Research in recent years has shown that mechanical properties of die-cast AZ91 magnesium alloy can be significantly improved after modification by adding a small amount of rare earth, it has good high temperature resistance and corrosion resistance. However, when rare earth content further increases, larger rod-shaped and massive Al-RE phases will be formed in alloy to split matrix structure, resulting in a significant decrease in plasticity of material and a reduction in strengthening effect. How to improve plasticity of rare earth-containing magnesium alloys, further optimize microstructure of material, improve mechanical and process properties has attracted attention of researchers. Combined effect of rare earth elements, alkaline earth elements and transition metal elements is an effective means to improve strength and corrosion resistance of magnesium alloys. Alkaline earth element Sr is valued for its good grain refinement and high temperature resistance. Combined addition of Sr and rare earth elements has a better effect than single addition of rare earth elements in improving alloy structure, strength, and heat resistance. In addition, trace amounts of Ti can also significantly improve corrosion resistance of magnesium alloys, and form new phases such as Al3Ti to refine matrix structure. It can be used as a multi-element alloying component of rare earth elements to improve mechanical properties of material.
Based on this, influence of trace amounts of Sr and Ti on microstructure and properties of die-cast AZ91D alloy was studied. Through improvement of microstructure, plasticity and corrosion resistance of AZ91+RE magnesium alloy were significantly improved while ensuring higher strength. Comprehensive properties of AZ91D alloy can be effectively improved through multi-element microalloying method and its application scope can be expanded.

Test uses AZ91 alloy, high-purity magnesium ingots, zinc ingots, Mg-20RE (75% Ce + 25% Y), Mg-2Mn, Al-10Sr and Al-10Ti-1B master alloy in proportion. Use resistance furnace and graphite crucible for smelting, and use CO2+0.2% SF6 mixed gas protection. First, AZ91 alloy ingot is heated and melted in a resistance furnace, then intermediate alloy is added in proportion. After stirring, refining, and standing, it is produced using a Frech-DK580 die-casting machine. Die-casting process parameters: mold temperature is 210℃, die-casting temperature is 690℃, and injection speed is 5m/s. Obtained specimen is a single-shoulder cylindrical tensile specimen with a diameter of φ6mm and a gauge length of 60mm.

Table 1 Chemical composition of test alloy (%)

 

Figure 1 AZ91 alloy with different RE and Ti contents
Optical microstructure

 

Figure 2 XRD analysis of AZ91 alloy with different RE and Ti contents

 

Figure 3 SEM structure of AZ91 alloy with different RE and Ti contents
Al-RE phase in structure is mainly rod-shaped with different lengths and a small amount of massive Al11RE3 phase. High-temperature stable Al-RE phase can hinder dislocation slip and improve room temperature and high-temperature properties of alloy. However, it may also split matrix and cause stress concentration, resulting in a significant decrease in plasticity of alloy. After adding Sr (see Figure 4b), length of Al11RE3 phase becomes shorter, shape becomes rounder, number increases, and brittleness characteristics of alloy are improved. After adding Ti (see Figure 4c), volume of Al11RE3 phase increases and shape becomes irregular, but unfavorable rod-like morphology is further reduced.

 

Figure 4 SEM structure and EDS analysis of Al11RE3 phase in AZ91 alloy with different RE and Ti contents

 

Figure 5 SEM structure and EDS analysis of Al10RE2Mn7 phase in AZ91 alloy with different RE and Ti contents

Table 2 EDS analysis results of AZ91 alloy with different RE and Ti contents (%)

Table 3 EDS analysis results of AZ91 alloy with different RE and Ti contents (%)

Table 4 Room temperature mechanical properties of four alloys

 

Figure 6 Stress-strain curves of four alloys
Elastic modulus and yield strength of Alloy No. 2 are significantly increased due to increase of Al-RE precipitated phase, and elastic deformation stage is significantly shortened. After adding Sr, grains are further refined, rod-shaped Al11RE3 phase becomes shorter, number increases, and distribution is dispersed. Its strengthening effect and unfavorable factors are weakened, thus leading to an increase in plasticity of alloy and a decrease in strength. After adding Ti, degree of alloy refinement reaches maximum and tendency of structural inhomogeneity increases, but brittle characteristics such as sharp shapes and stress concentration in microstructure are significantly reduced, so optimization effect is ideal. In addition, it can be seen from Figure 6 that tensile curves of No. 4 alloy and No. 3 alloy are relatively close, that is, elastic-plastic deformation process of material has not changed significantly, indicating that composition and quantity of main strengthening phase of alloy have not changed significantly. Reduction of brittle characteristics such as defects and stress concentration in alloy structure is main factor for improvement of strength and elongation.

 

Figure 7 Macrosurface morphology of four alloys after salt spray corrosion

 

Figure 8 Corrosion rates of four test alloys

 

Figure 9 Polarization curves of four alloys

Table 5 Electrochemical data of four alloys

(1) After adding rare earths, microstructure of AZ91+RE alloy is significantly refined and Al11RE3 phase is formed at the same time, which leads to a significant improvement in strength and corrosion resistance of alloy, but also results in a decrease in elongation of alloy.
(2) After adding Sr, microstructure of AZ91+RE+Sr alloy is further refined, morphology and quantity of Al11RE3 phase and β-Mg17Al12 phase change greatly, resulting in a relative decrease in alloy strength and corrosion resistance, but plasticity is significantly improved. .
(3) After adding Ti, weakening effect of Sr element on strength and corrosion resistance is suppressed. Strength of alloy is equivalent to that of AZ91+RE, good elongation is maintained, and corrosion resistance is optimal. Through multi-element microalloying, AZ91+RE+Sr+Ti alloy has the best overall performance.