problem, the Reduced Pressure Test (RPT), a procedure of examining a solid sample, is a commonly used and effective tool. The procedure is easily understood and followed. The equipment is simple, rugged and inexpensive and the results usually correlate to casting quality. However,
pressure must be controlled during solidification.
Either the solidified RPT test sample is cut and polished to yield a qualitative or semi-quantitative measure of gas content or the sample density is determined by measuring the sample weight when it’s dry and
then suspended in water. As long as there is no shrinkage in the sample, measuring the density is the preferred method as it eliminates the subjectiv- ity associated with visual examination of the cut surface. With in situ techniques, the sampling process causes no errors, so these measurements have the potential to be the most accurate and reliable. The majority of in situ methods feature a system that involves recirculating inert gas introduced into the melt by a sample probe. The inert gas is recirculated through the melt, collected and passed over a differential thermal conductivity sensor to determine the hydrogen content of the gas. This recirculation continues until the hydrogen content reaches a value in equilibrium with the melt. The new method for measuring
hydrogen is promising. The latest version of the electrochemical sensor is nearly equivalent to a thermo- couple. This would be the holy grail of gas measurement, allowing us to measure gas like we measure tem- perature with handheld devices.
Practical Considerations on RPT At least 90% of aluminum
metalcasters in North America use the RPT to determine metal qual- ity. A few practical approaches can improve performance. The necessary equipment for a
vacuum evaluation of an aluminum melt consists of a pump, a gauge to monitor the vacuum level inside the test chamber and a control system to regulate the pressure (Fig. 3). In practice, the molten sample is placed on the pedestal, a chamber with a viewing port is placed over the sample, the chamber is evacuated to the desired pressure, and the sample is allowed to solidify under the reduced pressure. The reduce pres- sure in the chamber causes dissolved hydrogen to come out of the solu- tion in the melt and either escape from the sample through the molten surface or form bubbles inside the solidifying sample. It is important that the operator
32 | MODERN CASTING August 2015
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