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Experiences in a New Degassing Concept


A new model can help identify ways to reduce cycle time and the amount of


consumables used for more effective degassing. BRIAN BEGAN, FOSECO (CLEVELAND); RONNY SIMON, FOSECO (BORKEN, GERMANY)


R


otary impeller degassing, or rotary degassing for short, is a well-established


practice for removing hydrogen from molten aluminum alloys. Te principle behind rotary degassing is to dis- seminate inert or active gasses within the aluminum melt. Te dissolved hydrogen seeks to equilibrate with the purge gas bubbles, which rise to the melt surface, carrying hydrogen out of the melt and into the atmosphere. Te principle is effective and time trusted. However, the effectiveness of the degassing process is greatly influ- enced by variables including ambient conditions, rotary degasser param- eters, and melt properties. Hence, a specific degassing process may work brilliantly one day, but fail to deliver the required quality the next day even if the degassing parameters are strictly maintained, due to a change in ambient conditions or minor fluctua- tions in the alloy composition (such as adding a little more Mg or Sr). Often degassing procedures are designed to negate these fluctuations by adding degassing time, increasing purge flow rates or the eliminating of potentially helpful alloy additions. Issues with hydrogen control when degassing become exaggerated when a process requires an intermediate level of hydrogen. Tis is common when controlled amounts of hydro- gen within a melt can help offset the problematic volumetric contraction


36 | MODERN CASTING February 2018


observed when pouring the same casting with a hydrogen-depleted melt. In applications where discernible amounts of hydrogen are desired, “best in class” aluminum foundries typically complete a full “degassing” cycle and then try to add “controlled” amounts of hydrogen via mixed gas additions. However, the ability for “controlled” amounts of hydrogen to go into solu- tion is greatly influenced by the same variables influencing the removal of hydrogen via rotary degassing. Te production of aluminum cast- ings globally continues to increase


rapidly. A significant portion of these production increases can be attributed to the automotive industry where lightweighting vehicles to lower emis- sions and improving fuel efficiency are paramount to meeting consumer and regulatory requirements. Pressed with these objectives, automotive design- ers hope to achieve targeted weight savings by replacing heavy structural steel components with lighter weight aluminum castings. However, to successfully meet the requirements of these new applications, the aluminum castings have to achieve mechani-


Fig. 1. A screen shot of degassing model is shown.


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