Simple Solutions That Work! Issue 18

58 Contact: BRAD HOHENSTEIN [email protected] the molten aluminum will combine with the inert gas bubbles and float to the top of the melt where they burn off. In order to determine if enough of the hydrogen has been removed to eliminate porosity in the part, a Reduced Pressure Test (RPT) is performed on the molten aluminum prior to pouring and evaluated by using a density test with a unit such as the Palmer PAS3000. Advancements in analysis and degassing equipment have made the Hydrogen Porosity defect the easiest of the porosity defects to control. Internal Pin Holes – As with external pin holes, internal pin holes are caused by a reaction between the molten aluminum and the mating surface. In the case of internal pin holes, the reaction is with an internal core. They can only be detected by viewing or analyzing the cast surface of the part adjacent the core. If you cut a cross section of the part through the core area, a “halo effect” can be seen, as shown in the drawing. Internal pin hole porosity is caused by a reaction between the molten aluminum and the core surface. Typically, moisture on the core, moisture in the core coating, or improper core material is the culprit of this defect. External Pin Holes – Pin holes are small porosities on the surface of the part caused by a reaction between the molten aluminum and the surface of the mold. This reaction can be due to improperly dried mold coating, moisture in the mold, improper sand additives or improper binder mixture. Pin holes can usually be detected visually, however fluorescent penetrant inspection is the best method of detection. Since Pin holes reside at the cast surface, they are sometimes removed in the machining process, but this will depend upon how deep they extend into the part. Shrink Porosity – Small irregular shaped porosity formed during the solidification of the casting. All aluminum shrinks around 6% during solidification. Shrinkage P-porosity forms in the last area of the casting to freeze. The key to eliminating this defect is to drive the shrinkage porosity to the riser where it will not be part of the final cast part. However, driving all shrinkage to the riser is not as easy as it sounds. Many times, casting features such as protrusions or adjacent thick and thin sections will create isolated pools of liquid within the solidified portions of the part. These isolated pools of aluminum also shrink around 6% during solidification creating shrinkage within the casting. The shrinkage porosity within this area is normally connected to each other. If the shrinkage is connected to both the outer and inner surfaces of the part, the part will not be pressure tight. If a customer casting reject is identified as a “Leaker,” it is likely due to shrinkage porosity. Shrinkage is very difficult to detect visually however, it is very easily detected in liquid penetrant testing. If shrinkage is connected to the surface, it can also be detected using a simple Stereo Microscope at 10 to 20 times magnification. When viewed in this method, what looks like small bb’s can be seen on the inner surface. These small bb’s are in fact, dendrites which are exposed as the aluminum shrinks. These exposed dendrites are a sure sign that the defect is shrinkage porosity. There are only two ways to cure the shrinkage, feed it or chill it. The goal is to eliminate the trapped pools of metal and drive the solidification of the aluminum to the riser. Changing the solidification profile to eliminate shrink porosity may require a change in the gating design, addition of chills in thick sections adjacent to the shrink, or both.