Study On Microstructure Evolution And Strengthen Mechanism Of Al/Ti Ultrasonic Assisted Friction Stir Lap Joints

Aluminum and titanium alloys are widely used as main structure-materials for weight reduction in transportation industries,which is attributed to advantages of low density,high specific strength and good corrosion resistance.In mechanical products that are more sensitive to their own weight,such as aircrafts and sports cars,the use of aluminum alloys and titanium alloys is kept increasing.,aluminum/titanium composite structural parts as a preferred solution have broad prospects in aerospace,rail transit,automobile and shipbuilding industries for excellent comprehensive properties.However,there are huge differences in physical and chemical properties of aluminum and titanium alloys,which make it difficult to obtain an ideal Al/Ti weld by conventional processes.This research uses the method of friction stir welding to achieve a reliable connection between aluminum alloy and titanium alloy,and ultrasonic assisted friction stir welding system is built to further improve the weld properties.Tool wear,welding process,joint structure,interface evolution and mechanical properties are explored in this study.W-25Re was selected as the tool material in this study.The structure and dimensions of the friction stir welding tool are reasonably designed.Optimal pin length and processing window for Al/Ti welding were determined through theoretical analysis and process test.Welding defects such as surface channel,tunnel defects and interface unweld were avoided with selected welding parameters.The defect-free Al/Ti friction stir lap joints are obtained with good weld surface formation,controllable test plate deformation and reliable connection.And this part of the work has laid the foundation for the follow-up in-depth study of the joining mechanism of Al/Ti friction sitr welding.The welding process,microstructure and mechanical properties of aluminum/titanium friction stir lap joints are deeply studied.The influence of welding parameters and tool size on the Al/Ti lap joint process and the microstructure and mechanical properties of the joint is explained.The research results show that the welding process directly affects the friction between the welding tool and the metal during the welding process and further affects the welding heat generation,microstructure evolution and joint performance.As the rotating speed of the welding tool increases or the welding speed decreases,the spindle torque decreases and the welding temperature increases during the welding process,which causes the joint grain structure to grow and interface reaction products increase.Moreover,when the interface temperature is high and the mechanical action of the welding tool is strong,there will be titanium alloys near the interface of the joint being extruded by the welding tool to form a hook-like structure during the welding process.Due to the large difference in melting point between aluminum alloy and titanium alloy,the formation of the hook-like structure is likely to be accompanied by internal defects in the weld.this process also aggravates the wear of welding tools.Therefore,a good joint structure and an appropriate amount of interface reaction products can be obtained under appropriate process parameters with optimal tensile performance.Ultrasonic assistance further optimizes the Al/Ti friction stir lap welding.Tool wear is avoided completely due to avoiding the direct mechanical action between the welding tool and the titanium alloy.Favorable microstructure and mechanical properties were obtained by optimizing process parameters.The research results show that Al/Ti ultrasonic-assisted friction stir lap welding can completely avoid the generation of hook-like structures,the joint structure is more uniform and predictable,the interface virtual welding can be bridged,and the interface structure can be optimized.Under different ultrasonic process conditions,the joint grain structure increases slightly with the increase of ultrasonic power.The fluidity of the material during the welding process is improved,and the weld appears to form continuous but smaller flash edges.A small amount of micropores are formed in the aluminum alloy structure due to material loss.Therefore,during the tensile-shear process,the crack propagation path gradually changed from the interface expansion to the expansion in the aluminum alloy.The thermodynamic and kinetic analysis of the interfacial reaction process in the Al/Ti friction stir welding process was carried out,and the influence of dislocations on the reaction process was calculated using material calculation software,and the interfacial reaction mechanism and compound formation sequence were proposed.The research results show that the Ti Al₃ sublayer is first formed at the Al/Ti interface during the aluminum/titanium friction stir welding process;when there is excessive welding heat input,the Ti Al sublayer will be formed at the Ti Al₃/Ti interface,that is,the formation Al/Ti Al₃/Ti Al/Ti laminated structure.Calculations show that during the friction stir process of aluminum/titanium,the aluminum/titanium interface is controlled by element diffusion,and the presence of dislocations can induce pipeline diffusion,which then significantly reduces the diffusion activation energy and reaction activation energy of the elements.Due to the introduction of a large number of dislocations in the friction stir process,the interface reaction rate during the friction stir welding process can be significantly increased.