Ultrasonic vibration-assisted incremental forming

Incremental forming (IF) is the process of shaping products gradually by applying local deformation, and has been introduced as an innovative applicable process for producing complex shapes with high quality and lower cost. However, some limitations—such as forming high-strength materials, surface quality and dimensional accuracy—have hindered its applications in industry. To solve these problems, this study introduces ultrasonic vibration (UV) to IF—titled ultrasonic vibration-assisted incremental forming (UVAIF). Material softening occurs under UV, known as the acoustoplasticity phenomenon, which can highly increase the material formability. However, the science behind this phenomenon is still a mystery. Thus, the aim of this thesis is to clarify the material formability under UV and the mechanisms of the novel UVAIF. To describe the material behaviour under UV, two different studies was undertaken: (i) material modelling under a wide range of strain rates, yet peeling off the effect of UV, and (ii) material modelling under UV by combining both the effects of strain rate and UV. For the case without vibration, a new constitutive model was successfully proposed by considering the dislocation drag mechanism. This model successfully linked the constitutive parameters with the characteristics of the material microstructures. To accurately predict the material behaviour under UV, this research also developed a novel comprehensive physics-based constitutive model by considering the dislocation dynamics and acoustic energy transformation mechanisms in the material. Experimental tests, numerical modelling, and statistical analysis were undertaken to investigate the UVAIF. The experimental tests confirmed the abilities of UVAIF, such as high formability with more uniform thickness distribution, low forces, low springback, finer surface quality, and enhanced friction condition. To clarify the forming mechanisms, finite element modelling was then undertaken with a self-developed subroutine based on the new constitutive model. Following this, the current research performed a parameter study based on the Taguchi method and single-factor analysis, followed by a regression study and dimensional analysis to investigate the UVAIF process systematically. The results confirmed that the material softening in the form of flow stress reduction easily occurred under UV, which subsequently increased the material formability, surface quality, dimensional accuracy, and friction condition