Comparison of compression and torsion constitutive analyses of Al-Mg-Si alloy at elevated temperature

In the last decade several papers dealing with FEM simulation of bulk forming processes have been presented and discussed in literature [1-2]. FEM simulations became a very useful tool for process analysis and optimization, especially in such cases, like extrusion, where on-plant experiments are complex, expensive and their results are of difficult interpretation. In order to perform reliable and accurate FEM simulations, a correct definition of the flow stress dependence on thermo-mechanical parameters is an essential prerequisite. In this context several equations have been defined and included in FEM codes. In particular, the Garofalo relationship [3] is widely used for modeling the flow stress behavior of aluminum alloys. In this equation the peak flow stress (sigma)) dependence on strain rate (ε̇) is described by the Garofalo expression: ε= A sinhασ exp( Q / RT) (1) where n is the stress exponent, Q is the activation energy for high-temperature deformation, R is the gas constant, T is the absolute temperature, A and  are material parameters. Other equations were developed recently taking into account the strain dependence on the flow stress behavior[4]. Several testing methods, both in cold or hot conditions, can be used in order to determine the flow stress. Tensile testing is the most popular but can be used only when small true strains (below 0.5-0.8) are accounted for, since the limit of this technique is the occurrence of necking which limits the maximum attainable strain. Compression testing requires an excellent lubrication of the specimen, since severe barreling of the specimen usually occurs at strain around 1. Torsion is a pure shear mechanical testing, permits to reach a high strain before rupture and frequently exceeds 5; these very high strains can be favorably compared with those typical of bulk processes like extrusion, where the strain field can reach values up to 10. The aim of the present paper is thus to study the hot deformation behavior of Al AA6063 alloy by means of hot compression and torsion tests in the 350-550°C temperature range, typical of real extrusion temperatures. The behavior of the alloy, in terms of equivalent stress vs equivalent strain, was modelled by different constitutive equations: a modified and strain dependent form of the Garofalo’s equation and the Hensel and Spittel equation. The experimental flow stress curves obtained by compression and torsion were compared with the curves predicted by the developed models and discussed in order to define the goodness of the models in predicting the materials hot flow stresses.