Magnesium casting alloys can conveniently be divided into two basic groups zirconium-free and magnesium-zirconium alloys. The first commercially available group of zirconium-free alloys were Mg-Al-Zn based, the most widely used alloy being AZ91 (Mg 9%Al 1 %Zn 0.2%Mn). Other alloy systems include Mg-Al-Si, Mg-Al-rare earth and Mg-Zn-Cu alloys. Mg-Al-Zn alloys are cheap but are also difficult to cast successfully. Sand castings are prone to outcropping, microporosity, and achieving pressure tight castings can be a problem unless extensive sealing treatments are carried out. AZ91 is used primarily in high pressure die cast form particularly by the automotive industry, as car makers worldwide aim to reduce vehicle weights in order to improve fuel economy.
Sand cast Mg-Al-Zn alloys have reasonable properties in the T6 condition up to 120°C, although strength then decreases rapidly with increasing temperature and mechanical properties can vary substantially depending on section thickness. Despite these drawbacks, AZ91 castings are used in some aerospace applications, including complex aerospace sand castings such as the Westland Sea King helicopter gearbox.
Introduction of high purity variants of these alloys with lower levels of heavy metal impurities (iron, copper, and nickel) has significantly improved corrosion performance. The sand casting alloy AZ91C has now been largely replaced by its high purity variant AZ91E which has a corrosion rate around 100 times better in salt fog tests.
ZC63 alloy (Mg 6.0%Zn 3.0%Cu 0.5% Mn) was developed as an easy to process, commercial grade alloy. The North American automotive industry has shown particular interest, and ZC63 engine castings, including cylinder blocks and sumps, are being tested as a possible replacement for other materials.