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Close attention paid to gas testing, MMCs.


Subjects ranging from micro- and macrostructure studies to HiPping and thermal analysis characterized the Aluminum Division's 1991 program. While nearly all of the nine sessions and 25 individual presentations made this year were well attended, it appears that those dealing with reduced pressure testing of aluminum and metal matrix composites garnered the most attention.

Four speakers comprised the panel, which discussed a practical approach to the reduced pressure testing (RPT) of molten aluminum (91-068). With the RPT method, a sample of molten aluminum is pulled from a furnace and placed in a sample cup, which is then encased in a vacuum jar and allowed to solidify under a vacuum. After solidification, the sample is sectioned and the amount of porosity observed, which indicates the level of hydrogen in the melt. This allows the gas level in the furnace to be either increased or decreased, depending upon the casting's requirements prior to pouring.

In introducing the subject, W.M. Rasmussen, American Foundrymen's Society, said the panel presentation was put together to "help foundrymen better understand and use reduced pressure testing." He briefly discussed the effects of hydrogen gas on aluminum casting quality as well as several methods used for detecting it while the metal is in the molten state. But he zeroed in on RPT because it is the most commonly used technique of the various testing methods. This, according to Rasmussen, is because the equipment needed to perform the test is low-cost, rugged and fairly simple, and the test procedures are easily understood. And, he said, the test's results "usually correlate well with the final casting quality."

C.E. Eckert, Metallurgical Products and Technologies, followed Rasmussen to the podium and discussed sampling techniques and variables. He stressed the importance of establishing and auditing sampling procedures as well as reducing testing variables in order for RPT results to be meaningful. This test is more complex than it may seem," Eckert said. "So, it is imperative that you select your procedures carefully and then stick with them."

The results of RPT can be evaluated both qualitatively and quantitatively. R. Atkinson, Bodine Aluminum, described three qualitative techniques. The visual bubble method, he explained, calls for observing the test sample during solidification and determining when the first bubble or blister appears. This indicates the amount of hydrogen in the melt. A second technique is visual comparison in which the sample is allowed to solidify, is sectioned and then compared to other known samples. Atkinson also talked of using dye penetrant on a sectioned sample in order to show gas porosity much better than simply observing the piece.

Quantitative techniques, like bulk density and specific gravity measurements, were described by S.K. DeWeese, Estalco Aluminum. "RPT," he said, "is a relative and semiquantitative method of gas measurement at best because of the hydrogen lost during transfer of the sample from the furnace to vacuum chamber and other influences such as oxides. But RPT can be an effective quality control tool if these types of influences are controlled and their effects understood."

In a separate presentation on quantifying the reduced pressure test, researchers W. La Orchan, M.H. Mulazimoglu and J.E. Gruzieski, McGill University, studied the A356 and 413 alloys (91-30). They reported that "The true hydrogen content of A356 and 413 melts is always significantly higher than that calculated from the density of standard RFT samples. In the case of A356, this value can be corrected to give a reasonable measure of melt hydrogen by using either a constant correction factor over a narrow hydrogen range or a hydrogen-dependent correction factor over a wider range." In their work, the researchers also studied how silicon modification may affect RPT.

Unfortunately, according to the authors, standard RPT is much less sensitive to hydrogen concentration and less repeatable when used with the 413 alloy. They explained that the difference was due to the freezing mechanisms of each alloy, and proposed that risered samples instead of the standard cup sample would provide a much more accurate reading of hydrogen in molten aluminum.

The subject of castable metal matrix composites (MMCS) continues to be of high interest to aluminum foundrymen. D.O. Kennedy and J.C. Church, Lester B. Knight & Assoc., reported on their work in gating MMCs (91-87). To determine if differences exist between conventional aluminum alloys and castable composites, the same base-line gating system used to produce commercially acceptable aluminum 356 alloy castings was used to pour F3A.15S composites. This proprietary alloy has an A356 matrix and 15% by volume silicon-carbide reinforcement.

According to Kennedy and Church, "Modifications to normal aluminum [gating] systems, which could produce satisfactory composite castings, was the desired objective [of the work], as compared to developing a whole new gating approach for these types of castings." But they found that the need to gate MMCs differently than unreinforced castings was evident.

In other presentations on MMCs, A. Saigal, Tufts University, studied the tensile properties of silicon-carbide reinforced aluminum cast composites using finite element analysis (9163); and J. Singh, S.K. Goel, V.N.S. Mathur and M.L. Kapoor, University of Roorkee, india, examined the wear behavior of squeeze cast MMCs (91-124).
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Title Annotation:95th AFS Casting Congress, May 509, 1991 - Birmingham, Alabama; metal matrix composites; A Technical Review: Aluminum Division
Publication:Modern Casting
Date:Jun 1, 1991
Previous Article:Program highlights products, processes.
Next Article:Sand reclamation issues under study.

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