Advantages of colour etching in quality control of recycled Al-Si cast alloys

Colour Metallography is a set of light metallographic microscopy methodologies that utilize phase colour contrast as a source of new structure information compared to conventional techniques. In many cases, colour contrast can be induced where no identifiable phase contrast is obtained by conventional techniques. With the help of colour contrast, we gain new information on the structure of Al-Si alloys, especially intermetallic phases, in which the benefits of its use in casting quality control are of benefit. Blue-yellow contrast is definitely more pronounced than dark grey or light grey contrast. There is no substantial reproducible colour reproduction, since it is irrelevant whether the two phases are distinguished by blue-yellow or green-ocher contrast. When using the colour contrast, the structural components differ not only according to their microscopic appearance, but also based on the knowledge of the chemical composition of alloys, interactions between structural components, knowledge of the possible influence of Al-alloys structure (e.g. refining, modifications, heat treatment). Colour contrast in quality control of Al-Si alloys was applied to AlSiMg cast alloy (AlSi7Mg0.3; AlSi7MgTi; AlSi10MgMn; etc.).