Master alloy

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Master alloy is a trade term for those alloys that are not used to manufacture end products, but are added to molten metal in order to improve their properties. They are available as bars, pigs (also toothed, well portionable, small pigs), granulates or powder.

A master alloy always consists of a base metal such as iron (Fe), aluminum (Al), copper (Cu) or nickel (Ni) and, as a commercial product, contains alloying elements in the prescribed composition, which in their pure form are difficult to alloy with the base metal because their melting points very high or its boiling temperature are low or only slowly in solution or go slightly oxidized.

This does not exclude the possibility that foundries melt the master alloys they need themselves, but as a rule master alloys are manufactured and brought onto the market by specialized companies; they save having to deal with alloy-specific problems and make it easier for foundries to produce casting alloys with the properties that are most useful for the intended use. This also includes the compensation of oxidation or burn-off losses, which is absolutely necessary for AlSi alloys containing Mg, in order to maintain the alloy properties.

Composition of master alloys

Iron-based master alloys

Based on iron are the ferroalloys like ferro-chromium (FeCr), ferro-manganese (FeMn), ferrosilicon (FeSi), ferrosilicon-magnesium (FeSiMg), ferrotitanium (FeTi) or ferrophosphorus (FEP), the latter a master alloy, also known as structure-influencing additive is used in the production of castings from hypereutectic AlSi alloys. Other master alloys are used in steel and cast iron production in order to achieve specified mechanical values ​​and the structural conditions typical for the various types of cast iron.

An example of the effect of such alloy additions is spherulitic cast iron (ductile iron), which always requires an addition of magnesium for desulfurization, because only sulfur-free melts allow the graphite to be spherical.

The same composition as master alloys also have materials used for inoculation, such as e.g. B. Ferrosilicon.

Aluminum based master alloys

The majority of metals - with the exception of magnesium - that are desired as an alloy component of aluminum can only be added to the alloy if they are not added in their pure form, but rather "diluted with aluminum". Pure copper is not soluble in an aluminum melt, but a master alloy made from 50 parts of aluminum and 50 parts of copper. AlSr 10 consists, for example, of 90% aluminum and 10% strontium, which is used in eutectic and near-eutectic alloys (<12% silicon) for refinement that influences the structure. The optimum temperature for solubility in the base melt, which can be seen from the solidification diagram of such mixtures, is the yardstick for the composition of a readily soluble master alloy.

Copper-based master alloys

AlCu50 is known in aluminum foundries among numerous master alloys based on copper , such as those required for the production of special bronzes. In heavy metal castings, i.e. non-conductive copper, bronze and also gunmetal , a generally used master alloy is CuP 10 (also used as CuP15), which is intended to deoxidize the melt.

Alloy testing

A spectrographic determination of the alloy components does not guarantee optimal quality. For this, micrographs and solidification diagrams of the multi-component systems are required, as well as the determination of which types of primary and mixed crystals were formed during solidification.

See also