximum tensile strength of the joint is 468.5 MPa,and the joint strength coefficient is 73.8%.The post-weld heat treatment test shows that after the post-weld artificial aging at 120℃for 12 h（designated as AA）,the hardness of the NZ and BM increased.But the hardness of the LHZ,tensile properties and fracture location were unchanged.Solution treatment at 470℃for 1.5 h+water quenching+artificial aging at 120℃for 12 h（designated as AA T6）do not change the grain structure of the joint under low welding speed of 100mm/min.However,abnormal grain growth at the bottom of the NZ under high welding speed of 250 mm/min was found.Moreover,T6 heat treatment resulted in dissolution of the original precipitates and the re-precipitation of fine and uniformη′andη（Mg Zn₂）phases,which significantly improved the hardness of the joints.The T6 joint cracked along the“S”line during tension with severe reduction of plasticity and joint efficiency of 87%.The DR–FSW test of 7B56–T4 aluminum alloy with thickness of 21 mm shows that tunnel defect appears in the joint at a welding speed of 100 mm/min,with rotation speed of 100 r/min for the shoulder and 250 r/min for the pin.This leads to premature fracture of the joint during tensile test.Fracture surface shows brittle fracture.For the no defect DR–FSW joint,NZ was characterized as fine and equiaxed grains due to dynamic recrystallization.The grain size in the NZ of DR–FSW joints gradually decreases along the thickness direction.The tensile strength of the joint was 359 MPa,and the strength coefficient was 59.5%.