Modelling of ultra-thin steel sheet in two-stage tensile deformation considering strain path change and grain size effect and application in multi-stage microforming

Multi-stage plastic deformation of metallic sheets for fabrication of complex micro structures with high aspect ratio features in different industrial clusters has been prevailing due to product miniaturization and integrated manufacturing. Owning to the varying strain path and the miniaturized scale of work pieces, both the strain path change (SPC) and size effect (SE) significantly affect the micro-scale deformation behaviors. To have a scientific insight and understanding of the influences of SPC and SE, two-stage tensile tests were conducted using the 0.1 mm thick SS 316L sheets with different grain sizes and EBSD was employed to characterize the microstructure evolutions. The results showed that the yield stress and elongation rate in the second tensile stage were decreased with the increase of pre-strain and the intersection angle between two tensile directions, while the hardening rate was found to be solely dependent on pre-strain. Changing the tensile direction in the second stage reversed the orientation distribution preference and raises the percentage of high Schmid factors, resulting in lowering the yield stress and hardening rate. On the other hand, more intra-granular mismatching boundaries accumulate in the coarsened grains, which impedes the dislocation movement and increases the deformation resistance. These two confronted mechanisms of SPC and SE interactively influence the deformation behaviors. A constitutive model for describing the flow stress affected by SPC and SE was established based on the micro-mechanism, which provided a basis to support the multi-stage microforming.