Friction stir processing for selective laser melted maraging 18Ni-300 steel: microstructures and properties

18Ni-300 maraging steel is well suited for selective laser melting (SLM) owing to its low contact angle and favorable mechanical properties. However, as-built SLM parts typically exhibit microstructural heterogeneity and residual porosity. In this work, friction stir processing (FSP) was applied as a post-treatment to selective laser melted (SLMed) 18Ni-300 to compare the microstructure and mechanical response before and after processing. The as-built microstructure is characterized by cellular and columnar dendritic structures, whereas the stir zone (SZ) after FSP undergoes complete dynamic recrystallization, resulting in fine equiaxed grains. Both 2D metallography and 3D X-ray computed tomography (CT) reveal a pronounced reduction in porosity after FSP, with the porosity decreasing from 0.19% to 0.02% in metallography and from 0.09% to 0.01% in CT, corresponding to reductions of approximately 89% in both cases. Microhardness mapping indicates an approximately 80 HV decrease in the SZ compared with the as-built SLM state, while the heat-affected zone exhibits a hardness peak. Nanoindentation results further show that the nanohardness of the SZ is lower than that of the SLM state by around 1.0 GPa, whereas the elastic modulus in the SZ is slightly higher. These variations in mechanical response are mainly attributed to the intense thermomechanical coupling during FSP, which promotes dislocation rearrangement and partial re-solution of nanoscale precipitates.