Significant Grain Refinement in 17-4PH Stainless Steels by In Situ Alloying with TiB₂ Additives During Laser Powder-Bed Fusion

Owing to the additive nature, the process of laser powder-bed fusion (LPBF) additive manufacturing typically results in epitaxial growth of elongated columnar grains across multiple layers. Such an anisotropic grain structure often causes undesired mechanical anisotropy. This study demonstrates that an addition of 3.0 wt pct TiB₂ leads to fully equiaxed grains, pronounced grain refinement to a submicron scale, and in situ elimination of phase heterogeneity in as-built LPBF 17-4PH stainless steel. The complete dissolution of TiB₂ particles through eutectic reaction during the LPBF process brings in multiple effects. The dominant matrix phase, δ-ferrite, forms from the liquid and remains at room temperature, resulting from rapid cooling inherent to LPBF and the stabilizing effect of Ti on δ-ferrite. Boron (B) element from TiB₂ decomposition leads to its rapid B segregation due to its negligible solubility in iron matrix and the formation of continuous (Fe, Cr)₂B precipitates at the δ-ferrite grain boundaries in the as-built state. The in situ formation of (Fe, Cr)₂B as the leading phase in the melt pool acts as primary nucleation sites to cause significant grain refinement. Additionally, the solute drag effect of boron and the pinning effect of (Fe, Cr)₂B at grain boundaries significantly contribute to the pronounced grain refinement, as they help resist grain growth during subsequent thermal cycling. The adverse impact of continuous (Fe, Cr)₂B at grain boundaries is the loss in tensile ductility (a total strain of only 0.9 pct) despite of high tensile strength. To overcome the embrittleness of the as-built TiB₂/17-4PH, post-annealing at 700 °C for 45 minutes can dissolve grain boundary (Fe, Cr)₂B, converting continuous (Fe, Cr)₂B into discrete precipitates. This leads to enhanced mechanical properties that significantly surpass ASTM standards and exceed the performance of most 17-4PH materials produced through additive manufacturing processes reported in the literatures. Moreover, with solution treatment at 1038 °C for 15 minutes, the ultrafine δ-ferrite grains achieved in as-built state tend to transform into ultrafine martensite, offering a viable pathway tailoring microstructures through multi-step heat treatment.