Size effect on plastic anisotropy in microscale deformation of metal foil

Abstract

Metal foils are extensively used for manufacturing microparts with a high depth-to-thickness ratio by microforming processes. Since there are few grains participating in the deformation of metal foil, the mechanical response of each individual grain intensively affects the deformation behavior and the quality of the formed micropart, which leads to strong anisotropy in microforming. To clarify the size effect on material anisotropy and the impact of scale-dependent plastic anisotropy on the formability of metal foil, a series of tensile experiments and geometrically scaled-down deep-drawing tests of SUS304 foil with different thicknesses of 50–200 μm and grain sizes of 10.2–80.5 μm were performed. The experimental results indicate that the anisotropy of flow stress, yield tensile ratio (i.e., ratio of the yield strength to the tensile strength) and elongation at angles of 0°, 45° and 90° to the rolling direction is prominently enhanced with decreasing thickness and that the anisotropy generally shows a substantial sensitivity to grain size. In addition, the abnormally increased hardening rate of flow behavior for metal foil with few grains in thickness direction arises from the deformation-induced transformation from austenite to martensite. The Lankford values in different orientations, the planar anisotropy and normal anisotropy coefficients decrease with increasing grain size and decreasing foil thickness. The foil manifests strong plastic anisotropy when the grain size is comparable to the foil thickness due to the dominant response of individual grains in terms of crystal orientation, texture, size and shape at a low value of t/d ratio (i.e., the ratio of the thickness to the grain size). Furthermore, several macroscale yield criteria are adopted to characterize the anisotropic phenomenon in the multi-scaled deep drawing process. The results suggest that the Yld2000-2d criterion is more suitable than Hill’48 and Von Mises functions to predict the anisotropic behavior of metal foil with a large number of grains involved in microforming. However, the prediction precision is worsened with decreasing t/d ratio, and the use of Yld2000-2d becomes problematic when the foil thickness is of the same magnitude as grain size, which is closely linked to the interactive effects of grain size, foil thickness and crystallographic texture in microscale plastic deformation.