Mechanical Investigation of CuZn30 Brass Under Conditions of Variable Axisymmetric Deformation

This paper presents the results of investigations on CuZn30 brass subjected to cyclic oscillatory torsion. The authors hypothesized that by applying specific deformation conditions under which a 180° change in the orientation of the principal components of the strain state occurs, it is possible to induce a change in the flow stress and strain. Experiments with various strain rates (0.01, 0.1 and 1 s^-1) and different strain amplitudes (0.03, 0.06 and 0.3) were conducted at room temperature. Monotonic tests, carried out at the same temperature and under the same strain rates, served as reference results for the cyclic torsion tests. In metal forming processes, it is often assumed that elastic strains are so small compared to plastic strains that they can be neglected. However, during cyclic oscillatory torsion tests, elastic strain occurs in each deformation cycle and hinders the accurate determination of the true plastic strain. Therefore, the authors developed a dedicated method for separating elastic and plastic strains, which they applied to determine the plastic strains in individual cyclic hysteresis loops. In the cyclic torsion tests with the two smaller strain amplitudes (0.03 and 0.06), the increase in the limit strain was remarkably large-up to more than 80-fold. Such large strains were achieved at a flow stress significantly lower than that in the monotonic torsion tests, with a maximum stress drop of two and a half times. Conversely, in the tests conducted across all applied strain rates for the amplitude of 0.3, the stress was always distinctly higher, and the strain was much smaller than for the two smaller amplitudes. In all investigated cases, the results showed that for each applied strain rate, an increase in the strain amplitude leads to a very pronounced increase in the flow stress value and a reduction in the strain level, which is driven by the enhanced work-hardening of the material. The experimental results clearly demonstrate that precise control of cyclic strain parameters (strain rate and amplitude) allows for flexible tailoring of the flow stress and limit strain of the CuZn30 brass during cold forming. The authors demonstrated that forcing a 180° change in the deformation direction can lower the yield strength and, consequently, improve the deformability of the CuZn30 brass. The obtained results may contribute to the development of energy-efficient, advanced metal forming processes, as an appropriately selected cyclic deformation scheme can significantly influence both the manufacturing process and the final properties of the product.