Modifying silumins
What is silumin modification?
Under modification silumins imply special melt processing, to get fine eutectic silicon in the cast structure. Such a structure eutectic silicon increases the mechanical properties of the casting, including, relative extension, as well as in many cases - the casting properties of the aluminum melt. Usually, modification of silumin produced by adding small amounts of sodium or strontium.
The essence of modification
The essence of modifying silumins - the effect of sodium content on the possible forms of eutectic silicon in silumin Al Si11 - is shown in the figures 1-4.
Figure 1 — Lamellar structure of eutectic silicon [1].
Conditions for the formation of lamellar silicon arise in casting alloys in the complete absence of phosphorus or modifying additives, for example, sodium or strontium.
Figure 2 — Granular structure of eutectic silicon [1]
Conditions for the formation of the granular structure of eutectic silicon arise in the presence of phosphorus, but no sodium or strontium. Silicon crystals exist in the form of coarse grains or plates.
and)
b)
Figure 3 - a) "Unmodified" structure of eutectic silicon;
b) Modified structure of eutectic silicon [1]
In the "undermodified" and to a greater extent in the modified microstructural state, for example, with added sodium or strontium, granules are significantly reduced in size, get a rounded shape and are evenly distributed. All this has a beneficial effect on the plastic properties of the material., in particular, on the value of the relative elongation.
Figure 4 – "Remodified" structure [1]
In case of "re-modification", for example, excessive sodium, vein-like ribbons with coarse silicon crystals appear in the structure. This means a deterioration in the mechanical properties of silumin..
Modification of silumins by sodium
In silumin with silicon content more than 7 % eutectic silicon occupies most of the metallographic sample area. With a silicon content of 7 to 13 % type of eutectic structure, for example, granular or modified, significantly affects the mechanical properties of the material, in particular, plasticity or elongation. therefore, when a higher elongation is required when testing a specimen, aluminum alloys with silicon content from 7 to 13 % modified by adding approximately 0,0040-0,0100 % sodium (40-100 ppm).
Modifying Silumins strontium
In silumin with a silicon content of about 11 %, especially for low pressure casting, strontium is used as a long-term modifier. The difference between strontium and sodium as a modifier is, that it burns out much less from the melt, than sodium. Strontium is added in quantity 0,014-0,040 % (140-400 ррм). Strontium modification is usually carried out at the stage of producing ingots from the corresponding alloys., therefore, the modification is no longer performed at the foundry. At low casting cooling rates, strontium modification is much less effective and therefore it is not recommended for use., for example, in sand casting.
Features of the processing of modified melts
To avoid strontium burnout, all melt treatments, including degassing, are conducted without the use of chlorine-containing materials, and using, for example, argon or nitrogen. Strontium modification is not lost even when the return metal is remelted, for example, profitable parts of castings. If necessary, the loss of strontium is replenished with ligature additives, containing strontium, according to the instructions of the supplier of the original modified alloy ingots.
Re-modification of silumins
Since sodium burns out from the melt relatively quickly, the subsequent modification of silumins with sodium should be carried out at the foundry at regular intervals. In melts, modified with sodium, no materials should be used in all melt operations, containing chlorine. Chlorine reacts with strontium and sodium, takes them out of the melt and, thereby, prevents its modification.
Sources:
- Materials from Aleris International, 2014
- ASM Handbook, Volume 15: Casting (2008)
