| Due to the outstanding advantages of light weight,high strength and toughness,excellent corrosion resistance and good damage tolerance,Al-Zn-Mg-Cu alloys are quite indispensable materials for manufacturing light structural parts in important industrial fields such as aerospace,national defense,transportation and so on.Recently,with the continuous development of science and technology,the better overall performance of structural parts in the design and manufacturing process of airplane,spacecraft,ship and weapons have been highly expected.Thus,new aluminum alloys with higher comprehensive performance are urgently needed to meet the demands of new aircraft design.Micro-alloying of transition or rare earth elements combined with the high alloying design of main constitutes,have been proved to be one of the most promising ideas to adjust the microstructures and properties of aluminum alloys.In this work,trace amount of Sc element were added to the Al-Zn-Mg-Cu-Zr alloy.Some alloys with different Sc contents,of which nominal composition are Al-9.2Zn-2.2Mg-1.5Cu-0.11Zr-xSc(x=0.00,0.15,0.17,0.20,0.25),were designed and prepared.Based on the above alloys,the effects of Sc on as-cast microstructures of alloys were systematically studied.The nucleation,growth and coarsening behaviors of Sc-containing dispersoid during homogenization and its retarding effect on static recrystallization during solution and quench were investigated in detail.The effects of Sc on the microstructures and properties of friction stir welded joints were also analyzed.The aim was to provide theoretical basis and fundamental support for the research and development of new super-high strength aluminum alloy containing Sc.The main research work and results included:The as-cast microstructures of Al-9.2Zn-2.2Mg-1.5 Cu-0.11Zr-(Sc)alloys were studied.The grain morphology,grain size and coarse second phases in as-cast alloys were related to the addition of Sc elements.When Sc contents were 0.00 wt.%and 0.15 wt.%,the grains were typical dendrites with sizes of 100μm and 87 μm,respectively.The second phase was mainly AlZnMgCu quaternary phase,accompanied by a small amount of iron-rich phase.Sc atoms mainly solidified into the α-Al matrix to form supersaturated solid solution.When Sc contents were 0.17 wt.%,0.20 wt.%and 0.25 wt.%,the grains were fine and uniform equiaxed ones with average grain size of 25μm.In addition to AlZnMgCu quaternary phase and iron-rich phase,the Al3(Sc,Zr)primary phase distributed at grain boundaries or inner grains,which could significantly change grain morphology and refine grain size.The nucleation mechanism of Sc-containing dispersoids in Al-9.2Zn-2.2Mg-1.5Cu-0.11Zr-0.17Sc alloy during homogenization annealing was studied.During the non-isothermal annealing process at 250-470 ℃,the supersaturated solid solution containing Sc began to decompose at 250 ℃,forming Sc-rich clusters and gradually developing into Al3Sc nuclei.The concentration of Zr atom at the interface between A13Sc nuclei and matrix increased at 440 ℃ and above.A13Sc was the precursor of A13(Sc,Zr)dispersoid.A13(Sc,Zr)was centered on A13Sc and surrounded by a thin shell rich in Zr.The precipitation sequence of Sc-containing dispersoid could be described as:Sc-rich cluster→Al3Sc nucleus→A13Sc nucleus with Sc-rich shell→mature Al3(Sc,Zr)particle.The growth behavior of Sc-containing dispersoids in Al-9.2Zn-2.2Mg-1.5Cu-0.11Zr-0.17Sc alloy during homogenization annealing was investigated.Treated by isothermal annealing at 3 70～470 ℃ for 10 h,with the increase of annealing temperature,the morphology of dispersoids changed from square to spherical or near spherical,and the average size increased gradually.The average diameters at 400 ℃,440 ℃ and 470 ℃ were 10.8 nm,15.4 nm and 18.7 nm,respectively.The precipitate size distribution(PSD)presented a Gauss model.In complex multicomponent systems containing Zr and Sc,the process of Al3Sc nucleus transforming to mature Al3(Sc,Zr)particle was controlled by diffusion.In the early stage of nuclei growth,Zr atoms did not have long-range diffusion capacity,but only Sc atoms diffused independently.With the increase of temperature,Zr atoms began to diffuse over long distances.Limited by the low diffusion ability of Zr atoms and decline of Sc concentration gradient in front of the interface,the growth rate of Al3(Sc,Zr)dispersoids would slow down gradually.The coarsening behavior of Sc-containing dispersoids in Al-9.2Zn-2.2Mg-1.5Cu-0.11Zr-0.17Sc alloy during homogenization annealing was yet discussed in detail.During isothermal annealing at 440 ℃ for 1～100 h,the Sc-containing particles presented more regular spherical morphology with slightly larger average size.The average diameters of dispersoids after 1 h,10 h,36 h and 100 h were 10.9 nm,15.5 nm,14.3 nm and 16.7 nm,respectively.The PSD curve still showed a symmetrical Gauss distribution.Under the same condition,the coarsening rate of Al3(Sc,Zr)particle was slower than that of Al3Sc particle,and the former had higher thermal stability than the latter.The effect of Al3(Sc,Zr)dispersoids on the static recrystallization process for hot rolled sheets during solution and quench was researched.The static recrystallization of Al-9.2Zn-2.2Mg-1.5Cu-0.11Zr alloy began af:ter heat treatment at 370 ℃/2 h.The recrystallization initial temperature of Al-9.2Zn-2.2Mg-1.5Cu-0.11Zr-0.17Sc alloy was about 400-440 ℃.Under the same heat treatment,the alloy with Sc containing showed fewer recrystallized grains than the other.Sc element could significantly increase the initial temperature of recrystallization and inhibit the recrystallization process.The microstructures and properties of friction stir welding(FSW)joints and base metals of Al-9.2Zn-2.2Mg-1.5Cu-0.11Zr-(Sc)alloys were studied comparatively.The FSW joint for each alloy could be divided into nugget zone(NZ),thermal-mechanical affected zone(TMAZ),heat affected zone(HAZ)and base metal(BM).NZ showed a dynamic recrystallization feature consisting of fine and uniform equiaxed grains with high density dislocations.Most of the aged precipitates dissolved.TMAZ grains were elongated with higher dislocation density,and residual aged precipitates coarsened remarkably.HAZ retained the same grain morphology as the base metal.Most of the aged η’precipitates grew up and a few coarsened to be η phase.The welding coefficient of Al-9.2Zn-2.2Mg-1.5Cu-0.11Zr alloy was 71%,and that of Al-9.2Zn-2.2Mg-1.5Cu-0.11Zr-0.17Sc alloy was 74%.With Sc addition,the ultimate tensile strength and yield strength of the alloy welded joint increased by 43 MPa and 23 MPa respectively,and the elongation improved by 2.3%.The fracture positions of the two alloys were both located in NZ,but the fracture modes were quite different.The former was brittle intergranular fracture,while the latter seemed mixed fracture modes. |
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