Understanding aluminum degassing.
This article discusses hydrogen absorption during aluminum melting and casting and assesses methods to evaluate and remove hydrogen.
Role of Solidification
The solubility of hydrogen in molten aluminum increases with temperature (Fig. 1). A degree of 'alloy specificity' also exists--at a given molten metal temperature, less hydrogen is soluble in 319 alloy than in 356 alloy, for example. However, both exhibit substantially reduced solubility in the solid state.
Solidifying metal must reject the hydrogen or resultant castings will suffer from porosity. Here, solidification plays a role. For a given hydrogen concentration in the liquid state, the faster the solidification rate, the fewer problems with resultant porosity. Because not all sections of the casting solidify at the same rate, ample opportunity exists for hydrogen buildup in the remaining liquid metal during solidification. If the hydrogen partial pressure in this remaining liquid exceeds a critical level, a gas molecule forms from the association of hydrogen atoms, creating porosity.
Shrinkage Porosity vs. Gas Porosity
Not all porosity can be blamed on hydrogen. Figure 2 portrays a classical distinction between shrinkage porosity and gas porosity. Shrinkage porosity occurs as the metal solidifies; a 6% volume change occurs when an aluminum alloy transforms from a liquid to a solid. This change is accommodated in normal casting practice through the use of risers. Shrinkage porosity indicates a lack of feed metal reaching the place where it is needed--the last portion of the casting to solidify. Consequently, the resultant porosity follows the solidification path and the dendrite boundaries, and shrinkage porosity displays a ragged, irregular edge.
By contrast, gas porosity shows a more distinctive regular contour, often round. Casting porosity is often a combination of the two. Porosity also may be due to a reaction from molding materials and coatings or simply air entrapment due to inadequate venting.
Identifying Hydrogen Presence
Hydrogen enters molten aluminum easily. Major sources include absorption from the atmosphere, products of fossil fuel decomposition (hydrocarbons), adherent condensation on dirty tools, chemical flux materials and alloy additives. Hydrogen absorption also increases with higher ambient relative humidity.
Assessing the level of hydrogen in the melt can be done in several ways. Hydrogen determination methods fall into two categories--real time and laboratory. In real time, the main method employed is closed-loop recirculation. In this technique, a probe introduces a small amount of carrier gas into contact with the molten aluminum. Hydrogen in the melt diffuses into the carrier gas and is allowed to equilibrate.
The resultant increased partial pressure is related to the concentration of hydrogen in the melt. Through computerized data analysis, results are reported in actual hydrogen content expressed as ml [H.sub.2]/100g Al.
Laboratory techniques are more time-consuming and use solid samples from a chill mold. The sample is machined and subjected to a vacuum sub-fusion, nitrogen carrier fusion (LECO) or vacuum fusion analysis in the laboratory.
Actual hydrogen concentration is calculated from hydrogen partial pressure and thermal conductivity measured by these methods. Such laboratory methods are useful in process control studies but cannot be utilized as decision points in real time.
One other measure is available to foundry personnel on the shop floor-- the Reduced Pressure Test (RPT) (Fig. 3). RPT can be conducted in 5 min and makes two types of determinations. The density of the sample can be determined using Archimedes principle and then compared to the theoretical density (a specific property of a given alloy composition). The sample also can be sectioned and compared with a standard chart of specimens generated from a range of test conditions/results to give a visual comparison rating.
Decisions then can be made to further treat the melt or to proceed with casting based on the quality level desired. However, the RPT result does not give a quantitative hydrogen concentration result--it is a semi-quantitative test that reveals the overall metal cleanliness of the melt sample. Inclusions present will nucleate easier pore formation during the reduction in pressure.
Several methods exist to reduce hydrogen. As seen in Fig. 1, allowing the temperature to drop permits for natural outgassing--the first and simplest method to remove hydrogen.
Historically, a number of other techniques that rely on an inert, insoluble purge or collector gas have been employed. The inert gas collects the soluble hydrogen atoms, allowing a hydrogen molecule to form inside the lower pressure of the collector gas bubble.
Tablet products were among the first techniques to have been widely utilized (especially in smaller melt furnaces) to degas the melt. The tablets usually are based on the decomposition of hexachloroethane ([C.sub.2][Cl.sub.6]).
In the melt, the tablet forms aluminum chloride, an insoluble metastable gaseous phase. The aluminum chloride gas bubble serves as a collector and allows the hydrogen to be absorbed onto the bubble surface and into the bubble itself. The bubble then rises to the surface, delivering the hydrogen to the atmosphere.
Static lances, wands or flux tubes also have been used to degas the melt.
The difficulty with these methods is that the resulting gas bubble is large with a low surface area-to-volume ratio. Consequently, the gas bubbles rise rapidly to the melt surface with minimal reaction time to collect the hydrogen. Further, unless the lance is moved vigorously around the melt furnace--which is often difficult--little coverage or mixing of the entire furnace volume results, reducing the efficiency of this degassing process. Porous refractories also have been used to insert inert gas into aluminum melts, and while the bubble size is small, very little mixing of the gas bubbles and metal takes place.
Rotary Impeller Degassing
Rotary impeller degassing, a technique borrowed from the chemical process industry that improves mixing capability, was introduced into aluminum foundries in the mid-80s. In this technique, purge gas is introduced to the melt through a rotating shaft and impeller, or rotor (Fig. 4). This provides increased kinetic mixing of the melt with the purge gas. In addition, the action of the rotor creates bubble shear, giving rise to a broader swarm of smaller bubbles over a wider area, which increases surface-area-to-volume ratio. These finer bubbles have a longer residence time in the metal, allowing for a higher capability of collecting the hydrogen atoms present.
Figure 5 compares the relative degassing efficiency of the three techniques discussed--lance, porous plug refractory and rotating shaft/rotor.
For the operating foundry person, several important variables must be considered in developing a suitable degassing process with a rotor. The parameters that must be integrated include:
* initial hydrogen level versus desired final hydrogen level (as determined by evaluation);
* available time for melt treatment;
* vessel size/volume;
* the relationship between rotor configuration and rpm, gas volume, surface effects (vortexing, splash, etc.) and the time necessary and available to achieve desired degassing results.
The interplay among these variables must be determined on a case-by-case basis by the individual foundry to achieve the optimum combination of process and equipment parameters. In general, the optimum result achieves the necessary specifications in as short a time as possible, at the lowest cost and without excessive turbulence.
Adding Flux Injection
Another option for the rotary impeller concept is the introduction of flux injection (Fig. 6). Combining both flux injection and rotor dispersion creates a one plus one equals two-plus benefit.
Successful flux injection requires a properly constituted flux with a morphology that is granular and flows, and that melts only when it finally enters the melt. A carrier gas delivers the flux, and it is largely the carrier gas itself (usually nitrogen) that does the degassing. The rotary degasser provides the kinetic mixing between the flux, carrier gas and the metal to create full-vessel reaction.
The flux itself serves two key functions. Properly constituted, the flux can assist not only with hydrogen reduction, but also with partial removal of inclusions from the melt by virtue of the flotation action of the collector gas that is the flux carrier. Proper flux chemistry affects de-wetting for easier separation of solid inclusions from the melt through surface energy effects. Flux application by submerged injection also has the following advantages:
* a more controlled flux consumption/utilization
* less spillage and waste;
* better environmental compliance.
With or without flux injection, any degassing process creates dross. This dross is metal-rich, containing up to 85% aluminum that can be recovered. An appropriate flux composition can treat this dross in its original place, not only reducing the dross volume but also substantially decreasing the metallic content of the dross from 85% to 30% or less.
With any degassing process chosen, the specific purge gas used also affects the resulting dross.
Choosing a Purge Gas
There are discernible differences in the kind of purge or collector gas utilized in a degassing process.
Nitrogen--Nitrogen gas is the most commonly employed and is the least expensive. Nitrogen also creates a 'wet' dross, one that is rich in metallic aluminum.
Argon--Argon, while significantly more expensive than nitrogen, produces a less metal-rich dross. Argon is more inert, and being heavier, it provides a protective cover over the melt during degassing, precluding further oxidation and hydrogen absorption. It also is easier to keep the argon supply drier and cleaner as fittings and hoses deteriorate.
Active halogens--Active halogens also have been used to assist the inert (nitrogen, argon) gas in achieving greater degassing efficiency. In the past, these have included freon ([CCl.sub.2][F.sub.2]), chlorine and sulfur hexafluoride ([SF.sub.6]). However, with a small purge gas bubble--as produced by an efficiently operated rotary degasser--the effect of an active halogen on hydrogen reduction may be minimal.
While selecting a gas to use is important, it is not the primary factor in degassing success. In addition to obvious environmental concerns with halogen gases, the specific degassing process (lance, rotor, etc.) affects the degassing performance more than which gas is used.
For a free copy of this article circle No. 344 on the Reader Action Card.
For More In formation
"Measurement and Removal of Hydrogen in Aluminum Alloys," MM. Makhlouf, L. Wang and D. Apelian, AFS Special Report (1998).
"The Treatment of Liquid Aluminum-Silicon Alloys," J.E. Gruzleski and B.M. Closset (1990). Available from the AFS Library at 800/537-4237.
"RPT Gauges Aluminum Porosity," W. Rasmussen and C. E. Eckart, MODERN CASTING, March 1992.
About the Author
David Neff holds a Ph.D. in Metaullurgy from Case Western Reserve Univ. and has worked in the nonferrous metals field for 32 yr. He has been at Metaullics Systems for 18 yr and is currently the Manager, Molten Metal Treatment.