Authors: Talant Ryspaev, M. Janecek, Robert Kral, Volker Wesling, Lothar Wagner
Abstract: The grain refinement after thermo-mechanical treatment (TMT) was investigated in AZ91, AE42, und QE22 magnesium alloys. The optimal over-aging temperature was determined to be 300 °C in the case of AZ91 and AE42 alloys and 350 °C for QE22 alloy.
After optimized TMT, the average grain sizes were 13.5 µm (AE42), 11.1 µm (AZ91) and 1.9 µm (QE22). The QE22 alloy exhibited the superior superplastic properties, with maximum elongation to failure 750 % and strain rate sensitivity parameter m=0.73.
The Friction Stir Welding showed that the original base material grain structure of the alloys AZ31 and AZ91 replaced by ultrafine grains in the stir zone.
The purpose of the present paper is to present the results of the grain refinement in magnesium alloys by thermo mechanical treatment and stir welding.
220
Authors: Martin Vlach, Ivana Stulikova, Bohumil Smola, Tomáš Kekule, Veronika Kodetova, Jaroslav Malek
Abstract: The effect of hot rolling on mechanical and electrical properties, microstructure and recrystallization behaviour of the AlMnScZr alloy was studied. The mould-cast alloy and the alloy after hot rolling at 300 °C was studied during step-by-step quasilinear annealing from 200 °C up to 600 °C with heating rate 100 K/h followed by subsequent isothermal annealing at 600 °C/5 h. Precipitation reactions were studied by electrical resistometry, differential scanning calorimetry and hardness measurements. Transmission electron microscopy and electron backscatter diffraction examination of specimens quenched from temperatures of significant resistivity changes were used to identify microstructural processes responsible for these changes. Only occasional irregular sharp-edged polygonal particles of the AlMnFeSi system were found in the as-prepared state of the mould-cast alloy. The as-prepared state of the hot-rolled alloy was characterized by a dispersion of fine coherent Al3Sc and/or Al3(Sc,Zr) particles and furthermore a fine (sub) grain structure was observed. The hardening effect in the alloys is due to presence and/or precipitation of the Sc,Zr-containing particles with L12 structure. The distinct resistivity changes of the alloys are mainly caused by precipitation of Mn-containing particles. Two-stage development of the Al6Mn phase (in (sub) grain interiors and at (sub) grain boundaries) in the hot-rolled alloy was observed. The presence of Sc,Zr-and Mn-containing particles has an anti-recrystallization effect that prevents recrystallization minimally up to 600 °C and annealing of 1 hour in the hot-rolled alloy. The apparent activation energy for the Al3(Sc,Zr)-phase and Al6Mn-phase precipitation was also determined. The activation energy values obtained in the hot-rolled AlMnScZr alloy are comparable to those observed in the hot deformed AlMnScZr alloys prepared by powder metallurgy.
18
Authors: Martin Vlach, Bohumil Smola, Ivana Stulikova, Veronika Kodetova, Hana Kudrnova, Jaroslav Malek, Vladivoj Ocenasek
Abstract: The mechanical, thermal and electrical properties and recrystallization behaviour of the cold-rolled AlMgScZr alloy prepared by powder metallurgy were studied. The materials were investigated during isothermal annealing (400 and 550 °C) and during step-by-step linear annealing from room temperature up to 570 °C. The observed results were compared with microstructure observation by transmission electron microscopy and electron diffraction from a previous study of the Al–Mg-based alloys with Sc and Zr. The precipitation sequence of the Al–Mg system and coarsening of the Sc,Zr-containing particles caused electrical and heat flow changes during the annealing. The presence of the Al3(Sc,Zr) particles has an anti-recrystallization effect that prevents recrystallization at temperature minimally up to ~ 400 °C. A partial recrystallization of the alloy was registered after annealing at 550 °C already for 30 min. The cause of the anti-recrystallization effect is precipitation of the Mg-containing particles as follows from a comparison to the alloy without Mg. But the Mg-addition to the Al–Sc–Zr alloy prepared by powder metallurgy has a poorer anti-recrystallization effect than a Mn-addition.
161
