Simple Solutions That Work! Issue 13

I t is frustrating to see so many aluminum foundries struggle with hydrogen (gas) porosity in their castings, when it is one of the easiest defect to prevent. BRAD HOHENSTEIN President - Porosity Solutions Course Instructor - The Foundry Way Learning Center ARTICLE TAKEAWAYS: • Eliminate gas porosity with sound degassing techniques • Use RPT testing to develop and control the degassing process • Avoid the common degassing mistakes STOP MAKING EXCUSES. PREVENTING GAS POROSITY IN YOUR CASTINGS IS SIMPLE! Elimination of hydrogen porosity in aluminum castings is simple. All it takes is proper foundry degassing techniques and proper implementation of degassing controls. Unfortunately, many foundry floor personnel lack the basic understanding of hydrogen porosity and the process controls required to prevent it. The goal of this article is to impart this knowledge in a simple, down-to-earth manner which can be used to develop a foundry degassing procedure for eliminating gas porosity as a casting defect in your foundry. HYDROGEN IN THE MELT There is a continuous reaction between air and molten aluminum. The oxygen in the air reacts with the molten aluminum to form aluminum oxides which manifests as the surface skin on the melt. This surface skin or oxide layer is also called dross and typically skimmed from the surface prior to pouring. The hydrogen in the air reacts with the molten aluminum to separate from the oxygen and become dispersed throughout the melt. Key points to understand concerning the reaction between molten aluminum and air are: • For all practical purposes, it is impossible to stop the oxygen and hydrogen from combining with the molten aluminum. Whenever molten aluminum contacts air, there will be oxides and hydrogen in the melt. • Higher humidity results in higher concentrations of hydrogen throughout the melt. This means that effective degassing may take longer on high humidity days as there is more hydrogen to remove from the molten aluminum. • The hotter the molten aluminum, the faster hydrogen is absorbed into the melt. For 300 series alloys such as 356, it is best, if possible, to melt and pour at temperatures under 1400° F (760° C) as the solubility of hydrogen in the molten aluminum increases exponentially above 1400 degrees. The effect of a high melt temperature could be that a degassing time which is normally 15 minutes at 1350° F could become 45 minutes at 1450° F. 36