Simple Solutions That Work! Issue 18

57 HOW TO ISSUE BRAD HOHENSTEIN President Porosity Solutions ARTICLE TAKEAWAYS: • Not all casting porosity is the same • Learn the many types of casting porosity • Identify > Understand > Fix Continued on next page How to Eliminate Porosity in Aluminum Castings P orosity is one of the most common causes of customer rejects in aluminum castings. For the foundry customers, porosity is easy to identify. The customer sees a hole or dimple on either the cast or machined surface of the part and then scrap the part as a porosity reject. For the customer it is just that simple, if it has a hole in it—it’s porosity! For the aluminum foundry however, fixing a porosity defect can be quite complicated. There are many unique types of porosity defects in aluminum castings such as hydrogen or gas porosity, external pin holes, internal pin holes, sponge shrink, centerline shrink, shrink, blows, chill blow, air holes, and micro-porosity, to name a few. For the foundry to correct the porosity defect on the casting, they must be able to 1) identify the type of porosity, 2) understand the root cause, and 3) address the root cause in the casting process. In many cases the customer expects the foundry to eliminate the porosity issue by issuing a Corrective Action Request to the foundry without returning the defective part. However, if the casting porosity reject is to be fixed, it is critical that the foundry insists on the customer returning the defective part for analysis. Below we look at the characteristics and causes of the most common porosity defects in aluminum castings. filling, pouring down the riser or improper gating and metal flow. Since the air bubbles float upward in molten aluminum, many times blow holes are more prominent in the cope section of the part. Blow holes are most often an internal defect that is detected during machining of the part, however at times they can be seen on the cope (top) surface of the part. Blow Holes – Blow holes, also known as blows, air holes, or gas holes are larger smooth wall shaped bubbles which may be spherical or elongated. They are typically caused by entrapment of air or gasses during the filling of the mold. The causes of the air entrapment can be due to an interrupted pour, turbulent Hydrogen Porosity – Hydrogen porosity, also known as gas porosity, are small round smooth surfaced internal cavities evenly distributed throughout the aluminum casting. Hydrogen porosity is always a subsurface defect. That is to say if you have surface porosity on the casting, it is not hydrogen porosity. Hydrogen porosity is caused by hydrogen in the molten aluminum. Hydrogen is the only gas with significant solubility in molten aluminum. In fact, the hydrogen would rather be in the molten aluminum than in the air. For all practical purposes, the only way to eliminate the hydrogen in the melt is by degassing. Degassing is a process in which an inert gas, typically nitrogen or argon, is inserted into the melt by use of a rotary degasser or a porous degassing wand. The hydrogen in