| With the rapid development of automobiles,ships,aerospace and other fields,environmental and energy issues have become more prominent.Lightweight is undoubtedly an effective way to reduce energy consumption and pollution,improve fuel economy.Since 2000 series and 7000 series high-strength aluminum alloys have many advantages,such as high specific strength,good fatigue performance and strong corrosion resistance,they have been increasingly used in the complex structural parts in aerospace and other manufacturing fields.When the traditional fusion welding process is used to weld high-strength aluminum alloys,brittle dendrites,eutectic structures and welding defects,such as pores,cracks and segregation,are produced during fusion welding process,resulting in significant decrease of mechanical properties of the joints.Friction stir welding(FSW),as a new type of solid-state welding method,has attracted wide attention in the aerospace industry due to its unique advantages such as fewer defects,small deformation,low residual stress,and excellent comprehensive performance.In this study,2024-T351 and 7075-T651 high-strength aluminum alloy sheets for aerospace have been used to carry out the systematical investigation on microstructure and properties of the dissimilar AA2024-T351/7075-T651 FSW joints.The effect of tool shoulder profiles,material position,and welding processing parameters on microstructure and properties of the joints and the optimal design of welding processing parameters have been performed.The influence of post-weld heat treatment(PWHT)process on microstructure and mechanical properties of the joints has been investigated.The evolution law of welding processing variables on material flow behavior of the joints has been analyzed,and the relationship between the mixing degree of material flow and corrosion behavior has been discussed.The main research contents and conclusions are as follows:(1)Different tool shoulder profiles(concentric circle shoulder-CCS and three-helix shoulder-THS)and base materials(BMs)position reciprocation were adopted for welding,the microstructure and mechanical properties of the joint are analyzed.The results indicated that under the same welding conditions,the welding heat input produced by CCS tool is lower than that of THS tool,resulting in lower average grain size of the nugget zone(NZ)and width of heat-mechanical affected zone(TMAZ)in the CCS joints than THS joints,and its mechanical properties are better than THS joints.Changing the position of base materials(BMs)affects the width of TMAZ and mechanical properties of the joints,and the mechanical properties of the joints are higher when 2024 aluminum alloy is located on the advancing side(AS).(2)The effect of different welding processing parameters(rotational speed,welding speed and plunge depth)on microstructure,mechanical properties and corrosion behavior of the joints has been systematically studied,and the optimal design of different welding processing parameters has been carried out.The results showed that dynamic recrystallization occurs in the NZ of the joints,forming finer equiaxed grains,and the average grain size increases with the increase of rotation speed and plunge depth,and the decrease of welding speed.The NZ of the joints experiences the highest peak temperature and the most severe plastic deformation,resulting in the dissolution of initial precipitation in the BMs and partially re-precipitation.The peak temperature and the degree of plastic deformation in the TMAZ are lower than those in the NZ,so the precipitated phases are coarsened and partially dissolved in the TMAZ,and the dissolution degree is low compared to the NZ.The heat-affected zone(HAZ)is only affected by the welding thermal cycle,its peak temperature is the lowest,and the evolution of the precipitation phase is mainly the coarsening process.The mechanical properties of the joints first increase and then decrease with the increase of rotation speed and plunge depth,and decrease with the decrease of welding speed.This is mainly related to the coarsening and dissolution of the precipitation strengthening phase in the welded zone of the joints under different welding processing parameters.Galvanic corrosion occurs in the NZ of the joints,resulting in lower corrosion resistance than the BMs,and it varies with the welding processing parameters.According to the evolution law of mechanics and corrosion performances of the joints with the welding processing parameters,response surface methodology(RSM)based on central composite design(CCD)is employed to establish the mathematical regression model between the comprehensive performance of the joints and the welding processing parameters,and the optimum combination of welding processing parameters is obtained:rotational speed of 1495 rpm,welding speed of 187mm/min,plunge depth of 0.03 mm.After the verification of welding test,the comprehensive performance of the optimal joint is acquired:yield strength of 306 MPa,tensile strength of 439 MPa,tensile elongation of 7.77%,and corrosion rate is 7.43mm/year.Compared to 2024 aluminum alloy,the welding efficiency of the joints is improved to 93.4%.(3)Different PWHT process are performed to systematically analyze the influence of three different welding heat input conditions(the ratio of rotation speed to welding speed)on microstructure and mechanical properties of the joints.The research found that the tensile properties of the joints at lower welding heat input have been significantly improved.After aging treatment at 480 ~oC for 1 h and 120 ~oC for 24 h,the dissolved precipitates in the welded zone of the joints are re-precipitated to be fine precipitates,and evenly distributed in the Al matrix,the tensile strength of the joint increases from 395.1MPa in the welded state to 413.3 MPa,and the elongation increases from 3.2%in the welded state to 10.9%.After PWHT,the tensile strength and elongation of the joints at the optimized welding heat input are basically the same as those of the welded state.This is mainly because the size,distribution and quantity of the precipitated phase in the welded zone of the joints after aging treatment at 480 ~oC for 1 h and 120 ~oC for 24 h are basically the same as the welded state.Due to the weak connection defect at the bottom of the NZ of the joints at higher heat input,the mechanical properties of the joints are not improved by using PWHT.(4)The influence of different welding processing variables such as tool shoulder profiles,material position,rotational speed,welding speed and plunge depth on material flow behavior in the NZ of the joints is systematically analyzed.The evolution law of material flow in the joints is revealed,and the relationship between the mixing degree of material flow and the corrosion behavior is discussed in detail.The results indicated that under the same welding conditions,material flow of the THS joints is more sufficient than that of the CCS joints,when 7075 aluminum alloy is located on the AS,more sufficient material flow occurs in the NZ of the joints.The degree of material mixing increases with the increase of rotational speed and plunge depth and the decrease of welding speed.The materials mixing degree of shoulder interface zone(SIZ),vortex zone(VZ),bottom zone(BZ),and bottom interface zone(BIZ)in the joints from high to low is:BZ>VZ>SIZ>BIZ,which is the same as the order of the corresponding corrosion resistance from high to low.Galvanic corrosion takes place in the SIZ and BIZ,resulting in poor corrosion resistance,while material mixing of BZ is more sufficient,which leads to higher corrosion resistance,it indicates that higher degree of material mixing can effectively weaken galvanic corrosion and improve corrosion resistance. |
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