Effect Of La₂O₃ On The Cast Structure And Mechanical Properties Of AZ31 Magnesium Alloy

Magnesium alloys have great predominance for the application of aerospace, automobile and electronics due to their high physical and mechanical properties. Compared with die-casting magnesium alloys,wrought magnesium alloys with higher strength,scalability and diversified mechanical properties,can be satisfied with the requirement acting as structural parts.Mg-Al-Zn alloy is the most commonly used alloys for its good strength,ductility,corrosion resistance performance,and lower prices.It had been paid high attention by the material searchers who had made lots of useful investigations on how to further improve its comprehensive properties for more wildly application.In this paper,the testing magnesium alloy AZ31 was made by adding different contents La₂O₃ into Mg-Al-Zn matrix alloy under the protecting of SF₆/CO₂ mixed gas shielded.The microstructure and mechanical properties were investigated before and after homogenization,and the effect of La₂O₃ on the microstructure and mechanical properties of AZ31 magnesium alloy were studied.Microstructure observation included metallographic analysis,scanning electron microscope analysis and X-ray diffractometer analysis.Mechanical properties included micro-hardness, tensile properties and toughness properties.The following conclusions:The grain is refined effectively by adding the La₂O₃,and the average grain size decreased 157μm to about 100μm.The morphology of phase Mg₁₇Al₁₂ transformed from continuous and coarse net into small and dense discontinuous: The mechanics of modifying effect of La₂O₃ on AZ31 were as follows:the grain growth was restrained because Al₁₁La₃ phase appearing on the grain boundary and inhibiting the growth of phase Mg₁₇Al₁₂,which is leading to dispersion strengthening.Furthermore,the mechanical puppetries of AZ31 alloy were improved by Al₁₁La₃ phase inhibiting the grain boundary;The mechanical prosperities of AZ31 alloy was improved by the addition of La₂O₃,the as-cast microstructure and mechanical properties would be best after adding mass 0.6%La₂O₃ into Mg-Al-Zn matrix alloy.There were large amount of tearing ridges,minute dimples and cleavage steps appearing on the tensile and impact fractures.The fracture shows typical quasi-cleavage fracture;In the homogenization treatment process,the effect of temperature increment on the microstructure of AZ31 with 0.6%La₂O₃ was more remarked than that of holding time prolongation.The predominance of the homogenization treatment was temperature;The homogenization treatment technics was insulation keeping 10 hours at 400℃,which can decrease the segregation of cast ingot efficaciously,and improve its plasticity properly.The microstructure and plasticity of cast ingot would not change much but just add energy consumption when the holding time was over about 10 hours.For AZ31 +0.6%La₂O₃ magnesium alloy,suitable homogenization annealing treatment technics was 400℃×(10h-12h);During the homogenization annealing of heating and insulation,the coarse Mg₁₇Al₁₂ phase distributed on the grain boundary and interdendritic grain dissolved in the a-Mg matrix.During cooling,Al exsoluted from supersaturated a-Mg matrix in the form of Mg₁₇Al₁₂ precipitated phase,and the small granules homogeneous distributed in thea-Mg matrix.The acicular phase Al₁₁La)₃ with high thermal stability appeared on the grain boundary.