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Aluminum Degassing: Methods & Measurements


To produce quality aluminum castings, metalcasters must remove excessive hydrogen in the molten metal, while conducting tests to be sure they did. A MODERN CASTING STAFF REPORT


air or wet tools, the water decom- poses to release hydrogen in the melt. Excessive quantities of this dis- solved 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 accom- plish two things: 1. Prevent and minimize intro-


M


duction of hydrogen in the melt. 2. Measure and remove the


hydrogen prior to pouring. Tis article will focus on mea- surement and removal.


Gas Removal Te ability to degas molten aluminum is generally accomplished by using a purge gas, typically introduced into the melt by a rotary degassing unit. Tis degassing pro- cess is limited by thermodynamic laws; when purge gas bubbles are


30 | MODERN CASTING August 2015


olten aluminum is extremely reactive, so when it comes in contact with moist


introduced to the melt, they collect hydrogen as they float toward the surface. Te best possible situation is these hydrogen-saturated bubbles leave the melt and reduce hydro- gen levels. In this case, the process efficiency is 100% from the thermo- dynamic point of view. But as the gas content in the melt drops, so does the equilibrium pressure of hydro- gen in the bubbles, so the amount of purge gas required to remove the remaining hydrogen must increase.


Te equilibrium gas removal ratio


is shown in Fig. 1 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. Tis behavior limits a metalcaster’s ability to degas to a very low level of hydro- gen. Te 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. Te effect of alloying elements is characterized by changes in the alloy correction fac- tor, 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 cast- ings can be achieved without undue difficulty in most cases.


Practical Degassing Procedures


Fig. 1. Gas removal rates increase as the hydro- gen content nears 0.1 cc/100g.


Degassing is usually accom- plished in one of three areas of the


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