The use of pulsed lasers for noncontacting generation of ultrasound in solid materials is expanding rapidly [1], as is optical detection of ultrasound [2]. The noncontacting nature of laser ultrasonics is opening new areas of research where physical contact of transducers to the material under study is impossible or inadvisable. One example is in the titanium melting industry. Currently, vacuum arc remelting (VAR) is used to produce much of the nation’s titanium from Kroll process sponge. However, the process provides only limited means of removing oxynitride and carbide inclusions from the melt, which can become stress intensifiers in the ingot. VAR of titanium can be replaced with plasma or electron beam hearth melting, both of which have the potential to eliminate these stress-intensifying inclusions by increasing the residence time of the molten titanium in the hearth so that the oxynitrides dissolve and the carbides settle out of the melt. This process is so important that industry is starting to replace VAR with hearth melting for titanium to be used in critical applications such as rotating turbine parts. The new process has other advantages as well. Processing steps will be eliminated because sponge will no longer need to be consolidated into electrodes and fewer melting steps will be required. The improved quality of the melted product will result in less scrap, and the ability to recycle scrap into high value products will also be a major improvement. The most important aspect, though, is the capability to produce superior ingots with the potential of allowing turbine engines to be lighter and more efficient. However, industry has identified a critical requirement for these hearth melting processes: measurement of the volume of molten metal to ensure sufficient residence time in the melt. Ultrasonic sensing is one possible way for locating the interface between molten and solid metal so that the depth of the molten metal, the volume, and thus the residence time may be determined. Because the titanium hearth operates at high temperatures (1650°C), contacting transducers with buffer rods are not practical; it is also a potential source of melt contamination. Therefore, a totally noncontacting sensor system is needed. This sensing technology would also be widely applicable to other metals, including other reactive and refractory metals, superalloys, and steel.</p
