| To design reasonable parameters of dies and produce a better extrudate at a lower cost and in a short period, the metal plastic forming process simulation is the key technique. It can describe the whole forming process. But achieving the numerical analysis successfully depends on various factors: including constitutive relationship, the initial L/D ratio of the billet, the shapes of the profiles and tools, the friction condition, temperature and velocity of the punch etc. Among them, constitutive relation is one of the most important factors having an effect on the precision of FEM simulation.Compression test is a common method to determine material flow stress. However, even on the lubricated conditions, the friction on the interface between the die and the sample inevitably leads togenerate bulging of the sample, furthermore, the precision of the flow curve is affected owing to the above-mentioned bulging. Because the final forming dimensions of the specimen are easily measured, the definition of the bulge is defined based on the final maximum diameter and the minimum diameter. In this paper, a method combining FEM simulation with compression tests is used, The friction factor is evaluated by the bulged (bulge method), then reduce the error caused by friction, and the constitutive equation of AA6063 obtained in the end.Firstly, The upsetting tests were carried out using the DEFORM-3D program based on the finite element method, the cylindrical samples of AA2017 alloy are 15 mm in diameter, 15 mm and 22.5mm in height, that means the ratio of the initial diameter d0 to the initial height h0 are 1 and 1.5 respectively. Several simulations were done with various values of friction factor. Obtained two groups of relationship curves among bulge, strain and ratio of initial diameter to height with ten stepwise friction factor changes from 0.1 to 1.0, and the regression equations are given. Then several simulations were done using different materials andtemperatures in duplicate. Results show the relationship curves between bulging and friction factor calculated by FEM is not dependent on the material and temperature, it can be widely utilized as the calibration curves to evaluate friction factor. Furthermore, the bulge data is utilized to correct the flow stress.Secondly, the Gleeble-1500 hot simulator is used to conduct 17groups AA6063 specimens (H/D=1.5), then the calibration curves (H/D=1.5) is used to determine the friction factor values respectively, and the slab method was applied to reduced the error caused by friction. The flow stress curves direct from compression at strain rate 2.5, 4, 6.4, 10 and temperature 400 C, 440 C, 480 C, 520 C can be corrected, and as a result the true flow curves are obtained. Studies show the largest difference between the experimental stress and the true stress is about eighteen percent, so the correction of the flow stress values directed from experiments is essential, or a big error can be generated for the following simulation; It also can be shown that whatever the lubricated or unlubricated interface conditions is used, the true flow curves are very similar, so the influence caused by lubrication condition onthis method can be neglected.Thirdly, on the basis of the former work, the rigid-viscoplastic constitutive equation of the AA6063 is regressed, and the correlative coefficient between the constitutive curves and the experiment data is highly 0.99.The constitutive relationship obtained from the above-mentioned studies will be used as the material model in the FEM simulation. The coupled thermo-mechanical effects between the temperature and mechanical properties are taken into account. The porthole die hot extrusion process of a AA6063 tube is simulated by using the DEFORM-3D software, through analyzing the development of stress, strain, velocity and temperature distributions, the rules of metal flow are described in the extrusion process. The studies are significant for determining the parameters of the process and the die design.
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