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, чем в твердом: 0,65 and 0,034 ml / 100 g, respectively. 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, он (водород) может выделиться в молекулярной форме, resulting in the formation of primary or secondary pore.
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.
The hydrogen in aluminum castings
Location hydrogen in hardened aluminum depends on the level of its content in the liquid aluminum and conditions, at which solidification. Since the presence of hydrogen porosity is the result of nucleation and growth mechanisms, which are controlled by diffusion, the reduction of the hydrogen concentration and an increase in solidification velocity are overwhelmingly on the nucleation and growth of pores. For this reason, casting, made in by injection into the split metal mold, more prone to defects, associated with hydrogen, than casting, who produced, for example injection molding.
Sources of hydrogen in aluminum
Hydrogen enters the aluminum from many sources, including the furnace atmosphere, charge materials, a flux, Melting tools and the reaction between molten aluminum and the mold.
furnace atmosphere. If the melting furnace is fueled with 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.
a flux. 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.
The reaction of aluminum with hydrogen
It is believed, that aluminum, Like most metals do not react directly with hydrogen. Typically metals form compounds, by the loss of electrons, which are accepted by other elements. Hydrogen also forms compounds, теряя электроны (или делясь электронами). Therefore, usually hydrogen atoms not accept electrons, that render metals to form compounds. Only a few very reactive metals, such as sodium, potassium, Calcium and magnesium can "forcing" of hydrogen atoms take their electrons to form a solid ionic compounds, which are called hydrides of these metals.
For direct synthesis from hydrogen aluminum hydride and aluminum requires a pressure of about crazy 2000000000 atmospheres and temperatures above 800 TO. Meanwhile, such as aluminum hydride compound exists. aluminum hydride – This unstable compound, which is easily decomposed at a temperature above 100 ° C. It is prepared by an indirect route, as a result of reactions of other compounds.
Источник: Aluminum and Aluminum Alloys, ASM International, 1993.