Abstract
The objective is to propose an accurate method for determining the forming limit curves (FLC) for ultra-thin metal sheets which are complex to obtain with conventional techniques. Nakazima tests are carried out to generate the FLCs of a pure copper and a copper beryllium alloy with a thickness of 0.1 mm. Because of the very small thickness of the sheets, the standard devices and the know-how of this test are no longer valid. Consequently, new tools have been designed in order to limit friction effect. Two different methods are used and compared to estimate the necking: the position-dependent measurement method (ISO Standard 12004-2), and the time-dependent method based on the analysis of the derivatives of the planar strain field. It is shown that the ISO standard method underestimates the forming limit curves. As the results present non linear strain paths, a compensation method is applied to correct the FLCs for the tested materials, which combines the effects of curvature, nonlinear strain paths and pressure. The curvature effect for such thickness and punch diameter on the FLCs is weak. The results show that this procedure enables to obtain FLCs that are close to those determined by the reference Marciniak method, leading to a minimum in major strain that converges to the plane strain state.
Highlights
The considerable increase in even smaller and lighter products is pushing several industrial sectors to move toward the field of miniaturization [1]
As the results present non linear strain paths, a compensation method is applied to correct the forming limit curves (FLC) for the tested materials, which combines the effects of curvature, nonlinear strain paths and pressure
The results show that this procedure enables to obtain FLCs that are close to those determined by the reference Marciniak method, leading to a minimum in major strain that converges to the plane strain state
Summary
The considerable increase in even smaller and lighter products is pushing several industrial sectors to move toward the field of miniaturization [1]. Forming by plastic deformation of metal parts with very small dimensions using ultra-thin sheets is playing an increasingly important role in the connectivity and electronics industry in particular [2]. The thickness of a sheet is what determines whether it is called thick, thin or ultra-thin. A sheet is ultra-thin if its thickness varies between 0.013 mm and 0.2 mm. The use of ultra-thin sheets in the manufacture of small parts is limited by several factors such as material, tools, friction, etc. The issues observed in micro-forming are strongly associated with miniaturization, since in their forming processes, some intrinsic dimensions of material such as the grain size and the surface roughness are not scaled down with the geometric dimensions [4]
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