For release: April 10, 2000

Contacts:                                                           
Jim Foley, Metallurgy and Ceramics, (515) 294-8252
Dave Rehbein, Metallurgy and Ceramics, (515) 294-8215
Danelle Baker-Miller, Public Affairs, (515) 294-5635
   
Ames Lab researchers developing technique to guarantee strength in metal parts

AMES, Iowa - Automotive manufacturers may one day be able to guarantee the strength of mass-produced parts made from powdered metals with research conducted at the U.S. Department of Energy's Ames Laboratory.

Powdered metal components are common today as mass-produced parts made of low-alloy steel, stainless steel, copper, brass and low-strength aluminum. The parts range from gears in an automobile steering system to  mountings for rear-view mirrors. To make these parts, a powdered metal is poured into a die, pressed and heated, or "sintered," to bond the powder particles together as a solid mass.

By merging the fields of nondestructive evaluation and powder metallurgy, known as P/M, scientists at Ames Laboratory have developed a new technique for observing the sintering process using a commercial, custom-built electromagnetic acoustic transducer.

According to associate scientist Jim Foley and metallurgist Dave Rehbein, P/M has needed an in-process sensor for a long time, though scientists have struggled to find something that works effectively. The ultrasonic, nondestructive technique Foley and Rehbein developed may satisfy industry's need to guarantee the strength of metal parts without destroying them to examine their properties.

The electromagnetic acoustic transducer enables Foley and Rehbein to evaluate bonds in powdered metals in real time as the parts sinter in a furnace. The prime 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 the EMAT could turn guesswork into formulas that engineers could use to predict correct processing conditions when mass producing parts.

Currently, the work of developing those formulas can be inefficient and time-consuming. Foley said 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 said. "In sintering, the major factors are the properties of the metal, time and temperature. But as with any other science, we need to know how these factors interact to create the strongest parts."

A standard EMAT can be used on any substance that conducts electricity, though it is difficult to operate at high temperatures because heat deteriorates the strength of its permanent magnets. Foley and Rehbein use ultrasonic transducers that were custom-built to tolerate temperatures up to approximately 600 C (1112 F). Their EMAT unit uses two pairs of permanent magnets and electrical coils, one on either side of a pressed P/M part as it rests in a sintering furnace. The magnetic and electrical fields produce eddy currents that create sound waves inside the metal. Once 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. But when the bonds become stronger, the wave output more closely resembles the input," Foley said. 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 that 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," Rehbein said. "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 working with other scientists at Iowa State University to better understand this relationship."
   
The pair's research offers a new approach in a much-needed area. "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 such as EMAT could be expanded for use in production, even evaluating sintered parts on an assembly line," Foley said.

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 said. The team plans to publish the sintering models that they develop.

Research on the inspection technique has been funded by the Department of Energy's Basic Energy Sciences Office. One of the DOE's primary missions is to engage in breakthrough research in materials science to improve the efficiency, economy, environmental acceptability and safety of energy sources.

Ames Laboratory is operated for the DOE by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.

View graphics of the EMAT unit and the sintering process.

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