Real-time Sensing of Metallurgical Transformations by Laser-ultrasound

The ability of laser-ultrasound to monitor in real-time metallurgical transformations is reviewed in this paper. Although the technique is appropriate for online industrial materials characterization, this paper addresses laboratory characterization of metallurgical transformations. Examples are presented on the real-time sensing of grain size and grain growth, phase transformation and recrystallization of aluminum and steel. Results show good sensitivity of the laser-ultrasonic technique to microstructural parameters. The comparison with traditional in-situ (eg. dilatometry) or ex-situ (eg. metallography) characterization techniques is discussed. Experimental configurations for a cost-effective laser ultrasonic system coupled to a laboratory furnace and more specifically to a Gleeble thermo-mechanical simulator are discussed. Introduction: The use of laser-ultrasonics to the monitoring of metallurgical transformations in real-time is a natural application of the technique due to its remote sensing character. Ultrasonic waves are generated by a pulsed laser (typically of a few nanoseconds of duration and with energy below one Joule) and the detection is assured by another laser (long pulse or continuous) and an interferometer that, together, probe ultrasonic displacements at the material’s surface [1]. This all-optical ultrasonic method allows ultrasonic probing of materials at high temperature at a convenient distance (typically tens of centimeters) on the outside of the furnace. In addition to the non-contact character of the laser-ultrasonic method, the technique is broadband (this is especially interesting for frequency domain analysis and for high resolution in the time domain) and has fewer constraints on material geometry (material surface is the transducer: curved surfaces are allowed). The present paper discusses the use of the laser ultrasonic technique to real-time monitoring of metallurgical transformations and reviews selected results obtained at IMI/NRC using a laser-ultrasonic system coupled to a Gleeble thermo-mechanical simulator. Description of the laser-ultrasonic system coupled the Gleeble furnace: Resistance heating thermo-mechanical simulators have been widely used on metallurgical research to do physical simulations of industrial thermo-mechanical cycles. But only a few hot-stage-adapted sensors are able to monitor rapidly changing microstructures. Among the slow response techniques are the hot stage electron microscopy (TEM, SEM) and X-Ray and neutron diffraction techniques. Dilatometry is a fast, simple, and widely used technique to sense metallurgical transformations but provides very limited information. Ultrasonic techniques are well known to be highly sensitive to many microstructural parameters like grain size, texture, phase fraction, porosity, inclusions, etc. and can be applied to fast transforming microstructures. Figure 1a illustrates the principle of the laser-ultrasonic technique. The generation laser pulse heats the material’s surface and vaporizes a small volume (a few nm of material) that, by a recoil effect, generates an ultrasonic wave that propagates through the material. Simultaneously the detection laser illuminates the sample surface and the reflected (or scattered) light, phase-modulated by the surface motion caused by acoustic disturbances, is demodulated by an interferometer. Figure 1b shows a signal obtained at high temperature with the laser-ultrasonic technique. Detection LASER Generation LASER