Effects of vibration direction on the mechanical behavior and microstructure of a metal sheet undergoing vibration-assisted uniaxial tension

Abstract Ultrasonic vibration can cause softening and hardening during metal deformation, and this concept has been widely exploited in micro-forming. Recently, it was found that low-frequency vibration with micro-amplitudes caused the similar effects and it also varied with the vibration direction. This study investigated the effects of applying vibration in three directions (transverse, normal, and rolling denoted as TD, ND, and RD, respectively) on the mechanical behavior and microstructure of a commercial steel sheet during tensile deformation. Uniaxial tension test results showed that superimposed vibration in TD caused a significant stress reduction, and superimposed vibration in ND/RD resulted in a modest stress increase. Residual softening was observed in all three cases when vibration was removed and the tensile test was continued. Strain field estimation and electron backscatter diffraction revealed that vibration in TD caused localized plastic deformation, aggravated the deformation process, and the formation of a large number of low-angle grain boundaries. Vibration in ND/RD don’t have any effect on the localized plastic deformation. But the oscillatory stress was able to enhance the dislocation movements, promoted dislocation annihilation, and decreased dislocation density and the fraction of low-angle boundaries. Vibration softening and hardening effects were proved by the crystal plasticity theory and variation of activation energy and shear strain rate. Residual softening induced by vibration in TD was attributed to the plastic deformation in the “softer” region of the specimen. Compared with the normal tension, the decline of the dislocation density caused the residual softening in ND/RD. These results provide a fundamental understanding of low-frequency vibration-assisted forming.