Hot Deformation of AA6082 Containing Fine Intermetallic Particles
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DUCTION
A. Hot Deformation in Aluminum Alloys
HOT deformation of Al-Mg-Si alloys is of industrial importance due to their increasing use in structural light parts as a consequence of their high specific bending stiffness and strength. During its processing, large ingots are hot rolled in a multistep mill after homogenization and before aging. Homogenization of AA6082 is carried out in order to spheroidize plate-like particles and to precipitate dispersoids.[1] Afterward and during hot deformation, dynamic recovery (DRV)[2] is the main restoration mechanism that takes place in aluminum alloys due to the high stacking fault energy[3] of these materials. If large strains are reached, dynamic recovery can be followed by grain pinching off which produces new grains by geometric dynamic recrystallization (gDRX).[4–6] All these dynamic processes are also influenced by the interaction between dispersed particles and dislocations or boundaries. In this way, small precipitates can pin or retard the movement of dislocations, grain and subgrain boundaries by means of the Zener-drag effect as shown in many works such as in Reference 7. On the other hand, large particles can CECILIA POLETTI, Associate Professor, is with the Institute for Materials Science and Welding, Graz University of Technology, Kopernikusgasse 24/I, 8010 Graz, Austria. Contact e-mail: [email protected] TOMASZ WO´JCIK, Project Assistant, is with the Institute of Materials Science and Technology, Vienna University of Technology, Favoritenstrasse 9-11, 1040 Vienna, Austria. CHRISTOF SOMMITSCH, Professor, is with the Institute for Materials Science and Welding, Graz University of Technology, and also with the Christian Doppler Laboratory for Materials Modeling and Simulation, Graz University of Technology. Manuscript submitted March 15, 2012. Article published online October 23, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
promote dynamic recrystallization,[8] although this effect is not commonly observed in aluminum alloys. The DRV occurs by developing a substructure and reaching a constant subgrain size (diameter) dss during hot deformation when steady state flow is reached. The size of the subgrains can be correlated to the Zener– Hollomon (Z) parameter as used in Reference 9 ln dSS ¼ B C ln Z
½1
where B and C are empirical constants. B. Constitutive Equations The phenomenological constitutive equations are used to describe the interdependence of the stress r with the strain rate e_ and temperature T during hot deformation. The Z parameter is defined as Z ¼ e_ expðQ=RTÞ
½2
where Q is the apparent energy of activation and R is the universal gas constant. Furthermore, Z can be related with the flow stress and temperature using the Eq. [3] for low stresses or the universal Eq. [4] for a wide range of stress values.[10] AP rnP ¼_e expðQP =RTÞ ¼ ZP
½3
A sinhðarÞn ¼ e_ expðQ=RTÞ ¼ Z
½4
with A, b, a, and n as material constants and P as a subscript related to the power law. In this work, the Eqs. [3] and [4] were replaced by the modified ones as used
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