A novel cryogenic thermomechanical treatment for refining second-phase particles in 6061 aluminum alloy to improve surface quality in ultra-precision diamond turning

Abstract

Second-phase particle pull-out, embedding, and scratching are critical factors limiting the surface quality of 6061 aluminum alloy during single-point diamond turning (SPDT). In this study, a novel cryogenic thermomechanical treatment was proposed to regulate the fragmentation, dissolution, and subsequent precipitation behavior of second phases, thereby improving the ultra-precision machinability of 6061 aluminum alloy. Three processing routes, namely annealing–cryogenic deformation–solution treatment (ACS), solution treatment–cryogenic deformation–solution treatment (SCS), and annealing–cryogenic deformation–solution treatment–cryogenic deformation–solution treatment (ACSCS), were designed, and the evolution of second phases, precipitation behavior, and their effects on surface roughness were systematically investigated through microstructural characterization, composition analysis, hardness testing, and SPDT experiments. The results show that cryogenic deformation after annealing markedly promoted the mechanical fragmentation of coarse Mg₂Si and AlFeSi phases, which was attributed to dislocation pile-up and interfacial stress concentration caused by the suppression of dynamic recovery at 77 K. Solution treatment accelerated the dissolution of soluble Mg₂Si into the matrix, while the ACSCS route achieved the finest and most homogeneous second-phase distribution through the synergistic sequence of pre-fragmentation, accelerated dissolution, re-fragmentation, and re-solution treatment. During artificial aging, the ACSCS sample developed denser nanoscale precipitates owing to the higher supersaturation of Mg and Si atoms and abundant dislocation-assisted nucleation sites. Consequently, its peak hardness reached 142.6 HV at 175 °C, and the precipitate-free zone (PFZ) width decreased to 87 nm. After SPDT, the surface roughness Ra of the ACSCS sample was reduced to 3.4 nm, representing a 55.8% decrease compared with the conventional T6-tempered sample. This work provides an effective microstructural strategy for enhancing the surface quality of aluminum alloy optical components.