Constitutive relationships for AlZnMg, AlZnMgCr, and AlZnMgZr alloys
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f the main advantages of AlZnMg alloys in comparison with other aluminum-base alloys is their high strength combined with high ductility. For example, typical 0.2 pct proof strength for alloy AA7108.70 is approximately 400 MPa and elongation to fracture (A5) is approximately 12 pct in the T6 condition. This alloy contains typically 5.4 wt pct Zn, 1.2 wt pct Mg, and 0.15 wt pct Zr. The high strength of these alloys makes them very suitable for structural applications where there are strong demands for high strength combined with minimum weight. One example is bumper beams in cars, which can be made of hollow or semihollow hot extruded profiles. One disadvantage of the AlZnMg alloys is that their high room-temperature strength is accompanied with a high deformation resistance at hot working temperatures. The high deformation resistance is mainly attributed to the presence of magnesium, copper, chromium, and zirconium.[1,2] High deformation resistance at hot working temperatures may result in a low extrusion speed due to limitations in available ram pressure and due to heat generation during extrusion. A high ram pressure causes large stresses in the extrusion tool, and the tool life may therefore be reduced. It has been shown that zinc does not influence the deformation resistance during hot working[1,3,4] but contributes significantly to the room-temperature strength after aging. From extrusion tests, it is found that chromium and zirconium, both in solid solution and precipitated as dispersoids, increase the ram pressure.[2,5] Numerous investigations show
BJØRN RØNNING, Postdoctoral Researcher, and NILS RYUM, Professor of Materials Science, are with the Department of Materials Technology and Electrochemistry, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. Manuscript submitted July 31, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
that magnesium increases the flow stress during high-temperature deformation and thus the ram pressure during extrusion.[1–4,6,7,15] During extrusion experiments, temperature and strain rate vary with time and with position within the material being extruded, and are thus not easily measured or defined. Therefore, such experiments do not give any precise information about how the effects of zinc, magnesium, chromium, and zirconium on the flow stress are influenced by strain rate and temperature. It is also difficult to establish a relationship between flow stress, temperature, and strain rate on the basis of extrusion experiments. In this investigation, hot torsion tests will be carried out to establish a relationship between these quantities, and also to determine the effects of the aforementioned elements on the flow stress for different strain rates and temperatures. It has been shown earlier that the hyperbolic sine law successfully describes the relationship between flow stress, strain rate, and temperature at hot working conditions:[10,11]
冢 RT冣
˙ ⫽ A ⭈ [sinh (␣)]n ⭈ exp ⫺
Q
[1]
where Q is the activation energy for hot deformation; R is the gas c
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