Experimental validation of the simulation of single-point incremental forming of AA7075 sheet with Yld2004-18P yield function calibrated with crystal plasticity model

Abstract

Incremental sheet forming was developed several decades ago as a cost-effective forming process for low volume production. The deformation mechanism of this process is completely different from the conventional sheet metal–forming processes. Applying a three-dimensional (3D) yield function such as Yld2004-18p for the simulation of incremental sheet forming is necessary to account for the significant anisotropy and out-of-plane shears that develop in the sheet metal. Since it is difficult to experimentally measure out-of-plane shear stresses, a set of virtual experiments was conducted with crystal plasticity finite element method (CPFEM) to obtain the required data. To that end, five different representative volume elements (RVEs) were constructed and simulated with a rate-independent CPFEM model to assess which model best predicts the anisotropy of the AA7075 aluminum sheet. Of the four CPFEM models based on the associate flow rule, three used RVEs accounting for grain orientations and grain size distributions, while the fourth model used only the grain orientation information (Taylor’s assumption). The fifth CPFEM model was modified based on the Hill’s 1948 non-associated flow rule and used the Taylor’s assumption. It was verified by experimental data that the fifth CPFEM model (Taylor + Hill) provides the most computationally efficient and accurate prediction of flow stresses and R-values as a function of the accumulated plastic work. The results from the Taylor + Hill model were then used to calibrate the Yld2004 yield function used in the simulation of the single-point incremental forming (SPIF) of 45° and 67° conical shapes with AA7075 sheet metal. It was found that simulation results obtained with Yld2004 yield function well predicts the deformation characteristics of both cone shapes when compared with experimental results. Also, the yield locus of AA7075 sheet metal after the SPIF process was predicted based on evolved crystal orientations and the critical resolved shear stress.