Flow Modelling of AA 6082 Aluminium Alloy
The flow behaviour of AA 6082 aluminium alloy has been studied by means of torsion testing carried out at temperatures ranging from 425 to 500°C, with strain rates varying from 1 to 20 s −1. For a given temperature and strain rate, flow curves exhibit a p
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TRODUCTION The deformation processing technology depends on both advanced process modeling techniques, such as the Finite Element Method, and precise characterization of the plastic flow behaviour of the material under conditions of strain, strain rate and temperature, representative of the process. Equations describing the flow stress dependence on strain, strain rate and temperature are required as input to computer process models to predict the deformation history of any material element during forming [1]. Additionally, there are many practical hot forming operations where materials are subjected to high strain rates, so that the knowledge of a high strain rate response of the material is also required. The aim of the present paper is to analyze the hot forming behaviour of AA 6082 alumini um alloy, by means of torsion tests performed at high temperatures (425 to 500°C) and strain rates (1 to 20 s- 1), in order to obtain the material parameters in the constitutive equation describing the dependence of flow stress on strain, strain rate and temperature. The predictions, in terms of equivalent stress-strain curves, are analyzed and compared with experimental data.
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THE CONSTITUTIVE MODEL
The most used constitutive equation to model the hot working behaviour of metallic materials, proposed by Seilars and Tegart [2], correlates the equivalent flow stress a to the equivalent strain rate t: and to temperature T as follow:
Z
=e·exp( ir )= A. [sinh( aa J]n'
Published in: E.Kuljanic (Ed.) Advanced Manufacturing Systemsand Technology, CISM Coursesand Lectures No. 437, Springer Wien New York, 2002.
(1)
368
C. Bruni, A. Forcellese, F. Gabrielli
where Z is the Zener-Hollomon parameter representing the temperature modified strain rate, Q is the activation energy related to the deformation mechanisms taking place during the process, R is the universal gas constant, A, n~ and a are material parameters. Such relationship applies across a broad range in hot working reducing to a power law at low stresses and to an exponential law at high stresses. Equation (1 ), that is generally used only for particular points of the flow curves, for example at the peak or at the steady-state regime of the a-e curves, has the advantage of simplicity. However, since equation (1) neglects the strain effects, it results in a very coarse approximation of the true behaviour of materials. An alternative method to obtain constitutive equations in hot working conditions is based on a phenomenological approach and consists in determining the relationships giving Q, A and n' as a function of strain. By substituting such relationships into equation (1), the flow stress is obtained:
(2)
Such procedure was utilized in the present work in order to supply a flow stress data bank for the wrought aluminium alloy under investigation. Equation (2) accounts for strain rate and temperature sensitivity by means of Zener-Hollomon parameter. Moreover, the sensitivity of A, Q and n' values to the various restoration mechanisms taking place during deformatio
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