Tailoring B2 phase precipitation via LDED and annealing to enhance tribological properties of low-density steel coatings

Abstract

To integrate lightweight design with wear resistance, Fe-Mn-Al-C low-density steel coatings were fabricated via laser directed energy deposition (LDED). Unlike conventional methods involving cold rolling and annealing to precipitate B2 phases, this study employed a novel approach whereby polygonal B2 phases were precipitated by controlling the laser power, followed by a post-annealing treatment at 900 °C for 15 min to form needle-type B2 phases. This strategy incorporated two distinct B2 phase morphologies into the coating, and their effects on wear property were systematically investigated. The results indicate that during the deposition process, polygonal B2 phases preferentially nucleated and progressively covered the ferrite matrix. As laser power increased from 1200 W to 1600 W, the content of polygonal B2 phases increased progressively, accompanied by a rise in BCC phase fraction from ∼19.2% to 26.0%. Concurrently, the size and content of κ-carbides and D0₃ phases also increased, leading to enhanced wear resistance. After annealing, submicron-scale needle-type B2 phases precipitated in the austenite. Together with the polygonal B2 phase containing nanoscale D0₃ phase, they formed a reinforcing skeleton, reducing the wear rates at room temperature and 550 °C to 7.647 × 10⁻⁵ mm³N⁻¹ m⁻¹ and 0.304 × 10⁻⁵ mm³N⁻¹ m⁻¹, respectively. The annealing process also promoted the dissolution of κ-carbides and a reduction in size of D0₃ phases. The superior wear resistance is attributed to a multiscale synergistic mechanism involving the strengthening effect of the dual-morphology B2 phases and the protective role of the oxide layer. This study provides a novel pathway for designing lightweight wear-resistant coatings for high-temperature applications.