Fundamentals of The Reduced Pressure Test (RPT)
The Reduced Pressure Test is an inexpensive and effective method for checking aluminum melt quality and is used by thousands of aluminum foundries across the world to control gas porosity in their castings. Controlling hydrogen porosity (gas porosity) in aluminum castings is the main use of the RPT test, however it can also be a good indicator of melt cleanliness.
How it works: A small sample of the molten aluminum is taken from the melt and immediately placed under a vacuum dome. The aluminum sample is then allowed to solidify for approximately 7 minutes at 25 to 28 inches-Hg vacuum. The process of solidifying under vacuum expands the volume of hydrogen gas approximately 10 times greater than solidification at normal atmosphere. This expansion of hydrogen gas magnifies the porosity in in the RPT sample making it possible to measure the porosity potential of the melt. Since thin film oxides (bifilms) also influence hydrogen gas bubble formation, the RPT test can also be used as an indicator of melt cleanliness.
– Methods of Evaluation –
Visual Comparison to Gas Chart
The most common method for evaluating the RPT sample is by sectioning the sample and comparing to a gas sample chart. A typical gas sample chart will display several photos of various cut and prepared RPT samples. Each photo will typically reference a corresponding specific gravity, density, density index, or percent porosity number. However, in addition to being the most common method, visual comparison is the least accurate, least safe, and most expensive method for evaluating a sample. To prepare the sample for visual evaluation, it first must be cut in half, ground on a sander, and polished on a buffing wheel, all of which includes labor and consumables. Once the time-consuming task of preparing the sample is complete, a judgement call is made by comparing the sample to a chart. Frequently, different operators will report different results when comparing the exact same sample to the chart.
Specific Gravity (Density)
Specific gravity or density (same number) is a much better method of analyzing a RPT sample than visual comparison to a gas chart. Very accurate results can be obtained in less than a minute with no consumables involved. The RPT sample is simply weighed in air and weighed in water using a specific gravity apparatus. A calculation is performed using the two weights to determine the density of the RPT sample. Foundry floor units such as the Porosity Solutions, PAS3000 and the PAS5000 porosity analysis system will guide the operator through the weighing process, perform the calculations and display the specific gravity (density) of the sample. Typically, a foundry will assign a target density number to an alloy or process that the operator must meet in order to ensure a low porosity casting. For example, the book density of 356 is 2.68 which is the specific gravity of a porosity free sample. For 356, a foundry may assign a target specific gravity (density) of 2.6. If the RPT sample meets or exceeds the target (2.6 to 2.68), it is OK to pour the melt. If the specific gravity of the RPT sample is below 2.6, too much gas is present and additional degassing must be carried out.
Percent Porosity Measurement
Percent Porosity is determined in the same way as the specific gravity/density test above, however an additional calculation is performed using the RPT sample density and the theoretical maximum density of the alloy. Foundry floor machines like the Porosity Solutions PAS5000 Porosity Analysis System automatically provide this result along with the specific gravity (density) number for the RPT test sample. Percent Porosity is helpful when multiple alloys are cast. For example, if a foundry casts both 356 and 319 alloys, they would need to assign a unique specific gravity target number for each alloy due to the difference in book density between the two alloys. 356 alloy has a book density of 2.68 while the density of 319 alloy is 2.79. In order to achieve the same low level of porosity, 356 would require a target specific gravity number of 2.6 while 319 would require a target number of 2.7. Using a Percent Porosity target number instead of a specific gravity target number allows the same target number to be used for all alloys. For the example above, assigning a 3% Percent Porosity target to both alloys would yield the same results as assigning a 2.6 specific gravity number to the 356 alloy and a 2.7 specific gravity number to the 319 alloy.
Aluminum Density Index
The Aluminum Density Index method is typically used to evaluate both the hydrogen content and the cleanliness level of the RPT sample, providing an excellent indication of the overall cast quality of the melt. To determine the aluminum density index number, two samples of the melt must be produced. An RPT sample solidified under vacuum and a sample of the same size solidified in normal atmosphere. A calculation is then performed using the density measurements of both samples. A foundry floor machine such as the Porosity Solutions PAS5000 Porosity Analysis System will guide the operator through the weighing process and output the density index number. A density index number less than 1 indicates excellent melt quality with an extremely low hydrogen and oxide content while a density index number of 10 and above indicate poor melt quality.