Titanium alloys manufactured via laser powder bed fusion for motorcycle applications: effect of post-process heat treatments on microstructure and mechanical properties

Titanium alloys are strategic materials in the aerospace, automotive, and biomedical sectors due to their exceptional specific strength and corrosion resistance. While Powder Bed Fusion-Laser Beam (PBF-LB) enables the fabrication of complex, lightweight geometries, the extreme cooling rates inherent to the process result in metastable martensitic microstructures that often lack the ductility and fatigue resistance required for industrial applications. This doctoral thesis provides a comprehensive investigation into the microstructural evolution and mechanical optimization of three PBF-LB titanium alloys: the α-β alloys Ti-6Al-4V and Ti-6Al-2Sn-4Zr-6Mo, and the near-α alloy Ti-6Al-2Sn-4Zr-2Mo. Unlike conventionally wrought alloys, AM components lack the prior plastic deformation necessary to form a globular bimodal microstructure. Consequently, this research focuses on the development of a unique bilamellar microstructure—characterized by elongated primary α (αp) laths within a fine secondary α (αs)-β matrix—achieved through tailored Solution Treatment and Aging (STA). Experimental results demonstrate that achieving an αp volume fraction of 45–50% is critical for an optimal balance of tensile strength, ductility, and fatigue limits. Using Kitagawa-Takahashi diagrams and the El-Haddad model, it was confirmed that this microstructural state significantly enhances tolerance to inherent PBF-LB defects, such as Lack of Fusion. Tribological assessments of Ti6246 further revealed that optimizing the hardness-toughness compromise facilitates the formation of a protective mechanically mixed layer (MML) during dry sliding. For the near-α Ti6242 alloy, an innovative high-pressure STA process was developed, integrating HIP with solution treatment to minimize porosity while maintaining high-temperature performance. Tests at 550 °C showed that while standard water-quenched STA provides higher strength via Ti3Al precipitation, the HIP-integrated process offers superior ductility. This research concluded with the successful production of functional prototypes for Ducati Motor Holding S.p.A., bridging the gap between fundamental materials science and high-performance engineering.