Study On The Heat Treatment Process And Cold Rolling Property Of Mg-Li-Al-Zr Alloys

With the implementation of energy-saving and emission-reduction in the industry and the enhancement of environmental protection consciousness in the public,there is an increasing requirement for materials to decrease the weight of the apparatus.As the metal material with the lowest density,Mg-Li alloys will be used in a wide field.Even though the alloys possess an outstanding plastic formability at room temperature,their low strength severely limits the alloys’,especially the β-single Mg-Li alloys,practical application.In this paper,we adopted a new method by comprehensive utilization of alloying,heat treatment and plastic formability to obtain the perfect match of strength and plasticity.As the sharp dropping of the solid solubility in Mg-Li alloys,Al is added.Besides the strengthening during heat treatment,strain-strengthening is involved in cold rolling.Meanwhile,a comparative analysis of the changes in the structure evolution and mechanical properties of Mg-11Li-3Al-xZr(x=0,0.1)alloys in as-cast,extruded,heat-treated and rolled conditions is carried out to reveal the role of Zr in the alloy during the processing.Our research results are summarized as below.1.Mg-11Li-3Al-x Zr(x=0,0.1)alloys own a near-equiaxed microstructure and consist of β-Li 、 α-Mg 、 θ-MgLi2 Al and AlLi phases in as-cast condition;besides,Mg-11Li-3Al-0.1Zr alloys contain a Al3 Zr phase due to the addition of the Zr.The two alloys show no changes in phases after extrusion.However,their grain is refined because of the dynamic recrystallization,especially with the Zr addition.Moreover,the tensile strength of the two alloys is significantly enhanced by 27% and 33% after extrusion when compared with that of the alloys in an as-cast state.3.The Mg-11Li-3Al-xZr(x=0,0.1)extrusion plates remain the same grain size under the solution treatment in the range of test temperature.With the increase of the temperature,θ-MgLi2Al、AlLi、α-Mg are dissolved into the β-Li;the dissolution of Al3 Zr makes the β-Zr appear;thus,a fully β-Li phase matrix is obtained at 400℃ and 450℃,respectively.Meanwhile,the peak in hardness is achieved.Due to the mechanism of solution strengthening,the yield strength and tensile strength are largely enhanced,while the elongation drops sharply.3.The Mg-11Li-3Al-xZr(x=0,0.1)extrusion plates were gone through an aging treatment at 70 ℃ after the solution treatment at 400 ℃ and 450 ℃,respectively.θ-MgLi2 Al precipitates preferentially in the alloys.α-Mg precipitates at the same time in Mg-11Li-3Al,and Al Li phase is retained due to the decomposition reaction: θ-MgLi2Al→Al Li+α-Mg.However,there is no evidence of the precipitation of α-Mg and the decomposition of θ-MgLi2 Al in Mg-11Li-3Al-0.1Zr alloy.With the increase of aging time,the yield strength and tensile strength of the two alloys gradually decrease;Mg-11Li-3Al alloy shows up-and-down in elongation,while the elongation of Mg-11Li-3Al-0.1Zr increases gradually.4.The heat treated Mg-11Li-3Al-x Zr(x=0,0.1)alloys are cold rolled in different degrades of deformation without dynamic recrystallization.With the increase of rolling degrades of deformation,the precipitation of α-Mg、θ-MgLi2Al、Al Li phases are promoted,meanwhile,the hardness of the alloys drop sharply and increase again gradually.The yield strength,tensile strength and elongation of Mg-11Li-3Al alloy increase first and then decrease,resulting in the perfect match of comprehensive mechanical properties at 60% deformation.Mg-11Li-3Al-0.1Zr alloy shows the same trend in elongation,while the yield strength and tensile strength decrease gradually,leading to the perfect match of comprehensive mechanical properties at 40% deformation.5.In addition to the effect of grain refinement,Zr can enhance the microstructural stability of the Mg-11Li-3Al-0.1Zr alloy by reducing the diffusion rate of solution atoms,enhancing the stability of α-Mg and compounds at high temperature and inhibiting the precipitation of α-Mg and compounds during ageing and rolling.Moreover,the effect of solution strengthening,which highly sacrifices the elongation but enhances the yield strength and tensile strength,can be greatly improved with Zr addition.