ECAP vs. direct extrusion—Techniques for consolidation of ultra-fine Al particles

In this study equal channel angular pressing (ECAP) with accommodated backpressure was used as a technique for consolidation of fine atomized monocrystalline (d50 = 1.3 μm) Al 99.7% powder. The effect of ECAP on consolidation behavior of powder, microstructure and mechanical properties of subsequent compacts in comparison to conventional direct extrusion (DE) is presented. Consolidation was realized via simple processing route, where complicated step of gastight encapsulating of vacuumed powder was avoided. Both ECAPed and extruded compacts featured sound ultra-fine grained (UFG) microstructures, consisting of elongated grains of intensively sheared powder particles. Grain boundaries of compacts were stabilized with homogenously redistributed nano-scale particles of torn oxide surface layers of initial Al powder particles. UFG character of ECAPed and extruded compacts resulted in their relatively high strengths at room temperature (up to Rm = 316 MPa) and especially at elevated temperatures (up to Rm = 188 MPa at 300 °C). Inferior ductility of ECAPed compacts (A ∼2%) compared to extrudants (A ∼10%) was due to their disadvantageous microstructure texture reflecting deformation along ECAP shearing plane. Owing to grain pinning effect of broken oxide particles, compacts exhibited excellent structural stability with no major microstructural changes and deterioration of mechanical properties determined after 350 °C/20 h annealing. The trade-off between simple processing route and effect of residual porosity on high temperature mechanical properties is discussed.