Interlayer time as a robust, geometry-agnostic predictor of microstructural and mechanical properties evolution in PBF-LB/M Ti6Al4V alloy

Process optimisation during laser-based powder bed fusion of metals (PBF-LB/M) has in the past been shown to offer a large amount of control over microstructure and mechanical properties of materials, especially in the ability to utilise the in-situ thermal treatments, facilitated by heat accumulation during the fabrication process. Interlayer time ($\mathrm{ILT}$) in PBF-LB/M of Ti6Al4V titanium alloy was previously shown to play a significant role in controlling the final microstructure, however, was also prone to occasional impurity pick-up facilitated by an increase in part temperature during the fabrication. However, a systematic understanding of the interplay between $\mathrm{ILT}$, process parameters, build size, heat accumulation and impurity pick-up was lacking. Here, we evaluated the effect of $\mathrm{ILT}$ in a wide range (10 – 73 s) as well as part geometry on the evolution in microstructure, mechanical properties and residual stresses generated in PBF-LB/M Ti6Al4V. This study confirms that $\mathrm{IL}T$ < 40 s is linked to dramatic heat accumulation, significant microstructure coarsening as well as reduction in strength and residual stresses, while further reduction to as low as $\mathrm{IL}T$ ≈ 10 s led to sample glow, surface discoloration and impurity pick-up with consequent increase in hardness and embrittlement, manifesting a crucial limitation on the in-situ heat treatment implementation through $\mathrm{ILT}$ control. We further demonstrate that this limitation can be overcome by reducing the cumulative track length per unit volume (via using thicker powder layers and/or wider hatch spacing), which, despite achieving higher overall average temperatures, acquired less impurities due to fewer melting/exposure cycles during fabrication of identical size parts. Further, by virtue of minimising the number of expansion/contraction cycles per part, this approach was shown to be highly effective in reducing the magnitude of residual stress generated in parts. Most importantly, this work demonstrated that the effect of $\mathrm{ILT}$ on the microstructure and mechanical properties of PBF-LB/M Ti6Al4V is largely agnostic to the global part geometry and/or a number of parts in a build, even for vastly different process parameter combinations. This indicates a significant potential for the future development of a new $\mathrm{ILT}$-derived process optimisation metric for direct and informed design of microstructure and properties in PBF-LB/M Ti6Al4V.