Etude du comportement en fatigue traction-compression de l'acier H13 obtenu par fusion laser sur lit de poudre en préchauffage faible/élevé du substrat

Fatigue resistance is a critical limitation for the service reliability of LPBF-produced H13 tool steel in die-casting molds, where cyclic thermomechanical loads promote crack initiation at process-induced defects. This work proposes an ultra-high substrate preheating strategy (500°C) to reconfigure microstructural defect landscapes and residual stress states—key factors governing tension–compression fatigue performance (R = –1).A fundamental study of compressive behavior under elevated preheating, benchmarked against conventional 200°C, reveals that 500°C significantly reduces vertical mechanical fluctuations (10.6–17.8% vs. 24.3–54.1%). This improvement arises from martensite-to-bainite transformation, carbide precipitation, and an 86.8% reduction in residual stress (487 → 64 MPa), collectively enhancing structural stability and homogeneity.In fatigue, machined samples exhibit dual transitions in microstructure and defect morphology. μ-CT shows that defects at 500°C are predominantly spherical (>0.8), while 42% remain irregular at 200°C (0.4–1). Meanwhile, the microstructure evolves from a biphasic martensite/retained austenite matrix to a bainite-dominated multiphase configuration, shifting fatigue crack initiation from irregular defects to a competition between spherical-defect-induced and defect-free mechanisms. Consequently, machined specimens preheated at 500°C achieve 2–5× longer fatigue life under 500–1200 MPa stress amplitudes.Surface quality is also identified as a critical factor. Solidification valleys (primary roughness, PRR) act as dominant crack initiation sites in as-built samples. Compared to semi-polished samples, fully machined specimens (PRR removed) reach >17× higher fatigue life, despite only moderate roughness reduction (Sa: 25.3 → 3.5 µm). Kitagawa–Takahashi analysis shows a 3.8-fold increase in the critical defect size threshold due to residual stress relaxation, facilitating a transition from multi- to single-crack propagation. In high-cycle fatigue, preheating at 500°C enhances lifetime up to 73×, highlighting the decisive role of surface morphology.Key innovations include: (1) introducing bainitic transformation for mechanical homogenization; (2) establishing links between preheating temperature, defect morphology, microstructural evolution, and crack initiation mechanisms; (3) identifying PRR as hidden yet dominant fatigue initiators; and (4) proposing an integrated process strategy that minimizes post-processing requirements.