RPT gauges aluminum porosity: the rising demand for top quality aluminum metal castings puts emphasis on reducing test variables to improve accuracy.
A major component of that effort is the production of "clean metal" by eliminating inclusions and neutralizing the hydrogen gas porosity. Hydrogen gas dissolved in liquid aluminum foundry alloys is one of the most significant problems facing metalcasters and is a main cause of the rejection of cast aluminum parts.
Paramount among the support tools available to foundries to detect excess amounts of hydrogen gas in molten aluminum is the venerable, widely-used reduced pressure test (RPT). It is sometimes referred to as the vacuum density test (VDT), the vacuum solidification test (VST) or the Straube-Pfieffer test, but the broadest foundry designation is RPT.
RPT has been used by foundrymen for decades to detect and measure hydrogen gas and its popularity stems from its simplicity. The test uses a sample of molten alloy that is allowed to cool under a reduced pressure (usually in the range of 1-100 mm Hg). The reduced pressure encourages pore and gas bubble formation so that a sample solidified from gassy metal will contain an exaggerated amount of porosity.
An assessment of the hydrogen content of the liquid is obtained usually by visual examination of the solidified specimen. A puffy or convex top surface indicates a metal with a high gas content while a depressed surface or shrinkage pipe suggests a low gas content. Samples may be sectioned and ground to reveal gross interior porosity.
The test operator has a choice of qualitative and semi-quantitative methods to assess the hydrogen level. These include looking for the first bubble as the sample solidifies, measuring the specific gravity of the solidified sample, visual examination, use of dye penetrants, etc.
When the hydrogen content is about 0.15 cc/100 g or below, the traditional RPT can give results that indicate the alloy is essentially gas-free.
Hydrogen, highly soluble in aluminum, can cause serious casting defects such as decreased tensile strength and elongation. Depending on the alloy and its cooling rate, hydrogen can produce blisters and porosity sufficient to reduce the cast metal's mechanical properties, resistance to corrosion and crack propagation.
Hydrogen is ubiquitous; it results from the continuous dissociation of water vapor which is everywhere in the atmosphere. It is present on foundry tools, in charge materials and refractories, even in the atmosphere surrounding the furnace. It is absorbed during melting, pouring, transfer and all other processing operations and its consequences can be devastating to a casting's usefulness.
Aluminum readily holds hydrogen in solution as long as the bath is molten. As the metal solidifies after pouring, it is unable to hold the hydrogen and attempts to rid itself of it. In doing so, it leaves voids, or pores, that can cause deterioration of the final casting's properties and result in poor surface finish when the parts are machined or polished.
The RPT is the most widely used foundry procedure to determine excessive hydrogen present in a melt, but other methods for determining gas concentrations also are useful. They include direct measurement tests such as recirculating gas, extracted sampling, first bubble and subfusion.
Recirculating gas--In this test, nitrogen gas is recirculated through an aluminum alloy melt, picking up hydrogen until the hydrogen concentration comes into equilibrium with the melt. The hydrogen content is then determined by measuring the thermal conductivity of the mixture of the two gases. It is an accurate test, but costly in comparison to the RPT.
Quantitative RPT--This test involves taking a sample from the melt, placing it in a vessel inside a vacuum chamber and quickly reducing the pressure. As the sample solidifies, hydrogen is expelled from the metal, increasing the pressure inside the chamber. The total pressure is then read after a fixed time period (typically five minutes) and the hydrogen content of the alloy calculated with the aid of a computer programmed for an optimum hydrogen content value scale.
This relatively costly instrument is not accurate to measure melts with a low gas content. It also is mechanically complex and requires recalibration to detect and measure small amounts of hydrogen.
First bubble method--Yet another reduced pressure test, this one requires a small melt sample to be placed in a vacuum chamber. The pressure is reduced until the first gas bubble breaks the surface of the sample. At this point, both temperature and pressure are noted and the hydrogen content of the melt calculated. The drawbacks of this test include its high cost and complexity, skewed results due to inoculation of gas bubbles by inclusion contamination, and inaccurate readings because of low gas concentration.
Subfusion and Vacuum fusion--These are mainly laboratory techniques capable of providing precise hydrogen analysis. In both subfusion and vacuum fusion, the hydrogen is extracted under vacuum from a rapidly chilled sample which must be chilled fast enough to allow the hydrogen to be entrapped in the solid without surface porosity formation.
In the subfusion analysis, the sample is heated to about 50|degrees~C, below its melting point; in vacuum fusion, the sample must be melted. Several hours are required to measure the hydrogen from the solid sample and 10-15 minutes for the melted sample. During sample preparation it is important to limit hydrogen contamination of the sample surface.
The RPT Process
This form of vacuum testing for hydrogen porosity is the one most often used by foundries because it has been proven quick, inexpensive and effective over the years it has been in use. The test equipment is inexpensive and some foundries have fabricated their own RPT apparatus from readily available parts.
The test equipment also is rugged, simple to operate and well-suited to withstand the severest foundry environment. Its procedures are easily understood and the results usually correlate satisfactorily with actual casting quality.
The familiar test has been around so long and its usefulness to aluminum foundry needs so ingrained that its use is subject to wide operating variations from foundry to foundry. However, as with the computer truism, garbage in, garbage out, the accuracy of RPT is only as good as the consistency and accuracy of specific operating procedures, simple though they may be.
Areas of Concern
Briefly, three factors affect RPT results as they apply to successful use in production or as a process development tool. They include vacuum measurement, pressure reduction and solidification.
Vacuum measurement is an absolute essential to the success of RPT. Frequent calibration of the vacuum system (vacuum gauge, regulator, release valve and pump) is important to assure constancy. Accuracy of the test means that the process relies on establishing as many constants as the process allows to obviate operator error. To be consistent from melt to melt, variables, such as the start-up vacuum, positive bell chamber seal, sample accuracy and solidification time, should be as constant as possible to limit chances for test and operator errors.
Reducing the pressure causes the hydrogen in the sample to try to oppose the vacuum formed; the hydrogen will then precipitate out of the metal in an attempt to fill the vacuum. It's much like opening a can of carbonated beverage. The dissolved carbon dioxide gas reacts to the released pressure caused by breaking the seal of the can. Now the C|O.sub.2~ gas begins effusing, or bubbling out of the liquid in which it is dissolved, as it seeks equilibrium with its surrounding environment.
As the RPT progresses, some of the hydrogen gas will escape the metal sample as it solidifies, but some will nucleate around inclusions and become entrapped as internal pores. The time the sample spends between the liquidus and solidus phase is important as to whether or not a bubble is captured and manifests itself as a pore. Hydrogen is rejected from the solidifying to the liquid stages and then to the atmosphere but the RPT depends on the gas bubbles to be manifested in the sample rather than being effused off.
Solidification, in the presence of hydrogen gas precipitation and inclusions, is complex and not well understood, but it is one of the most important variables of the test. It is yet another reason why consistent pressure regulation and sample interpretation is vital to RPT repeatability.
It has been demonstrated that for a pure aluminum metal sample, there is approximately 18 times more hydrogen contained in the liquid than in the solid phases of the metal as it solidifies. But a gradient begins to develop between the two as the solidification proceeds. This enriches the solidifying metal in hydrogen to the point where the solubility conditions are no longer satisfied. The hydrogen begins to precipitate at the reduced pressure and manifests itself as pores in the solidified metal.
RPT Technique, Apparatus
The basic elements of the RPT apparatus include a vacuum chamber, crucible (sample cup), chamber base, vacuum gauge, vacuum regulator, release valve and vacuum pump. The gauge and regulator are key elements that assure reduced pressure in the chamber is constant from sample to sample.
The cup is preheated to melt temperature by immersing the outside diameter of the cup into the melt itself. The melt is then skimmed and the crucible filled carefully to limit the possibility of turbulence because turbulence can increase hydrogen and inclusion content resulting in artificial test results.
The sample is rapidly transferred to the vacuum chamber to minimize temperature loss prior to the start of the test. A vacuum is then pulled (generally to 1-100 mm Hg absolute pressure) and the sample allowed to solidify. The process is observed and evaluated by density measurement or sectioning. All pertinent stages of the RPT should be charted carefully and verified as part of the melt record and reference.
As noted above, RPT is most effective if the sampling is done in a repeatable sequence that eliminates or reduces variables. The same test standards apply for most aluminum alloys and, although the test criteria may change, the test procedures remain unchanged.
Reduced pressure--This is vital because pressure has a major impact on bubble formation and gas migration. The vacuum must be drawn in the shortest possible time and be constant from sample to sample.
A higher vacuum is not always best. For most applications, the lower the vacuum, the greater RPT sensitivity. One of the pitfalls of too high a vacuum level is the danger of degassing the sample. The same would apply to too low a vacuum that allows the sample to solidify over a long period.
Selecting one vacuum level over another is difficult to equate because vacuum is a function of the objectives of the test. If the test is to be used as an initial bubble test, then generally it is the higher vacuums (2 mm Hg) that are the most desirable. For tests that will be sectioned for examination, lower vacuum (50 mm Hg) is the more preferable. It is advisable for accuracy to use absolute pressure rather than atmospheric pressure because the latter will vary as the barometric changes (typically between 28-31 in. Hg or 711-787 mm Hg) occur with local weather conditions.
Sampling--Furnace sampling should be done quickly for two reasons. First is to limit the effects of temperature loss between the melt and the start of the test. The higher the sample temperature, the greater the impact it will have on the apparent hydrogen level inferred from the RPT. Second is because there is a wide temperature gradient for hydrogen in a quiescent bath during which ordinary diffusion is slowed. The vacuum, density and temperature all have a major impact on test results.
Sampling should also be done to limit turbulence caused by the physical act of pouring the sample cup. Turbulence can entrain hydrogen at this point sufficient to change the hydrogen content of the sample from the melt itself to its position in the vacuum chamber.
Generally, the larger the sample, the more accurate the test results. Qualitatively, the larger sample size is more sensitive, but accuracy is difficult to define. Using too large a sample could lead to the vacuum degassing of the sample during solidification, resulting in inaccurate test results.
Solidification rate--Solidification is the primary RPT variable and is linked to other variables such as temperature of the melt, the thermal conductivity of the base on which the cup is placed and the sample size.
The rate is important in terms of hydrogen distribution and the period that the alloy spends between the liquidus and solidus temperatures. Isolating the sample crucible on an insulated base prevents the sample from cooling too rapidly. However, cooling the sample too slowly can cause degassing and give an erroneous hydrogen test.
Vibration--If vibration or agitation is used to promote nucleation, the rate of vibration must be minimal but constant from test to test.
Alloy type--This is important because alloying elements change the solidification rate. Volatile elements such as calcium or sodium, which have high vapor pressures at low absolute pressures, can distort hydrogen porosity because they volatilize and form pockets of their own gases.
The addition of alloying elements to aluminum changes the aluminum's hydrogen solubility. Silicon, zinc, manganese and copper decrease the solubility of hydrogen while alloys with magnesium, titanium, nickel and lithium increase it.
Sample contamination--Metal cleanliness is important to the testing process. Inclusion and hydrogen contamination presence in the sample can change the parameters by introducing problems with the test procedure.
Data analysis--The test is capable of both qualitative and semi-quantitative evaluation. Qualitatively, the methods of evaluation include visual bubble examination, visual comparison and dye penetrant inspection. Bulk density and specific gravity represent two quantitative evaluation methods.
A serious problem for many RPT users is the failure to adequately establish sampling and apparatus testing procedures and the lack of proper auditing procedures to assure their compliance. The absence of control charts for consistent vacuum levels, sample weights, vacuum time, seal testing and regulator and gauge accuracy can introduce sampling error and the subsequent alteration of RPT repeatability parameters.
Editor's Note: This is an edited version of the panel presentation on RPT presented at the AFS Casting Congress in Birmingham, Alabama, May 5-9, 1991. Part 2 of this two-part series will focus on the qualitative and quantitative aspects of RPT.
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|Title Annotation:||reduced pressure test|
|Author:||Eckert, C. Edward|
|Date:||Mar 1, 1992|
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