A new hybrid process to produce ultrafine grained aluminium plates

The present study concerns the issues of the detailed microstructure evolution, development of grain boundaries and mechanical properties of commercially pure aluminium processed using multi-turn Equal Channel Angular Pressing (mtECAP) and upsetting processes. Both processes cause a refinement of microstructure in coarse-grained materials, though the deformation paths differ. This results in differences in microstructure, the distribution of grain boundaries, and mechanical strength and fractures. Four passes of mtECAP with a channel angles of φ = 90°, route C, caused a grain refinement of 1-1.2 µm and a fraction of HAGBs in a range of 41.3-53.1%, depending on the plane examined. Upsetting from a height 26 mm to 3 mm caused higher average grain size and a lower fraction of HAGBs in particular planes. The mechanical strength was lower, but the applied strain in this sample was also much lower than for the sample processed with mtECAP. Only combining the two processes resulted in an ultrafine grain structure with an average grain size of below 700 nm in the X and Y planes. The results show that this hybrid process results in a plate with low anisotropy of tensile properties and with the tensile strength close to 200 MPa.