Research On Microstructure And Mechanical Properties Of A Low-alloyed Mg-Al-Sn-Ca Alloy With High Strength And Extraordinary Formability

As the lightest structural metallic material,magnesium(Mg)and its alloys are kept hoping to dramatically reduce the mass of vehicles,allowing for better fuel efficiency and lower CO₂ emissions.Hence,there has been an increasing demand for lightweight low-alloyed Mg alloys with high strength and good formability to meet the requirements of stamping components in the transportation industry.Unfortunately,the development of strong basal texture during rolling leads to poor room temperature formability of most Mg alloy sheets.In recent years,the combined effect of adding Al and Sn has been reported to be of great help to the activation of non-basal slips and weakening texture of Mg alloys,which makes Mg-Al-Sn(AT)based alloys with high forming potential.However,low-alloyed AT system alloys tend to exhibit low strength due to the lack of strengthening phase.Not only that,high strength and high formability often have a certain incompatibility in Mg alloys with hexagonal close-packed structure,that is,the strategies used for improving formability usually result in a reduction of strength.Therefore,it is still a challenge to develop a low-alloyed Mg alloy with both high strength and extraordinary formability.Aiming to the above questions,this paper takes Mg-2.0Al-0.8Sn(wt.%)based alloy as the research object,realizing the grain refinement and the introduction of the second phase through alloying of Ca,developing a low-alloyed Mg-2.0Al-0.8Sn-0.5Ca(ATX2105,wt.%)alloy with high mechanical properties;proposing a new method to promote the precipitation by multi-directional rolling,revealing the mechanism of multi-directional rolling to promote precipitation and further improving the strength of ATX2105 alloy;optimizing the multi-directional rolling and annealing process,processing a fine-grained,ring weak textured ATX2105 alloy sheet with high strength,large ductility and excellent formability at room temperature.In this paper,the development and mechanical properties mechanism of a high-strength and high-formability low-alloyed Mg alloy were studied by means of alloy composition design,process optimization and microstructure control,providing theoretical basis for breaking the barrier of incompatible formability and strength.The main conclusions are as follows:(1)With the addition of Ca in Mg-2.0Al-0.8Sn(wt.%)alloy from 0.0 wt.%to 0.5 wt.%,the strength,ductility and isotropy improve simultaneously.The Mg-2.0Al-0.8Sn-0.5Ca(wt.%)alloy shows the best room-temperature tensile properties with a yield strength(YS)of?226.0 MPa,an ultimate tensile strength(UTS)of?282.4 MPa and a fracture elongation(EL)of?20.2%.The improvement of strength benefited from the combination of finer grains and more precipitates,and the enhanced ductility can be contributed to the promotion effect of texture weakening on basal slip in all directions.(2)Comparing with unidirectional rolling,multi-directional rolling can promote the precipitation of particles and improve the precipitation strengthening effect due to the introduction of higher density dislocations.And the ATX2105 alloy thereby obtains excellent room temperature tensile properties of?251 MPa for YS,?302 MPa for UTS,and?23.4%for EL.(3)By optimizing the multi-directional rolling and annealing process for microstructure control,a fine-grained(?3.1?m)ATX2105 alloy sheet with ring weak texture was prepared.This alloy exhibited high strength(YS:?188.5 MPa),large elongation(EL:?28.1%)and extraordinary room temperature formability(IE value:?7.8 mm);the high strength is mainly attributed to its sufficiently fine grains and high density of precipitated phases,the high elongation and formability are due to its unique ring weak texture,which promotes the activation of dislocation slip and the formation of low-angle grain boundaries during deformation.