Development of a VGRAIN system for CPFE analysis in micro-forming applications

To aid crystal-plasticity finite-element (CPFE) simulation for the forming of micro-components, a VGRAIN system has been developed to generate the microstructure of materials. Based on Voronoi tessellation and probability theory, grains and grain boundaries for micro-materials can be generated. A gamma-distribution function with three physical parameters of a material, which are the average, minimum, and maximum grain sizes, is used to describe the grain-size distribution of the material and a numerical method has been developed to link the gamma-distribution variables and the physical parameters of the material. The distribution of generated virtual gains using the developed VGRAIN system is compared with that of microstructural-examination data for a number of engineering materials, close agreements being obtained for the cases studied. Grain orientations, which are defined using two angles, related to the global coordinate system, have been assigned in the VGRAIN system according to probability theory. The crystal orientations for the virgin and deformed materials are represented using pole figures, so that the grain orientation before and after deformation can be compared and analyzed. The generated virtual microstructure, with grain-orientation information, is then input the commercial FE solver, ABAQUS/CAE, for further pre-processing for CPFE micro-forming simulation. A crystal-viscoplasticity material model for face-centered cubic metals is implemented in the FE code through the used-defined sub-routine, VUMAT/UMAT. CPFE analyses have been carried out to investigate the grain-size and orientation effects on the distortion of formed micro-pins.