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

Abstract

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 mm, 100 mm, 50 mm, and 20 mm) heat treated at four temperatures (900 uC, 950 uC, 1000 uC, and 1050 uC) 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 mm. 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 mm.

Investigations of size effect on formability and microstructure evolution in SS304 thin foils

“…9 Decrease of tensile flow stress and increase in elongation were observed in SS304 sheets with increasing thickness from 20 to 150 mm. 10,11 Gau et al 12 observed a similar size effect in flow stress on tensile testing of aluminium and brass sheets. They found that the mechanical properties worsen with decreasing specimen thickness to grain size ratio (T/D ratio) till T/D ratio approaches 1.…”

“…The authors proposed two new approaches for forming limit prediction and systematically investigated necking and fracture strain of sheet foils at different temperatures to identify the effect of T/D ratio and temperature on formability. 11 Bulging test experiments were conducted by Michel and Picart 13 for microscale brass sheets to investigate the miniaturization effect and it was found that flow stress decreases with miniaturization. Hydraulic bulging tests of 51-mm-thick SS304 with varying grain size and punch diameter showed that flow stress decreases with decreasing T/D ratio from 5.5 to 3 and decreasing ratio of bulge diameter to thickness from 1961 to 191.…”

Softening Effect on Fracture Stress of Pure Copper Processed by Asynchronous Foil Rolling

“…It was found that the dynamic recovery at boundaries at large strain eliminates grain size effect on work hardening. Chen et al [9] proposed two models to predict the forming limit of stainless steel 304 foils in micro-forming with the consideration of size effect and ductile fracture criterion. Fan [10] established a composite model to describe the size effect on fracture behavior by treating polycrystalline material as a composite consisting of grain interior and grain boundary.…”

Effects of grain size and heterogeneity on the mechanical behavior of foil rolling

“…The metal forming in macro-scale has been developed for many years and a series of classic theories have been established. However, when the part's geometry size is scaled down to micro-scale, the design of micro-parts fabricated by micro-forming cannot be conducted based on the knowledge transfer from macro-forming to micro-forming due to size effect [1][2][3][4][5]. For these micro-products, the size of the microstructure is generally as the same order as the geometry dimensions, leading to different size effect.…”