Effects of thermomechanical history on hardness of aluminium

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Abstract
A viscoplastic constitutive model with an evolving internal state variable, called hardness, has been developed for commercially pure aluminium. One application of such a constitutive model is in process modelling where hardness distributions may be predicted throughout the workpiece. This paper assesses the accuracy with which microhardness measurements of quenched specimens correlate with the hardness predicted by the constitutive model for various imposed thermomechanical histories. Using axisymmetric compression, different hardness values are achieved by various tests, both underdeveloped (increasing hardness) and overdeveloped (decreasing hardness) structures being produced during deformation. The steady state flow stress and hardness for a particular strain rate and temperature may be achieved with less strain by first deforming at a high strain rate and then decreasing the strain rate. The constitutive model accurately predicts the amount of prestrain required at the higher strain rate. Differing initial structures are achieved by using as-extruded and fully recrystallized materials. Using microhardness measurements as a method of characterizing the initial structure, the constitutive model is able to predict the subsequent flow curve and evolution of microhardness. MST/572