Simple Solutions That Work! Issue 17

41 BACK TO BASICS BRAD HOHENSTEIN President Porosity Solutions ARTICLE TAKEAWAYS: • Poor melt control cannot be fixed later in the process • Many casting defects start prior to pouring the casing • To finish with a good casting, focus on the beginning processes Continued on next page For a Good Casting, Don’t Forget the Processes Prior to Pouring A luminum foundries often spend the bulk of their resources trying to detect and fix casting problems downstream of the pouring process while paying little attention to what is going on prior to the pour. In fact, the key to eliminating many casting defects is controlling what happens prior to the pour. The list of casting defects caused by not controlling the processes prior to the pour include: • Hydrogen Porosity (gas porosity) • Surface Reaction Porosity (pin holes) • Oxide Inclusions • Low Mechanical Properties (Brinell Hardness, Tensile, Yield, Elongation) HYDROGEN POROSITY Hydrogen porosity (gas porosity) is an internal defect. It is not exposed until the part is machined at which time it can easily be seen with the naked eye. Hydrogen porosity is typically round in shape and evenly distributed throughout the part. Causes – Hydrogen porosity is caused by excess hydrogen gas in the molten aluminum. Hydrogen is ever present within the moisture in the air. The higher the humidity level, the higher the hydrogen content of the air and the faster the gas content of the molten aluminum rises. Hydrogen 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 air. While the aluminum is molten, hydrogen stays in the solution state within the melt but it quickly comes out of solution forming gas holes as the aluminum solidifies. For all practical purposes it is impossible to prevent hydrogen from entering the melt. Detection Methods – To detect hydrogen in the melt a sample of the molten metal is solidified under vacuum and then analyzed. This method is called a Reduced Pressure Test (RPT). Once a RPT sample slug is solidified under vacuum, the specific gravity of the sample is measured and compared to the book value of the alloy. This measurement method is extremely accurate and takes less than 30 seconds using equipment such as the Palmer Porosity Analysis System PAS3000. For example, the book value specific gravity of 356 alloy is 2.68. If the RPT sample taken after degassing is 2.60 or higher, the casting should be hydrogen porosity gas free. Cures - Hydrogen is easily removed from the molten aluminum with proper degassing techniques. Degassing is a process where inert gas (typically Argon or Nitrogen) is inserted near the bottom of the crucible through a graphite shaft and dispersed throughout the melt.