Work-hardening Characteristics and Forming Limit of Clad Sheet Metals.

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For recent demands to obtain products with light weight, high quality and high aded value, clad sheet metals are increasingly used in the press forming of various parts such as car, furniture and electronic components. Since a clad sheet is generally composed of 2 or 3 layers of different materials, the plastic properties are different from those of a single sheet.In the present investigation, the effects of the combination of sheet materials and tha clad ratio on the work-hardening characteristics and forming limit of a clad sheet are discussed. First of all, uniaxial tension, uniaxial compression through thickness and plane strain compression tests are carried out to know the plastic properties of each sheet metal composing a clad sheet. The Hill, Gotoh and Bassani anisotropic yield functions are compared with the experimental results.Secondly, an upper bound analysis of the uniaxial compression of (Al-steel) and (Al-Cu) clad sheets is made based on the stress-strain curves of each material measured above. From this analysis, it has been predicted how the stres-strain curve, yield locus and forming limit of the clad sheet metal change with the clad ratio.As an additional experiment related to the press forming of clad sheet, ironing of 3-layer (Cu-Al-Cu, Al-Cu-Al) tubes is carried out and the effects of ironing conditions on the thickness strain of each layer are clarified.Further investigations should be made of the constitutive equation and the factors for estimating formability of clad sheet metals. An analysis taking into consideration the plastic anisotropy of each sheet metal is also necessary to predict the onset of de-lamination which is one of the most important factors affecting the deformation behaviour of the clad sheet metal.