Microstructures Evolution And Mechanical Properties Of Friction Stir Welding Joints Of 6082 Ultra Thick Plate Aluminum Alloy

6082 aluminum alloy has the characteristics of moderate strength,high impact toughness,and good welding ability,etc.It has been widely used in the industrial filed.In recent years,the application of large heavy-duty aluminum alloys in the industrial filed is rapidly increasing.But it is difficult to meet the quality requirements of the new manufacturing multiple unites technology by adopting the traditional arc welding technology.Friction stir welding(FSW)is a solid-phase joining method with unique advantages in the connection of thick plate aluminum alloys.Therefore,it is of great significance and application value to study the microstructure and properties of 84 mm thick 6082 aluminum alloy FSW joints.In this paper,double-sided butt joint of FSW test was conducted on 84 mm thick plate of 6082 aluminum alloy.It focusing on the microstructure and mechanical properties of the thickness at thick plate FSW joint each region.In addition,the influences of welding progress parameters and heat treatment progress on the FSW joint microstructure and mechanical properties was researched.The results show that the weld zone(WNZ)of the FSW joint has an equiaxed crystal structure and the precipitated is a dotted GP zone.As the thickness increases the grain size decreases,the GP zone concentrates and the size increases.The thermo-mechanically affected zone(TMAZ)also evolved into equiaxed crystal structures.With the increase of thickness,the crystal grain distortion increases.There are a higher density of dislocations and a few GP zone in TMAZ.In the heat affected zone(HAZ),the degree of recrystallization gradually decreases with the increase of thickness,and the main precipitate inside the HAZ zone is black rod-shaped??.The hardness of the center layer of the 6082-T6 aluminum alloy FSW joint was “V” shaped.And the hardness curve of both the front and the back surfaces and the near surface of the joints show a “W” distribution.The minimum value of weld hardness appeared in the overlapping area of the two welds.The main reason for the decrease in hardness is that the strengthening element not fully dissolved;The FSW joint tensile results show that the surface layer strength is 209 MPa,and the average intensity is the lowest at 180 MPa in the center layer.As the thickness increases,the tensile strength decreases.The fracture location of each sample of the joint occurred at intersection of the TMAZ and HAZ on the advancing side;The number of fatigue cycles of FSW joints increased with decreasing load,and fatigue fractures mainly occurred at ATMAZ.The second phase particle exfoliation is the main reason for the decrease in the fatigue performance of the joint ATMAZ.The study found that as the welding speed increases,the welding peak temperature,the TMAZ width narrows,the WNZ grain size,and the HAZ recrystallization degree decreases.In addition,the hardness value and tensile strength of the joint along the thickness direction decrease as the welding speed increases;As the rotational speed of the stirring head increases,the grain size of WNZ,the width of TMAZ,and the grain size of HAZ increases.While,the hardness slightly and the tensile strength of the joint decreased.Artificial ageing before welding increasing the width of the TMAZ zone,and increases the grain size of WNZ and HAZ.The artificial ageing hardness of the joints prior to welding in the surface layer and subsureface hardness trend and nature aging the same,but the hardness increases.In the center layer,the hardness curve is “V” after manual treatment before welding.The artificial aging before welding has no obvious effect on the strength of the FSW joint;Artificial aging after welding makes the grain size of WNZ increasing,the surface hardness of the joint improving,and the center layer hardness of the joint decreasing.The artificial aging after welding increases the strength of the surface layer and the near surface layer of the joint by about 15 MPa,and the strength value of the center layer has no obvious change.As a whole,artificial aging after welding has a certain influence on the strength of the joint.