High power laser powder bed fusion of 18Ni300 maraging steel: Processing optimization, microstructure and mechanical properties

Abstract

A 2000 W high power laser powder bed fusion technique (HP-LPBF) was used to fabricate 18Ni300 maraging steel in this study. The densification feature, microstructure and mechanical properties of the as-fabricated specimens were studied, as well as the influence of solution-aging heat treatment on the microstructure and mechanical properties. The results show that dense samples (relative density >99.5%) can be obtained when the laser energy density (Ev) is higher than 80 J/mm3. A large quantity of columnar prior-austenite grains (PAGs) are observed within the as-fabricated samples, and a typical dendritic microstructure is shown inside the PAGs. Some ordered Ni(Al, Fe) intermetallics precipitates with diameters of 3–5 nm have formed as the result of the intrinsic heat treatment effect. With the increase of Ev, the average width of the PAGs and the average dendrite arm spacing are increased. The sample fabricated with the Ev of 111 J/mm3 (short for HP-AF) has the best trade-off between build rate and mechanical properties among the as-fabricated samples. The build rate of the HP-AF sample is up to 18 mm3/s, which is about 3 times higher than that of the previous studies based on low power laser powder bed fusion. The yield strength (YS) (983 ± 10 MPa), ultimate tensile strength (UTS) (1100 ± 11 MPa) and elongation (El) (9.8 ± 0.7%) of the HP-AF sample are comparable to those of the wrought 18Ni300. After solution-aging treatment, the austenite fraction increases from 4.4% (HP-AF) to 5.3%, whilst the Ti/Al rich nano precipitates with size 30–45 nm can be observed in the matrix. After solution-aging treatment, the YS and UTS of the sample (short for HP-ST + AT) are dramatically increased to 1945 ± 12 MPa and 1865 ± 8 MPa, though the elongation is decreased to 5.4 ± 0.5%. The mechanical property of the HP-ST + AT samples is comparable to that of the wrought-aged 18Ni300. The high strength of the HP-ST + AT 18Ni300 sample is mainly attributed to precipitation strengthening as well as dislocation strengthening, followed by grain boundary strengthening.