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Formability of sheet metals is often assessed by means of forming limit diagrams (FLD). In this paper, performing three different series of experiments, the effect of forming velocity on FLDs is investigated experimentally for Al 6061-T6 and AISI 1045 sheets. In addition to determining the conventional FLDs at quasi-static condition ( _ " 6 0.01/s), FLDs were acquired in the case of forming by low impact ( _ " 6 50/s) as well as explosive free-forming ( _ " 6 1000/s). Samples were deformed in different strain states, thereby generating data for both sides of the FLD. Numerical simulating of stand-off explosive forming tests, the dies and specimens were designed accurately. The Johnson-Cook constitutive model for metal sheets and the Jones-Wilkins-Lee (JWL) model for the explosive charge were used. In order to optimize the sheet and explosive mesh size, a sensitivity analysis was carried out by using the response surface method.The results show that while a substantial improvement in high strain rate formability of the aluminium sheet can be obtained, this improvement is not considerable for the steel sheets. Also, under the effect of contact pressure between tool and sheet, the formability increases for both steel and aluminium sheets on impact.

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The incremental sheet metal forming is a flexible process producing various parts with no need for dedicated dies. In this article, the upper-bound approach is used to study the deformation zone of single point incremental forming of truncated cones. The velocity field and the dissipated power of the process are achieved using an assumed deformation zone and streamlines defined by Bezier curves. The tangential force acting on the tool is attained by optimizing the presented upper-bound solution. Then, influences of the effective parameters including vertical pitch, initial thickness, tool diameter, and wall angle on the tangential force and the predicted equivalent strain are investigated. A strain hardening law is utilized to consider the work hardening behavior of sheet metals. In order to validate the presented upper-bound solution, predicted tangential forces are compared with experimental data reported in literature for forming AA3003 truncated cones. The comparison shows an appropriate agreement between experimental and predicted tangential forces.

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Forming processes of metal sheets are generally limited by plastic instability phenomena and flow localization. The occurrence of these phenomena is dependent on the material properties such as strain-hardening exponent, strain rate sensitivity, anisotropy parameters and grain size and is also dependent on the strain path. The formability of the sheet metals can be assessed by the forming limit diagram (FLD). In this study, a theoretical model using the ‘many slices’ approach is introduced to simulate the neck growth. The effects of changing strain path and grain sizes on the limit strains are then investigated both theoretically and experimentally. The low carbon steel ST12 and austenitic stainless steel 321 are used in the experimental approach. The theoretical and experimental FLDs of these sheets are obtained for different grain sizes and after pre-straining in uniaxial and biaxial tension parallel to the prior rolling direction. It is shown that the limit strains are quite sensitive to the grain size and strain path. Thus, by selecting the proper strain path and grain size, better formability properties can be achieved. Also, good agreement is obtained between theoretical and experimental results.

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