Use of Magnesium alloys has seen rapid growth over the last 10-15 years for High Pressure Die Cast (HPDC) automotive applications and enjoys sustained use in the sand cast form for aerospace and speciality applications.
The primary reason for this success is the lightweight of Magnesium, being only two-thirds the density of Aluminium. In addition alloy development has generated Magnesium alloys with excellent corrosion resistance and for some alloys, elevated temperature performance to 250ºC (482ºF), which is superior to most Aluminium alloys.
In contrast, precision cast Magnesium is comparatively limited. The reason for this lies primarily with the potential reaction of Magnesium with the mould material - Silica reacts with Magnesium to produce Magnesium Oxide and Magnesium Silicide. Silica is present in most slurry mixes as either a constituent of the binder or refractory.
Several foundries have overcome this reactivity issue by proprietary techniques, successfully producing Magnesium castings from Plaster block moulds. Ceramic shell moulds have also been produced in more recent times by careful process control and/or modification of primary shell compositions.
Suitable alloy selection for precision casting has proved to be important in recent work. Two alloys are favoured, namely AZ91 and Elektron 21. Both alloys are easy for the foundry to melt and handle. AZ91 is suitable where a cast shape is simply required, and Elektron 21 where good mechanical properties are required at either room or elevated temperatures (200ºC/392ºF). This latter alloy offers the potential to improve prototyping capabilities and increase the use of Magnesium in property critical components.
This paper provides an insight into the potential of the aforementioned alloys and includes a view of the complementing investment technology, developed through Cast Technology International (Cti) High Technology Materials programme*.
Mechanical property data is provided for both prototyping and final use applications.