Influence of soaking treatments on hot ductility of Al-4.85 pct Mg alloys containing Mn

Abstract

Interrupted hot torsion tests at 450 °C have been performed on homogenized as-cast specimens of Al-Mg alloys. The chemical compositions are approximately Al-4.85 pct Mg-0.74 pct Mn-0.22 pct Cr. The testpieces have been homogenized by three different heating schedules, and the number of revolutions to fracture in hot torsion tests has been measured. The tests lead to the finding that minor Si impurity, as little as 0.05 pct, is very harmful to the hot ductility. On the other hand, Fe impurity up to 0.34 pct has no influence. It is also found that homogenization schedule is very important to hot ductility. A two-stage heating consisting of a preheating at 430 °C for one and a half hours plus a soaking at 530 °C for eight hours results in the best hot ductility. Three different kinds of interdendritic precipitates of the compositions (Fe, Mn)Al_x, (Cr, Mn, Fe, Ti)Al₇, and Mg₂Si have been identified by electron microscopy. After homogenization submicron dispersoids of the composition MnAl₆, containing Fe and Cr as alloying elements, precipitate within the dendrites. These dispersoids are shown to exert major influences on hot ductility.