Editorial Feature

Magnesium Alloys - Zirconium Containing Casting Alloys

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The discovery of the powerful grain refining effect of zirconium was a huge step forward in the development of magnesium alloys back in 1937. The presence of zirconium provides sand castings a fine, equiaxed grain structure with standard grain sizes of 30-50 µm, compared to the comparatively coarse dendritic structure of Mg-Al-Zn alloys.

Since zirconium forms stable compounds when combined with manganese and aluminum, it could not be applied to grain refine Mg-Al-Zn alloys. Therefore, a completely new series of Mg-Zr alloys was needed.

While the earlier Mg-Zr alloys had zinc as strengthening element, the successive alloys contained silver, thorium, and the recent alloys have yttrium. The majority of Mg-Zr alloys comprise rare earth elements such as neodymium, cerium, and lanthanum, which form uncomplicated eutectic systems with magnesium. This improves the castability since grain boundary networks of comparatively low melting point eutectics are formed.

Continuous development of alloys has resulted in huge improvements in the mechanical properties of Mg-Zr alloys at ambient temperature. But the substantial optimization of the high-temperature properties enables the latest alloys to be used under temperatures of up to 300 °C compared to 150 °C for previous Mg-Zr alloys.

Mg-Zn-Rare Earth (RE)-Zr Alloys

For a number of years, Mg-Zn-RE-Zr alloys containing lanthanum or cerium rare earth elements have been the most extensively used alloys for gravity casting and sand casting. RZ5 (ZE41) is the single most widely used alloy for gearbox casings. It has the ability to retain its mechanical properties up to about 150 °C.

Another alloy, ZRE1 (EZ33), has been used widely in jet engines, which necessitate optimal creep properties at about 150 °C. Both the alloys exhibit exceptional castability, enabling complicated castings to be produced with almost no microporosity.

A few Mg-Zn-Zr alloys without rare earth elements, such as ZK51 (Mg 4.5%Zn 0.7%Zr), were created but were not extensively used because of poor weldability and castability.

Thorium-Containing Alloys

Magnesium thorium alloys—ZT1 (HZ32) and TZ6 (ZH62)—were created for high-temperature creep resistance up to 350 °C. They are mainly used in aero engines, where optimal creep properties are crucial, as lightweight substitutes to titanium or high-temperature aluminum alloys.

However, issues relating to the production and foundry handling of Mg-Th alloys have resulted in complicated and expensive production processes, and environmental aspects have considerably raised the disposal costs. Hence, Mg-Th alloys are not being suggested for new projects while yttrium-containing alloys may probably be used for similar applications in future.

Silver-Containing Alloys

These alloys were created for optimized ambient and high-temperature properties compared to Mg-Zn-RE-Zr alloys. Silver is the key strengthening element, apart from praseodymium or neodymium rare earth elements. Optimal tensile properties are retained at temperatures beyond 200 °C in the T6 heat-treated condition; however, the creep properties are low-grade compared to those of thorium-containing alloys.

MSR (QE22) is the most common alloy that contains 2.5% silver. In a later advancement, EQ21—an economical, low-silver alloy—was developed. This alloy had comparable ambient temperature and moderately better high-temperature mechanical properties. In general, these alloys are used in engine casings and aerospace gearbox where enhanced high-temperature performance is essential, such as the Rolls Royce Tay intermediate casing.

Yttrium-Containing Alloys (WE Alloys)

The Mg-Y-Nd-RE-Zr alloys WE43 and WE54 were created mainly for better mechanical properties than currently available magnesium alloys, specifically at higher temperatures and as a substitute to thorium-containing alloys. WE43 alloys have less rare earth elements and yttrium, than WE54. Thus, they exhibit lower strength but higher ductility.

A key feature of WE alloys is their intrinsically exceptional corrosion resistance than other magnesium alloys. During salt fog tests, corrosion rates of WE alloys were found to be similar to that of high-purity AZ91 alloy and various aluminum casting alloys.

Due to its blend of ductility, strength, and corrosion resistance, WE43 has been widely used in the aerospace sector. This alloy exhibits strength superior to that of MSR at higher temperatures and that of RZ5 at all temperatures. Moreover, its properties compare really well with those of typical aerospace aluminum alloys, A203 and A356, specifically above 200 °C.

Furthermore, the strength of WE43 does not decrease much after prolonged exposure to elevated temperature. By contrast, some aluminum casting alloys exhibit considerable loss of properties.

WE54 alloy, which is more robust than WE43 but less ductile, has been developed specifically for applications where high strength is crucial or ductility is less vital. Until now, WE54 has gained the most attention as a substitute to aluminum alloys in high-performance racing car market, where it is used in large engine castings (heads, cylinder blocks, and sumps) and smaller parts such as pump housings and covers.

At present, foundries in North America, Europe, and Japan have substantial expertise in casting and supplying WE alloy castings as part of their product range.

Key Properties

  • Good to outstanding corrosion resistance
  • Light weight
  • Good high-temperature mechanical properties
  • Low density (two-thirds of that of aluminum)

Applications

The Mg-Zr alloys are used in aerospace applications such as intermediate compressors, transmissions, auxiliary gearboxes, canopies, generators, castings for gearboxes, and engine components.

The light weight and mechanical properties of these alloys allow them to be used in motor racing applications to lessen the weight of vehicles.

Nuclear applications, sporting goods, electronics, flares, office equipment, flash photography, sacrificial anodes, and tools are other applications of these alloys.

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