Laser-Directed Energy Deposition of Low-Carbon Low-Temperature Ultra-Fine Bainitic Steel: Multi-Physical Modelling, Metallurgy and Performance Studies

An innovative iron-based alloy powder characterized by reduced carbon content and elevated Mn content was developed, leading to the fabrication of iron-based ultrafine bainitic steel with exceptional mechanical properties utilizing laser-directed energy deposition (L-DED). The layers' morphology, microstructure, phase composition, and elemental distribution were analyzed, indicating that the melting depth of the layer was deep and ultra-fine dendrite formed in the top region. A multi-physical powder and melt pool simulation was conducted to analyze the melt flow and pool solidification characteristics, showing that the strong downward flow inside the molten pool caused by the powder impingement leaded to the increase of the melting depth. The cooling rate G×R, where G was the temperature gradient and R was the solidification rate, was higher at the top pool boundary, which facilitated the ultra-fine dendrite formation at this location. Following subsequent low-temperature isothermal heat treatment, a refined bainitic microstructure emerged within the deposited layer. The bainitic plates in the 573K isothermally transformed bainite were interspersed with film-retained austenite (RA). Conversely, the bainitic plates formed at a lower temperature (523K) exhibited a coalesced morphology, stemming from the diminished stability of super-cooled austenite. The bainite plates underwent growth and coalescence, ultimately giving rise to the bainitic microstructure. The 523K transformed coatings demonstrated superior hardness (509.2 HV), strength (1435 MPa), and elongation (11.5%) compared to the 573K transformed coatings (456.5 HV, 1241 MPa, 10.1%). This research holds significant implications for the advancement of low-carbon bainitic materials with remarkable comprehensive mechanical properties in additive manufacturing applications.