and ambient air. At sea level, the average atmo- spheric pressure will support a column of mercury 29.92 in. (760mm), or 30 in. for simplic- ity’s sake. Most metalcasters will use a gauge pres- sure of 26-28 in., which is called vacuum level or pressure. A356 alloy can
Fig. 4. The sample on the right shows fewer oxides which implies a lower gas content.
be chosen as an example to show what happens dur- ing solidification. Te metal contains a hydrogen gas content of 0.12cc/100g, a fairly low gas content. When a sample of this metal is taken from the crucible and placed in a RPT, the pressure of hydro- gen increases because of two factors:
1. The sample cools to a lower
temperature. 2. As freezing progresses, hydro-
gen and silicon segregate and accu- mulate in the remaining liquid. The gas pressure continues to grow during solidification until it
ALUMIN UM ME L TE RS!
-25 “Hg, S.G.: 2.47
-26 “Hg, S.G.: 2.36
QCD-1 Porosity Measurement System
• Automatic measurement and recording of Specific Gravity / Sample Porosity in 15 seconds.
• Eliminates the need to cut and polish the sample to permit a very subjective estimation of gas level.
T.R.P. Vacuum Tester (True Reduced Pressure)
• Measures the absolute vacuum level, eliminating the need for daily/weekly/monthly calibration of the differential gage used on most RPT units.
• Fully automatic operation to pull the Absolute Vacuum to a target level of (+/- 0.20”Hg), run a timed cycle, and dump the vacuum.
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dan@qcdesignsinc.com •
www.qcdesignsinc.com
Q.C. Designs, Inc. (269) 983-6859 • Fax: (269) 982-0823
34 | MODERN CASTING August 2015
reaches the equal of the average atmo- spheric pres- sure when 70% of the sample had solidified. For a cast- ing of this metal freezing on your shop floor, porosity cannot form until 70% of the material is frozen solid. However, when a vacuum
is applied, the situation is differ- ent. First consider the case where a gauge pressure of 28 in. is applied to the solidifying sample. The gauge pressure of 28 in. corresponds to an absolute pressure of 50 Torr. This is slightly below the equilibrium gas pressure in this sample at the start of solidification, so gas bubbles may form on the surface of this sample before it starts to freeze. Receiving accurate readings from
reduced pressure tests is essential to creating quality castings. Here are a few of the many ways of ensuring proper RPT procedures: • Use a thin-walled cup for sample collection.
• Skim oxides from the melt surface.
• Preheat the cup in an adjacent surface area.
• Transport sample rapidly to a vacuum unit with proper seals and pump capacity.
• Cover the chamber that allows visual observation.
• Control vacuum to target level. • Adjust cycle time to allow for full solidification of the sample.
• Measure specific gravity of the sample and compare to process specifications (or count bubbles on the surface).
Tis article was excerpted from the 2014 book, “Best Practices in Aluminum Metalcasting,” by Geoffrey K. Sigworth, published by AFS and available for pur- chase at
www.afsinc.org/store.cfm.
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