Study On Microstructure,Properties And Deformation Of The Joint Prepared By High-speed Friction Stir Welding On Thin Walled Plate Of Aluminum Alloy

Due to the existence of Zn and Cu elements,the weldability of 7075 aluminum alloy（Al-Zn-Mg-Cu）is reduced.Therefore,defects such as porosity,crack,joint softening and serious residual deformation are easy to occur when the traditional fusion welding method is adopted.Friction stir welding（FSW）,as a new solid-phase joining technology,is considered to be one of the most effective welding methods for7 series aluminum alloys.With the development of lightweight and miniaturization in the manufacturing fields of aerospace,railway vehicles,automobile industry and electronic appliances,the demand for thin walled welded parts ia also increasing,and the joint performance and deformation control become the key to promote its engineering application.During the FSW of thin walled plate of aluminum alloy,kissing bond defects at the root of the weld,thickness reduction of the weld and serious residual deformation are easy to appear,which severely restricts the engineering application of FSW structures of thin walled plate of aluminum alloy.High-speed（high rotational speed+fast welding speed）FSW can not only significantly improve the welding efficiency,but also beneficial to obtain thin walled plate welded joint with good performance.However,the current research on high-speed FSW of thin walled plate is still in its infancy.In this thesis,the high-speed FSW process of 0.5 mm thick 7075-T6 aluminum alloy thin walled plate was systematically explored.The heat production mechanism of high-speed FSW of aluminum alloy thin walled plate was revealed by a combination of numerical simulation and experiment.The influences of welding parameters and tool structure on the temperature field,microstructure and mechanical properties of high-speed FSW of aluminum alloy thin walled plate were discussed.The influences of welding parameters and process conditions on the residual stress and residual deformation of high-speed FSW of aluminum alloy thin walled plate were systematically studied.The numerical simulation results of temperature field show that the heat source in the high-speed FSW of aluminum alloy thin walled plate mainly came from the friction heat between the tool and workpiece,and the plastic deformation heat of the weld metal accounted for a small proportion.Tool structure had a significant impact on the heat generation.Under the high-speed conditions of 6000/1200（rotational speed of 6000 rpm and welding speed of 1200 mm/min）,the heat mainly came from friction heat,and the plastic deformation heat was very little when the pinless tool was used.While the proportion of friction heat decreased and the proportion of plastic deformation heat increased significantly when the pin tool was used under the same shoulder penetration depth of 0.14 mm.Compared with the pinless tool,the peak temperature was higher,the elevated temperature exposion time was longer and the temperature gradient under the shoulder range was smaller when the pin tool was used under the same shoulder penetration depth.When the ratio ofω/v was constant at 6.67rad/mm,the peak temperature was slightly higher but the elevated temperature exposion time was signifficantly shorter when the high-speed（8000/1200）FSW was used than that of conventional（2000/300）FSW.The test results of weld microstructure show that kissing bond defects were absent in the joint fabricated by the pinless toolunder the high-speed of 6000/1200,while they were observed at the root of the weld fabricated by the pin tool,and the degree and scope of the kissing bond defects dereased with the increasing of penetration depth.The loss of weld material caused by flash was the main reason for the weld thickness reduction.The thickness reduction ratio of the FSW weld of 7075-T6aluminum alloy thin walled plate increased with the increasing of rotational speed and decreased with the increasing of welding speed.The thickness reduction ratio of the weld fabricated by the pinless tool was significantly lower than that fabricated by the pin tool under the same shoulder penetration depth.When the ratio ofω/v was constant at 6.67 rad/mm,the proportion of recrystallized grains,subgrains and high-angle grain boundaries in the nugget zone（NZ）of the weld fabricated by high-speed（8000/1200）FSW was decreased,and the proportion of deformed grains was increased.For the weld fabricated by the high-speed（8000/1200）FSW,from top to bottom along the weld thickness direction,the grain size and the proportion of deformed grains were gradually increased,the proportion of recrystallized grains and high-angle grain boundaries were gradually decreased.The upper NZ was dominated by recrystallized grains,and the lower NZ was dominated by deformed grains.The test results of mechanical properites show that the hardness distribution of the joints produced using the pinless tool presented a W shape.With the increase of rotational speed,the hardness profile became wider and the hardness value decreased.While as the welding speed increased,the hardness profile became narrower and the hardness value increased.The hardness distribution of the joints produced using the pin tool presented a U shape.The softening degree of the joints fabricated by the pin tool was greater than that fabricated by the pinless tool under the same shoulder penetration depth.When the ratio ofω/v was constant at 6.67 rad/mm,the tensile properties of the joints fabricated by the high-speed（8000/1200）FSW were significantly improved,and the tensile strength,yield strength and elongation of the joints were increased by 22.8%,23.3%and 59%compared with the jonits fabricated by conventional（2000/300）FSW.The tensile properties of the aluminum alloy thin walled plate joints fabricated by the pinless tool were better than that fabricated by the pin tool under the conditions of high-speed FSW.The tensile properties of the joints were improved with the increasing of welding speed under the high rotational speed of 6000 rpm when the pinless tool was used,and the maximum tensile strength of 482MPa（equivalent to～87%of BM）was obtained at the welding speed of 1200 mm/min.The tensile specimensfractured at the boundary between the thermomechanically affected zone（TMAZ）and the NZ on the RS due to local thickness reduction,showing a shear fracture mode.A large number of deep dimples and thick tear ridges were observed in the fracture surface,andηphase（Mg Zn₂）precipitates were distributed at the bottom of some dimples.The tensile specimens produced using the pin toolunder the same shoulder penetration depth fractured at the NZ due to the kissing bond defects,showing a wedge-shaped fracture mode.The axial force increased with the increasing of welding speed and decreased with the increasing of rotational speed when the pinless tool was used.The axial force during the FSW of 7075-T6 aluminum alloy thin walled plate using the pinless tool was lower than that using the pin tool under the same shoulder penetration depth.The longitudinal residual stress profile of the joint fabricated by the pinless tool showed a typical M shape.The maximum value of the longitudinal residual stress was measured in the area close to the shoulder edge,and the weld zone exibited tensile residual stress.The transverse residual stress of the joint was much smaller than the longitudinal residual stress,and presented transverse compressive residual stress.The residual stress of the joints decreased with the increasing of welding speed and increased with the increasing of rotational speed.When the ratio ofω/v was fixed,the residual stress of the joints fabricated by the high-speed FSW was slightly lower than that fabricated by the conventional FSW.The out-of-plane maximum deformation Z_max,the longitudinal maximum bending deformation z_max and the transverse angular deformationαof the workpiece were decreased with the increasing of welding speed and increased with the increasing of rotational speed when the pinless tool was used.When the ratio ofω/v was fixed at 6.67 rad/mm,the minimal residual deformation of the workpiece was obtained at the welding parameter of 6000/900.As the penetration depth increased,αwas gradually increased,while Z_max and z_max were decreased first and then increased.The Z_max and z_max of the joint fabricated by the pinless tool were decreased by 20.83%and 23.69%respectively compared with the jonits fabricated by the pin tool under the same shoulder penetration depth,andαwas increased by14.32%.The Z_max and z_max of the joint fabricated by the copper backing plate were decreased by 24.55%and 40.66%respectively compared with the jointfabricated by the steel backing plate,whileαwas increased by 53.29%.As the clamping distance increased,the Z_max and z_max were gradually increased,andαwas decreased first and then increased.The influence of welding parameters and tool structure on microstructure,mechanical properties and residual deformation of the joint prepared by high-speed FSW on thin walled plate of aluminum alloy was clarified in this thesis,and the influence of tool structure on the heat production mechanism was also revealed.The research results can provide theoretical basis and experimental data for the engineering application of high-speed FSW technology of aluminum alloy thin walled plate,which has important scientific significance and engineering application value.