Abstract
The use of ultrasonic vibrations in manufacturing has been demonstrated its potential pathway to replace heating-assisted forming with significant benefits, including lowering the carbon footprint and increasing cost-effectiveness. This technology utilizes high-frequency mechanical vibrations, typically in the ultrasonic range (20 kHz or higher), to enhance the deformation and flow of metals during drawing, tube bending, punching, equal channel angular pressing, and incremental sheet forming. The utilization of high-frequency vibration in these manufacturing methods has revealed a fascinating interplay between the mechanical properties of metals and the dynamic forces introduced by the ultrasonic vibration. While the application of ultrasonic vibration in metal forming processes has shown promise in surface finish and reducing forming forces, there is still a need for more in-depth studies on the macroscopic response of materials during conventional mechanical testing for optimizing the process and ensuring the quality and reliability of formed components. In this study, the cyclic effect of ultrasonic vibration on the macroscopic material response of commercially pure aluminum, namely Al1100-O, was investigated through the implementation of ultrasonic-assisted compression (UAC) tests. Cyclic vibrations were applied to the compression of Al1100-O around a strain of 10%, 20%, and 30% with increasingly higher vibration amplitudes. Acoustic softening results presented in this work were calculated at each engineering strain. Both acoustic softening and residual effect due to ultrasonic vibration are experimentally verified in commercially pure aluminum in the range of 1μm and 3μm amplitude with 20 kHz vibration frequency. The mechanism of acoustic softening is centered on the localization of acoustic energy at defect sites, such as dislocations. This localization reduces the critical energy required for slip. In contrast, acoustic residual hardening is primarily attributed to the increase in dislocation density, resulting from the multiplication of dislocations induced by ultrasonic vibration. To validate the proposed mechanism, electron backscatter diffraction (EBSD) tests were carried out on formed samples. Comparing the misorientation maps, the overall low angle grain boundaries (LAGB) misorientation density increased for the UA sample, implying a smaller subgrain size. The formation of LAGB is a common method to reduce the overall internal defect density. Greater intensities in the kernel average misorientation (KAM) and geometrically necessary dislocations (GND) maps support the supposition that greater subgrain boundaries are formed during forming. The findings confirmed the significant effects of high-frequency vibration on metal plasticity and provided a basis for understanding the underlying mechanisms of vibrationassisted forming.
