Refining Al/Si melts with BINAL®
Modification of the Al/Si eutectic in gravity mold and sand casting.
Sodium is used to refine an Al/Si melt. The Si content of the melt is 5 – 13.5%.
This is a targeted melt treatment, whereby a fine-fiber eutectic silicon is to be produced in the cast structure. The refinement improves the mechanical and casting properties of the melt. It is refined by adding sodium or strontium.
The refinement of an aluminum melt can be done using thermal analysis. The behavior of a cooling melt is observed. This means that you plot the current temperature against time. When sodium is added, the melting point of the eutectic (Al/Si) is significantly lowered (approx. 10° C).
The originally lamellar structure of Al-Si alloys is transformed into a fine-grained, refined structure with significantly better properties for the casting by adding sodium or strontium.
The calculated amount of sodium is placed in a diving device (e.g. a basket made of coarse wire) and quickly immersed below the surface of the melt.
Alternatively, the BINAL piece is placed on a bed of salt and immediately submerged using a diving device.
Example: 25 g of sodium for 250 kg of aluminum melt corresponds to a content of 0.01% Na.
After the reaction is complete, the diving device is slowly moved up and down two to three times to gently stir the metal bath. The melt then remains for another 5 to 10 minutes before pouring.
BINAL® (sodium in aluminum composite foil) refines the aluminum alloy by reacting with the melt. It should be noted that this reaction does not cause any bath movement. In most foundries, a sodium release of approx. 50 to 100 ppm should be released into the melt, although this depends on the sodium content of the initial material.
In different foundries the treatment temperature varies between 680ºC and 810ºC. It should be noted that the reactivity of BINAL® increases at higher temperatures. You should therefore place BINAL® on a layer of covering salt and submerge it as quickly as possible.
The degree of refinement can be checked using thermal analysis.
| quantity added:0,008% by metal weight | 50 – 60 ppm |
|---|---|
| quantity added:0,01% by metal weight | 80 – 100 ppm |
* These are empirical values (however, it should be noted that these values depend on the speed of immersion – the faster → the better the sodium transfer)
Packaged sodium can be stored indefinitely if stored correctly. There must be no open water points or pipes carrying water ar prohibited in storage area.”. Sprinkler systems are not permitted.
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We will be happy to advise you on the right BINAL® piece sizes and quantities for your melt.