Processing–structure–property relationships in Ti–6Al–2Sn–4Zr–2Mo alloy processed via different direct additive manufacturing technologies

Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near-α titanium alloy designed for high-temperature aerospace applications, was fabricated using electron beam powder bed fusion (EB-PBF), laser powder bed fusion (L-PBF), and laser powder directed energy deposition (LP-DED). A systematic comparison was conducted to elucidate the relationships between processing route, defect population, microstructure, mechanical response, and corrosion behavior. X-ray computed tomography revealed distinct porosity characteristics, with relative densities of ~99.95%, 99.97%, and 99.99% for L-PBF, EB-PBF, and LP-DED, respectively. L-PBF samples exhibited finer but more numerous lack-of-fusion and keyhole-type pores, whereas EB-PBF showed larger, predominantly gas-induced porosity. LP-DED produced the lowest defect severity with reduced pore size and volume fraction. The superior density of LP-DED is attributed to larger, more stable melt pools that facilitate gas escape, whereas the higher defect count in L-PBF stems from keyhole instabilities and lack-of-fusion associated with its smaller beam spot and higher scan speeds. Microstructural analysis revealed that while columnar prior-β grains (9–53 μm wide) were common to all processes, the final microstructure characteristics was strongly process-dependent: L-PBF resulted in refined acicular α′ martensite with lath widths below ~0.4 μm due to rapid cooling, EB-PBF produced coarser α laths (~0.5–0.8 μm) within a basketweave morphology owing to elevated build temperatures, and LP-DED exhibited the coarsest α+β microstructure with lath widths exceeding 1 μm. These variations are driven by the extreme cooling rates in L-PBF which trigger a displacive martensitic transformation, contrasted by the in-situ thermal aging of the EB-PBF heated powder bed and the high heat input of LP-DED, which promote diffusional growth of alpha laths. Correspondingly, nanoindentation hardness decreased from ~5.24 GPa (L-PBF) to ~5.12 GPa (EB-PBF) and ~4.87 GPa (LP-DED). This trend correlates with the Hall-Petch strengthening effect derived from refined lath scales and the significantly higher dislocation density observed in L-PBF samples due to severe thermal gradients. Electrochemical testing in 3.5 wt.% NaCl solution indicated superior corrosion resistance for L-PBF and LP-DED, with corrosion current densities approximately one order of magnitude lower than EB-PBF. The reduced performance of EB-PBF is attributed to the presence of larger LOF defects and the simultaneous presence of distinct alpha and β phases; this heterogeneous phase distribution likely facilitates the formation of micro-galvanic cells, accelerating localized corrosion. These findings highlight the critical role of additive manufacturing strategy in tailoring the microstructure and performance of Ti-6242 for demanding structural applications.