Non-contact ultrasonic study on thixotropic alloys

Thixotropic metals have better fluid flow behaviour in the semi-solid state when compared to conventional metals and have been increasingly used commercially for the production of high quality engineering components using the technique of thixoforming. However in industrial production, there is no proper quality control system available at present to monitor the softness or to estimate the percentage of solid/liquid fractions of the metal before it is forged into the required shapes. Due to the existence of a temperature gradient within the sample when heated in the RX furnace a bulk measurement is needed. This thesis examines the capability of non-contact ultrasound to make acoustic wave measurements in to the semi-solid state. In particular we have identified pulsed laser generation and EMAT reception as being the most appropriate approach. The Nd: YAG laser has proved capable of generating broadband ultrasound and a water-cooled spiral coil permanent magnet EMAT an efficient detector of both shear and longitudinal waves. The system enables both the longitudinal and shear waves to be detected simultaneously. Investigations have been conducted on various thixotropic metals ranging from lower melting temperature Al-Si alloys (≃570°C) to the higher melting point M2 tool steel (≃1200°C). A Cu-Sn alloy that has a melting temperature of around 830°C was also used for the investigation. Due to the high price and difficulty in obtaining the thixotropic materials preliminary measurements were carried out on non-thixotropic Al-Si alloys and a commercial mild steel. A water-cooled shear EMAT, furnace systems and an ultrasonic cell have been designed and constructed for the thesis. The experiment was carried out in a through transmission arrangement with the laser and the EMAT on epicentre. The experimental work is presented in chapters 4 to 6. Chapter 4 covers the investigation on the Al-Si alloys, chapter 5 covers investigations on the higher melting point materials, and chapter 6 presents investigations into the efficiency of the chosen laser-EMAT system and analyses systematic errors. The laser-EMAT system has proved capable of making measurements not only in the partially molten state but also in the fully molten state with sample temperatures up to 1140°C. The sudden drop in the longitudinal wave to shear wave transit time ratio indicates that the onset of melting could be detected. Since the ratio depends only on the temperature and the percentage of solid and liquid present in the sample and is independent of the sample dimensions it could be a suitable parameter for monitoring the percentage of liquid/solid fractions necessary for thixoforging. However work has yet to be carried out in the industrial environment to prove the capability of the system in monitoring the softness of the metal during thixoforming