The Effect of AZ31 Alloy Flow Stress Description on the Accuracy of Forward Extrusion FE Simulation Results

Abstract

Hot compression tests of the AZ31 magnesium alloy, performed for wide ranges of temperature and strain rate, revealed two different flow curve types for the material investigated. At higher strain rates and lower temperatures, flow curves exhibit a distinct peak. At lower strain rates and higher temperatures, flow stress values change less rapidly. This makes it difficult to find a single function able to accurately describe the deformation behaviour of AZ31 alloy in the entire forming range.The present study discusses an effect of the AZ31 magnesium alloy flow stress description on the accuracy of extrusion force prediction by means of FE simulation. A number of forward extrusion trials were carried out in order to acquire experimental data on AZ31 alloy deformation behavior in various forming conditions. Cylindrical billets of 40 mm in diameter and the tooling were initially heated to temperatures in the range of 200 to 400• C and placed in the working space of the 1500 kN hydraulic press to produce extruded rods of 12 mm in diameter. Numerical models for conducting corresponding extrusion simulations were prepared in Forge 2009 software and the selected form of Hensel-Spittel function was applied for the material flow stress description. Function coefficients were calculated both for the entire forming range of AZ31 alloy as well as for the ranges of parameters specific to a certain extrusion trial conditions. The numerical results were compared to the experimental ones and the accuracy of both approaches were estimated. It was found that the selected flow stress function, determined for the wide ranges of temperature and strain rate, allows to achieves a tis factory accuracy of AZ31 alloy extrusion force prediction by FE simulations.Keywords: magnesium alloy, AZ31, flow stress, extrusion, FE simulation Dwa różne rodzaje krzywych płynięcia uzyskano w próbach ściskania na gorąco stopu magnezu AZ31, prowadzonych dla szerokiego zakresu temperatury i prędkości odkształcenia. Dla wyższych prędkości odkształcenia i niższych temperatur krzywe płynięcia wykazują wyraźne maksimum naprężenia uplastyczniającego. Dla niższych prędkości odkształcenia i wyższych temperatur zmiany wartości naprężenia uplastyczniającego są mniej gwałtowne. Z tego względu trudno jest znaleźć jedną funkcję opisującą zależność naprężenia uplastyczniającego dla całego zakresu warunków odkształcania stopu magnezu AZ31.W artykule przedstawiono wpływ zastosowanej funkcji naprężenia uplastyczniającego stopu AZ31 na dokładność wyznaczania siły wyciskania poprzez symulacje MES. Przeprowadzono szereg prób wyciskania współbieżnego w celu uzyskania danych doświadczalnych charakteryzujących zachowanie się stopu AZ31 w różnych warunkach kształtowania. Próbki walcowe o średnicy 40mm wraz z przyrządem do wyciskania były nagrzewane do temperatury w zakresie od 200 do 400• C i umieszczane w przestrzeni roboczej pionowej prasy hydraulicznej o nacisku 1500kN. Następnie wyciskano z nich pręty o średnicy 12mm. Próby wyciskania zostały zamodelowane w programie FORG...

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