Hot deformation behaviour and flow stress prediction of 7075 aluminium alloy powder compacts during compression at elevated temperatures

In the present study, the hot deformation behaviour of 7075 aluminium alloy powder compacts was studied by performing hot compression tests on a Gleeble 3800 machine. The main objectives were to evaluate the effect of the relative green density on the hot deformation behaviour and to model and predict the hot deformation flow stress of powder compacts using constitutive equations. For this purpose, powder compacts with relative green densities ranging from 83 to 95%, which were prepared by uniaxial cold pressing a commercial pre-mixed powder, were hot compressed at temperatures ranging from 350 degrees C to 450 degrees C and at true strain rates ranging from 0.01 s(-1) to 10 s(-1). The true stress-true strain curves of the powder compacts exhibited a peak stress at a critical strain after which the flow stress remained nearly constant. As the deformation temperature increased or the strain rate and green density decreased, a decrease in the peak stress level was observed. The relationship between deformation temperature, strain rate, and the peak flow stress of powder compacts was described by the Zener-Hollomon parameter in an exponential equation containing relative green density compensated material constants and the deformation activation energy. The peak flow stresses calculated from the proposed formula were in good agreement with the experimental results, which confirms the applicability of the employed method for the prediction of the hot deformation flow stress of porous materials with different relative green densities. (C) 2011 Elsevier B.V. All rights reserved.