Aluminum casting plants are using significant amounts of primary, secondary and master alloys in order to produce die casted parts of high quality. The quality of cast products directly depends on the quality of molten metal from which the products are cast. Comprehensive understanding of the melt quality is of vital importance for the control and prediction of actual casting characteristics. Any defect added or created during the melting stage will be carried to the final microstructure, and certainly, affect the quality of cast products. Therefore it is apparent that the control of the quality of the cast products begins with the control of the quality of the melt.
The High Pressure Diecasting process is very attractive to the casting buyer, offering fast production rates coupled to optimised production costs. Over the past 15 years there has been considerable growth in the HPDC process and now highly stressed castings are being manufactured by this process. As castings become more complex and some wall sections become much thinner, the need to produce alloys of a higher quality level and metal cleanliness has become vitally important.
It is well known that molten aluminium alloys have wo inherent characteristics: the tendency to absorb hydrogen gas from the atmosphere, and the ability to readily oxidise. On melting an alumina ﬁlm is instantaneously formed and this will act as a protective layer as long as it is left in place. However metal movement and breaking of the alumina ﬁlm during metal treatment, transfer and pouring cause oxide ﬁlms and inclusions to be formed and included within the melt. Over time there is also a tendency for oxide to form and build on refractory and crucible walls. These hard dense inclusions will break off after time, and result in hard spots in castings.