Inclusions in aluminum melt

The parameters that can significantly influence the cast metal quality are microstructure, homogeneity, chemical composition and cleanliness [15]. The factors which determine the cleanliness and quality of molten aluminum are amount of alkali contaminates, undissolved impurities (inclusions) and dissolved hydrogen. The inclusions in aluminum melt contain various forms such as films, individual particles or agglomerated inclusion of oxides, nitrides, borides, carbides and other combination of elements (chlorides and CaS). The size of these inclusions can vary from a few microns to half a millimeter. The numbers of inclusions that can be found in one kilogram of aluminum melt ranges from hundreds to thousands. The inclusions can cause different defects, influencing both mechanical properties and surface quality of products [12,13,15,16]. If these inclusions are not removed from the melt, they will appear after solidification and lead to many defects in final products such as porosity in foil, surface defects in rolled sheets, edge cracking in slabs, poor machinability, streaks, negative effect on plastic deformation of billets during extrusion, increasing the wear in extrusion dies, causing breakage in drawing wire, tearing in can stock etc. Even smaller inclusions, for example less than 10 microns, can be visible as a hole in products demanding high surface quality [5,6, 12].

As described earlier, there are various types of inclusions in aluminum metals: metallic and non-metallic inclusions. Normally they can be recognized by their appearance using a optical microscope. The oxides ( Al2O3), carbides (Sic, Al4C3), spinels (MgAl2O4), and borides (TiB2) are known as typical non-metallic inclusions [13,17].

Furthermore, two types of precipitation can be observed in aluminum metals: pure precipitated metallic phases like ( Al-Si-Mg, Al-Fe-Si) and precipitation of metallic elements in non-metallic inclusions such as MgO. The morphology of these precipitations, which is determined by their composition, include different shapes such as needle shape, plate shape, dendrite, rod shape, random clusters and lumps [13]. Figure 8 shows morphology of some metallic and non metallic inclusions by SEM.

Figure 8. Morphology of some inclusions by SEM