Effect of pulse frequency on the morphology, microstructure, and corrosion resistance of high‑nitrogen steel prepared by laser directed energy deposition

Abstract

Quasi-Continuous-Wave laser direct energy deposition offers a number of advantages, such as a lower heat accumulate, a higher cooling rate and improved mechanical properties over Continuous-Wave counterpart. However, how pulse frequency of Quasi-Continuous-Wave affects sample's morphology, microstructure and corrosion resistance is not clear. In this work, high‑nitrogen steel samples were fabricated by Quasi-Continuous-Wave laser directed energy deposition with optimized process parameters to investigate the effect of pulse frequency on coating's surface morphology, secondary dendrite arm spacing, micro-hardness, and corrosion resistance based on experiments and temperature field simulation by Finite Element Method. The results indicated that as the pulse frequency increased (5–20 Hz), the surface profile curve of the sample tended to be stable and the surface roughness decreased. Meanwhile, when the pulse frequency increased from 20 Hz to 80 Hz, the maximum peak temperature in the melt pool decreased from 2860 °C to 2690 °C. Correspondingly, the minimum peak temperature increased from 1210 °C to 1480 °C. Once the pulse frequency exceeded 60 Hz, the melt pool can keep continuous during the laser-off period. Furthermore, a lower pulse frequency resulted in a refined equiaxed dendrite microstructure in samples and a more even distribution of nitrogen in the melt pool due to an improved cooling rate and reduced shape control factor. As a consequence, the samples fabricated at a lower pulse frequency exhibited higher micro-hardness and corrosion resistance.