Abstract
HT9 ferritic–martensitic steel is a critical structural material for advanced nuclear systems and high-temperature applications due to its excellent swelling resistance, irradiation tolerance, and thermal stability. However, achieving near-full density during Laser Powder Bed Fusion (LPBF) remains challenging because densification is highly sensitive to processing parameters. In this study, a systematic process parameter optimization was conducted to investigate the influence of heat input on porosity formation in LPBF-fabricated HT9 steel. A parametric matrix consisting of laser powers ranging from 250 to 300 W and scan speeds between 750 and 900 mm/s was manufactured on a single build plate using a stripe scanning strategy. Volumetric heat input was calculated to correlate processing conditions with densification behavior. Porosity was quantitatively evaluated through metallographic image analysis using optical microscopy. The results demonstrate a clear U-shaped relationship between heat input and porosity content. At low heat input (< ~70 J/mm³), insufficient energy led to lack-of-fusion defects and elevated porosity levels exceeding 2–3%. In contrast, excessively high heat input (> ~85 J/mm³) promoted gas-induced spherical pores. An optimal heat input range of approximately 78–83 J/mm³ resulted in relative densities above 99.6% with minimal and uniformly distributed porosity. This study establishes a robust processing window for LPBF-HT9 and provides practical guidelines for producing high-density components suitable for demanding nuclear and high-temperature structural applications.