This study develops a new method to measure internal stress in materials. By using critically refracted longitudinal wave transducers with frequency of 1 and 5 MHz and relevant critically refracted longitudinal ultrasonic technique, we investigated how stress influence ultrasonic propagation time and critically refracted longitudinal wave propagating velocity along different directions in pre-stretched 7075 aluminium alloy plate. The experimental results from this study indicate that when specimen is axially tensioned and the angle between the direction of stress and the propagation orientation of critically refracted longitudinal wave increases, stress has decreasing effect on critically refracted longitudinal wave velocity. Besides, the velocity of longitudinal wave propagating perpendicular to axial hardly changes in stressed state and this demonstrates that the velocity of critically refracted longitudinal wave propagating perpendicular to the direction of stress has no relation with the applied stress. Therefore, critically refracted longitudinal ultrasonic technique has high potential to measure the stress component along one direction in the stressed material. This study presents a non-destructive method that can be used for measuring and evaluating internal stress of the material of interest.
To analyse the effects of loading pass and heating temperature on the deformation behaviour and microstructure evolution of 7075 cast aluminium alloy, multi-pass high temperature loading experiments on Gleeble-3500 thermal simulation compressor were carried out. The results indicated that under each loading, all the true stress-strain curves are typical dynamic recrystallization curves. As strain increases, the stress increases sharply and then basically remains unchanged when it reaches a maximum value. As temperature rise, the peak value of flow stress slightly decreases when the temperature is less than 350°C, whereas it increases slightly when the temperature is larger than 350°C. The grain was elongated perpendicular to the axis. Only a small amount of fine recrystallized grains were found to nucleate and grow at grain boundaries at a lower temperature (250°C). As temperature increased, the recrystallized grain gradually increases, and its proportion gradually also increases until the recrystallization is basically completed. The achievements can provide a guideline to select processing parameters for the processes of metal forming of 7075 cast aluminium alloy, such as ingot blooming, spinning and extrusion.
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