Constitutive relationships for AlZnMg, AlZnMgCr, and AlZnMgZr alloys

The effects of zinc, magnesium, chromium, and zirconium on the steady-state flow stress during hot working of both as-cast and homogenized AlZnMg(Cr/Zr) alloys were determined by means of torsion testing. The equivalent strain rates varied between 0.01/s and 10/s and the temperatures ranged from 450 °C to 560 °C. The zinc and magnesium concentration varied from 4.5 to 7.5 wt pct and from 0.8 to 1.8 wt pct, respectively. In addition, alloys containing typically 0.15 wt pct zirconium or 0.20 wt pct chromium were investigated. Magnesium, zirconium, and chromium were found to increase the flow stress, whereas zinc had practically no effect. The flow stress in the homogenized material was in most cases higher than in the as-cast material. Fitting of the coefficients in the hyperbolic sine constitutive equation to the experimental results showed that some of the coefficients could be related to concentrations of magnesium and zinc in solid solution, whereas others might be regarded as constants. The following relationship was determined between the coefficients α and n and the magnesium and zinc concentration: α=a·Mg] ^b and n=c·Mg] ^d +e·Zn]+f. The coefficients a, b, c, d, e, and f were determined by fitting of these relationships to the experimental data. The steady-state flow stress calculated by means of the constitutive equations was in good agreement with the experimental steady-state flow stress.