Simple Solutions That Work! Issue 16

FLEXIBLE MANUFACTURING & ENGINEERING TRENDS 51 BRAD HOHENSTEIN President Porosity Solutions ARTICLE TAKEAWAYS: • Why degassing is a required process • Differences in degassing media • Types of degassing lance’s • Key degassing parameters Continued on next page Understanding Degassing of the Aluminum Melt T he process of degassing the aluminum melt is performed to remove dissolved hydrogen from the molten aluminum and eliminate the resulting hydrogen porosity in the castings. When the aluminum melt contains significant levels of hydrogen, it can be assured the castings will contain internal porosity as shown in Figure #1. The typical detrimental effects of hydrogen porosity in the castings are: 1. Reduced Tensile Strength 2. Reduced Elongation 3. Visual Rejects on machined surfaces HOW HYDROGEN ENTERS THE MELT Most hydrogen in the melt enters via moisture in the air, however wet tools and moisture in cores or molds also contribute. Hydrogen gas is the only gas with significant solubility in molten aluminum. Unfortunately, molten aluminum has an affinity for hydrogen. That is to say, the hydrogen would rather be in the aluminum than in the moisture in the air. For all practical purposes it is impossible to prevent hydrogen from entering the melt however, it is not impossible to slow the rate of absorption. By understanding the factors that influence the absorption rate of hydrogen into the molten aluminum, foundries can implement practices to reduce the hydrogen content in the melt. The three main contributors to the absorption rate of hydrogen in molten aluminum are: 1. Humidity 2. Air temperature 3. Melt temperature Most aluminum foundries understand that humidity plays a major role in increasing “gas” in the melt but usually humidity alone is not the culprit. It is the combination of humidity and air temperature that is the true problem. For example, at 50% relative humidity, the partial pressure of water vapor in the air is more than ten times greater at 100 degrees F than at 32 degrees F. Therefore it is more difficult to control casting porosity on hot humid days than on cold rainy days. Melt temperature is another major contributor of hydrogen porosity. However, this is a contributor that can and should be controlled. If possible, aluminum foundries should control the melt temperature to a maximum of 1400 degrees F. At temperatures over 1400F the solubility of hydrogen in aluminum greatly increases. In fact, the solubility doubles every 200 degrees over 1400! For this reason it is better for foundries to change their gating to improve metal flow instead of just increasing the metal temperature. Figure 1.