Cross-route comparison of additive manufacturing of a low-carbon steel: Microstructure and mechanical properties for high-strength structural applications

This study investigates how additive manufacturing routes influence the microstructure and mechanical properties of a low-carbon steel for high-strength structural applications. A direct comparison was performed for laser powder bed fusion (PBF-LB), laser directed energy deposition with wire and powder feedstocks (DED-LB/w and DED-LB/p), and arc directed energy deposition (DED-Arc), using multiscale characterization by optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Distinct thermal histories produced markedly different microstructures. PBF-LB resulted in lath martensite, whereas the DED routes produced bainitic microstructures due to their lower cooling rates. No retained austenite was detected, and prior austenite grains were elongated along the build direction in all conditions. These microstructural differences strongly affected mechanical properties. Vickers hardness ranged from 271 ± 6 HV0.5 for DED-Arc to 366.3 ± 11 HV0.5 for PBF-LB, with similar trends in yield and ultimate tensile strengths. Among the bainitic conditions, DED-LB/w showed the highest mean tensile strength, while DED-Arc exhibited the greatest ductility, demonstrating clear route-dependent differences in the strength–ductility balance. Overall, additive manufacturing route selection enables substantial tailoring of microstructure and mechanical performance within a single low-carbon steel composition, providing access to a broad property range relevant to high-strength structural applications.