Microstructure evolution of AZ91 alloy during hot extrusion process with various ram velocity

Hot extrusion experiments of AZ91 alloy were carried out at various ram velocities of 0.1, 0.5 and 1.0 mm/s. The results showed that the dead metal zone, shearing zone, inflowing zone and slight deformation zone of the part remaining from extrusion exhibited varied microstructure characteristics. The complete dynamic recrystallization (DRX) occurred in the extruded AZ91 alloy, while the average grain size was increased with the increase of ram velocity. The fine Mg17Al12 precipitations were dynamically formed during hot extrusion process, and the amount of Mg17Al12 precipitates was decreased with increasing ram velocity. The precipitates have particle stimulated nucleation effect and retarding effect on grain growth, resulting into much finer DRXed grains. Moreover, it was found that the {10-12} tensile twins existed in samples extruded at ram velocity of 0.5 and 1.0 mm/s. All extruded samples showed strong basal texture perpendicular to transverse direction, and the texture intensity was weakened with increasing ram velocity. This weak texture intensity is attributed to the existence of {10-12} tensile twins.