Influence of scanning speed and hatch distance on defects, microstructure, and mechanical performance of Fe–Mn–Al–Ni–C low-density steel fabricated via directed energy deposition

This study examined the effects of the directed energy deposition (DED) process parameters on the microstructural evolution, defect formation, and mechanical properties of Fe–Mn–Al–Ni–C low-density steel. The samples were fabricated in an Ar atmosphere by adjusting the scanning speed (600, 1000, 1400 mm/min) and hatch spacing (1.32, 1.11, 1.00 mm), and the influence of these process parameters on the porosity, microstructural characteristics, and compressive yield strength was analysed systematically. The experimental results showed that increasing the scanning speed and reducing the hatch spacing (1400 mm/min, 1.00 mm) decreased the amount of unmelted powder on top of the melt pool, reduced surface roughness, and significantly lowered the overall porosity. Under these conditions, however, the increased heat input reduced the cooling rate of the melt pool, promoting the growth of the ferrite/B2 (BCC) phase. This weakened the secondary phase strengthening effect, reducing the compressive yield strength (1058.02 ± 17.33 MPa). In contrast, a lower scanning speed (600 mm/min) and a larger hatch spacing (1.32 mm) accelerated the cooling rate of the melt pool and suppressed the growth of the ferrite/B2 phase, resulting in a fine and uniformly distributed microstructure and enhanced compressive yield strength (1183.02 ± 23.56 MPa). This study revealed the complex relationship between the DED process parameters, microstructure, porosity, and yield strength, providing a theoretical foundation and practical guidance for further optimising the forming process and improving the overall performance of this alloy.