Numerical analysis of the formability of an aluminum 2024 alloy sheet and its laminates with steel sheets
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I.
INTRODUCTION
THE precipitate-hardening aluminum 2024 alloy has high strength, superior to that of normal steels, and is commonly used as a light structural material. However, the forming products of the 2024 sheet are still limited. The press forming of the 2024 sheet is difficult, although the ductility itself is not so bad. In forming processes of the 2024 sheet, fracture often occurs without any obvious necking phenomenon.[1] In order to find the forming method and conditions suitable for a certain sheet, the forming limit, i.e., the fracture initiation in sheet-forming processes, has to be correctly predicted. Although many studies on fatigue cracking have been carried out for the 2024 sheet,[2–6] few studies have been found where the forming limit in practical forming processes of the 2024 sheet is numerically analyzed. In sheet metal forming, the forming limit is generally determined by the onset of localized necking, because the sheet tears immediately after the onset. Therefore, the conventional analytical methods used to predict the forming limit are based on the tensile instability or bifurcation theories.[7–10] However, these analytical methods are not applicable to the 2024 sheet, which breaks suddenly without any necking phenomenon preceding the forming limit. Needless to say, it is even more difficult to predict by conventional approaches the forming limit of the laminates composed of the 2024 sheet. In the present study, therefore, another approach is applied to the prediction of the forming limit of the 2024 sheet. The fracture initiation in sheet-forming processes is predicted by the finite-element simulation combined with a criterion for ductile fracture. The calculations are carried out for axisymmetric deep drawing of the 2024 sheet and for its laminates clad by mild steel sheets. The validity of the predictions is examined by comparing them to the experimental results. Furthermore, the possibility of improvHIROHIKO TAKUDA, Associate Professor, and NATSUO HATTA, Professor, are with the Department of Energy Science and Technology, Kyoto University, Kyoto 606-8501, Japan. Manuscript submitted December 23, 1997.
METALLURGICAL AND MATERIALS TRANSACTIONS A
ing the formability of the 2024 sheet by laminating it with steel sheets is examined.
II.
MATERIALS AND LAMINATED COMPOSITE SHEETS
The chemical composition of the aluminum 2024-T4 alloy sheet used in this study is indicated in Table I. The sheet thickness is 1.0 mm. Uniaxial tension tests were carried out at 0, 45, and 90 deg to the rolling direction. The gage length and width were 50 and 12.5 mm, respectively. The relations between the tensile force and the elongation are shown in Figure 1. The decrease in the tensile force before fracture is not observed in Figure 1, and it shows fracture initiation in the tensile specimen with no obvious necking phenomenon. The tensile properties are indicated in Table II, with average values in the three directions. The true stress-strain (s-ε) relation of the 2024 sheet can be approximated by the wor
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