Effects of process parameters and loading direction on the impact strength of additively manufactured 18%Ni-M350 maraging steel under dynamic impact loading

The microstructural and damage evolution of 18%Ni M350 maraging steel fabricated by laser-based directed energy deposition (DED) process under dynamic impact loading are investigated in this study. The influence of additive manufacturing (AM) processing parameters, including laser power, powder feed rate, and energy area density (EAD) are discussed. The mechanical testing of the additively manufactured alloy was done using the split Hopkinson pressure bar (SHPB) system. The dynamic impact test was conducted using strain rates ranging from 100 to 4000 s−1. The test was conducted on cylindrical specimens (7 mm diameter and 8 mm long) with axes parallel to the build direction (Z-axis) and perpendicular (X- and Y-axis) to the build direction to evaluate the effects of loading directions on mechanical responses and microstructural evolution in the alloy under the impact loading conditions. The test results revealed that the materials processed using the lowest EAD parameters exhibit the greatest impact strength under dynamic impact loading for all directions. Specimens impacted along the build direction (Z-axis) exhibit a continuous drop in peak flow stress once the strain rates exceeded 2300 s−1 indicating the greatest susceptibility to thermal softening at high strain rates. Strain localization leading to the formation of adiabatic shear bands (ASB) was observed in some specimens. Cracks developed along the ASBs, while some AM-process-related defects, such as the lack-of-fusion, interacted with adiabatic shear bands, promoting cracking inside the shear bands. Despite possessing the highest impact strength, the materials produced by the lowest EAD parameters are the most susceptible to the development of adiabatic shear bands leading to cracking and fracture.