Prediction of Surface Roughening and Necking Behavior for Metal Foils by Inhomogeneous FE Material Modeling

Abstract

Micro metal forming with metal foils is one of the promising approaches to fabricate micro parts. In this study, a finite element (FE) model for metal foil considering material inhomogeneity due to different flow stresses for each crystal grain to predict free surface roughening and necking behavior is suggested. Material used is pure copper C1020-O, pure aluminum 1N30-O and pure titanium TR270C-O with thickness of 0.05mm. Material inhomogeniety parameter of variation in α value is determined by parameter fitting between uni-axial tensile test and FE analysis considering material inhomogeneity under uni-axial tensile state. Standard deviation σsd of variation in α value of 0.28 for C1020-O is obtained by parameter fitting process. In addition, free surface roughening behavior is observed by FE analysis considering material inhomogeneity and confocal laser microsope. As a result, the increase in surface roughness with uni-axial tensile deformation can be observed for both FE analysis and experiment. In addition, it is considered that the generation of concave parts in free surface roughening is due to grains with low flow stress by quantitative measurement of FE analysis and confocal laser microscope. Surface roughening behavior of FE analysis considering material inhomogeneity is in good agreement with that of experimental results. Thus, the validation of FE model considering material inhomogeniety for metal foils can be verified. Furthermore, the effect of material properties for metal foils such as grain size, material inhomogeneity parameters and strain hardening sensitivity on necking behavior is investigated. As a results, it is found that the ratio of surface roughening to thickness strongly affects necking behavior for metal foil. In particular, in case of large n-value, the concave part generated by surface roughening during plastic deformation would cause the onset of necking for metal foils. Therefore, it is found that the factor of surface roughening is very strong in micro metal forming with metal foils.