Effect of Preheating Temperature on the Optimal Processing Windows of Inconel 718 Processed by Laser Power Bed Fusion

Laser Powder Bed Fusion (LPBF) of Inconel 718 (IN718) enables near-net-shape fabrication of complex components but is limited by narrow processing windows, crack susceptibility, and defect formation. In this work, the influence of substrate preheating on LPBF processability, densification, microstructure, and hardness of IN718 is investigated. Cuboid samples (10 × 10 × 10 mm³) were fabricated at three preheating temperatures (80 °C, 300 °C, and 500 °C), while laser power was varied between 100 W and 200 W with fixed layer thickness (30 μm) and hatch spacing (80 μm). Density was assessed using helium pycnometry and optical microscopy, while both optical and scanning electron microscopy (SEM) were used to characterize melt pool (MP) geometry, cellular substructure, cracking behavior, and oxide inclusions. Vickers hardness (HV10) measurements were performed to assess as-built mechanical response under high load of 10kg, whereas micro hardness under a load of 0.3kg was used to evaluate the hardening and/or softening phenomena occurring during LPBF processing. The results show that increasing preheating temperature significantly widens the full-density processing window, suppresses cracking, stabilizes MPs, and promotes partial in-situ ageing, leading to enhanced as-built hardness. Nevertheless, to high preheating temperatures appear to promote both the occurrence of large porosities and the formation of oxides inclusions. These findings highlight the need for preheating-aware LPBF process metrics beyond classical volumetric energy density.