Effect Of Al-Si Addition On The Microstructure And Mechanical Properties Of Mg-Li Alloys

Mg-Li alloys have low density, high specific strength and specific modulus, excellent superplasticity and shock absorption, small anisotropy, good processing formability, but it has crucial drawbacks of low strength, poor high temperature performance, poor corrosion resistance, which restrict its application in engineering field. Al element has good solid solution strengthening and second phase strengthening in Mg-Li alloys. Moreover, when Si is added to the Mg-Li alloys, Mg2Si phase with high temperature stability can be formed, which plays a positive role on strengthening the Mg-Li alloys. But magnesium alloy containing Si by the common casting process can generate corase Mg2Si phase and lead to the deterioration of the mechanical properties. Vacuum suction casting can fill the mould quickly and smoothly, and the alloys fabricated by vacuum suction casting have better mechanical properties because of its less casting porosity and slag, finer and more uniform grains. Mg-8Li alloy is selected as the matrix alloy in this article. Mg-Li-Al-Si alloy is frabricated by vacuum suction casting with adding Al-Si. The size, shape, distribution of Mg2Si phase were improved significantly because of rolling and Sr modification, which gives crucial strengthening effect in the preparation of high-strength lightweight Mg-Li alloy.In this paper, Mg-8Li alloy is chosen as the matrix, the3%,6%of Al-12.6wt.%Si and Al-12.6wt.%Si-0.06wt.%Sr alloy are added respectively to Mg-8Li alloy as the matrix, for enhancing the mechanical properties. The microstructures of as-casted and as-rolled Mg-Li-Al-Si alloy are observed by the optical microscope (OM) and the scaning electron microscope equipped with a spectrometer (SEM and EDS). The phases were analysied by means of the transmission electron microscope (TEM) and X-ray diffraction (XRD). Using micro hardness tester and universal experimental tensile machine respectively, the hardness mechanical properties of the alloy are studied, the main conclusions are as follow:(1) The microstructure of as-casted Mg-8Li alloy is mainly composed of a-Mg phase and P-Li phase. After adding Al-Si alloy, the dendritic Mg2Si phase and white AlLi particle phase were observed. With the increase of Al-Si content, except for the volume of Mg2Si and AlLi phase increasing, MgLi2Al phase was formed in the matrix.(2) The microstructures of as-cast and as-rolled Mg-Li alloy are refined with the addition of Al-Si, and the refinement effect is increasing with the increase of Al-Si content. After adding Sr modified Al-Si, the microstructure of Mg-Li alloys including Mg2Si phases was further refined, have been refined. The dendritic Mg2Si phase and AlLi phase embedded in the a-Mg phase are fragmentd in the rolling process. (3) With addition of Al-Si alloy, the tensile strengths of as-cast alloys and as-rolled alloys are significantly improved, while the ductilities are markedly reduced. When6wt.%Al-Si alloy is added to the Mg-8Li alloys, the tensile strength of the as-rolled alloy reachs up to370.09MPa, increased by77.8%compared with that of the as-cast alloy. And its tensile fracture mode is cleavage fracture and local ductile fracture.(4) The tensile strength and ductility of as-casted alloys were further improved after adding the Sr modified Al-Si alloy. The tensile strength of Mg-8Li-6(Al-Si) alloy increases from208.2MPa to224.7MPa, the elongation rate also increases from10.9%to12.3%before and after Sr modification.(5) The microhardness of as-cast and as-rolled alloy increases significantly with the increase of Al-Si content. When6wt.%Al-Si is added to Mg-Li alloy, the microhardness of the alloy reaches114.3HV. Compared with the microhardness of as-cast alloy, the microhardness of as-rolled alloy was significantly improved due to work hardening.