Effects of Ambient Oxygen Concentration on Microstructural Evolution and Mechanical Properties of Wire Arc Additively...

Ti-6Al-4V thin-walled specimens were fabricated by gas tungsten arc welding-based wire arc additive manufacturing under controlled oxygen concentrations of 1, 500 and 1000 ppm, with ambient air used as a severe oxygen-exposure reference. The effects of oxygen concentration on oxygen uptake, microstructure, oxidation behavior and mechanical properties were investigated. Within the controlled range, the internal oxygen content increased from 0.07 to 0.15 wt.%, remaining below the ASTM B381-2013 limit. These specimens retained sound interlayer bonding and were mainly composed of α-Ti with a small amount of β-Ti, without detectable crystalline TiO₂ by X-ray diffraction. Controlled oxygen uptake refined the α lamellae and increased deformation resistance through interstitial solid-solution strengthening, increasing hardness from approximately 320 HV to 330-350 HV and tensile strength from 880 to 940 MPa, while reducing elongation from 11.5% to 9.5%. In contrast, the ambient-air specimen reached an oxygen content of 0.36 wt.%, developed an approximately 90 μm oxidation-affected layer and showed TiO₂-related oxides, α-colony aggregation and interface weakening. Its tensile strength and elongation decreased sharply to 295 MPa and 1.9%, respectively. These results indicate that atmosphere control in WAAM Ti-6Al-4V should prevent the transition from controlled oxygen strengthening to excessive oxygen-induced embrittlement.