The degassing device for aluminium can remove impurities in the melt to provide cleaner and better quality metal. It is one of the most commonly used and most effective cleaning methods in foundries.
There are two main impurities in molten aluminum: dissolved hydrogen and solids
Non-metallic inclusions. When the metal cools, the dissolved hydrogen escapes from the solution and forms harmful pores. Such porosity and non-metallic solid inclusions will reduce strength and adversely affect the final properties of aluminum castings.
In aluminum, the reaction with water vapor is as follows:
·2 Al + 3 H2 O = Al2 O3 + 3 H2
Molecular hydrogen then dissociates in the molten metal:
·3 H2 = 6 hours
Molten aluminum also interacts with oxygen in the atmosphere, so in addition to the oxidation reaction shown in equation (a), the following reactions also occur:
·4 Al + 3 O2 = 2 Al2 O3
This reaction results in the formation of scales on the surface of the molten metal during the melting process, and any transfer of the molten metal follows.
The oxides produced are trapped in most of the molten material and then transferred to the finished casting.
Other non-metallic inclusions, such as carbides, nitrides or borides, can come from sources such as crucible materials or other refractory materials.
Therefore, degassing device for aluminium must be used to remove dissolved hydrogen and non-metallic inclusions from molten aluminum before casting to achieve the best quality.
The process developed for cleaning metals is a physical process involving the use of inert gas flux.
The hydrogen dissolved in the molten material diffuses into the rising bubbles of the flux gas and is transported to the surface of the molten material. This process depends on two main steps:
The rate at which hydrogen diffuses into inert bubbles through the diffusion boundary layer
Concentration of hydrogen in inert bubbles
Diffusion is the determining stage of the degassing rate. Therefore, in order to obtain the best degassing efficiency, the following requirements must be met:
The bubbles of the inert gas are smaller and the residence time in the molten metal is longer. This ensures a large surface contact area between the inert bubbles and the molten metal, thereby ensuring a higher mass transfer coefficient relative to the diffusion layer.
The distribution range of inert bubbles on the entire cross section of the molten metal is very wide
The full speed of the molten material accelerates the transfer of hydrogen to the inert bubble
The static surface of the molten pool of molten material to avoid reacting with the atmosphere to absorb fresh hydrogen.