| As the lightest structural metal materials,magnesium alloy has a huge market demand in the field of automotive lightweight because of its excellent mechanical properties,especially their high specific strength.Automobile weight loss will improve fuel efficiency and reduce exhaust emissions,so we can achieve green development.However,the wide application of magnesium alloys is limited by its poor plasticity,processing difficulties,casting defects and corrosion resistance.In this study,the effect of calcium oxide(Ca O)particle size on microstructure and properties of AZ91 alloy was investigated firsly based on commercial AZ91 alloy using Ca O particles with different size(nanosize:50 nm,sub-microsize:200 nm,500 nm and microsize:1000 nm)as grain refiners.The Al element(0.5,1.0,1.5,2.0 wt.%)were further added to explore the mechanism of synergistic promotion of Ca O particles and high Al content in the mechanical and corrosion resistance of the alloy.In order to strengthen the theoretical basis for the practical application of the experimental alloy,the high-cycle fatigue behavior of several alloys under the specified stress was explored,and the reasons for the different properties of alloys in different systems were also analyzed.The experimental results are as follows:(1)AZ91 alloy with different size Ca O particles(C1 C2 C3 C4)have different effect on microstructure,properties and corrosion resistance.As the particle size increases,the performance increased and then decreased,when the size is 500 nm,the reaction is most abundant in the melt,so the grain size of C3 alloy is the smallest(33.7?m),the volume fraction of second phase is least(19.9%)and distribute uniformly.This results in the highest hardness,compressive strength and yield strength of C3 alloy,which is increased by 36.7%,94.8%and 35.9 respectively relative to the matrix;After the salt spray test,the corrosion level was improved four levels.(2)The further addition of Al elements on the basis of C3 alloy increases the effect of solid solution strengthening and second phase strengthening of AZ91 alloy due to the synergistic effect of Ca O particles and Al element.The strength and corrosion resistance of A1?A2?A3?A4 alloys are further improved compared with that of AZ91 and C3 alloys.When Al content was 1.0wt.%,it reached the maximum value again,the hardness,compressive strength of the alloy enlarged by 19.5%and11.5%respectively compared with C3 alloy;The corrosion grade after salt spray was the same as C3 alloy.(3)After high cycle fatigue experiments on the above alloys,it was found that both Ca O and Al could improve the fatigue life of the matrix under specified stress level,among which C3 alloy could withstand cyclic loading without fracture failure under medium and low stress,10~6cycles under high stress.The addition of Al did not further enhance its fatigue performance compared with Ca O particles,but the fatigue life in the system enlarged primary and then reduced with the expand of Al content.(4)After analyzing the influence mechanism of Ca O and Al on the properties of the alloy,it was found that grain size and the number,size,distribution of the second phase were the most important reasons.For Mg alloy,the maximum effect is achieved when the grain size is around 30?m and the volume fraction of the second phase is within the range of 18-20%. |
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