Aplicação do ensaio de compressão em discos empilhados para produção de moedas de prata esterlina Ag925 e cuproníquel Cu75Ni25

Abstract

The production of a coin is the result of several mechanical forming processes and the most important of them involves cold forging in a closed die, where a disc of raw material is pressed between two dies and a retention ring called a collar. This part of the production process has in common the application of high compressive loads with a low metal flow. And the result is the filling of multiple microcavities of the tool with the raw material and the consequent formation of reliefs on all faces of the disc. Only the topography of the part is affected on the order of micrometers, unlike traditional forging, and dimensional tolerances must be strictly controlled. Pressure can increase without limits during the process until the die resistance or press capacity is exceeded, requiring an appropriate choice of work parameters. Access to the flow curve of the material is fundamental in this sense and the standardized compression test is one of the ways to obtain it, however, the availability of sheets and plates of the raw material with dimensions compatible with those of the test specimen is restricted, which is usually a solid cylindrical. This study addresses the application of the compression test on stacked discs as an alternative to the traditional test for determining the flow curve for sterling silver and cupronickel. In the case of silver, the curves were initially obtained using the traditional method for a solid cylinder 20 millimeters high and 40 millimeters in diameter and then compared with others acquired by stacking 9 discs of the same material. Also for silver, the curves of the hardened and annealed metal were analyzed. In the case of cupronickel, the curves were obtained for discs measuring 6.4 millimeters in height and 13 millimeters in diameter and the correlations were carried out by varying the geometry of the stacks with 4 and 6 discs. The experimental result showed that the flow curves outlined with both stacked annealed sterling silver and cupronickel discs were compatible with similar literature, respecting deviations of the order of 10% and 5% respectively. The analysis also indicated that the representation of the curves of the solid silver cylinders did not converge with the stacks of unannealed discs of the same material, making it impossible to use them as a reference. In another test, the curves were able to illustrate the difference in terms of heat treatment between samples. Finally, the present study identified a series of influences on the modeling of the curves, related to the preparation of the tooling, the assembly of the stacks and the determination of the deformation parameters.