Precipitation Processes in Al-Mg-(Mn,Cu) Type Alloy Sheets Evaluated Through Electrical Resistivity Variations

Precipitation/dissolution processes were followed by electrical resistivity variations in Al-Mg-(Mn)-Cu and Al-Mg-Mn type alloy sheets after different thermo-mechanical treatments (TMTs). In order to get an insight into the precipitation processes during processing of Al-Mg type alloys, some samples were solution treated at 535°C/1 h, and some of them were recrystallization annealed at 320°÷350°C/3 h. After that all the samples were treated in a same manner: cold rolling to 50–60% and final annealing at temperatures in the range of 220°÷470°C/3 h. It was supposed that the structure of samples pre-treated by recrystallization annealing at 320°/350°C is characterized with S type (Al₂MgCu) phases in the Al-Mg-(Mn)-Cu alloys and with β′/β (Mg₅Al₈) phases in the Al-Mg-Mn alloys. These structure features are rather unchangeable during the subsequent cold rolling and annealing treatments in the temperature range of 220°÷320°C. During annealing at higher temperatures (such as 470°C) precipitation of Mn-bearing particles (MnAl₆ or (Fe,Mn)Al₆) might occur in both type of alloys. After solution treatment at 535°C most of the alloying elements are dissolved into solid solution, thus inducing a high potential for precipitation processes during annealing at 220°÷470°C. The cold deformation was found to contribute the electrical resistivity intensively at the beginning of deformation (in the range up to ∼20%), and it was not influenced by the chemical composition of tested alloys. The resistivity variations with the cold deformation were fitted by power-law equations.