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Hydrogen: 1/3 of the porosity battle.

A quarter of a century after Elwin Rooy presented his award-winning paper "Control of Aluminum Casting Quality by Vacuum Solidification" with co-author Edwin Fischer, he stood once again and reapplied it to today's aluminum foundry.

The original paper noted that porosity stems from the interactions between dissolved hydrogen, the presence of heterogenous nucleation sites (oxides) and solidification conditions. Reiterating his findings 25 years earlier, Rooy, now a consultant, reported, "Hydrogen in solution before casting is only one-third of the solution to porosity problems."

He said foundrymen want a real-time test to determine melt acceptability. This entails measurements of temperature, chemistry, oxide/inclusion content and dissolved hydrogen concentration.

In addition to the suggestions he and Fischer made for using the vacuum solidification test in 1968, Rooy offered two new guidelines for the test. First, evolution of gas from the sample at the point of pressure reduction indicates contamination by non-metallics that heterogeneously nucleate precipitation of an unknown amount of dissolved hydrogen. The number of locations at the sample surface where bubbles emerge crudely approximates the relative concentration of oxides present.

Second, bubbles during the final stages of cooling and solidification indicate a clean melt with an appreciable, yet unknown, amount of dissolved hydrogen.

"This testing method provides direction in choosing the most effective form of additional melt processing to correct an unacceptable condition," he said.

Noting the developments in the last 25 years, Rooy said the work he and Fischer presented in 1968 remains valid.

He noted that dissolved hydrogen in aluminum can be accurately and quantitatively measured, but measurement alone is of little value to foundries. No practical quantitative control test exists for measuring entrained nonmetallics in aluminum.

The interactive effects of dissolved hydrogen and oxide contamination may be assessed by the vacuum solidification test, he said. The density of samples solidified under vacuum can be used to control melt quality, improve casting quality, reduce scrap and simplify melt preparation.

"Instead of focusing on melt quality," concluded Rooy, "foundries should direct their energy to the control of specific process features that assure consistent aluminum melt quality."
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Title Annotation:CasteExpo '93: 97th AFS Casting Congress, Chicago; aluminum casting
Publication:Modern Casting
Date:Jun 1, 1993
Previous Article:A talk about change.
Next Article:Practical working information marks technical sessions.

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