Oxidation characteristics of Ti-6Al-4V machining scraps and their influence on microstructure and fracture behavior in recycled alloys

• The colors of oxidized Ti-6Al-4V scraps reflects both film thickness (via 10th-order optical interference) and oxide composition. • This selective sorting mechanism removes Al-rich/V-deficient oxides, resulting in a slightly reduced in Al and elevated in V of the recycled alloy. • The oxygen content remains well controlled, with only a minimal increase of 19 ppm. • The recycled alloy exhibits enhanced elongation and slightly reduced tensile strength, which is attributed to the increase in β phase and decrease of internal stress. During the machining process of Ti-6Al-4V alloy, a significant amount of scraps are generated, which cannot be directly recycled and remelted for reuse. Effective sorting of oxidized scraps is critical, as it determines the quality of recycled material. This study examines the correlation between oxide film characteristics and scrap coloration, revealing that film thickness and composition vary with surface color. A thicker oxide film has a layered structure: an outer layer of TiO 2 + VO 2 and an inner layer of Al 2 O 3 . Color differences are primarily attributed to the composition of the oxide film and the effects of thin-film interference, which provides a theoretical foundation for the color-based sorting of high-oxygen-content scraps. Furthermore, to evaluate the characteristics of recycled alloy processed through color-based oxide sorting, microstructural evolution and fracture mechanisms were compared between virgin and recycled Ti-6Al-4V alloy. Nevertheless, the recycled alloy maintains exceptional mechanical properties. These compositional differences give rise to an increase in the β phase content and a reduction in the internal stress within the recycled alloy, leads to a different fracture behavior. This study demonstrates the effectiveness of color-sorting technology in controlling oxygen content, enabling efficient titanium alloy recycling with substantial industrial application potential.