The Structural Refinement of Commercial-Purity Copper Processed by Equal Channel Angular Pressing with Low Strain Amplitude

Abstract

Commercial purity (99.8%) copper is processed by up to 12 equal channel angular pressing (ECAP) passes at room temperature via route Bc, using a low strain amplitude (Δε ≈ 0.48 per pass) lower than typical values reported for copper. The first pass results in a 121% increase in microhardness (from ≈47 ± 2 HV to ≈104 ± 5 HV), followed by moderate increments reaching ≈127 ± 8 HV after 12 passes. Electron backscattered diffraction analyses reveal progressive grain fragmentation through the formation of low-angle grain boundaries (LAGBs), which gradually evolves into high-angle grain boundaries (HAGBs) as the accumulated strain increases. This process leads to a heterogeneous microstructure with localized regions of recrystallized polygonal grains after 12 passes. The reduced strain amplitude delays the LAGB–HAGB transition, leading to relatively large average grain sizes, which decrease from ≈29.5 ± 16 μm in the annealed state to ≈8 ± 4.2 μm after ε ≈ 5.76, larger than those typically obtained through ECAP under similar strains, but with higher Δε. Furthermore, the adoption of a lower strain amplitude substantially reduces the processing load, which, based on classical wear theory and literature evidence, is expected to improve die durability due to decreased contact pressures at the die–material interface.