This study evaluates the effect of friction stir processing (FSP), with and without carbon nanotube (CNT) reinforcement, on the microstructure and tensile behavior of selective laser melted (SLM) AlSi10Mg alloy. The as-built alloy exhibits melt pool boundaries, a continuous silicon network, and approximately \(\:3.3\%\) porosity, which restricts ductility despite relatively high strength. FSP performed under optimized conditions ( \(\:1000\:\text{r}\text{p}\text{m}\) rotation and \(\:100\:\text{m}\text{m}/\text{m}\text{i}\text{n}\) traverse speed) produced a defect-free stir zone in both conditions. Severe plastic deformation and dynamic recrystallization during FSP eliminated melt pool features and reduced porosity to \(\:\sim1.2\%.\) Introducing CNT further enhanced densification, lowering porosity to \(\:\sim0.8\text{\%},\:\) corresponding to an overall porosity reduction of approximately 75% compared to the SLM sample. The CNT-reinforced stir zone showed significant grain refinement, with an average grain size of \(\:\sim2.2\:{\upmu\:}\text{m}\) and a high fraction \(\:(\sim0.78)\) of high-angle grain boundaries, corresponding to an overall grain size reduction of approximately 76%. This refinement is attributed to dynamic recrystallization promoted by Zener pinning, particle-stimulated nucleation, and localized strain concentration caused by CNTs. The silicon network was fragmented into fine particles, contributing to Orowan strengthening and grain boundary stabilization. X-ray diffraction confirmed successful CNT incorporation. Microhardness decreased from \(\:119\:\text{H}\text{V}\) (SLM sample) to \(\:103\:\text{H}\text{V}\) after FSP due to silicon network breakdown and thermal softening, but increased to \(\:128\:\text{H}\text{V}\) with CNT addition. Tensile results showed optimal performance for CNT-reinforced friction stir processed samples (UTS \(\:353.3\:\text{M}\text{P}\text{a}\) , elongation \(\:35.73\:\text{\%}\) ), corresponding to an improvement in tensile strength of approximately 13% and elongation of approximately 184% compared to the SLM, demonstrating an effective solid-state approach for achieving improved strength ductility balance in SLM AlSi10Mg.
