Synergistic control of multiscale microstructure and ultra-precision machining performance in 6061 Al alloys via cryogenic processing

Achieving ultra-homogeneous and ultrafine microstructures in 6061 Al alloys remains a critical challenge for high-end optical mirror applications, where coarse second-phase and grain-scale heterogeneity severely deteriorate single-point diamond turning (SPDT) performance. Cryogenic deformation enables the synergistic control of grain size and second-phase evolution and is a widely adopted approach for the fabrication of high-quality optical aluminum mirrors. In this study, the effects of cryogenic deformation combined with tailored heat-treatment strategies on microstructural evolution and surface formation behavior were systematically investigated in both conventional and high-purity 6061 Al alloys. The results show that, for the conventional 6061 Al alloy, a processing route consisting of cryogenic deformation followed by solution treatment and aging (Cryo-T6) produces fine equiaxed grains (∼10–20 μm). However, residual micron-scale second-phase particles persist and induce local cutting instabilities, limiting the achievable surface roughness (∼3.6 nm). In contrast, the high-purity 6061 Al alloy exhibits a markedly reduced second-phase fraction but undergoes severe grain coarsening (∼39–48 μm) under the Cryo-T6 condition due to insufficient pinning, accompanied by abnormal grain growth and a characteristic “patchy” surface morphology (∼3.7 nm). To overcome this limitation, a novel strategy combining high-purity alloy design with cryogenic deformation followed by direct aging (Cryo-DA) is proposed. This approach preserves deformation-induced substructures, leading to the formation of a nanoscale subgrain structure (∼300 nm) with an extremely low fraction of second-phase particles. As a result, highly uniform plastic flow during machining is achieved, yielding a superior surface roughness of ∼1.8 nm which is reduced by approximately 50% compared with conventional 6061 Al alloys processed by Cryo-T6. The findings reveal that the synergy between alloy purification and process design governs microstructural uniformity, thereby dictating surface formation mechanisms in SPDT. The proposed high-purity 6061 Al alloy coupled with the Cryo-DA route provides an effective pathway for fabricating high-performance optical Al alloys.