Vodorod Aluminum
The solubility of hydrogen in aluminum
Hydrogen gas is the only, which significantly dissolves in aluminum and its alloys. Its solubility is proportional temperature value and the square root of pressure. As it shown on the picture, the solubility of hydrogen in liquid aluminum significantly higher, than in solid 0,65 and 0,034 ml / 100 g, respectively (Fig.. 1). These values slightly vary depending on the chemical composition of the alloys. Upon cooling and solidification of the molten aluminum with the hydrogen content considerably higher, than its solubility in the solid state, it (hydrogen) can be released in molecular form, which will lead to the formation of primary or secondary pores [1, 2, 3].
Fig. 1 – Solubility of hydrogen in 99.9985% pure aluminum at 1 atm hydrogen pressure [3]
Reaction of aluminum with water vapor
Aluminum reacts with water vapor at high temperature This hydrogen gas is the main source of hydrogen in aluminum.
3H2O(g) + 2A1 = Al2O3 + 3H (g) (1)
On pic. 2 a model of hydrogen capture from water vapor over aluminum melt is presented [3].
Fig. 2 – The mechanisms for hydrogen dissolution into the molten aluminum from moisture in the atmosphere [3]
Sources of hydrogen in aluminum
Hydrogen enters the aluminum from many sources, including the furnace atmosphere, charge materials, a flux, melting tools and reactions between molten aluminum and mold [1].
furnace atmosphere. Moist air enters the furnace atmosphere – water vapor reacts with aluminum. Moreover, if the melting furnace is powered by natural gas or, let us say, heating oil, the possible incomplete combustion with the formation of free hydrogen.
charge materials. ingots, foundry scrap and return can comprise oxides, corrosion products, foundry sand and other contours, and grease, which are used for machining. All of these pollutants are potential sources of hydrogen, which is formed during reduction of organic substances or chemical decomposition of water vapor.
Fluxes. Most of flux - it's salts and all salts are hygroscopic,, ie ready "with pleasure" to absorb water. Therefore, the wet flux is inevitably introduced into the melt, hydrogen, which is formed by chemical decomposition of water.
melting tools. melting tools, such as peaks, scrapers and spades can also be a hydrogen source, if you do not keep them clean. Oxides and flux residues on these instruments are particularly "tricky" sources of pollution, as they absorb moisture directly from the air. furnace refractories, gutters and distribution channels, lime and cement slurries, Buckets for sampling - are all potential sources of hydrogen, especially if they are not dried.
The interaction between the molten aluminum and the mold. If in the process of filling the mold the molten metal is flowing turbulently overly, it may trap air in the internal volume. If the air is not able to or do not have time to get out of there before the start of solidification, the hydrogen will hit the metal. The reason of air entrapment can also correctly executed feeders mold. Another source of hydrogen is excessively moist sand molds.
Hydrogen aluminum porosity
The formation of bubbles of hydrogen in aluminum is highly dependent on the cooling and solidification rate, as well as the presence of nucleation centers for the hydrogen evolution, such as oxides entrained into the melt. Therefore, for the formation of porosity significant excess of dissolved hydrogen is required as compared with the hydrogen solubility in solid aluminum. In the absence of nucleation sites for the release of hydrogen requires relatively high concentrations - about 0,30 ml / 100 g. In many commercial alloys do not exhibit porosity and at such a relatively high hydrogen content, as 0,15 ml / 100 g.
Hydrogen porosity adversely affects the mechanical properties of the material, depending on the type and chemical composition of the aluminum alloy. In Fig. 3 shows the relationship between the actual hydrogen content and the observed porosity. In figures 4 and 5 shows the effect of hydrogen porosity on the tensile strength of aluminum and some cast alloys [1].
Fig. 3 – Porosity as a function of hydrogen content in sand-cast aluminum and aluminum alloy bars [1]
Fig. 4 – Ultimate tensile strength versus hydrogen porosity for sand-cast bars of three aluminum alloys [1]
Fig. 5 – Influence of gas content on the tensile and yield strengths of aluminum alloy 356 [1]
Hydrogen removal (degassing)
Most often, melt degassing is performed by the following methods [1]:
- Lance gas purging
- Hexachloroethane degassing
- Rotary degassing
- Porous plug degassing
Gas purge
The simplest method for degassing molten aluminum is to apply a pressurized purge gas or gas mixture through a suitable tube (lance) made of ceramic-coated cast iron or graphite.. Hydrogen diffuses into the purge gas bubble, which rises to the surface of the melt and is released into the atmosphere (Fig.. 6). Purge gas can be either inert (argon or nitrogen), as well as reactive, for example, chlorine. Reactive gases are used in small concentrations up to 10% together with inert gas. Chlorine reacts with molten ammonia to form gaseous AlCl3, which then serves as a purge gas [1].
Degassing with hexachloroethane
Maybe, The most common degassing method in foundries is the use of roethane hexahlounder (C2Cl6). The tablet decomposes in the aluminum melt with the formation of gaseous AlCl3. Rising bubbles of AlCl3 gas then collect hydrogen gas and deliver the gas to the surface of the melt to release.
Rotary degassing
The principle of operation of the rotary injection system (Fig.. 7) is, that the gas is injected into the shaft of the rotating element and released through small holes in the rotor. When rotating at a speed of 300 to 500 rpm, the formed gas bubbles are broken, forming scattered and very small bubbles for degassing. The high surface area to volume ratio of the degassing bubbles provides a significant increase in contact area and, Consequently, increased reaction kinetics, resulting in more efficient degassing.
Hydrogen removal rate depends on gas flow rate and initial hydrogen content, as well as from the consumption of metal, but in most commercial systems, the final hydrogen content can be significantly lower 0,15 ml/100 g [1].
Degassing through a porous plug
Another method of degassing by creating fine bubbles is to use a porous dispersant at the end of the tube., supplying purge gas (Fig.. 8). Porous plugs are graphite or ceramic materials with very fine interconnected porosity., through which gas can pass. These plugs can be installed in bucket bottoms, ovens or auxiliary processing tanks. The fine porosity of the cork material allows the creation of small bubbles, comparable to the dimensions of rotary degassers.
Fig. 6 – Schematic of phenomena taking place during gas purging with argon containing chlorine gas [4]
Fig. 7 – Schematic of an in-line rotary degassing unit [1]
Fig. 8 – Porous plug degassing [1]
On pic. 9 comparison of degassing results, achieved using identical gas flows for porous plugs, lances and rotary degassing systems. Porous cork is more efficient, than a lance, and satisfactory degassing results can be obtained in many cases by adjusting the size of the vessel, gas flow rate and processing time.
Fig. 9 – Comparison of lance, porous pIug, and rotary degassing efficiencies for identical gasflows [1]
1. Aluminum and Aluminum Alloys, ASM International, 1993
2. Aluminum Alloy Castings Properties, Processes, and Applications /J. Gilbert Kaufman, Elwin L. Rooy – ASM International – 2004
3. Molten Metal Processing /Ryotatsu Otsuka //Fundamentals of aluminium metallurgy – Ed. R. Lumley – 2011
4. Ingot casting and casthouse metallurgy of aluminium and its alloys / J .F. Granfield //Fundamentals of aluminium metallurgy – Ed. R. Lumley – 2011
