Corrosion resistance of additively manufactured Ti-6Al-4V titanium

Sammanfattning

Titanium alloy Ti-6Al-4V (Ti64) offers excellent properties for biopharmaceutical applications but remains less studied compared to stainless steel 316L (SS316L). This thesis characterized the corrosion resistance of additively manufactured Ti64 produced by Laser Powder Bed Fusion (LPBF) and Electron Beam Melting (EBM), using L-PBF SS316L as reference. Samples were manufactured in different orientations (0°, 45°, 90°) and L-PBF Ti64 underwent heat treatments including stress relief (595°C/1h), solution treatment (900°C/1h), and aging (500°C/4h). Immersion tests were conducted in 1M NaCl and 1M NaOH solutions for 24 hours and 21 days at 37°C. Characterization included microstructural analysis (SEM), porosity measurements, mechanical testing (tensile, impact, microhardness), surface profilometry, and weight difference. All samples showed high density (>99.92%). L-PBF Ti64 exhibited α' martensitic microstructure transforming to lamellar α+β after heat treatment, while as-printed EBM Ti64 displayed coarse lamellar α+β structure. In NaCl, all materials developed pitting corrosion. In NaOH, stress-relieved L-PBF Ti64 formed a protective oxide layer (~200 nm), while EBM Ti64 showed localized oxidation at defects. SS316L exhibited smaller but higher amount of pitting. Microhardness decreased from ~375 HV (stress-relieved) to ~355 HV (heat-treated) for L-PBF Ti64 but did not change after immersion. Impact testing revealed Ti64's brittleness (7-13 J) versus SS316L (153-243 J). Galvanic corrosion occurred between stainless steel washers and Ti64 plates. L-PBF Ti64 demonstrated superior corrosion resistance in NaOH through uniform oxide formation. The finer microstructure of L-PBF provided better passivation than EBM. Despite excellent corrosion resistance, Ti64's lower impact toughness may limit certain applications. Heat treatment is essential for optimizing both mechanical properties and corrosion resistance of LPBF Ti64 for biopharmaceutical use.