Structural changes within the graphite particle-AlSi6Cu4 alloy interface zone in the recycled metal composite
Marian Mitko, Szczepan Tomczyński Politechnika Częstochowska, Katedra Odlewnictwa, al. Armii Krajowej 19, 42-200 Częstochowa
Annals 4 No. 10, 2004 pages 159-163
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abstract Composite material obtained by squeeze casting of the AlSi6Cu4 (AK64)alloy mixed with graphite particles has been recycled by subsequently repeated cycles of melting and diluting with the matrix alloy. Finally a composite containing 5% of graphite particles has been produced. Structures of composite before and after being recycled have been shown in Figure 1. The graphite particle-matrix alloy interface has been examined by means of optical and electron microscopy, and X-ray microanalysis using the thin foil method. SEM images and results of field and linear microanalyses are presented in Figures 2-5. The adhesive-mechanical bonding between graphite particles and metal matrix achieved in the initial composite obtained by mixing and squeeze casting has been retained even after several hours’ course of melting and diluting, and the gravity casting of the resulting composite has been still possible. No intermediate phases have been found in the matrix zone enveloping the graphite particles. However, some graphite particles are crushed due to the recycling process and a sort of impurities gathers near their surface. Crushing and lamination of graphite particles is also induced by changing stresses which accompany temperature changes during repeated melting and solidifying of composite. The zone close to the graphite particles more frequently reveals the presence of oxygen-rich regions, and within them also Mn-, Fe-, or Cu-rich areas are found. This indicates the tendency of the oxide precipitates to concentrate at the reinforcing particle/matrix interface. Despite strictly observing the technological regime the quantity of these impurities grows with the growth of the number of melting and diluting cycles, because rafination processes cannot be employed. This process leads to the degeneration of the intermediate zones and in consequence to the significant loss of the serviceability of these composites. Therefore the composite can be subjected only to a restricted number of subsequent melting and diluting cycles. Key words: aluminium alloys, composites, graphit particles, structure of the composites, recykling