Effect of substrate temperature on in-situ precipitation during laser powder bed fusion of Fe-TiB₂ high modulus steel

Fe-Ti-B high modulus steel (HMS) fabricated via laser powder bed fusion exhibits in-situ precipitation of nanostructured TiB2 particles within a ferritic Fe-matrix. However, porosity and cracking are common challenges associated with this process. This study systematically varies process parameters, specifically volume energy density and substrate temperature, to analyse macroscopic defects formation and propose methods to prevent their occurrence through detailed microstructure characterisation. For substrate temperatures of 400, 600, and 800 degrees C, an optimal combination of laser power and scan velocity was determined, resulting in minimised specimen porosity (< 1%). Yet, pronounced cracking occurred at 400 and 600 degrees C substrate temperature, most likely attributed to the presence of hard and brittle non-equilibrium microstructure constituents. Increasing the substrate temperature to 800 degrees C further reduces porosity and promotes the formation of the equilibrium constituents Fe-alpha and TiB2. These phases are desirable as they improve the stiffness-to-density ratio while reducing hardness and brittleness. By mitigating thermal gradient and resulting lower stresses, the successful fabrication of HMS samples with the desired microstructure and defect-free macrostructures becomes feasible. Potential future steps, such as incorporating in-situ heat treatments between layer depositions, are outlined and discussed as means to lower the substrate preheating temperature.