Effect of ultrasonic vibration on stress relaxation in micro-compression test with step motion

In this study, to promote stress relaxation effect by the servo actuated step motion that stops the die motion at regular interval, a novel microforming process with combining ultrasonic vibration and the step motion was proposed. To investigate the effect of ultrasonic vibration on stress relaxation by step motion, a micro-compression test was carried out in different scale dimension with the specimen size of 0.5, 1.0 and 2.0mm in diameter and height. Stress relaxation by step motion was repeated for 6 times at constant strain interval. Ultrasonic vibration with amplitude of 1.2 and 2.4 μm was applied on the axial direction during the stress relaxation by step motion. As results, in the all process conditions, the stress drop increased with decreasing the specimen size. To investigate the size dependency of the effect of ultrasonic vibration on stress relaxation, the stress drop in the surface and inner grains was calculated based on the surface grain theory. The result showed that the stress drop in the surface grains was larger than that in the inner grains in similar scale dimension. Additionally, the stress drop in the surface grains increased with decreasing the specimen size. The possibility of improving the material formability by this process was experimentally demonstrated