Regulation Research On Microstructure And Properties Of Al-5Mg-xSn Alloy By Thermo-Mechanical Treatment

Al-Mg series alloys are widely used in aerospace,marine,automotive and other applications for their high tensile strength,low density and excellent corrosion resistance.However,the poor plasticity and low deformation properties of Al-Mg alloys with high magnesium content have always been the key problems to be solved in the research field of this series of alloys.Alloying is an effective way to improve the microstructure and mechanical properties of Al-Mg series alloys.Sn element,which can improve the microstructure,mechanical properties and formability of the alloys,is one of the most potential alloying elements for the research and development of aluminum alloys with high performance.It is expected to solve the problems of strength insufficiency and plastic processing difficulty of Al-Mg series alloys with high Mg content.Therefore,the existence form of Sn in Al-Mg alloys as well as the effect and mechanism of Sn alloying on microstructure and mechanical properties of Al-Mg alloys were studied in this research,expecting to solve the above problems and providing guidance for the development of new type Al-Mg alloys with high performance.The crucial results were as follows:(1)The addition of Sn could result in the grain refinement in Al-5Mg alloy.The main reason was that Sn element converged at the front of solid/liquid interface during solidification,which extended the range of undercooled region and increased nucleation rate.Meanwhile,the formed Mg2Sn phase could blocked the growth of dendrite and inhibited the interface migration of the alloy,thus impeding the growth of?-Al primary phase.(2)Sn could combine with Mg to form Mg2Sn phase in the Al-5Mg alloy as the Sn addition ranged from 0 to 1 wt.%.The morphology of Mg2Sn phase was mainly spherical and rod-like with Sn addition from 0 to 0.3 wt.%,and the average diameter of small spherical phase was 200 nm and 400 nm the average diameter of large spherical phase was 2-4 ?m,the average length and width of rod-like phase was 20 ?m and 2.5 ?m,respectively.The morphology of Mg2Sn phase began to change into lath-shape or bone-like with Sn addition from 0.4 to 1 wt.%,which mainly distributed at the grain boundary.(3)The strength and plasticity of the Al-5Mg alloys first increased and then decreased with the Sn addition ranged from 0 to 1 wt.%,and when the Sn content increased to 0.3 wt.%,the tensile strength and elongation of the alloys reached the peak,and the values were 210.8 MPa and 25.9%,which were 4.7%and 30.2%higher than those of as-cast Al-5Mg alloy,respectively.The main reason was that the small and deformable Mg2Sn soft phase formed in the alloy could bring about second phase strengthening and crack propagation inhibition.With the further increase of Sn content,the strength and elongation of the alloy began to decrease,mainly due to the formation of coarse bone-like Mg2Sn phase in the alloy,which reduced the strengthening effect.(4)The optimal heat treatment schedule of Al-Mg-Sn alloy was aging treatment at 200?× 2 h after solution treatment at 500?× 60 h.The morphology of Mg2Sn phase in the Al-5Mg-0.1Sn alloy after heat treatment was mainly spherical and dispersed distribution with the average diameter of 20 nm,70 nm and 2 ?m,leading to further improved the strength and plasticity of the alloy.The values of tensile strength and elongation were 227.8 MPa and 31.1%,which were 13.1%and 56.3%higher than those of as-cast Al-5Mg alloy,respectively.(5)During the rolling deformation process preheated at 300? for 0.5 h,the grains of Al-5Mg(-0.2Sn)alloys were lengthened along the rolling direction and gradually formed to fibrous structure.With the increase of rolling deformation,the Mg2Sn phase in Al-5Mg-0.2Sn alloy without heat treatment was broken up and formed into some irregular Mg2Sn phase particles.On the contrary,the second phases in Al-5Mg-0.2Sn alloy after heat treatment showed good stability.The average size maintained at 2?m and no obvious crushing fracture of the second phase was observed.The grains of the alloys were recrystallized after annealing treatment,the tensile strength and elongation of Al-5Mg-0.2Sn alloy after heat treatment were 275.2 MPa and 33%,respectively,which were 6.9%and 24.5%higher than those of Al-5Mg alloy without heat treatment,respectively,and were 13.3%and 17.9%higher than those of Al-5Mg-0.2Sn alloy without heat treatment,respectively.