Oxide inclusion

Structural defect where the oxides in the casting are present as non-metal inclusions, usually in the form of thin, film-like skins.

They are insoluble and cause local interruptions in the microstructure (see also Structural anomaly). As a result, they have a negative impact on static and dynamic strength characteristics and elongation (Figs. 1 to 4).

 

In highly vulnerable AlSi casting alloys (see also Aluminum casting, Aluminum gravity die casting alloys) in particular, the oxides have extremely fine pores (pore radius of 2 to 7.5nm), resulting in a high specific surface and capillarity with a high diffusion efficiency. This leads to the formation of oxide agglomerates with different apparent densities.

In addition to mixed oxides of different compositions and structures with Si, Mg, Na and Sr, the following two oxide forms are very frequent:

  • γ-Al2O3, formed at temperatures of up to 750°C, has a distorted Spinel lattice, is soft and thermally unstable (density: 3.64 to 3.96 g/cm3; spec. surface of 150 to 400 m2/g)
  • α-Al2O3, formed at temperatures over 750°C depending on time and nucleation and has a hard hexagonal corundum lattice that is thermally stable (density: 3.96 to 4.02 g/cm3; spec. surface of approx. 1 m2/g)

The oxide inclusions in cast steel formed by deoxidation products generally consist of MnO-Al2O3-SiO2 compounds. Depending on their composition, they differ by appearance and form. When they are rich in SiO2, they have a round shape and glass-like appearance. Under polarized light, these inclusions can show black extinction crosses. The inclusions increase in size with a rising MnO content, growing ever more heterogeneous in structure.



Oxides in the form of non-metal inclusions, often in connection withgas blisters or gas porosity, primarily occur in cast steel, nodular graphite cast iron (Fig. 5) and light- and heavy-metal casting.



The risk of oxide formation, and thus the risk of oxide inclusions, is particularly high in aluminum casting alloys. The occurrence of this defect is independent of the molding process, it can appear in sand and gravity die castings as well as in pressure die castings. Oxide inclusions can be detected in the microstructure of metallographic specimens and can also be visible to the naked eye at the casting surface.

Aluminum, silicon, magnesium and, for example, chromium and alloys containing these metals in high concentrations exhibit a particular tendency to form insoluble oxide films during melting, ladle filling and casting due to their high affinity to oxygen. In practice, the degree of oxidation with atmospheric oxygen is not only dependent on concentration and pressure but also, to a large extent, on temperature and time.

This temperature and time dependence must be taken into account during melting and holding since the oxidation propensity increases rapidly as the temperature rises.This is especially true for aluminum melts with high magnesium contents (e.g. Al Mg5Si2Mn, Al Mg9). In addition to aluminum oxides, these generate spinels which prevent the formation of an air-tight self-cover as this would be possible with aluminum oxide alone. This causes atmospheric oxygen and atmospheric hydrogen to diffuse into and react with the melt.

  • Fig. 1: Oxide film inclusions in the fracture structure of a gravity die casting (GK-AlSi12), magnification 12:1
  • Fig. 2:  Micrograph of section 1 in Fig. 1, magnification 64:1, not etched
  • Fig. 3: SEM image of section 1 in Fig. 1 (overview)
  • Fig. 4: EDX analysis (overview) of the sectionhighlighted in Fig. 3
  • Fig. 5:  Oxide inclusions in nodular graphite cast iron; here, the residual magnesium content of 0.057% is too high which is also consistent with the graphite degeneration in the vicinity of the defect (see also Dross), magnification 50:1, not etched
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