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

Abstract

A novel microforming process with combining ultrasonic vibration and the servo actuated step motion is proposed. To investigate the effect of ultrasonic vibration on stress relaxation by the servo actuated step motion, a micro-compression test is carried out in different scale dimension with the specimen size of 0.5 to 2.0mm in diameter. Stress relaxation by step motion was repeated for 6 times at constant strain interval. Ultrasonic vibration with amplitude of 1.2 and 2.4 ?m was applied on the axial direction during the relaxation time. As results, in the all process conditions, the stress drop increases 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 shows that the stress drop in the surface grains is larger than that in the inner grains in similar scale dimension. Additionally, the stress drop in the surface grains increases with decreasing the specimen size. The possibility of improving the material formability by this process was experimentally demonstrated.