Protective atmospheres for the heat treatment of magnesium alloys

Most magnesium alloys are used for die castings which, due to the presence of porous cores, cannot be heat treated at present without blistering. However, as technology improves, pore-flee castings will become available whose properties can be optimised by heat treatment and environmentally friendly protective atmospheres will be required for mass production processing. The remaining sand cast magnesium-zirconium alloys, mainly used for aerospace components, are currently heat treated. The most common alloy, ZE41, is usually only treated to the T5 temper whereas the increasingly popular WE43 is treated to T6 temper. During that heat treatment there is a potential fire hazard which can be solved by the correct application of a protective atmosphere. There is always a risk of fire in any magnesium heat treatment due to equipment failure or local furnace hot spots. The risk of ignition is particularly acute for alloys which must be solution treated at over 400 degrees C at which temperature some form of protective atmosphere has historically always been advised. The atmospheres recommended leave a great deal to be desired in terms of effective control, toxicity and environmental impact. A study of non-toxic environmentally friendly alternatives is presented for ZE41 and WE43. One route to pore-free castings is hot isostatic pressing (HIP). If HIP of magnesium castings is to become a standard process within the automotive industry, the cost must not be prohibitive. One way of reducing costs would be to replace the argon pressurisation medium with nitrogen and the effects of doing so are examined.