Experimental and theoretical study on minimum achievable foil thickness during asymmetric rolling.

Parts produced by microforming are becoming ever smaller. Similarly, the foils required in micro-machines are becoming ever thinner. The asymmetric rolling technique is capable of producing foils that are thinner than those produced by the conventional rolling technique. The difference between asymmetric rolling and conventional rolling is the 'cross-shear' zone. However, the influence of the cross-shear zone on the minimum achievable foil thickness during asymmetric rolling is still uncertain. In this paper, we report experiments designed to understand this critical influencing factor on the minimum achievable thickness in asymmetric rolling. Results showed that the minimum achievable thickness of rolled foils produced by asymmetric rolling with a rolling speed ratio of 1.3 can be reduced to about 30% of that possible by conventional rolling technique. Furthermore, the minimum achievable thickness during asymmetric rolling could be correlated to the cross-shear ratio, which, in turn, could be related to the rolling speed ratio. From the experimental results, a formula to calculate the minimum achievable thickness was established, considering the parameters cross-shear ratio, friction coefficient, work roll radius, etc. in asymmetric rolling.

Extreme extensibility of copper foil under compound forming conditions.

A copper foil with an extreme extensibility up to 43,684% was obtained without any intermediate annealing by means of asynchronous rolling with high tension. It was found that under the combination of compression, shearing and tension, the copper foil represents a wonderful phenomenon. As the reduction increases, the specimen hardness increases up to a peak value 138 HV0.05 when the foil thickness rolled to around 100 µm, and then it decreases down to 78 HV0.05 when the foil thickness rolled to the final size 19 µm. It tells us that the strain-softening effect occurs when the foil thickness is rolled down to a threshold level. The experimental results bring us some fresh ideas different with the traditional understanding on the strain-hardening mechanism of metals, which provides an experimental basis to establish the forming mechanism of the thin foil.

Fabrication of ultra-thin nanostructured bimetallic foils by Accumulative Roll Bonding and Asymmetric Rolling.

This paper reports a new technique that combines the features of Accumulative Roll Bonding (ARB) and Asymmetric Rolling (AR). This technique has been developed to enable production of ultra-thin bimetallic foils. Initially, 1.5 mm thick AA1050 and AA6061 foils were roll-bonded using ARB at 200°C, with 50% reduction. The resulting 1.5 mm bimetallic foil was subsequently thinned to 0.04 mm through four AR passes at room temperature. The speed ratio between the upper and lower AR rolls was 1:1.3. The tensile strength of the bimetallic foil was seen to increase with reduction in thickness. The ductility of the foil was seen to reduce upon decreasing the foil thickness from 1.5 mm to 0.14 mm, but increase upon further reduction in thickness from 0.14 mm to 0.04 mm. The grain size was about 140 nm for the AA6061 layer and 235 nm for the AA1050 layer, after the third AR pass.