| As a typical lightweight and high-strength materials,Al-Li alloys are widely used in the aerospace field.However,fusion welding of Al-Li alloys is prone to cracks,pores and burning of alloying elements,which seriously reduces the mechanical properties of the joints.Friction stir welding(FSW),as a solid-phase welding process,can overcome the shortcomings of fusion welding process,and is widely used for welding of Al-Li alloys.However,if a higher welding speed is employed in friction stir welding of high-strength Al-Li alloys,it is easy to cause insufficient plastic material flow in the weld nugget zone(WNZ),and lead to the formation of void defects in the joint.In addition,higher welding heat input and welding temperature at a lower welding speed will also lead to the coarsening of the grains and the dissolution of the precipitates in the joint,which will seriously deteriorate the mechanical properties of the joint.The addition of external energy field in the FSW process can improve the thermo-mechanical coupling process in FSW,and has the potential to overcome the shortcomings of conventional FSW.However,the influence mechanisms of the additional external fields on the microstructure and mechanical properties of friction stir welded Al-Li alloy joints is still unrevealed.In this paper,the conventional friction stir welding,ultrasonic vibration enhanced friction stir welding(UVeFSW)and splat cooling assisted friction stir welding(SCaFSW)of the third generation 2195 aluminium-lithium alloy were carried out.The effect of superimposing ultrasonic energy field and rapid cooling field on the microstructures and properties of Al-Li alloy FSW joints was studied.Firstly,the weld formation of 2 mm thick 2195-T6 Al-Li plate by conventional FSW,UVeFSW and SCaFSW under different welding process parameters were systematically studied.The results show that compared with conventional FSW process,superimposing ultrasonic energy in FSW can improve the joint surface,promote the plastic material flow,eliminate the S-line and void defects,and enlarge the width of the WNZ.While the application of an external splat cooling in FSW reduces the welding heat input and high temperature duration time which lead to a rough weld surface,a shrinking width of the WNZ in the top and bottom of the weld.In addition,the application of an external splat cooling in FSW is likely to cause tunnel and hole defects in the joint at a low friction stir welding heat input.Subsequently,the microstructures of the WNZ under the three welding processes was systematically analyzed by electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM).It was found that the application of ultrasonic external field in the FSW of 2195 Al-Li alloy increases the degree of recrystallization of the grains and refined the grains size in the WNZ.While the application of an external splat cooling in FSW reduces the welding heat input,which could inhibits the grain growth and reduces the degree of the recrystallization in the WNZ.The analysis of the microtexture of the joint shows that the superimposing of ultrasound field in FSW does not change the main texture type of the WNZ,but the strength of microtexture is slightly increased;while the application of an external splat cooling in FSW induces more shear texture in the middle of the WNZ.TEM results shows that there are local aggregation of precipitates in the WNZ of the FSW joints;while the density of dislocations and precipitates in the WNZ of the UVeFSW joints are higher than those of the conventional FSW joints.The precipitates are also more dispersed and finer in UVeFSW joint.Compared with FSW and UVeFSW,the dislocation density in the WNZ of SCaFSW joint is the highest,but the density of nano-scale precipitates is lower than that of UVeFSW,and there are a certain number of large-sized precipitates in the WNZ.Finally,the influence of superimposing ultrasonic field or external splat cooling field on the mechanical properties of friction stir welded 2195 Al-Li alloy joints was systematically studied.The results show that superimposing ultrasonic vibration can improve the tensile strength and elongation of the joint,and also expand the welding process window.In addition,the effect of ultrasound vibration on the tensile strength of the joint is more obvious at the low welding heat input.The application of ultrasonic vibration in FSW significantly increases the hardness values in the weld.The application of splat cooling in FSW at the condition with high welding heat input significantly increased the tensile strength of the joint but reduced its elongation.Within the range of welding parameters studied in this paper,the maximum tensile strengths of the FSW,UVeFSW and SCaFSW joints were respectively 459.9 MPa,467.4 MPa and 502.7 MPa,which were 85.0%,86.4%and 93.0%of the base material respectively.However,the application of an external cooling field in FSW at the condition with low welding heat input could lead to the formation of tunneling defects in the SCaFSW joints,which reduce its tensile strengths.In conclusion,the application of ultrasonic vibration energy in FSW of 2195 Al-Li alloy can promote the plastic material flow in the WNZ,resulting in better joint formation and mechanical properties with a broader welding process window.The application of an external splat cooling in FSW will lead to a narrower welding process window but inhibit the softening of the joint at the welding conditions of high welding heat input which refine the grain in the WNZ and significantly improve the tensile strength of the joint.However,the application of splat cooling in FSW is likely to cause void defects at the case of low welding heat input which resulting in a deterioration of the mechanical properties of the joint. |
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