Consolidation behaviour of AZ80 magnesium chips: influence of compaction pressure and holding time on porosity, interfaces and mechanical response

Solid-state recycling of magnesium alloys relies on effective pre-compaction to convert loose machining chips into dense precursors suitable for downstream processing. This study investigates the consolidation mechanisms of AZ80 Mg chips containing residual water-based lubricants, compacted in a single-action hydraulic press without prior cleaning. The compaction pressure, holding time, and pressure evolution were analyzed to determine their influence on briquette quality. The findings demonstrated that holding time, rather than peak pressure, was the governing factor for densification. Extended holding promoted internal stress redistribution and geometric adaptation, facilitating pore collapse and yielding green densities of ~ 91-92% with uniform axial distribution, while short holding times limited effective stress transmission, resulting in heterogeneous density gradients. Microstructural analysis revealed that while native oxide barriers persisted, preventing full metallurgical bonding, extended holding achieved sub-micron interfacial spacing and effective geometric closure. Hardness mapping indicated that shorter holding times retained higher hardness values (~ 110-116 HV), consistent with high localized work hardening and incomplete stress redistribution. In addition, compression tests revealed that stiffness was governed by interfacial integrity rather than bulk density. Therefore, samples with superior geometric sealing exhibited significantly higher stiffness (2964 MPa) compared to denser samples with poorer interfacial locking. These results indicate that optimizing interfacial contact through load maintenance is critical for producing stable briquettes, providing a pathway for robust solid-state recycling of AZ80 alloys.