| 2060-T8 Al-Li alloy is widely used in aerospace industry because of its low density,high specific strength and high low temperature performance.It is the main skin material of domestic large aircraft.When the fusion welding method is used to weld it,there will be some welding defects,such as pores,cracks,alloy element evaporation and so on.Friction stir welding(FSW)is a solid phase bonding technology,which has the advantage of low heat input in the welding process,which can effectively avoid welding defects caused by fusion welding.Aircraft skin is a three-dimensional curved surface component,which has obvious advantages when welding with highly flexible robot FSW.In recent years,scholars from all over the world mainly focus on the traditional FSW equipment when they study the FSW of 2060 Al-Li alloy,but there is little research on the FSW process of 2060-T8 Al-Li alloy robot.Therefore,this paper carries out the research on robot FSW process technology and 2060-T8 aluminum alloy robot FSW process and microstructure and properties,in order to provide technical and theoretical basis for its application in the aerospace field.In order to adapt to the robot FSW,with low stiffness,three kinds of stirring heads with different shoulder sizes were designed,and according to the principle of the same heat input per unit area,the robot FSW comparative tests of 2060-T8 aluminum alloy were carried out with different shoulder sizes.The pressure change curve in welding process and the effect of shoulder size on weld shape,microstructure and mechanical properties were studied.The results show that the welding pressure decreases nonlinearly with the decrease of shoulder size,and the weld with good forming quality is obtained when the shoulder size is not less than6 mm.When the shoulder size is 6mm,the tensile strength reaches the maximum value of 396 MPA.With the decrease of shoulder size,the grain size decreases,the microhardness is "U" distribution,the fracture position is the welding nugget zone,and the fracture mode is ductilebrittle mixed fracture.In this paper,the effects of welding parameters on the weld shape,macroscopic morphology and mechanical properties of FSW joint of 2060-T8 Al-Li alloy were studied by single factor test.The results show that the flying edge defect will appear on the weld surface when the heat input is too large.When the heat input is too small,holes and impermeable defects appear inside the joint.The tensile strength increases at first and then decreases with the increase of rotation speed,welding speed and welding pressure.Through the response surface method,the regression model equation with the process parameters as independent variables and the tensile strength as the response value is constructed,and it is concluded that the optimum process parameters are 1554rpm-230mm/min-3850 N,corresponding to the best joint mechanical properties of 447 MPa,and the true value of the test is 444.6MPa.The microstructure of 2060-T8 aluminum alloy FSW joint was studied.The results show that the grain size of the base metal is lath-shaped,the average size is 17.92μm,and the proportion of small-angle grain boundary is 68.4%.The average grain size in HAZ is 25.74μm,and the proportion of small-angle grain boundary is 59.06%.The grains in the heat-affected zone are distorted and some grains can be dynamically recrystallized,the average size is3.43μm,and the proportion of small-angle grain boundaries is 85.31%.All the grains in the weld core zone are dynamically recrystallized,which is equiaxed grain morphology,the average size is 0.54μm,and the proportion of small angle grain boundary is 35.28%.With the decrease of welding speed and rotation speed,the average grain size of nugget increases.The welding nugget zone is shear texture.The main strengthening phases in the welding nugget zone are T1 and δ’,and the T1 in the welding nugget zone dissolves and precipitates a large amount of Al_xCu_xMn phase. |
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