Softening-hardening mechanism in the direct hot-extrusion of aluminium compacts

Powder compacts from two different commercial aluminium powder grades have been densified by direct hot extrusion. The extrusion runs have been carried out at 425°C, with an extrusion ratio of 1:16 and a ram speed of 1.5 mm/s. Prior to extrusion, some green compacts were presintered (500°C). The evolution of the extrusion load during the process and the hardness of the final products have been investigated. Additionally, microstructural characterization by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD) was carried out in order to understand the softening-hardnening mechanisms taking place during the process. The obtained results evidence grain refinement. Additionally, inter-metallic precipitation, dynamic recovery and geometric dynamic recrystallization take place depending on the composition of the powder, the heat treatment before extrusion and the strain involved in the process.