The Effect of Laser Post-Heat Treatment of Laser Powder Bed Fusion High Nitrogen Steel on the Microstructure and Mechanical Properties

Laser powder bed fusion (LPBF) has been widely used in the additive manufacturing of complex metal parts, among which high nitrogen steels (HNS) have received favor due to the excellent properties and the low cost. Although the process of manufacturing HNS via LPBF has been explored in previous studies, the final results were not good, with low tensile strength and the low density. In this study, a novel approach is proposed to form HNS by a method of multiple lasers in LPBF, which reveals the relationship between process parameters, microstructure and mechanical properties of HNS formed via LPBF. The experimental results show that the mechanical properties of HNS obtained by multiple laser additive manufacturing are mainly affected by three aspects: (a): The rapid cooling characteristic of LPBF brings the possibility of forming more fine grains inside the sample. (b): The multiple laser process generates more dislocation density in the HNS via LPBF. (c): In the single laser melting, there are more columnar crystals inside the melt pools, whereas, with multiple laser melting, the existing columnar crystals inside the melt pools will be broken and more fine equiaxed grains will be formed. The HNS in this study achieved tensile strengths of > 1300 MPa and failure strains > 4 %, realizing the strength-ductility trade-off of the HNS formed via LPBF successfully.