Research on the microstructural differences and mechanical response mechanisms of martensitic stainless steel additive manufacturing under cross-scale heat input control

• DED offers 45 % tensile strain & 45 J impact toughness, doubling the performance of SLM. • Low DED cooling forms coarse martensite, minor austenite, sparse Cr 7 C 3 & low δ-phase networks for high toughness. • High cooling rate SLM forms fine martensite, δ-phase networks, and dense Cr 3 C 2 , inducing high strength but crack susceptibility via dislocation pinning. Additively manufactured precipitation-hardened martensitic stainless steels (PHMSS-AMs) exhibit significant brittleness, with microstructure primarily governed by cooling rate, which correlates positively with heat input. Different additive manufacturing (AM) methods involve heat inputs varying by orders of magnitude, leading to substantial differences in PHMSS-AM microstructure and mechanical properties. This study systematically compares microstructure and mechanical behavior of 17-4PH steel fabricated by laser-directed energy deposition (DED) and selective laser melting (SLM), which use the same heat source type but differ in heat input by approximately three orders of magnitude. The results revealed that high heat input (DED) significantly improves ductility and toughness, while low heat input (SLM) favors higher strength. DED samples achieved an average maximum tensile strain of 45 %—double that of SLM samples and aged wrought material—and higher impact toughness (45 J vs. 25 J for SLM). The slower DED cooling rate produced wider martensitic laths, reduced δ-phase networking, and lower Cr 7 C 3 carbide density, decreasing dislocation pinning and facilitating slip, thus enhancing ductility and toughness. Conversely, rapid SLM cooling promoted finer martensitic laths, pronounced δ-phase networks, and higher Cr 7 C 3 carbide density, causing strong dislocation pinning and resulting in high strength but low ductility and toughness.