Liquid metal quality

Castings are now becoming commonplace in highly demanding and safety critical applications, for example in the aerospace and automotive industries. It is, therefore, imperative that internal defect structures and potential failure mechanisms are fully understood if reliable castings are to be produced consistently by the foundry. Many alloy systems, such as those based on aluminium, magnesium, zinc and certain copper- and iron- and steel-based alloys suffer from the entrainment of double oxide films and dissolved gases. However, the presence of damaging oxide films is often only detected when a casting leaks or fractures at such inclusions. The ubiquitous presence of these very thin double films has not been widely accepted because no single metal quality test has been able to resolve such thin but extensive defects. We have developed a novel reduced pressure test that allows a detailed assessment of metal quality through the simultaneous visualisation of double oxide films and gas content. The technique will be demonstrated for use on aluminium alloys as this system provides clear examples and contains only a single dissolved gas.