Study on Microstructure and Properties of High-Speed Friction Stir Welded Joint of Aluminum Alloy Thin Plate

Author:Liu Fen Jun

Supervisor:fu li chen hai yan

Database:Doctor

Degree Year:2018

Download:23

Pages:147

Size:14104K

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Aluminum alloy sheets have been widely applied in automobiles,rail transportation and aircraft manufacturing,especially in the skin of unmanned aerial vehicles and general aircraft,due to their high plasticity,high strength and stiffness,outstanding corrosion resistance and machining performance.When the strength and stiffness of the structure is enough,lightweight can obviously improve the flight capacities.As a structural material,aluminum alloy thin plate welding is inevitable.It is usually considered that this alloy is very difficult to be jointed using conventional fusion welding due to the formation of metallurgical defect and poor mechanical properties.As a solid-state process,friction stir welding(FSW)can effectively prevent the formation of metallurgical defect and improve the mechanical properties.When the conventional rotational speed FSW is used to weld the aluminum thin plate,the process parameter selection range is narrow,and the welding efficiency is low.Moreover,it is very difficult to produce excellent joint due to improper selection of process paremeters.Whereas,compared with conventional rotational speed FSW,the process parameter selection range is wider and the welding efficiency is higher during high-speed FSW of aluminum alloy thin plate.In addition,the excellent FSW joint is easily produced using high rotational speed welding technology due to adequate welding heat input.In this paper,the temperature distribution and its influence on microstructure and mechanical properties of the high-speed FSW 6061-T6 aluminum alloy thin plate joint with 0.8 mm thickness are studied using numerical simulation and experimental methods.The mechanism of heat production and forming of high-speed FSW joint is discussed.And the tensile strength prediction model of the FSW joint is established based on the optimization of process parameters.The numerical simulation and temperature test results of the temperature distribution and heat production of the high-speed FSW aluminum alloy thin plate show that the highest peak temperature of the joint is located in the NZ,followed by the TMAZ,and the HAZ is the smallest.The heat production in the steady-state high-spped FSW process mainly comes from plastic metal deformation.The rotational speed and welding speed have a significant effect on the temperature distribution of the high-speed FSW joint.With the decrease of the rotational speed and the increase of the welding speed,the peak temperature in the nugget zone(NZ),thermal mechanically affected zone(TMAZ)and heat-affected zone(HAZ)decreases.The ratio of plastic heat production to total heat production is between 95.7%and 100%under the rotational speed of 8000 rpm and 10000 rpm,which is significantly higher than that of the conventional rotational speed of 2000 rpm(80.0%).At a fixed welding speed of 300 mm/min,the peak temperature in the NZ is reduced from 490.6℃to 444.8℃with increase the rotational speed from 8000 rpm to 10000 rpm.When the rotational speed is fixed at 10000 rpm,the peak temperature in the NZ is reduced from 490.6℃to 385.6℃with increase the welding speed from 300 mm/min to 1500mm/min.In addition,the peak temperature obtained at 8000 rpm and 300 mm/min in the NZ,TMAZ and HAZ using numerical simulation and temperature test are 449.5℃,397.6℃,284.6℃and 444.8℃,413.0℃,284.6℃respectively.The errors between calculated and test are no more than 5%.The electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM)test results of the microstructure of the high-speed FSW aluminum alloy thin plates joints show that the complete continuous dynamic recrystallization(CDRX)occurrs in the NZ.The CDRX in the TMAZⅠis incomplete and a moderate temperature recovery occurs.And the TMAZⅡonly experienced a low temperature recovery.The ration of the recrystallized grains and subgrains in the NZ obtained at10000 rpm and 1500 mm/min are 78.1%and 20.7%respectively,which increase by2.8%and 2.7%respectively compared to the joint obtained at conventional rotational speed of 2000 rpm and welding speed of 300 mm/min.The ration of recrystallized grains is significantly reduced compared with the NZ,while the content of the subgrains and low-angle grain boundaries(LAGBs)is significantly increased.The TMAZⅠis mainly composed of recrystallized grains and subgrains and little deformed grains.The deformed grains in the TMAZⅡare significantly increased compared to the NZ and TMAZⅠ,and the recrystallized grains and subgrains are obviously decreased.The HAZ is mainly composed of deformed grains and little recrystallized grains and subgrains.The grain size in the high-speed FSW aluminum alloy thin plate joint is affected not only by the peak temperature,but also by the strain rate of the plastic metal.The larger the strain rate is,the finer the grain size is.At a fixed welding speed of 1500 mm/min,the average grain size in the NZ decreases from 3.0μm to 1.8μm as the rotational speed increases from 8000 rpm to 11000 rpm.The precipitates distribution is greatly influenced by the rotational speed.The number and the size of the Mg2Si,Al8Fe2Si and Al2CuMg are obviously increased with the increase of the rotational speed,and the precipitates distribution is more similar to the base metal.The welding process optimization results of the high-speed FSW aluminum alloy thin plate show that the pin tool with three-helix concave shoulder can prepare the joint with perfect appearance,slight thinning and excellent mechanical properties.In the process of high rotational speed of 8000 rpm and 10000 rpm,in addition to form a large number of high-density dislocations and substructures,a large number of precipitates are reprecipitated in the NZ,which are significantly more than that of the conventional rotational speed of 2000 rpm.And the precipitates size and distribution in the high rotational speed FSW joint are similar to the base metal.In addition,the effective thickness of the joint produced by high rotational speed is thicker and the joint softening is smaller.The tensile strength of the joint obtained at 8000 rpm and1500 mm/min reaches 301.8 MPa,which is 85.8%of the base metal and 126.3%of the joint obtained at conventional rotational speed of 2000 rpm and welding speed of300 mm/min.The clamping distance has an obvious effect on the joint forming quality and effective thickness.With the increase of the clamping distance,the flash is bigger and the effective thickness is thinner.When the clamping distance increases from 10 mm to 35 mm,the effective thickness t decreases from 0.72 mm to 0.61 mm,and the tensile strength decreases from 301.8 MPa to 275.8 MPa.When the clamping distance is increased to 40 mm,the welded joint is not formed and the groove defect is distributed throughout the weld.The backing plate has a significant effect on microstructure and mechanical properties of the high-speed FSW aluminum alloy thin plate joints by changing the heat dissipation conditions.When the copper backing plate is used for FSW,the reprecipitation and growth of the precipitates are inhibited.The joint is severely softened,which further results in poor mechanical properties.The tensile strength of the joint obtained at 8000 rpm and 1500 mm/min with Cu backing plate is 249.0 MPa,which is 70.8%of the base metal.After artificial aging,a large number of precipitates,dislocations and substructures are formed in the joint,which is obtained at 8000 rpm and 1500 mm/min with Fe and Cu backing plates respectively.The softening of the joint is significantly reduced,and the tensile strength of the joint is 317.8 MPa and 304.0 MPa,which is 90.4%and 86.4%of the parent metal respectively.Furthermore,the process parameters are optimized by the quadratic regression orthogonal combination test,and the optimum process parameters for high-speed FSW of 6061-T6 aluminum alloy thin plate are the rotational speed of 8060 rpm and the welding speed of 1281 mm/min.The error between the calculated tensile strength value and experimental result of the FSW joint for the optimal parameters is 2.2%.The tensile strength error for calculating and testing obtained at 8000 rpm and 1500 mm/min is 0.8%.