Local deformation and processing maps of as-cast AZ31 alloy

The hot formability of many metals was studied using the processing map concepts. In this work, a modified dynamic material model was used to correlate the flow data of AZ31 in the as-cast condition. Compressive tests of two batches, a coarse and a fine grained AZ31 magnesium alloy were carried out using a Gleeble (R) 1500 device at temperatures between 260 and 420 degrees C, and in a range of strain rates between 0.03 and 3 s(-1). The actual local strain rate and strain distribution in the samples were obtained by finite element calculations. The local microstructures of the deformed samples were related to the local deformation parameters and correlated with the processing maps at 0.3, 0.4 and 0.5 of logarithmic strain in order to determine the mechanism of restoration and instabilities present during the deformation. Three domains of deformation could be distinguished. At low temperatures and low strain rates, relative large power dissipation eta could be correlated to recovery at the grain boundary, followed by geometric dynamic recrystallisation. Cracks formation at these zones due to flow localisation results in instabilities predicted by the parameter kappa < 0. The high temperature range was characterized by an intermediate of eta value, where recrystallisation was observed. Deformations at high strain rates were characterized by low eta and instabilities due to adiabatic flow, which produced wedge cracks and shear bands. An apparent activation energy of 147 kJ/mol at the peak flow stress was calculated by a sinh-type constitutive equation suggesting dynamic recrystallisation and twinning formation. (C) 2009 Elsevier B.V. All rights reserved.