Design of new 18Ni- <i>x</i> Ti maraging steel harnessing intrinsic heat treatment effect of additive manufacturing

A novel steel 18Ni-4Ti was designed by fine-tuning the composition of conventional maraging steel through in-situ alloying with titanium during laser-directed energy deposition (LDED), combined with processing parameters adjustment, to improve the intrinsic heat treatment (IHT) effect of additive manufacturing. Based on the initial 18Ni300 maraging steel, an interlayer dwell time of 120 s was introduced to cool the deposited sample into a martensitic state, thereby facilitating subsequent ageing reaction in the martensitic matrix during the following LDED process. Thereafter, titanium was added to 18Ni300 to accelerate the precipitation kinetics of the nano-Ni3Ti precipitates during IHT. Consequently, the ultimate strength of the IHTed maraging steel was improved from 1201 MPa (18Ni300) to 1546 MPa (18Ni–4Ni), rising by 28.7% and without sacrificing ductility. It is revealed that Ti addition provides increased solid solution strengthening combined with precipitation strengthening, which contributes to the improved IHT effect. Meanwhile, more retained austenite is printed, which results in a higher work hardening ability of the IHTed 18Ni-4Ti; this stabilises the plastic deformation at a higher stress level. The findings provide a fertile approach for new alloy development for the IHT of additive manufacturing and demonstrate the effect of Ti addition during IHT of the maraging steel.