Effect Of Strain-path Changes On The Prediction Of Forming Limit Curve In AA3104Aluminum Sheet

Abstract:AA3104is one of the most widely used materials for canned products. Stamping is frequently used during the production of canned materials and it involves formability under multiple strain paths. A forming limit curve (FLC) reflects limiting strains of materials before instability and has significant meanings for designing stamping processes properly.By means of different heat treatment, H22and O states of AA3104are acquired. Microstructures and formability of the sheets in the two states are evaluated through metallographic observation, tensile test, and Nakajima test. FLCs are determined experimentally to provide comparable data for simulations later. Finally, the FLCs of both H22and O states are predicted, and the strain-path condition during forming process is also studied using finite element method (FEM). The main results and conclusions can be summarized as follows:(1) Equiaxed grains are observed in the cold-rolled AA3104sheet followed by annealing at400℃for3min, which is considered as the O state. After annealing for10min at this temperature, the grain size shows no obvious growth. No remarkable changes are obtained in grain shapes after annealing at300℃for90min of the cold-rolled AA3104sheet. The grains are fiber shaped and this state is considered as H22. Compared with the H22state, the elongation and the height of FLC in O state are larger, while the hardness, the yield strength, and the ultimate strength are lower. In other words, better plasticity but poorer strength is obtained after annealing at400℃for3min.(2) Results of Nakajima test simulated by FEM show that, under the frictionless condition, extents of nonlinearity for strain paths at the domes of the samples vary with different regions in FLC. In the equi-biaxial stretching region, the strain path at the dome keeps relatively proportional; while in uniaxial stretching and plane strain regions, extents of nonlinearity are relatively larger. With increasing friction coefficients, the position where maximum strain occurs is away from the dome and the strain paths in the right half region of FLC are inclined to the major strain axis.(3) Based on simulation results using FEM, maximum strain acceleration necking criterion proposed by Pepelnjak and co-workers is evaluated. The step determining necking position is modified and related physical meanings are proposed, which makes this method possible to predict FLCs under frictional conditions where necking positions are driven away from the dome.(4) Using FEM combined with such necking criterion, numerical results of FLCs in both H22and O states can be predicted successfully compared with experiments. Computed FLCs based on the classical Marciniak-Kuczynski (M-K) method show more derivations from the experimental results, indicating methods considering strain-path changes can be used to predict FLC more accurately.