Scientists at Ames Laboratory, Ames, Iowa, have developed a nondestructive ultrasonic technique for observing the sintering of powdered metal components. Sintering involves pouring a metal powder into a die, which is then pressed and heated until the powdered particles bond together as a solid mass. By studying this process, Ames' scientists hope to better understand the interactions that influence a metal's strength. This would allow them to control the process to create stronger metals.
Ames' technique uses a commercial, custom-built electromagnetic acoustic transducer (EMAT) (see the figure). The EMAT unit is composed of two pairs of permanent magnets and electrical coils—one placed on either side of a pressed powder metal part as it rests in a sintering furnace. The magnetic and electrical fields produce the 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, they bounce back and forth in a pinball-like fashion when bonds are strong. As the sound waves exit the sample, they produce an electrical voltage in combination with the magnetic field. The voltage can be measured with an oscilloscope and observed on a computer monitor.
This technique lets scientists evaluate the bonds at work in powdered metals in real time. In turn, this helps them determine when the time and temperature are ideal to create powdered metal parts strong enough to handle maximum loads. While the technique is still in development, early tests prove that the scientists may be on the right track to uncovering and modeling the factors that affect a metal's strength.