Simple Solutions That Work! Issue 17

42 SURFACE REACTION POROSITY (PIN HOLES) Pin holes are small porosity cavities located on the surface of the part. Causes – A reaction between the molten aluminum and the mold or mold coating surface, typically, due to moisture in the mold or mold coating. Improper binder or sand additives can also cause a reaction with the molten aluminum. Detection Methods – Reaction porosity can usually be detected visually however the best method to determine the extent of the issue is by use of Fluorescent Penetrant Inspection (FPI) which will highlight the surface porosity showing exactly where the reaction occurred. Another detection method is to cut the casting in half, buff the cut surface, and inspect visually. Pin hole reaction porosity will only be detected at or near the cast surface. The internal exposed surface should be porosity free. Cures - Improved drying of the mold, improved mold coating drying, reduced moisture content in the sand mold. OXIDE INCLUSIONS “Inclusion” is a term used for an unwanted particle or element in the microstructure of the casting. Two common types of inclusions caused by poor melt practices are Aluminum Oxides and Magnesium Oxides. Causes Aluminum oxides – Aluminum oxides form instantly whenever molten aluminum is in contact with the air. In fact, without exception, the entire molten surface of the aluminum melt is covered with an oxide layer which thickens over time. If the molten surface is not skimmed properly to remove the oxide layer (dross) just prior to pouring, it can end up in the casting. Poor quality remelt is another common cause of high aluminum oxide content within the melt. Magnesium oxides – Magnesium oxides result from a reaction between molten aluminum and oxygen in the melt and grow over time. If magnesium additions are improperly made to the melt, magnesium oxides will result. Detection Methods – Due to the random distribution of inclusions in the melt, measurement and detection of any type of inclusion is difficult. The inclusions can be identified using metallographic techniques, however finding them in the casting by sectioning and polishing is like looking for a needle in a haystack. Typically, inclusion problems in castings are found during subsequent machining operations. When a machine tool comes in contact with an inclusion it can tear the machined surface resulting in a scrap part. In instances of large or heavy concentrations of inclusions, tool breakage can occur. Metal cleanliness measurement systems such as PodfA, Prefil, K-mold, and LiMCA can be helpful in developing proper metal handling processes and procedures to eliminate the source of the inclusions. Cures Aluminum Oxides – Care must be taken when removing the dross (oxide layer) from the melt prior to pouring. During the degassing process, the operator must not rotate the degassing shaft too fast as to create a vortex around the shaft. A vortex around the degassing shaft will suck air into the melt creating oxides. Clean, oxide-free remelt should be used to reduce the oxide content in the melt. Magnesium Oxides – Magnesium is a key element for increased mechanical properties in many alloys. When a high tensile and yield specification must be met, aluminum foundries often control the magnesium content to the upper portion of the specification. This requires periodic addition of Pin Hole Porosity on outer surface of Casting. This was caused by a reaction between the molten aluminum and mold coating. Magnesium Oxide Inclusion on the failure surface of this part.