Deep drawing behaviour of ultrafine grained copper : modelling and experiment

Ultrafine grained materials produced by severe plastic deformation methods possess attractive mechanical properties such as high strength compared with traditional coarse grained counterparts and reasonable ductility. Between existing severe plastic deformation methods the Equal Channel Angular Pressing is the most promising for future industrial applications and can produce a variety of ultrafine grained microstructures in materials depending on route, temperature and number of passes during processing. Driven by a rising trend of miniaturisation of parts these materials are promising candidates for microforming processes. Considering that bi-axial deformation of sheet (foil) is the major operation in microforming, the investigation of the influence of the number of ECAP passes on the bi-axial ductility in micro deep drawing test has been examined by experiments and FE simulation in this study. The experiments have showed that high force was required for drawing of the samples processed by ECAP compare to coarse grained materials. The limit drawing ratio of ultrafine grained samples was in the range of 1.9–2.0 with ECAP pass number changing from 1 to 16, while a higher value of 2.2 was obtained for coarse grained copper. However, the notable decrease in tensile ductility with increase in strength was not as pronounced for bi-axial ductility. The FE simulation using standard isotropic hardening model and von Mises yielding criterion confirmed these findings.

Size effects in micro cup drawing

Experimental studies into the effect of blank thickness on the deep drawing response of the coarse-grained and ultrafine-grained copper demonstrated the occurrence of a size effect: the dependence of the maximum load and the limit drawing ratio on the blank thickness in sub-millimetre range. A dislocation based constitutive model taking into account the thickness effects was used for numerical simulations of the process. It was demonstrated that the occurrence of the blank thickness effect is governed by the ratio of the blank thickness t to the grain size D of the material. Critical values of the t/. D ratio below which the size effect comes to bearing were determined. The obtained results can be seen as a demonstration of more general suitability of the model developed for predicting microforming operations with full account of the specimen or work-piece dimensions.