By Danelle Baker-Miller
Before scientists develop ultrasonic techniques to view fetuses in vivo, old wives' tales were as good a method as any for determining a baby's sex. Did the mother eat peanuts after conception? Does the pillow on her bed face north?
Just as nondestructive evaluation methods like ultrasound have enabled doctors to safely gather more information about a baby before birth, they have made it possible for other experts to eliminate guesswork and understand more about their discipline.
Two Ames Laboratory metallurgists, Jim Foley and Dave Rehbein, have merged the fields of nondestructive evaluation and powder metallurgy, known as P/M, and their expertise. Though not new, the two areas combine to put a new spin on the study of sintered metals. Sintering is a solid-state process, unlike melting, in which compressed metal powder is heated to form a solid mass. During heating, the atoms intermingle across the powder contact surfaces to "heal" gaps, bonding the powders together.
Foley and Rehbein explored options for applying ultrasonic techniques to study P/M sintering after Foley became leader of the Lab's Nondestructive Evaluation group a few years ago.
"P/M has needed an in-process sensor for a long time, but nothing works very effectively. When Jim became involved, we met to discuss using the two fields to improve on current techniques," Rehbein says.
Powdered metals are commonly used today in mass-produced parts made of low-alloy steel, stainless steel, copper, brass and low-strength aluminum. The parts range from gears in automobile steering systems to mountings for rearview mirrors.
But how do engineers know when these bonds are at their optimal strength? How, in the arena of mass-produced parts, can they guarantee that all of the gears produced in a day have a particular strength without breaking them apart to examine their properties?
Enter the electromagnetic acoustic transducer, the tool Foley and Rehbein are exploring for its potential in P/M.
Much like ultrasound on a fetus, EMAT enables them to evaluate bonds in powdered metals in real time, as they sinter in a furnace. The tool's primary benefit is the elimination of wasted time and materials required by the current destructive examination of prototype parts. The data the two researchers can acquire from using EMAT could turn guesswork into formulas engineers could use to predict processing conditions when mass producing parts.
Currently, the work of developing those formulas can be inefficient and time-consuming. Foley says EMAT is a mechanism for smoothing out the rough path of trial and error.
"In an ideal world, you wouldn't need to do any sintering experiments. You'd have an equation and plug in variables to determine the correct processing conditions. But to get to that stage, you need a method for measuring sintering. That's what EMAT is," he says. "In sintering, the major factors are the properties of the metal, time and temperature. But as with any science, we need to know how these factors interact to create the strongest parts."
EMAT has long been used to detect internal flaws in metals. David Hsu, a scientist at Iowa State University's Center for Nondestructive Evaluation, has used the technique to study airplane wings made of composite materials. To make the paper and resin wings electrically conductive, he covers them with aluminum foil.
"A standard EMAT can be used on anything that conducts electricity, but normally, the technology won't work under high temperatures because heat deteriorates the strength of its permanent magnets," Rehbein explains.
The commercial ultrasonic tranducers Foley and Rehbein use were custom-built by Ron Alers of Sonic Sensors, a company in San Luis Obispo, Calif., to tolerate temperatures up to approximately 600 C (1,112 F). Unlike other ultrasonic techniques that require a couplant of water or jelly, EMAT is noncontact and requires no couplant. Their EMAT unit uses two pairs of permanent magnets and electrical coils, one on either side of a pressed powdered-metal part as it rests in a sintering furnace. The magnetic and electrical fields produce eddy currents that create sound waves inside the metal.
Rehbein likens the ultrasound in EMAT to a microwave oven. "The magnetic and electrical fields come in combination from outside the sample but produce sound pulses inside the sample. Microwaves operate much the same way, by creating heat inside the food rather than by heating the oven itself," he says.
Inside the sample, sound pulses respond to the strength of the metal's bonds by dying out when bonds are weak or bouncing back and forth in pinball fashion when bonds are strong.
"The weaker the bonds, the more difficult it is for sound waves to propagate. The echo amplitude is low and output in the receiving end of the EMAT probe is weak," says Foley. "But when the bonds become stronger, the wave output more closely resembles the input. We can see much higher peaks on the monitor. In a 100-percent dense piece of sintered metal, the echo amplitude of the input wave would be nearly identical to the output wave," he explains.
On the way out, the sound waves, in combination with the magnetic field, produce electrical voltage that is measured with an oscilloscope and observed on a computer monitor.
Foley and Rehbein are confident they are on the right track -- tests show a nearly 100 percent correlation between measured output voltage and the load-bearing capacity of aluminum-copper alloys containing different percentages of silicon carbide after the alloys were cooled and removed from the furnace.
"The data showed an absolutely wonderful correlation," says Rehbein. "I had a hard time believing how well this technique works. Whatever it is in a metal's atomic structure that is controlling strength, it appears to also control echo amplitude. We're going to work with Bruce Thompson, director of ISU's Center for Nondestructive Evaluation, to better understand the connection."
Foley underscores that the EMAT-P/M combination is breaking new ground. "No one else has been able to evaluate P/M parts this way. This is very new science. With EMAT, we can experiment and determine when time and temperature are ideal to create powdered-metal parts strong enough to handle the maximum loads. Eventually, technology like EMAT could be expanded for use in production, even evaluating sintered parts on an assembly line," he says.
Though EMAT could eventually see the interior of automotive factories, it won't lose its value as an experimental tool. "The technique will still be important as an experimental tool for new alloys. It will always be used as a feedback mechanism to be certain that a metal's properties are as good as they should be," Foley says. The team plans to publish the sintering models they develop.
For more information:
Jim Foley, (515) 294-8252
foley@ameslab.gov
Research funded by:
DOE Office of Basic Energy Sciences
DOE Laboratory Directed Research and Development funds
Last revision: 9/15/00 sd
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