Density index

Characteristic number for the melt quality of an aluminum melt sample.

The melt sample pair densities (Fig. 1) - the one resulting from the vacuumdensity test (D80mbar) and the one from hardening under atmospheric pressure (Datm) - are related to each other on the basis of the following eq. 1:

 Eq. 1       DI =((Datm-D80mbar)/Datm)x 100 %

Calculation example:

Al Si9Cu3(Fe) melt density indices (VDS 226) before and after degassing treatment:

  • Before degassing:    DI1    =   (2,693-2,385)·100/2,693  =  11,44 %
  • After degassing:  DI2    =  (2,709-2,687)·100/2,709   =   0,81 %

The density index indicates by how much the sample which hardened in a vacuum is lighter than the one hardened under atmospheric pressure and can be used as direct meaure for the melt water content. Since germs and oxide gasses are released during the hardening process in the vacuum desntiy test, a low density index (lower than 1 percent) refers to a very good oxide-free melt quality. In case of highter hydrogen contents, a good density index correlation with the actual water content can be seen, e.g. how it can be determined on the basis of the Chapel process (Fig. 2).

  • Fig. 1: Vacuum density test pair, Source: mk Industrievertretungen GmbH
  • Fig. 2: Relation between the water content and the density index illustrated by a non-modified Al Si9Mg alloy, measured with a low pressure of 30mbarSource: Aleris Recycling (German Works) GmbH Al casting alloy brochure
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