Multi-stage finite element modeling of the deformation behavior during ultrasonic-assisted incremental sheet forming

The incremental sheet forming (ISF) has the characteristics of high flexibility, short lead time and enhanced formability. However, it still needs to be further improved in the aspect of forming quality (e.g. geometric accuracy and surface finish). The introduction of ultrasonic energy field may improve the deformability of sheets and the performance of parts, and reduce the load during sheet metal forming. The present work aims to meticulously investigate the influence of ultrasonic vibration on the deformation mechanism of material in several periods. First, based on an improved material constitutive model which considers both the thermal activation and dislocation density evolution process, a multi-stage finite element model of UISF was established. In particular, the multi-stage finite element model of UISF is divided into two stages, the full contour forming stage and the local forming stage, in which a transient simulation with high frequency output was realized. The influence of vibration parameters on the forming force as well as the distribution of stress and strain field in the forming process was comprehensively analyzed. The results show that the ultrasonic vibration reduces the forming force obviously (53% at amplitude of 10 & mu;m), which is due to the dynamic impact effect and acoustic softening effect. Four distinct stages, approaching, loading, unloading and departing, are observed in a single vibration period. The dynamic impact effect of ultrasonic vibration changes the contact state between sheet and forming tool and causes the continuous propagation of dynamic oscillatory stress wave inside the material (propagate to the entire plate at 0.002 s), which leads to the significant change of stress and strain field. At the same time, the maximum stress is significantly reduced (73% at amplitude of 10 & mu;m). By applying ultrasonic vibration on the stress-strain field, it is found that these vibrations mainly act on the contact region between the sheet and forming tool. The high frequency vibration makes smooth stress distribution along the path as well as spread over the whole sheet area to create a uniform distribution. Benefitting from the smooth stress distribution, the vibration also promotes greater formability with good forming quality and the plastic deformation mainly occurs in the side wall deformed area. The present work provides a practical simulation approach to study the transient deformation behavior of materials under the ultrasonic field.