Prediction of mechanical behavior in single point incremental forming with an advanced constitutive model based on the non-associated flow plasticity

Abstract

Single point incremental forming (SPIF) is one of flexible forming processes in the sheet metal forming sector. The SPIF produces sheet metals in 3D shapes by using CNC (Computer Numerical Control) machine flexibly. The tool path is designed to follow the desired geometry shape. It is known that the single point incremental forming has enhanced formability than the conventional sheet metal stamping process. Therefore, the traditional FLD prediction is not suitable to the SPIF. To evaluate the forming limit for the SPIF, four different shapes (cone, ellipse, pyramid, clover) were formed by the SPIF machine. Each shape showed a different deformation mode on the surface. The combined strain modes generated a new SPIF fracture forming limit (FFL) of Al6014-T4. In order to predict and study the single point incremental forming (SPIF) process considering the anisotropy in the large deformation range, the evolutionary associated and non-associated flow ruled 3D constitutive models based on the Hills 48 and YLD2000-3d were developed, respectively. The evolutionary constitutive models were implemented to the commercial finite element simulation software ABAQUS/Explicit via VUMAT (Vectorized User-defined MATerial) subroutine. The FE simulation results followed the SPIF experimental profile and thickness variation well. However, the prediction difference between constitutive models (von Mises, e-NAFR Hills 48, e-AFR YLD2000-3d) was insignificant. Because, in the SPIF, the strain in the circumferential direction is very small, the anisotropy of sheet metal is ineffective. Whereas it was confirmed that the predictive ability of the advanced constitutive model is essential in a process such as incremental tube forming (ITF), in which the amount of strain in the circumferential direction is significant enough.