Permanent modification

Modification of eutectic and nearly eutecticaluminum silicon die cast alloys (see also aluminum gravity die cast alloys) by adding weight loss and long-time resistant modification materials to the melt.

Compared to fading sodium, the modification effect lasts for a longer period (in general 2 to 12 hours, see Fig. 1). Strontium is mainly used as permanent modification material. Mold foundries often add it as AlSr pre-alloy to the melt, normally AlSi10 (rarely also AlSr90) in the form of bars or wafers.

Even after longer periods of standing or holding the melt, these “modified” or rather “long-time modified” alloys have a better structure, i.e. a fine (Al+Si) eutectic (Fig. 2).

The added amount in particular depends on the casting process and the casting wall thickness. Reference values for traditional casting processes are shown in Table 1.

Today, the Sr modification is broadly used for sand and gravity die casting, but also for pressure die casting, in particular for ductile pressure die casting alloys.

When used for sand casting, the mold sand water content should not exceed 3 percent due to the higher water absorption of Sr-modified Al melts. The Sr weight loss can be estimated with approx. 5 percent compared to the initial value. If permanently modified block material is used, the Sr weight loss can amount to 20 percent for the first melting process which is mainly caused by oxidation.

A dusadvantage compared to sodiummodification is the lower modification effect and the increased water absorption by the melt. An over-modification with Sr normally occurs with higher contents of more than 400ppm where strontium in most cases is bonded in Al2Si2Sr phases having no longer effects on modification. Melt control regarding its modification state occurs through thermal analyses.

  • Fig. 1: Sr and Na weight loss of a Silafont-13 (Al Si9Mg) according to Aluminium Rheinfelden Alloys GmbH
  • Fig. 2: Al Si7Mg0.3KT6 gravity die cast part structure, melt with 50 percent of cycle content, casting wall thickness of 20mm, modified using 135ppm of SrSource: Aluminium Rheinfelden Alloys GmbH
  • Table 1: Reference values for permanent modificiation using strontium