Flux injection/rotary degassing process provides cleaner aluminum.

Two methods for removing oxides and hydrogen can be bridged together to produce a higher rate of defect-free castings.

The demand for lighter, stronger and thinner wall castings is having a great impact on the foundry industry. As foundries adapt to these changing demands, reducing costs and improving quality require constant attention.

A recent report indicated that labor constitutes 46% of the cost in producing a finished cast component in the U.S. Because much of this labor cost is from cleaning, rework and weld repair, any foundry's competitiveness depends on its ability to economically produce good castings at a reduced man-hours per casting rate.

One element of producing a good casting is metal cleanliness. For castings, clean metal is defined as that which is not rejected due to porosity or inclusion defects. The scrap rate of the part being cast, resulting from porosity and inclusion defects, ultimately determines the level of metal cleanliness.

Two independent ways to achieve clean metal - fluxing by injection and rotary degassing - will be discussed in this article.

Two years ago, Wedron Flux experimented with marrying these two technologies into the Rotaflux process. By implementing this process as part of their aluminum treatment programs, foundries may be able to better meet their growing competitive needs.

FLUX INJECTION

Fluxes, for the purpose of this article, are solid halogen-containing salt blends in powdered form. Traditional methods of flux additions include equipment for blowing the flux onto the melt surface and furnace walls and/or charging the flux with the returns or scrap. The most commonly used method, however, is manually shoveling the flux onto the melt surface and walls, followed by manual stirring of the bath. These processes are operator dependent and labor intensive.

In recent years, however, flux injection has grown as a treatment method among aluminum foundries. In comparison with manual fluxing, the process offers: reduced flux consumption, lower inclusion and hydrogen levels, less aluminum content in the dross, reduced emissions, and lower treatment times/costs. It also provides better reliability and repeatability.

Illustrated in Fig. 1, the operation of the flux injection process is as follows. Specially formulated halogen-containing salt flux blends feed into a mixing chamber by way of a calibrated rotary feed table that measures the quantity of flux and ensures a repeatable process. The mixing chamber combines the flux with a carrier gas such as nitrogen or argon. The flux nitrogen mixture is conveyed by a ceramic coated lance and injected beneath the metal surface. The resulting chemical and mechanical reactions ensure that the efficiency of the flux is heady 100%.

The nature of the flux injection process enables an in situ (within the metal handling system) reaction to occur between the flux and molten metal bath, dramatically reducing the amount of flux required for metal treatment. Typical foundry applications show usage rates of 2 lb per 1000 lb of metal. In contrast, addition rates for manual flux applications are at levels of 3-5 lb per 1000 lb of metal.

The in situ reaction of a chemical or a chemical mixture with a molten aluminum alloy has proven more efficient than rabbling or plunging fluxes through the surface of the melt.

Oxide Removal

Oxides are considered to be the most commonly found inclusions in aluminum and its alloys. Aluminum oxides are similar in density to aluminum and don't readily segregate from the melt. Aluminum oxides tend to stay in suspension, while the heavier oxides such as corundum and spinel precipitate to the bottom.

Through flux injection, which introduces the fine flux particles below the metal surface, the flux nitrogen mixture comes into intimate contact with the metal. The halogen gas produced by the flux changes the surface tension of the metal and as the flux nitrogen mixture rises to the surface, it agglomerates the oxides. This process readily isolates the oxide from the unreacted aluminum. The oxide, now at the surface, is easily removed from the melt.

When nonmetallic particles are present, the result is an increase in the molten metal's surface tension. This increase in the surface tension results in a decrease in the fluidity of the metal. Indirectly, poor metal fluidity reveals unclean metal.

Hydrogen Removal

Flux injection is also used to remove dissolved hydrogen from aluminum and its alloys. A typical application for a 1500 lb crucible furnace is a 0.2% addition of flux by weight - with nitrogen as a carrier gas - added over 10 minutes.

A relatively small addition of flux reduces the length of overall degassing, saving time and money. The process of hydrogen removal with flux injection through a lance, due to the large gas bubble produced by the lance, is mass transfer controlled. The small addition of halogen gas released by the flux doubles the mass transfer coefficient for hydrogen. Flux injection also can offer degassing times similar as to when chlorine is used, without chlorine's environmental concerns.

Dross

Without any flux treatment, aluminum dross can have a metallic content of 95%. Manual fluxing will reduce this percentage to 50-70% metallic content. The aluminum content of the dross following the flux injection process is about 35%.

One foundry, after adopting flux injection, estimated $49,600 in savings in aluminum from the reduction in the metallic content of the dross over two years.

Environmental Concerns

Manual fluxing on top of the bath has been found to liberate the small or fine particulate salts into the atmosphere, resulting in high portions of chloride or fluoride compounds found in the flue gas and increasing the level of operator exposure. Because flux injection uses less flux, and introduces flux compounds below the melt surface where a near complete reaction takes place, these concerns are lessened.

ROTARY DEGASSING

Since its inception in the 1960s, the use of a rotating nozzle to introduce nitrogen, argon or reactive type gases such as chlorine for the removal of dissolved hydrogen from molten aluminum has proven to be an efficient and cost-effective treatment.

Rotary degassing operates by transporting a treatment gas through an impeller that rapidly rotates, producing fine bubbles and dispersing them throughout the metal. These small bubbles slowly rise to the surface, permitting a longer time to react. A water model depicting the rotary degassing process is shown in Fig. 2.

A number of configurations exist for rotary degassing: hoist hung units, floor mount degassing stations; units that mount on the external well of a furnace and mobile units that can be moved from station to station. Though the external configuration may vary, the mechanism for shearing the purge gas bubble remains the same. There are a number of impeller head designs from various manufacturers that control the bubble size from about 1/8 in. to 1/4 in. in diameter.

Hydrogen Removal

Through rotary nozzle technology, foundries have seen a more rapid degassing rate, consistently lower hydrogen levels and lower gas consumption than the systems it was designed to replace: pills, lances and porous plugs. The graphite shaft and impeller head are submerged in the molten metal bath and shear the gas bubbles into small, well-dispersed bubbles.

This reduction in size of the purging gas bubble dramatically increases the available purge gas surface area and the residence time. The stirring action created by the impeller head results in greater distribution of the purge gas. The process enables the purge gas to come into better contact with the hydrogen in the melt and facilitate its removal.

Oxide Removal

While improvements in the rate of hydrogen removal are clear, it is more difficult to define the rate of oxide removal. Studies have shown that the small purge gas bubbles of nitrogen or argon from the rotary degassing process can float the oxides to the surface.

The rate of oxide removal can be improved with a small percentage of chlorine present in the purge gas. The surface tension of the aluminum is changed by the chlorine, facilitating easy attachment of the oxides to the purging gas bubbles.

Dross

Rotary degassing creates additional dross and adds to existing dross on the surface. This is in part due to the flotation of inclusions and nonmetallic particles that may be present within the melt. Nitrogen has been shown to create a very wet dross, argon somewhat drier, with the driest dross created by halogen containing purge gases.

In this context, wet dross is synonymous with a high content of metallic aluminum. In one foundry application, the dross removed from a transfer ladle after treatment with a rotary degassing unit using nitrogen as the purge gas weighed on average 13 lb. After an additional treatment in the ladle with a drossing flux, the dross weighed an average of 5 lb.

COMBINED PROCESSES

The principal advantages of integrating flux injection/rotary degassing (FI/RD) technology to create the Rotaflux process was to complete an in situ dross treatment, facilitate the removal of oxides from the melt without the use of chlorine and maintain the same degassing cycle times as rotary degassing. The added benefit includes the ability to retrofit existing flux injection equipment and rotary degassing units without making either system obsolete.

Introduction of the purge gas is now done through the flux injection unit. The flux injection unit controls the flow of purge gas mixed with the halogen-bearing salt flux to the impeller shaft and head as well as the total cycle times. Figure 3 illustrates a hoist-hung FI/RD unit.

The most common obstacle when coupling the two units together centers around the rotary impeller head. The rotary impeller degassing head, in most cases, is designed to disperse the purge gas at volumes averaging 0.5 cu. ft per minute. This volume of gas isn't sufficient to maintain the solid flux particles in suspension, resulting in the buildup in the transfer line and a choke situation. To eliminate this situation, the volume of the purge gas for this combination process must be increased to about 0.75 to 1 cu. ft per minute, depending on the flux feed rate.

The added purge gas volume, combined with the halogen gases produced by the flux, quickly overwhelms the rotary degassing impeller head and decreases its efficiency. Thus, the rotary impeller head is operating in an envelope of purge gases, resulting in a larger diameter purge bubble and little contact with the molten metal.

For the combined process, a more aggressively designed graphite impeller head is needed (inset photo, [ILLUSTRATION FOR FIGURE E OMITTED]). Such a head design incorporates fewer but larger shearing tips.

Mechanical Properties

Evaluating the effects of the combined processes on mechanical properties and oxide removal involves three measurement techniques: percentage elongation, tensile strength and the K-mold notch bar test. The percentage of elongation and tensile strength of as-cast samples are methods of indirectly measuring the metal cleanliness. Improvement in the mechanical properties should correlate to improved metal quality, including the improvement of the metal's density.

Figure 4 compares the percentage of elongation and the tensile strength of the as-cast samples. The inert rotary degassing treatment used argon as the purge gas. The FI/RD treatment used a total flux addition equal to 0.75 lb or 0.05% addition by metal weight using an argon carrier. At equal treatment times, the combined process. resulted in a greater percentage of elongation and an increase in tensile strength over the metal treated with rotary degassing only.

Though the mechanical properties showed the greatest improvement when treated with FI/RD, these two tests don't isolate the effect of oxide removal within the metal. However, these tests do allow for the evaluation of the overall improvement in the mechanical properties that are affected by the combined process.

In addition, a K-mold notch bar test (which effectively evaluates the incidence of large inclusions that can be observed with the naked eye) revealed no inclusions in 32 fractures after treatment with the FI/RD process. Observation of before-treatment samples showed incidences ranging from one to three inclusions.

Hydrogen Removal

Specific gravity and the reduced pressure test both indirectly measure the amount of hydrogen present.

Reduced pressure test samples that have been cut are shown in Fig. 5. These cut samples compare the results from rotary degassing (at right) versus the FI/RD process for a total flux addition of 75 lb or 0.05%, with an argon carrier in a 1500 lb gas fired crucible holding furnace.

In the majority of foundries that operated previously with rotary degassing or flux injection, overall degassing times decreased an average of 30% with higher specific gravity results after converting to a FI/RD process.

Dross

If the dross is left untreated in rotary degassing alone, it can contain a substantial amount of aluminum. By introducing a small amount of flux with the purge gas through a rotary degassing unit offered by the combined process, metal loss can be reduced dramatically. The kinetic interaction imparted by the impeller head results in an in situ reaction, producing a dross that is low in metallic content.

Observations of one application showed that the dross from a ladle treated with a rotary degassing unit weighed 15 lb and was analyzed at 80-90% metallic content. In contrast, the same ladle, when treated with the combined process, delivered a dross containing less than 20% metallic content and weighing 3 lb. Dross metallic contents for flux injection alone are about 35%.

Cleaner metal with reduced degassing times, improved metal melt losses and reduced flux consumption can now be achieved simultaneously with the combined system. The FI/RD process combines the best attributes of flux injection and rotary degassing into a single treatment system.

Foundries can benefit from reduced treatment times, lower achievable gas levels, fewer incidents of inclusion defects, reduced overall treatment costs and ultimately, a greater percentage of good castings.