Control Mechanism For Microstructure And Mechanical Properties Of Al-Cu Dynamic Stationary Shoulder Friction Stir Welding

The Al-Cu welding structure is widely used in industries such as new energy vehicles,electric power,and communications.This is due to its ability to take into account functions such as heat dissipation,conductivity,cost reduction and achieving lightweight.As a solid-phase welding method,friction stir welding（FSW）has gradually become the main process for manufacturing Al-Cu composite structures in recent years.However,with the trend towards integrated design of structural functions,higher requirements are placed on the strength of Al-Cu FSW joints.Traditional FSW can only meet the welding requirements of low-strength aluminum alloy and copper in thin plates due to its shortcomings in controlling material mixing,reducing heat input and residual stress in joints.Therefore,it is urgently necessary to innovatively breakthrough in existing technologies to meet the common needs of various industries.Based on this,a new dynamic stationary shoulder friction stir welding（DSSFSW）method was proposed.This method optimized the material mixing and heat input by adjusting the dynamic and stationary shoulders ratio.Based on the above ideas,a split DSSFSW tool was designed with an external stationary shoulder and an internal dynamic shoulder.The external stationary shoulder ensured the weld formation,while the internal rotational shoulder promoted material flow and heat input during the welding.Meanwhile,the feasibility of the welding tool was verified through experiments.The optimal dynamic and stationary shoulder ratio was determined to be 7:12.The optimal shape of the pin was cylindrical with threads.Based on a developed welding tool,the DSSFSW numerical simulation study was conducted by using 2219-T6 aluminum alloy and C1100-T2 pure copper as the research objects.The study aimed to reveal key physical characteristics in the welding process,such as the temperature field,material flow behavior,and strain distribution.The results showed that compared to the traditional FSW,the new welding tool could reduce the heat input,inhibit the material flow,and reduce the strain.The material flow rate and strain gradually decreased from the top to the bottom of the joint.Furthermore,the tracer particle results showed the Cu particles on RS were mainly deposited in the region between AS and the centerline of the weld.This provided a theoretical basis for the development of a new Al-Cu DSSFSW process.The Al-Cu DSSFSW process was developed.Meanwhile,the microstructures and mechanical properties of the DSSFSW joints were studied,revealing the regulation mechanism of the DSSFSW on the microstructures and mechanical properties of the Al-Cu joints.The results showed that compared to the traditional FSW,the material mixing in stir zone was significantly suppressed by the new DSSFSW method,thereby eliminating the welding defects caused by severe material mixing.Meanwhile,the more diffuse distribution of Cu in Al was promoted.As the rotation speed increased from 1200rpm to 2100rpm,the average thickness of intermetallic compounds（IMCs）at the Al-Cu interface increased from310nm to 670nm.However,When the welding speed increased from 30mm/min to70mm/min,the thickness of IMCs decreased from 550nm to 470nm.Furthermore,the thickness of IMCs at the bottom of the joint was significantly higher than that in the middle and top.Based on the results of thermodynamic and kinetic calculations and combined with phase analysis using transmission electron microscopy,the mechanism of IMCs formation in Al-Cu DSSFSW joints was revealed.The Al₂Cu and Al₄Cu₉were the first generated phase on the Al and Cu sides,respectively.The first stage of the reaction process was the diffusion of Al and Cu atoms to form the Al₂Cu phase.Subsequently,after Cu atoms diffused to a specific concentration,they reacted with Al₂Cu in the second stage,forming the Al₄Cu₉ phase.The tensile test results indicated that with the increase of the rotation speed,the tensile strength first increased and then decreased;with the increase of welding speed,the tensile strength gradually reduced.The maximum tensile strength of the joint could reach 243 MPa,which was about 90%of the Cu base material,and the corresponding welding parameter was 1800 rpm-30 mm/min.For defect-free welded joints,the crack source first emerged in the IMCs at the Al-Cu interface and then fractured under tensile stress.The transverse ultrasonic was innovativelty applied to the stationary shoulder and directly transmited it to the welding zone,thus developing the new U-DSSFSW tool.The results showed that the ultrasonic could promote material flow and eliminate micro-cracks in SZ,thereby improving the weld formation.The ultrasonic could improve the mobility of dislocations,enhance the polygonal process of small-angle grain boundaries,refine grains,weaken the texture strength of SZ and increase the material strain.Meanwhile,the IMCs thickness could be reduced by ultrasonic vibration.In particular,the IMCs thicknesses in the top,middle,and bottom regions of the joints were reduced by 40nm,97nm,and 241nm,respectively.The tensile test results showed that compared with DSSFSW,the tensile strength of U-DSSFSW joints has been all improved under the same parameters.The maximum tensile strength of the joints has been increased to 247MPa,realizing the non-interface fracture.Meanwhile,the mechanism of ultrasonic improving the microstructures and mechanical properties of Al-Cu joints was elucidated:（1）The ultrasonic made the IMCs more brittle during shear deformation,resulting in a reduction in IMCs thickness and an enhancement in interface strength.（2）The dispersion of particles was enhanced in the SZ by ultrasonic,which increased the hardness and deformation resistance of the SZ.（3）The residual stress of the joint was reduced by ultrasonic,and the crack expansion was limited during the tensile test.