2017-05-15 12:03:14

Melting and Alloying Aluminum Alloys

Kinetics and mechanism of dissolving of metals in molten aluminum

Solubility of elements is dependant from three factors:
* similarity of crystalline packing of elements in metal-solvent and dissolving metal,
* size of component atom diameter change, and,
* chemical properties (electronegativeness).

Metals are well dissolved in aluminum if they are close to Al in periodic table of elements, isomorphic and not different in atom diameter more then 8-12%. Mg, Zn, Cu and Li dissolve well, without any difficulties in aluminum. If dissolving metal has significantly higher melting point than aluminum (Fe, Be etc.) dissolving is slow and it is necessary to additional heat the melt. If the components have very high melting point, dissolving is even slower (Ti, Zr).

Dissolving of metals in melt consists of destroying of their crystalline lattice and transferring of the atoms in the metallic bath. Driving force that determines dissolving kinetics is difference between free enthalpy of the element in the crystalline lattice and free enthalpy in the liquid state.

Formation of solid intermetallic layer forgoes dissolution of transition metals in aluminum, thus significantly lowering the solubility of these metals in aluminum. Diffusion mobility of AlnMem complex is very small. Time of alloying element dissolution is short and dependant on melt stirring rate, if the limitation stage of process is diffusion through the intermetallic layer.

Dissolution process of refractory metals in aluminum consists of three stages:
1. breaking of the atomic bonds in the crystalline lattice of the refractory metal and formation of the interrnetallic compound layer,
2. diffusion of the refractory metal atoms in the interrnetallic layer and transfer into melt,
3. diffusion of the refractory metal atoms into the boundary diffusion layer.

A number of kinetics researches were performed on dissolution of Fe, Si, Cu, Ti, Co, Cr, V, Mo, W etc.

Influence of material grain size on its dissolving rate

It is known that by pounding of materials rate of chemical processes increases as well as activity of reacting components. Usually for ensuring of suitable dissolving rate solid materials are pounded as much as the dissolving is harder. For intensification of dissolving rate mechanical or electromagnetic stirring could be used.

Dissolution of gaseous in molten aluminum

While smelting and casting non-ferrous metals and alloys, interactions with gases such as hydrogen, oxygen, nitrogen, water fume, carbon monoxide, carbon dioxide, and hydrocarbons occurs. Metals and gases form solutions through mechanisms of chemical reactions and mechanical mixing. Dissolution of gases in metals and formation of compounds is the final stage of their interaction. Mechanism of gas adsorption consists of (a) diffusion of gaseous on the melts surface, (b) chemical reaction on the surface, and (c) diffusion into the metallic bath. Desorption has inversed sequence of stages. Dissolving of gases in metal is directly dependent from absorption and diffusion.

Halogen elements salts, such as NaCl, NaF, KCl, NaBr, Na3AlF6 (cryolite) etc., are used for blanking and melt protection either individually or in combination. A salt must exhibit a small surface tension on the border towards oxides, but a large one on the border towards melt. Combination of two or more salts causes reduction of the melting point.

Nitrogen, chlorine and easily volatile chlorine based salts are used for Al and Al alloys degassing, through
* cooling
* vacuum degassing
* smokeless degasification
* application of ultrasound waves.

The application of easily volatile chlorine based salts is possible owing to reactions with aluminum, when AICl3 is produced, acting in the same way as pure chlorine and in accordance with the Dalton's law on partial pressures. There is a great number of salts with Cl2 connected to organic cation, such as:
* C6Cl6 - hexachlorobenzol, which is not in use any more due to its disturbing sharp smell
* C2Cl6 - hexachlorethane, which reacts violently, so it is combined with N2 after blowing
* CCl4 - carbon tetrachloride
* PCl5 - phosphorous pentachloride

The process of degasification is never complete owing to high hydrogen solubility in aluminum, which even increases with increase of temperature. Vacuum degasification is efficient and clean, but rather expensive and needs additional equipment.

Contrary to other light and non-ferrous metals, Al-Si alloys, which are widely used in practice, need to undergo the process of modification. Modification process implies a fine alloy structure pulverization (i.e. Si crystals pulverization) that consequently improves both mechanical and liquid characteristics, shrinks cavities, decreases hot cracking etc.

With Al-Si alloys the modification process is carried out either by means of metal Na or with the mixture of chloride and fluoride sodium salts, up to their eutectic composition i.e. around 13% Si. Nowadays metal Na is produced by the means of vacuum extraction system. Being easily inflammable, it is kept inside kerosene. Less used castings containing up to 8% Si do not need to be modified.

Recently some tests have been done with Ti and B, but the results were rather modest. The latest experiments are with Sr (0.65%) and Sb (0.15%) application, and both alloy types provided good results. Former is added in the form of liquid AI-Sr 10 alloy, whilst latter is used in a solid state, pulverized.

When Na is applied, the effects of modification and degasification often disappear. Na modification is time limited due to its volatility. Besides, the effect of modification is lost with remelting. All these effects do not appear with Sr application. It is a permanent type of modifier and there is no need for special storage conditions and handling. Its main disadvantage is disappearing of the effect if chlorine after-blowing is performed.

Al-alloys are mostly used as three-component and multi-component AI alloys with Si, Mg, Cu, Zn, Ni, Co etc.

Al-Si alloys. They are widely used owing to their very good mechanical (except elasticity), technological as well as anticorrosive characteristics. Especially in combination with other elements, mechanical properties are greatly improved by modification and heat treatment.

* Mg up to 1.25% improves mechanical and corrosive properties (except plasticity), especially after heat treatment. It creates intermetalic AI3Mg2 and Mg2Si phases.
* Mn, Ni, and Co in smaller quantities have a positive effect on mechanical properties. Ni and Co increase resistance at higher temperatures.

Al-Cu alloys with up to 5% copper are successfully applied in heat-treated conditions and show good mechanical properties. Alloys containing 4.5% copper have for long time been used in aircraft and defense industry, as well as for casting of pistons and engine heads with air-cooling. Properties of these can by further improved by adding 1-3% Si, depending on the casting mode.

AI-Mg alloys have low specific mass, good mechanical properties and high corrosion resistance, especially in seawater. They are therefore broadly applied in ship building industry, as well as in aircraft industry and other fields. They provide nice casting surface and are suitable for die-casting.

Mechanical properties of AI-Mg alloys can be increased by adding approximately 0.3% Zr. Minor addition of Si to these alloys can also have a positive effect on mechanical properties.

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