On the mechanical behavior of commercial-purity aluminum deformed under axisymmetric compression conditions

Abstract

Abstract The stress-strain behavior of commercial-purity aluminum deformed under axisymmetric compression conditions in the temperature range of 293–673 K is analyzed on a rational basis. The strain dependence of the flow stress at every temperature and strain rate is satisfactorily described by means of the exponential-saturation equation earlier proposed by Voce (1948, 1955). The temperature and strain rate dependence of both the initial flow stress and the saturation or steady- state stress is analyzed in terms of two different models. First, the hyperbolic-sine model advanced by Sellars and Tegart (1972) in terms of the Zener-Hollomon parameter, assuming that the activation energy for deformation of this material remains constant and equal to 156 KJmol−1 in the whole temperature interval. Second, the model proposed by Kocks (1976) in terms of a power-law considering that the stress sensitivity exponent of the strain rate is significantly temperature-dependent. This model leads to the introduction of a temperature-compensated strain rate parameter similar to the MacGregor-Fisher parameter. Under the consideration that the initial strain hardening rate should be independent of temperature and strain rate it is determined that the relaxation strain parameter involved in the Voce equation can be calculated as a linear function of the saturation stress. It is concluded that although both models describe the experimental data with similar accuracy, according to the statistical parameters calculated for every approach, Kocks model allows a somewhat better correlation with the present experimental data. The description of the stress parameters by means of the hyperbolic-sine relationship suggests a decrease in the activation energy for deformation with temperature below 473 K. Significant deviations in the computation of the stress-strain curves and in the description of the work hardening behavior of the material under some deformation conditions are still observed independently of the model employed. However, it is believed to be due to two severe requirements imposed simultaneously to the model: the description of experimental data determined under a wide spectrum of deformation conditions while maintaining a minimum of material parameters in the constitutive functions developed.