Microstructural and mechanical anisotropy of LPBF 17-4 PH stainless steel: Effect of powder composition

Laser powder bed fusion (LPBF) fabricated as-built 17-4 PH stainless steel (SS) is usually plagued by excessive soft ferrite and its undesirable heterogeneous distribution, resulting in pronounced mechanical anisotropy. In this work, the effects of powder composition on the microstructure and mechanical properties in horizontal and vertical orientations specimens were investigated. The results showed that the composition 1 with high Cr_eq/Ni_eq ratio promoted ferrite retention, exhibiting coarse columnar grain in X-Z plane and large equiaxed ferrite with limited martensite in X-Y plane. The pronounced microstructural variation resulted in the mechanical anisotropy: Horizontal samples exhibited higher strength (882 MPa) and lower ductility (15.7%) due to the densely distributed grain boundaries and sub-structures of columnar grains along the loading direction. The vertical specimens exhibited converse trend, attributed to the enhanced dislocation slip capability along the columnar grain growth direction. Composition 2 samples with low Cr_eq/Ni_eq ratio exhibited higher austenite stability through an expanded austenite stability temperature range. Promoted ferrite-to-austenite transformation during repeated laser heating resulted in the higher martensite and austenite content, enhancing the overall strength while preserving sound ductility. In vertical samples, a slower cooling rate prolonged duration within austenite stability temperature range, leading to elevated martensite content. The resultant ductility loss was compensated by enhanced reverted austenite formation and fine columnar ferritic grains, which contributed to nearly complete elimination of anisotropy in composition 2. This study demonstrated the potential of targeted compositional regulation to achieve optimized microstructure evolution and reduced anisotropy in LPBF as-built 17-4 PH SS.