Effect Of Li Content On Microstructure And Properties Of Mg-5wt.% Al Alloy

Magnesium-lithium alloys have the characteristics of light weight,low density,and abundant raw materials,so they have a very broad application prospect in the fields of aviation,aerospace and weapons.However,the strength of magnesium-lithium alloy is inversely proportional to the density.The smaller the density,the lower the strength.How to develop a high-strength and low-density magnesium-lithium alloy has always been a hot and difficult point at home and abroad.Aging strengthening is one of the most basic methods for strengthening metal structural materials.However,there are few studies on aging hardening behavior of magnesium-lithium alloys at home and abroad.There is no unified statement on the aging hardening behavior,microstructure and performance evolution of this series of alloys.This limits the further development of the alloy.In this paper,high-strength Mg-5wt.%Al alloy is selected as the research object,and different contents of lithium metal are added to prepare Mg-5wt.%Al-xwt.%Li alloy.The microstructure and mechanical properties of the alloy during metal casting,hot extrusion or solution aging were studied,as well as its hardening behavior during solution aging.An intermediate frequency induction electric furnace was used for smelting and casting Mg-5wt.%Al-?0?6?wt.%Li test alloy under the protection of high purity argon gas,and an ingot with a diameter of 180mm and a height of 300mm was cast at 700?.Next,the ingot was subjected to a homogenization treatment at 250?for 24 hours,and a hot extrusion treatment was performed at an extrusion ratio of 1:8 and an extrusion temperature of 300?.Both the as-cast alloy and the extruded alloy are solid-dissolved at 345?for 0?24h,and aged at80?120?for 0?216h to prepare samples with different heat treatment states.Optical metallographic microscope?OM?,scanning electron microscope?SEM?,X-ray diffractometer?XRD?,transmission electron microscope?TEM?,and tensile testing and hardness testing were used to analyze and study the microstructure and phase composition of the experimental alloy.And the evolution of microstructure and mechanical properties of the alloy during solid solution and aging are explored.The specific conclusions are as follows:1)The microstructure of Mg-5wt.%Al-xwt.%Li?x=1,2,4,6?as-cast alloys consists of coarse?-Mg phase,fine?-Li phase and dendritic Al₂Mg with different contents.As the Li content increases,the Al₂Mg phase content decreases,and short rod-shaped Al Li phases and lamellar Li₃Al₂phases precipitate at the same time.Li element is more active and has a lower electrode potential than Mg element,and the potential difference between Li element and Al element is larger than Mg element.As the Li content increases,it is easier to form Al Li phase and Li₃Al₂phase,so when the lithium content is At 6%,the content of Al₂Mg phase is the smallest.2)As the Li content increases,the tensile strength and elongation of the as-cast Mg-5wt.%Al-xwt.%Li ternary alloy gradually decrease.The hardness increases to 71.9 MPa,and the density decreases to 1.55g/cm3.With the increase of lithium content,the number and size of the second phase compounds increase at the same time,so cleavage planes,cleavage steps and dimples gradually decrease,resulting in crystalline fracture.3)The Mg-5wt.%Al-xwt.%Li alloy was subjected to a solution treatment at 345?for0?24h.With the increase of the solution time,the hardness of the alloy first decreased rapidly and then reached equilibrium.Under the same solution time,the hardness of the alloy gradually increases with the increase of Li content.With the progress of solution treatment,the small dendritic Al₂Mg phase,granular and linear Al Li phase,and lamellar Li₃Al₂phase in the alloy are dissolved into the?-Mg matrix,and a new phase Mg₁₇Al₁₂may be formed.4)When aging treatment temperature of Mg-5wt.%Al-xwt.%Li alloy is 80?120?,the aging hardening effect decreases with the increase of aging temperature,but increases first and then decreases with the aging time.As the Li content increases,the aging hardening effect gradually increases.The aging process of the alloy includes:In the initial aging?1?72h?,Al Li phase and Al₂Mg phase were precipitated in the?-Mg matrix;in the rapid precipitation period?72?180h?,Li₃Al₂phase was precipitated from the?-Mg.The peak aging was reached at 180h,the precipitated phases were Mg₁₇Al₁₂phase,Al₂Mg phase,and Al Li phase;after the aging period?>180h?,the diffraction peaks of the Mg₁₇Al₁₂phase disappeared,and the precipitated phases agglomerated and grew up,resulting in over-aging softening.5)The as-cast Mg-5wt.%Al-xwt.%Li alloy has undergone solid solution treatment at345?and peak ageing at 80?.When the Li content is 1%by weight,its tensile strength is the maximum,which is increased from 198MPa to 253MPa,an increase of 27.8%.When the Li content is 4wt.%,the tensile strength increases most significantly,from 123MPa to 165MPa,an increase of 34.1%.6)When the extruded Mg-5wt.%Al-xwt.%Li alloy is aging treated at a temperature of80?120?,the aging hardening effect decreases as the aging temperature increases,but increases with the aging time.At the same time,as the Li content increases,the aging hardening effect gradually increases,which indicates that the addition of Li element can effectively increase the content of precipitated phases during the aging process of the alloy.Natural aging can shorten the time required for peak aging of the extruded LA45?Mg-5Al-4Li?alloy?108h?,and a peak hardness zone?108h?180h?will appear.