Simple Solutions That Work! Issue 13

developing your initial degassing procedure and controlling the day to day degassing process. The most effective and simple way to do this is with the Reduced Pressure Test (RPT). The RPT test works by taking a small sample of the melt and solidifying it under a controlled vacuum (a RPT machine) and then measuring the density of the sample. The entire test takes about 8 minutes, 7 minutes under vacuum and about 30 seconds to get the density result. Many aluminum foundries use the RPT test but not in the best way. The traditional way to measure the RPT sample was by cutting the sample on a saw, sanding the surface, and comparing to a chart. This method is time consuming, operator dependent, and has too much variation in the results. Measuring the density of the vacuum cast sample is much quicker and extremely accurate. Just set a target density by alloy, measure the sample, and it is either good or bad. For example, the book density of 356 alloy is 2.67 g/cc. In this case, most foundries would set a target of sample density 2.58. This means if the vacuum sample measured 2.58 or higher, it is okay to cast and would result in a gas porosity free part. If the vacuum sample density measured 2.57 or lower, additional degassing would be required. New porosity analysis equipment on the market make measuring the density of the vacuum cast sample simple. Just solidify using the RPT vacuum. The sample can then be measured with the data recorded in just a few seconds. There are even advanced porosity analysis systems on the market, which combine automated vacuum, density measurements, and data collection into one all-inclusive unit. CONSIDERATIONS IN DEVELOPMENT OF DEGASSING PROCEDURE Whether developing or refining your degassing process, RPT testing should be used to guide your decisions. The following are some common issues I have witnessed over the years in foundry degassing practices. • Determine a separate degassing time for each size furnace. Many foundries will have a standard degassing procedure used across all furnaces and ladles. This is not good practice. For example, one foundry that I visited had a 10-minute degassing procedure which worked fine in their 500 lb crucible but not so good in their 2000 lb crucible. A quick check of RPT samples from each furnaced showed a density of 2.64 in the smaller crucible and 2.49 in the larger crucible. Of course, this resulted in porosity free castings from the 500 lb crucible while castings from the 2000 lb crucible contained Contact: BRAD HOHENSTEIN [email protected] gas porosity throughout the parts. Increasing the degassing in the 2000 lb crucible until the density measured over 2.60 solved this issue resulting in good parts from both crucibles. • The inert gas flow should be very low. The prevailing thinking seems to be that if a little inert gas is good, then a lot of inert gas must be better. A good inert gas flow results in a slow bubbling of the inert gas across the surface of the melt. Large rolling bubbles are no good. In fact, they are detrimental as the rolling metal folds oxides and air back into the melt. • Another common mistake is to spin the RID shaft as fast as possible to distribute a wider swath of bubbles. Unfortunately, when the RID shaft spins too fast it creates a vortex around the shaft sucking the surface metal and air down into the melt. The speed should be set to a point just below the start of the vortex, typically around 300 rpm or less. If you see molten aluminum sucking down around the shaft, back off the speed a little. • Develop a target density for each alloy and casting process. Start with the book density of the alloy. A good starting point for a porosity free target is about 10 points less than the book density. For example, the book density of 356 alloy is 2.67. In most cases a vacuum sample density of 2.57 will be sufficient to yield a gas porosity free part. However, solidification rate will affect the result. A permanent mold or die casting will solidify relatively quick so a density of 2.50 may be sufficient. A sand casting with no chills will solidify much slower and may require a target number of 2.60 or higher to yield a gas free part. Best practice corelates the RPT results to the casting. This allows for adjustment of the target density, if required. 38