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Striving for a better melt.

The aluminum sessions were the Congress' hottest ticket, topping all others in attendance, and generating over 25% of the total session attendance. Although the presentations focused on a variety of issues critical to aluminum foundries, improving melt characteristics headed many of the discussions.

G. Sigworth, Concurrent Technologies Corp., described a new method for gas measurement in aluminum.

"The control of dissolved gas in liquid metal is absolutely vital to the production of high-quality castings, and for this reason more than a dozen methods of gas analysis have been developed over the years," Sigworth said.

He said an ultimate method would be similar to how we measure temperature today: immersing a simple probe in the melt and after a short time of immersion for equilibrium to occur, an accurate numerical reading flashes onto a screen.

He noted that a new, simple technique has been developed: Professor Fromm's (Planck Institute in Germany) "direct pressure measurement"--which introduces a special porous material into the melt to act as an "artificial bubble," allowing the direct measurement of the hydrogen pressure in equilibrium to the melt.

The technique consists of a porous probe that is affixed to the end of an impermeable ceramic tube and placed in the melt. The valve opens to connect the measuring system to a vacuum, and any gas inside the volume is removed. When evacuation is complete, the valve closes. Gas pressure arises from the hydrogen dissolved in the melt. Hydrogen diffuses through the pore network in the sample probe until the gas pressure inside the measuring volume is the same as that in the melt.

This method, Sigworth said, provides an accurate and rapid in situ measurement of gas content. "Foundrymen know dissolved gas has an overriding effect on the distribution and amount of porosity and shrinkage in castings," he said. "In order to minimize scrap, therefore, one must carefully control dissolved gas levels."

In his presentation, "A Constant Volume Risered Mold for the Reduced Pressure Test (RPT)," J. Gruzleski, McGill University, noted the RPT is commonly used in aluminum foundries to obtain a qualitative evaluation of the melt hydrogen level. It is simple, inexpensive and, as presently used, is definitely nonquantitative.

Several attempts have been made to make the RPT quantitative. Gruzleski, along with McGill co-authors W. LaOrchan and M. Mulazimoglu, recently determined there's a relationship between density and the true hydrogen values that allow good quantification of hydrogen in a long freezing range alloy (356.2) from the density of a standard nonrisered sample.

"A constant volume risered mold allows the RPT to be used on short and long freezing range alloys," Gruzleski said. The authors tested it for quantification of hydrogen in 319, 356, 413 and 390 alloys.

They concluded a risered constant volume mold for the RPT can be used to determine the hydrogen concentration on alloys 319, 356 and 413, with good correlations between density, weight and hydrogen concentration and a reasonable reproducibility of data.

"Solidification shrinkage--one of the main sources of error in the RPT--has been minimized using the constant volume mold," Gruzleski said. "In order to develop a truly quantitative reduced pressure test that can be used on both short and long freezing range alloys, further work is in progress to assess the effects of melt cleanliness, sampling temperature, modification and grain refinement."

D. Rose, Duralcan USA, reviewed the status of procedures that have been developed to effectively remove gases and oxides from composite foundry returns.

For years, he said, the only metal matrix composite manufacturing processes using molten metal technology have been pressure infiltration and rheocasting, which experience limited commercial acceptance. Within the last five years, however, aluminum-silicon casting alloys reinforced with silicon carbide (SiC) particles have been available in foundry ingot form for subsequent remelting and shape casting.

All conventional casting techniques have been used to melt, process and cast the composite ingot. "Procedures specific to castable composite alloys, however, along with sound foundry practices, must be followed to maximize composite casting quality and internal structural integrity," Rose said.

One such procedure is the degassing and cleaning of foundry returns (gates, risers and scrap). "Recycling foundry returns is a necessary requirement if the economic viability of implementing castable composite materials into the marketplace is to be maintained," he said.

Rose said SiC particulate reinforced composite castings display significant increases in stiffness, wear resistance and elevated temperature strength over conventional aluminum alloy castings. They also have reduced coefficient of thermal expansion and increased thermal conductivity and, when combined with other advantages, have stimulated interest in numerous applications.

Rose concluded that the metal can be cleaned and degassed quickly and efficiently. In all trials, the hydrogen content was reduced about 50% or more. He also found that the levels of oxides and inclusions were reduced through the degassing/cleaning treatment as indicated by the K-mold data.
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Title Annotation:CastExpo '93: 97th AFS Casting Congress, Chicago
Publication:Modern Casting
Date:Jun 1, 1993
Previous Article:Pattern shop keys: QC, service.
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