Magnesium in its pure form has limited mechanical properties and is not suitable for structural applications. However, through alloying and heat treatment, magnesium's strength can be significantly enhanced, making it widely used in industries such as aerospace, automotive, machinery, and construction. The primary alloying elements in magnesium alloys include aluminum (Al), zinc (Zn), manganese (Mn), and rare earth elements (RE). These elements exhibit solubility changes in magnesium, enabling the use of heat treatment methods like solid solution and aging to improve the material's performance. Table 11-1 outlines the chemical composition of cast magnesium alloys in China.
One of the main impurities in magnesium is nickel (Ni), along with iron (Fe), copper (Cu), silicon (Si), and tin (Sn). Among these, Ni and Fe are particularly dangerous, as they severely reduce the corrosion resistance of magnesium. Although Ni has a higher melting point and density than magnesium, it dissolves easily in the metal. Therefore, during the smelting process, any equipment or tools used must not contain Ni. While Fe also negatively affects corrosion resistance, its impact is less severe compared to Ni. For instance, increasing the Fe content from 0.03% to 0.26% reduces the corrosion resistance by up to five times. Common alloying elements include Al, Zn, Mn, Si, RE, Ag, and Cu. Aluminum and rare earths enhance both mechanical and casting properties, while Mn forms compounds that reduce Fe content and improve corrosion resistance. Zinc boosts both corrosion resistance and strength, and silicon helps increase creep resistance. Silver improves high-temperature performance.
Cast magnesium alloys are generally categorized into three types: Mg-Al alloys, Mg-Zn-Zr alloys, and Mg-RE-Zr alloys. The first type, Mg-Al alloys, includes ZMgAl8Zn (ZM5), ZMgAl10Zn (ZM10), AZ91A, AM60A, and AS41A. The second group, Mg-Zn-Zr alloys, includes ZMgZn5Zr (ZMD), ZMgZn4RE1Zr (ZM2), and ZMgZn8AgZr (ZM1). The third group, Mg-RE-Zr alloys, includes ZMgRE3ZnZr (ZM3), ZMgRE3Zn2Zr (ZM4), and ZMgRE2ZnZr (ZM6). These latter two groups are often referred to as "grain-refined" or "boron-magnesium" alloys, while the first is known as aluminum-magnesium alloys.
The development of cast magnesium alloys can be divided into three stages. The first stage focused on adding aluminum and zinc to magnesium, resulting in Mg-Al-Zn alloys. These alloys offer tensile strengths comparable to those of cast aluminum alloys. Examples include ZM5 in China, L121 in the UK, and AM80A in the US. However, this type of alloy has several drawbacks, including a wide solidification temperature range (around 1570°C), which leads to micro-porosity and hot cracking during casting. Additionally, mechanical properties are affected by the thickness of the casting, often decreasing as the wall thickness increases.
The second phase involved improving the yield strength of Mg-Al-Zn alloys. In China, ZM1 was developed, while the US introduced ZK51A and the UK used 2970 MAG4P. Adding zirconium (Zr) significantly refined grain size, doubling the yield strength and improving fatigue resistance and notch toughness. Despite these improvements, the alloys still suffered from a wide solidification range and issues with porosity and hot cracking. To further enhance performance, some alloys were modified with rare earth elements, such as ZM2 in China, 2970 MAG5P in the UK, and ZE41A in the US.
The third stage focused on high-performance alloys for high-temperature applications. Alloys like ZM3, ZM6 (with rare earths as the main component), DTD5051 (UK), and HK31Ao (US) represent the advanced stage. Some alloys, such as Mg-Zr-Ag-RE, achieved excellent yield strength and creep resistance. Others, like those containing niobium, exhibited high specific strength but were limited by their high cost, restricting their industrial use.
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