Microstructural evolution and superplasticity of Al-5.8Mg-0.23Mn alloys processed by reciprocating extrusion

Abstract

This study developed the reciprocating extrusion method to refine the inclusions and grain structure of Al-5.8Mg-0.23Mn alloys to enhance their strength and superplasticity without prior homogenization treatment. Alloy cast billets were extruded with an extrusion ratio of 10:1 at 450 °C for one, five, or ten passes. The grain size was reduced to 4.6 µm, and the coarse inclusions refined to 2 µm, after ten passes. A subgrain structure was formed in the interior of the fine grains, indicating that dynamic recrystallization occurred during extrusion. In this study, dynamic recrystallization in the billet was repeatedly induced by a number of extrusion passes until a limiting grain size was obtained. Thereafter, dynamic recrystallization was no longer activated because grain boundary sliding, instead of dislocation gliding, accommodated the deformation strain required for extrusion. The alloys extruded in ten-passes extrusion were found to be stronger and more ductile than commercial Al-Mg alloys and showed improved superplastic behavior at 500 °C not only from low to high strain rate but also with a small flow stress of less than 30 MPa. These advantages demonstrate that reciprocating extrusion can produce Al-Mg alloys with improved mechanical properties making them good candidates for high-strain-rate superplastic forming.