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

Abstract

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.

Influence of Servo Motion on Forming Limit of Thin Metallic Foils Using Micro Bulge Test

“…This can be explained by the scale effect of the stress relaxation. In our former publication, the amount of stress relaxation increases with decreasing the specimen size in micro-compression test using the step motion or the combined motion [8]. Therefore, the effect of the formability enhancement would become more effective because the stress relaxation is promoted with decreasing the thickness from 0.08 to 0.05 mm.…”

“…This is attributed to the effect of stress relaxation, which suppresses plastic strains leading to localized thinning of the material by the homogeneous deformation. Additionally, authors developed a novel motion combined the step motion with ultrasonic vibration to enhance the stress relaxation [8].We have shown that the amount of stress relaxation increases by combining ultrasonic vibration and it becomes more significant with decreasing the specimen size.…”

Investigation on Material Deformation Characteristics of Ultrasound-assisted Microcompression with Dynamic Force Sensing Technology

“…From the last decade, the application of ultrasonic assistance has been attracting increasing attention in microforming. (1)(2)(3)(4)(5)(6)(7)(8) The benefits of ultrasonic vibration in microforming mainly include the surface and volume effects. The surface effect can lower the interfacial friction force between vibrated tools and specimens, (9) resulting in a better surface finishing, (10) and has been well studied in the past.…”