Improving aluminum melt quality.
In aluminum casting facilities, the presence of gas in the melt must be properly measured and controlled. In a 2005 study, Geoffrey Sigworth, Foseco, revealed improved melt treatment practices in U.S. metalcasting facilities had a considerable impact on aluminum tensile properties. Filtered and degassed castings showed a significant improvement over untreated castings. The use of a rotary impeller degasser for a longer degassing treatment further improved tensile properties (Fig. 1).
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"The [test bars in the study] were of the same alloy, mold and heat treatment," Sigworth said. "The only thing different is melt treatment. An eight-10-minute treatment can remove 80% of the porosity."
Sigworth recommends aluminum metalcasters invest in a good rotary impeller head that will produce small bubbles. Square, blocky heads do not work as well, he said.
In addition to degassing, flux or chlorine treatments can be used to reduce gas in your aluminum melt. When the bubbles from a small amount of purge gas come into contact with the metal, it forms a liquid salt. If the bubble bumps into an oxide, the oxide will stick to a salt layer and be removed with the bubble. Sigworth said some systems are designed for injection but perform degassing well, so it's important for metalcasting facilities to make sure they have the best equipment for both functions.
Metalcasters can control their melt treatment and quality with gas testing, according to Dan Groteke, Q.C. Designs, St. Joseph, Mich. "The higher the service of the application, the higher the risk and more testing that is required," Groteke said.
Along with chemistry analysis reports, stress and strength tests, reduced pressure testing can be used as a means of meta] control.
"Most operations run reduced pressure testing, but not as a control," Groteke said. "Foundries use it as a visual test, which is subject to error, and then don't do much with the information."
Metalcasting facilities can use reduced pressure testing to hold density, which is a reflection of gas content and metal quality, within a certain range, Groteke said.
Proper furnace maintenance also is critical to efficient melting.
"Good furnace practices, such as maintenance, repair and probably something in the way of dross recovery are important," Groteke said.
According to Sigworth, casting facilities should adhere to a regular furnace cleaning schedule that is set according to type of furnace, volume of metal and alloy.
"Aluminum is very reactive, and the oxides in aluminum are heavier and denser than the metal, especially in magnesium-containing alloys," Sigworth said. "If you don't do a good job cleaning, it will collect in the bottom and end up in your castings."
Furnaces with hydraulic lifts for tilting allow for easy cleaning, Sigworth said. "Those furnaces are more expensive, but worth the money, in my opinion."