Under thermally activated deformation conditions many engineering metals (steels, aluminum and magnesium alloys) exhibit much enhanced formability; thus, thermal forming has received increasing interests by automotive industries. The thermally activated material constitutive behaviors are not only strain dependent, but also strain rate and temperature dependent, and it is sensitive to in-situ microstructure evolution. In addition, non-steady-state deformation at a high strain rate (in the order of 10−2s−1 or above) introduces additional challenges in forming simulation. In this case, von Mises based macroscopic plasticity are often not sufficient to describe material behaviors with complex thermomechanical history. In this paper, the rate-dependent crystal plasticity model [1] was applied to the high temperature and high strain rate deformation that is dominated by dislocation creep. A user material subroutine was developed and used for FEA metal forming simulation using commercial ABAQUS/Dynamic code. In the simulation, material behavior was computed based on crystal plasticity at each strain increment without using von-Mises equation or a look-up table of material testing data. By inputting different slip systems or their combinations, and by matching the predicted crystallographic textures with experimentally obtained ones, the active slip systems responsible for the deformation was identified. Then, the material parameters were best fitted to the tensile curves obtained at various strain rates and temperatures. The model was applied for more complex multi-axial metal forming processes. The material behavior, along with its crystallographic texture development, was obtained and validated. As a demonstration, this paper also provides an analysis of a newly developed thrmal forming process [2] with this meso-scale crystal plasticity approach. This forming process involves diameter expansion of a tubular workpiece under combined internal pressure and axial loading and at elevated temperatures.
Superplastic Deformation Mechanisms at High Strain Rates in Mechanically Alloyed Aluminum IN905XL.
