Size effect and forming-limit strain prediction for microscale sheet metal forming of stainless steel 304

Owing to the extensive applications of stainless steel 304 on a microscale, a series of microscale tensile and dome height tests were conducted to investigate its size effects on mechanical properties and formability. Based on the experimental results and observations and the Oh et al. fracture criterion, two new models were proposed in this paper for predicting the forming limit of stainless steel 304 foils in microscale sheet metal forming. For the mechanical properties study, foils of four thicknesses (150 μm, 100 μm, 50 μm, and 20 μm) heat treated at four temperatures (900 °C, 950 °C, 1000 °C, and 1050 °C) were used for the experiments. For the formability prediction, the first proposed model includes the effect of the strain path while the second proposed model considers the coupling effects of both the strain path and the size effects. The first model is superior to the Oh et al. criterion with respect to predicting the forming-limit strain of the foils, but it is not suitable for foils that are thinner than 100 μm. However, the second proposed model can be used for stainless steel 304 foils irrespective of their thicknesses and thickness-to-average-grain-size ratios T/D. It can also be concluded that the size effects must be considered in microformability when the foil thickness is smaller than 100 μm.