RESUMO
Aluminum (Al) buffer rods, having circular, square, and rectangular cross-section shapes with and without cladding, were experimentally investigated to achieve high performance in ultrasound for industrial process monitoring. Focus was devoted to the signal-to-noise ratio (SNR) aspect in pulse-echo mode because high SNR can make many practical ultrasonic monitoring applications feasible. If the size of the rod cross section can be properly selected, spurious echoes induced in the rod having finite dimensions--due to mode conversion, wave reverberation, and diffraction--do not overlap the desired echoes during pulse-echo measurements. Thus, a sufficient SNR may be obtained using nonclad rods for certain applications, which is of low cost. Clad buffer rods, having the above three shapes and consisting of an Al core and a stainless steel cladding, also were manufactured through thermal spray method. These clad Al rods achieved a SNR of more than 40 dB and had better SNR than clad steel rods. It was shown that the rod with a larger cross-section area exhibited a wider ultrasonic beam. The Al rods are handy in real-time, nonintrusive and nondestructive ultrasonic monitoring.
RESUMO
In order to achieve net shape forming, processing of aluminum (Al) in the molten state is often necessary. However, few sensors and techniques have been reported in the literature due to difficulties associated with molten Al, such as high temperature, corrosiveness, and opaqueness. In this paper, development of ultrasonic techniques for imaging and measurements in molten Al using buffer rods operated at 10 MHz is presented. The probing end of the buffer rod, having a flat surface or an ultrasonic lens, was immersed into molten Al while the other end with an ultrasonic transducer was air-cooled to room temperature. An ultrasonic image of a character "N", engraved on a stainless steel plate immersed in molten Al, and its corrosion have been observed at 780 degrees C using the focused probe in ultrasonic pulse-echo mode. Because cleanliness of molten Al is crucial for part manufacturing and recycling in Al processing, inclusion detection experiments also were carried out using the nonfocused probe in pitch-catch and pulse-echo modes. Backscattered ultrasonic signals from manually added silicon carbide particles, with an average diameter of 50 microm, in molten Al have been successfully observed at 780 degrees C. For optimal image quality, the spatial resolution of the focused probe was crucial, and the high signal-to-noise ratio of the nonfocused probe was the prime factor responsible for the inclusion detection sensitivity using backscattered ultrasonic signals. In addition, it was found that ultrasound could provide an alternative method for evaluating the degree of wetting between a solid material and a molten metal. Our experimental results showed that there was no ultrasonic coupling at the interface between an alumina rod and molten Al up to 1000 degrees C; therefore, no wetting existed at this interface. Also because ultrasonic velocity in alumina is temperature dependent, this rod proved to be able to be used as an in-line temperature monitoring sensor under 1000 degrees C in molten Al.
