LINYI JIUHENG IMPORT AND EXPORT CO.,LTD , https://www.jiuhengwood.com
Magnesium-zinc (Mg-Zn) binary alloys have seen limited practical application due to their coarse microstructure and susceptibility to microshrinkage. However, these alloys offer a significant advantage: they can be significantly strengthened through age hardening. This makes further development of Mg-Zn alloys highly promising, provided that an appropriate third element is introduced to refine the grain structure and reduce the risk of microshrinkage.
Grain growth is a common issue in Mg-Zn alloys, but zirconium (Zr) has been recognized as an effective grain refiner, especially for as-cast Mg-Zn alloys. As a result, industrial Mg-Zn alloys typically contain a small amount of Zr. These alloys are age-hardened and are commonly used under direct aging or after solution treatment followed by aging. They exhibit high yield strength and good mechanical performance, making them suitable for structural applications.
Compared to Mg-Al-Zn alloys, Mg-Al-Zr alloys show higher yield strength, better density, and less wall thickness effect. Common alloys in this series include ZM1, ZM2, and ZM70. ZM1 and ZM2 possess good fluidity, though they tend to develop porosity. The casting properties of ZM2 have been significantly improved with the addition of rare earth elements, particularly when the zinc content is low and the rare earth content is high. ZM1 alloys are widely used in aircraft wheel castings and other simple-shaped components requiring high strength.
The degree of grain refinement directly impacts the mechanical properties of the alloy, and it is closely related to the amount of dissolved impurities. Therefore, precise smelting techniques and temperature control are essential. ZM2 has found use in critical parts such as front bearing housings, jet engine covers, and hydraulic pump casings. In ZM1 alloys, the zinc content greatly influences the mechanical properties. Alloys with lower zinc and higher rare earth content offer better casting and welding characteristics, although their tensile strength is lower. Conversely, higher zinc and lower rare earth content lead to better tensile properties, albeit with some trade-offs in casting and weldability.
Adding copper (Cu) to Mg-Zn alloys has been shown to enhance toughness and age hardening. A typical example is the sand-cast ZC63 alloy (w(Zn)=6%, w(Cu)=3%, w(Al)=0.5%), which exhibits superior tensile strength (240 MPa), yield strength (145 MPa), and elongation (5%) after aging—outperforming Mg-Al-Zn alloys like AZ91. Cu increases the eutectic temperature, allowing for higher solution temperatures and more effective aging strengthening. It also modifies the as-cast eutectic structure, changing the morphology of the Schröter phase from irregular blocks to plate-like structures. However, the addition of Cu reduces the corrosion resistance of the alloy.
Incorporating rare earth elements into Mg-Zn alloys can significantly improve their performance, particularly in terms of heat resistance. For instance, certain alloys can operate at temperatures up to 150°C, making them suitable for helicopter gearbox cases. Increasing the Zn content leads to the formation of large Mg-Zn-RE phases at grain boundaries, which may increase brittleness and cause local melting during solution treatment. Some studies suggest that these phases can be dissolved through prolonged heating in nitrogen, enabling their use in thin-walled magnesium parts like ZE63.
Silver (Ag) has also been found to enhance the age-hardening effect in Mg-RE alloys. Alloys such as QE22, QE21, and EQ21 have been developed accordingly. These alloys maintain excellent high-temperature strength and creep resistance from room temperature up to 473 K, making Ag one of the most effective elements for improving high-temperature performance in magnesium alloys. However, the use of thorium (Th) in Mg-Th alloys is limited due to its radioactivity.
The addition of selenium (Se) and yttrium (Y) has proven beneficial in magnesium alloys. Drits developed a series of high-strength, heat-resistant WE-type magnesium alloys. Y can be added in the form of mixed rare earths, with compositions such as W7 (75% Y, remainder heavy rare earths). These alloys exhibit excellent mechanical properties and are widely used in racing and aerospace applications, particularly in gearboxes.