Effects of process variables on the quality and mechanical properties of 18%Ni-M350 maraging steel produced by direct energy deposition laser additive manufacturing

The characteristics, defects formation and mechanical properties of 18 wt% Ni M350 maraging (18%Ni M350) steel produced by the powder direct energy deposition (DED-p) laser additive manufacturing process is studied. Using a response surface methodology (RSM) based on Central Composite Design (CCD), the relationships between the DED-p process parameters and the resultant single-bead characteristic of the alloy are established. Mathematical models based on second-order polynomials are developed for predicting the bead aspect ratio and the percentage of dilution between the deposit and the base material, which are crucial for producing the bulk-multilayer additive-manufactured material. Multi-layer deposits of the 18%Ni M350 material are produced using optimized process parameters. X-ray computed tomography is used to study the extent of defects formation in the multi-layer material. The results show that the morphology, volume percent and size of defects during the DED-p process are controlled by the laser energy density. In particular, the morphology of defects is found to change from irregular lack-of-fusion defects to spherical gas porosity and keyhole porosity as the energy density increases from similar to 14 J/mm(2) to 48 J/mm(2). A set of process parameters is achieved that significantly minimized the defects to produce high-density additive-manufactured material. Furthermore, the influence of the defects morphology in the 18%Ni-M350 material produced by DED-p on the tensile properties in as-processed and heat-treated conditions is evaluated and discussed in this paper.