Simple Solutions That Work! Issue 16

FLEXIBLE MANUFACTURING & ENGINEERING TRENDS 53 Continued on next page Figure 3. impeller lance is that it distributes a wider swath of fine bubbles which is helpful in degassing crucibles of 1,000 lbs and larger. However, in addition to distributing purge gas bubbles, many impeller heads can create a vortex at the melt surface around the rotating lance. A vortex around the degassing shaft can pull air into the melt, creating oxides and increasing the hydrogen content Figure 4. INERT DEGASSING MEDIA (PURGE GAS) The most common purge gases used for degassing molten aluminum are Nitrogen, Argon, and an Argon SF6 mixture. There are advantages and disadvantages of each. • Nitrogen Nitrogen is the most widely used degassing media in aluminum foundries. It is the least expensive of the group and while not quite as effective as Argon, it will get the job done. It may take a little longer to degas with Nitrogen than Argon so the foundry should weigh gas costs vs the cost of additional degassing time. • Argon Many aluminum foundries use Argon for degassing aluminum. It is more expensive than using Nitrogen gas but is slightly more effective at removing hydrogen. Standard degassing times can be shortened when selecting Argon over Nitrogen as the purge gas, however the end result can be achieved with either purge gas. • Argon SF6 Mixture This mixture contains mostly Argon with a small percentage of Sulfur hexafluoride (SF6) added. In addition to degassing, Argon SF6 will help remove impurities in the melt. When using this mixture, a much heavier layer of dross will develop across the melt surface and will need to be removed by skimming the surface. Argon SF6 is a popular choice for foundries when melting high levels of scrap or producing parts which require high purity aluminum. In addition to the added cost of this degassing media, a potential lowering of the Mg concentration in the chemistry of the melt can occur. If casting parts that require strict chemical control, the Mg level should be checked after degassing as adding additional Mg to the melt is sometimes required when degassing with an Argon SF6 mixture. Key Parameters of Rotary Inert Degassing (RID) Once the Aluminum Foundry chooses the degassing equipment and purge gas, development of a robust degassing procedure is critical. Fortunately, the key parameters that must be controlled, Inert Gas Flow, Lance Placement in the Melt, Lance Rotation Speed, and degassing time are easy to developed and controlled. • Inert Gas Flow Prior to inserting the RID lance into the melt, it must be pre-heated to prevent an adverse reaction between the melt and the lance. Once the pre- heat is completed, the inert purge gas must be turned on before inserting the lance into the molten aluminum. If the lance is inserted without purge gas flow, the lance opening will likely plug rendering the shaft inoperable. Figure 5. in the melt. During operation of the Impeller Head Lance, the operator should take care not to rotate the shaft so fast as to create a vortex. Figure #4 displays the working end of an Impeller Head RID Lance.