Study On Temperature Field,microstructures And Mechanical Properties Of Friction Stir Welding Joints Of 6082 Ultra-thick Aluminum Alloy

Friction stir welding,as a new solid-phase connection technology,has proven to be effective in joining aluminum alloy plates,and has been widely used in rail passenger car manufacturing in recent years.Most applications of friction stir welding are currently limited to aluminum alloy plates with a thickness of less than 35 mm.However,the thickness of many key components is much larger than 35 mm.In this case,the theoretical basis of friction stir welding of thin plates is not applicable to thick plate welding.Therefore,a systematic study of friction stir welding of thick plate aluminum alloy is needed to provide theoretical support for its practical production application.In this paper,80 mm thick 6082-T4 aluminum alloy commonly used in rail passenger car production is selected,and it is connected by friction stir friction double-sided welding.After welding,it is heat-treated at 180℃for 5 hours.The differences in temperature gradients along the thickness of the welds of ultra-thick aluminum alloy friction stir welding joints and their effects on the microstructure and mechanical properties of various regions were studied.The research results show that the temperature gradient in the thickness direction of the ultra-thick plate aluminum alloy friction stir welding joints is significantly different.The closer to the upper surface of the base material,the higher the temperature,otherwise the lower the temperature.Ultra-thick plate aluminum alloy friction stir welding joint is divided into:welding nugget zone（WNZ1³）,thermal-mechanical affected zone（TMAZ）,heat affected zone near the thermal-mechanical affected zone(HAZ_TMAZ),heat affected zone near the base metal(HAZ_BM)and base metal zone（BM）.The structure of 6082aluminum alloy base material is a typical extrusion shape,and the precipitation phase is mainlyα-Al（FeMn）Si phase andβ”phase;The grains in welding nugget zone are equiaxed,and the grain size gradually decreases along the thickness of the weld.The precipitated phases in WNZ1,WNZ2,and WNZ3 areβphase,β”phase and GP region,and a small amount ofα-Al（FeMn）Si phase;The grains in the thermal-mechanical affected zone are elongated,and there are higher density dislocations in this zone;The precipitated phase in the heat-affected zone is theβ’phase.However,compared with heat-affected zone near the base material zone,theβ’phase in heat-affected zone near the thermo-mechanical zone is severely roughened.The microhardness distribution of the surface layer and sub-surface layer of 6082 ultra-thick aluminum alloy FSW joints is a typical“W”shape.The micro-hardness of the nugget zone is basically the same as that of the base metal.The positions with the lowest hardness values are located in the HAZ_TMAZMAZ zone on the forward side,and the lowest hardness value is about 54 HV;The distribution of the microhardness curve of the center layer is“V”.The hardness of the nugget zone is the lowest,only 48HV.The micro-hardness of the weld nuclei along the thickness of the weld is gradually decreasing.The tensile results showed that the average tensile strength of the surface layer and the secondary surface layer reached 211 MPa and 201 MPa,respectively,while the central layer was 180 MPa,which is only 57%of the base material.The microhardness of the weld nugget in the center layer of the joint is significantly reduced,and the analysis may be due to two reasons:First,due to the use of double-sided welding,the weld nugget in the center layer of the joint has undergone two agitation;Second,the aging treatment after the welding is completed results in a decrease in its microhardness.The study found that the joints obtained by double-sided welding and single-sided welding have the same micro-hardness distribution trend,both of which are"V"type.The precipitation phase of the nugget in the center layer of the joint is the same under both conditions,indicating that the use of double-sided welding is not the reason for the decrease in the hardness of the nugget in the center layer of the joint.The effects of different aging conditions on the structure and properties of the center layer of double-sided welded joints are significantly different.The microhardness of joints maintained at 180℃for 30 hours is not much different from that of joints maintained at 5hours,and no obvious precipitates are precipitated in the joint structure;After 60 days of natural aging,there are a large number of clusters in the weld nugget area of the joint center layer,and the hardness of each area has improved;On the basis of natural aging,the joint at 180℃for 5 hours has a significantly increased hardness in the center layer base material area,no significant change in the hardness in the heat affected zone,and a decrease in the hardness in the nugget area;After the pre-aging at 70℃for 16 h and then at180℃for 5 h,the hardness of the welding nugget zone of the center layer increased most significantly.There are a large number of GP zone andβ”in the nugget zone of the joint.The finite element software ABAQUS6.14 was used to simulate the experiments in this paper.The results show that the numerical calculation results of the peak temperature of the edge of the nugget along the thickness of the weld during the welding process are548℃,510℃,499℃,478℃,and 425℃,which is the same as the actual measured peak temperature result（544℃,505℃,493℃,475℃,415℃）.Therefore,the model has certain reference significance.Peak temperature at 0 mm,8 mm,16 mm,24 mm,32mm,40 mm from the top surface of the weld is predicted using this numerical model,which is respectively 556℃,535℃,519℃,505℃,486℃and 447℃.