18 2月 Degassing Process In Aluminium
Degassing Process In Aluminium
Molten aluminum is extremely reactive, so when it comes in contact with moist air or wet tools, the water decomposes to release hydrogen in the melt. Excessive quantities of this dissolved gas have a well-documented detrimental effect on the mechanical properties of the final aluminum castings. What is also well known to anyone making castings is that dissolved gas has an overriding effect on the distribution and amount of porosity and shrinkage. Dissolved hydrogen levels must be controlled to minimize scrap. To control gas in aluminum, metalcasters must accomplish two things:
1. Prevent and minimize introduction of hydrogen in the melt.
2. Measure and remove the hydrogen prior to pouring.
This article will focus on measurement and removal.
The ability to degas molten aluminum is generally accomplished by using a purge gas, typically introduced into the melt by a rotary degassing unit. This degassing process is limited by thermodynamic laws; when purge gas bubbles are introduced to the melt, they collect hydrogen as they float toward the surface. The best possible situation is these hydrogen-saturated bubbles leave the melt and reduce hydrogen levels. In this case, the process efficiency is 100% from the thermodynamic point of view. But as the gas content in the melt drops, so does the equilibrium pressure of hydrogen in the bubbles, so the amount of purge gas required to remove the remaining hydrogen must increase.
The equilibrium gas removal ratio for pure aluminum above 1,400F (760C). A gas removal ratio of 200, for example, means it will take 200 liters of inert gas to remove one liter of hydrogen. This behavior limits a metalcaster’s ability to degas to a very low level of hydrogen. The solubility also increases exponentially with temperature, meaning an increase of 200F (111C) doubles the solubility. All things being equal, a higher temperature of an aluminum melt will increase the necessary degassing time.
Alloying elements also can have an effect on hydrogen solubility. The effect of alloying elements is characterized by changes in the alloy correction factor, with some common casting alloys show in Table 1. Alloys having greater values are more difficult to degas, so, for example, aluminum 535 will take four times longer to degas than pure aluminum. Fortunately, these factors can be controlled and the gas content and process required to eliminate excessive porosity in aluminum castings can be achieved without undue difficulty in most cases.
Degassing Process In Aluminium
ONLINE DEGASSING UNIT is usually accomplished in one of three areas of the metalcasting facility:
1. In the transfer ladle, used to convey metal between melting and holding furnaces.
2. In crucible furnaces, usually just before the molten aluminum is cast.
3. In an in-line system, when the metal is conveyed to holding furnaces through a launder.
The first two options are most common and the degassing operation for both is typically accomplished using a rotary impeller degasser (RID). In practical terms, all rotary degassers are not created equal. It is important to have an optimum head design to produce highly efficient, small bubbles. Significant cost savings may be realized from shorter treatment times and reduced gas usage. In the past, the metalcasting industry has gravitated toward simple head designs, which are less costly to machine but produce larger bubbles. This path presents a false economy due to reduced efficiency.