Iron is a common impurity of aluminum alloys – cast and wrought. It gets it from several sources and, at least, alloys Al–Si It considered harmful for various reasons. It should be noted, that iron is not always bad.
In some deformable aluminum alloys, ie alloys, intended for forging, pressing or rolling, iron may be deliberate alloying element for improving the technological properties of the alloy and / or increasing the strength of the final product. However, in the manufacture of castings do not apply these wrought alloys, and work with casting alloys, which have a much higher content of alloying elements in comparison to wrought alloys, and almost always – a certain amount of silicon. Nominal silicon makes these alloys casting.
Iron – natural adulterant
Iron It is a natural impurity, which occurs during the production of primary aluminum - conversion into alumina bauxite and alumina in the molten aluminum. Depending on the quality of the original ore, the degree of control technology and the quality of other starting materials, liquid primary aluminum typically comprises from 0,03 to 0,15 % gland, average – from 0,07 to 0,10 %.
There is no economical way to remove iron from aluminum (see. also Iron in aluminum), however, these "primary" values are typical of a lower level of iron content in cast and wrought aluminum alloys, and all subsequent melt processing can only increase it. Iron can fall into the melt during processing in the following two main mechanisms.
As iron enters the aluminum
1. A liquid capable of dissolving aluminum, iron is not protected from the molten steel smelting tool, as well as the steel furnace equipment. Prolonged contact of aluminum with steel level of iron in the melt can reach 2 % melt temperature 700 ° С (eutectic Al–Feoccurs when the content of iron 1,7 % and temperature 655 ° C). If the melt at a temperature to keep 800 ° C, the levels of iron can reach 5 %.
2. Iron can get into the aluminum melt by additives is not too pure alloying elements, for example, silicon, and adding scrap, which typically contains significantly higher iron content, than the primary metal.
For these reasons, the level of iron in aluminum alloys continues to grow each remelting. Therefore, the secondary aluminum alloys, particularly those alloys Al–Si, under high pressure are designed for casting, limiting the magnitude of the iron content 1,5 %. In the case of high pressure die casting high iron content just is not so bad, as it helps to minimize the costly problem of sticking to the metal matrix. However, typical secondary alloys Al–Si Casting non-pressurized typically contain much less iron, in the range of 0,25 to 0,8 %, and most of all - from the 0,4 to 0,7 %.
The reason for such a low iron content in alloys derived from commercial balance between the benefits of reduced cost and acceptable metal technological problems with its casting and / or mechanical properties of the final level.
levels of iron in aluminum casting alloys
1. As far as possible the level of iron in the casting of aluminum alloys Al–Si It should be as low as possible, to avoid its negative impact on the mechanical properties, particularly in ductility and fracture toughness. It means, which need to minimize contamination with iron aluminum alloys by careful selection and application of the charge of refractory protective coatings on all steel tool, which is used in the preparation and processing of melts.
2. It is necessary to avoid iron content above the critical level for the silicon content, as this may lead to a serious loss of ductility of the final cast product and lower casting performance due to the increase of marriage due to shrinkage porosity and, in particular, for tightness.
3. Iron limit (in %) for alloy Al–Si calculated approximately by the following formula: Fe≈0,075 x [%Si] – 0,05.
4. If the solidification / cooling rate is very high (e.g., when casting under high pressure), high iron content might be useful, but with a decrease in the cooling rate (die casting -> sand casting) the likelihood of problems with high iron content increases dramatically.
5. Traditional heat treatment regimes for aluminum alloys Al–Si, eg T6, do not change the nature of the iron-containing phases. In the structure remains the same although the intermetallic compound and as a result of heat treatment the overall level of mechanical properties of the alloy can be improved, it will be initially higher at a lower iron content.
6. manganese addition for the "neutralization" of the harmful effect of iron in the ratio of manganese to iron content is often used about 0,5. However, the benefits of this are not always obvious. The excess manganese may reduce the proportion β-phase and promote the formation of α-phase iron compounds. This may improve the plasticity, but can lead to the formation of so-called hard spots and the difficulty in machining. At a high content of iron manganese additives improve the fluidity of the casting is not always aluminum alloy and reduce its porosity. Moreover, addition of manganese in the melt with a high iron content can contribute to excessive dross formation.
The source: www.aomevents.com/conferences/…/Taylor