Evaluation of microstructure and tensile properties of grain-refined, Ti-alloyed ferritic stainless steel fabricated by laser powder bed fusion

Many alloys produced by Laser Powder Bed Fusion (LPBF) suffer from coarse grains and anisotropic mechanical properties. Here, we investigate how the addition of Ti can cause significant strengthening via grain refinement in a model ferritic stainless steel. We perform LPBF experiments using elemental Fe, Cr, and Ti powders (with significant O impurity in the Fe powder) and perform microstructural analysis by SEM, EDS and EBSD as well as tensile tests on the alloys Fe-19at.%Cr and Fe-19at.%Cr-5at.%Ti. The Fe-Cr alloy displays very large grains after LPBF and a strong cube texture, rendering its mechanical properties similar to a single crystal. In the Fe-Cr-Ti alloy, TiO particle formation in the melt causes strong grain refinement, leading to a texture-free microstructure consisting of equiaxed grains -1.6 mu m in diameter. The 0.2% proof strength of the ternary alloy is more than double that of the binary alloy (281 MPa -> 591 MPa), and the work hardening rate is also increased. While the elongation at fracture is reduced for the Fe-Cr-Ti alloy, at -15 %, it remains sufficient, and samples show ductile fracture surfaces. We estimate that the grain refinement accounts for the majority of the strengthening, however, solid solution strengthening and the effect of the texture are also significant.