Sustainable alloy design: Fe-enhanced Ti alloys for superior mechanical performance in additive manufacturing

The use of titanium (Ti) in the additive manufacturing (AM) industry has been steadily increasing. However, iron (Fe), a significant impurity originating from the sponge production process, is difficult to remove due to high energy requirements. Consequently, fully eliminating Fe from Ti is challenging and expensive. Understanding the role of Fe in Ti alloys and determining a maximum allowable threshold for AM is therefore critical. This knowledge could potentially relax current industry standards and enable the development of more sustainable Ti alloys. This study investigated the effect of Fe on the microstructure development and mechanical properties of Ti-Fe alloys, with a focus on their suitability for additive manufacturing. Through an in-situ alloying approach, Fe content was systematically increased from 0 to 3 wt%, leading to significant grain size reduction, from over 100 μm to less than 5 μm. These modifications in grain size and morphology, achieved by adjusting the Fe content in laser-based powder bed fusion (LPBF) fabricated Ti alloys, are directly related to substantial enhancements in the mechanical properties. The hardness increased from 171 to 366 HV, and the tensile strength effectively doubled from 426 to 1026 MPa while maintaining ductility. XRD analysis revealed the presence of a full α phase when the Fe content was ≤ 0.75 wt%. However, at higher Fe concentrations, small β peaks were observed. To identify the strengthening mechanisms, theoretical frameworks such as the Hall-Petch relationship and Labusch model were employed. Two significant inflection points were identified at Fe concentrations of 0.1 % and 1 wt%, indicating three important compositional ranges: Region #1 (Fe ≤ 0.1 wt%), Region #2 (0.1–1 wt% Fe), and Region #3 (1–3 wt% Fe). The strengthening mechanism in each region is discussed in detail.