| Abstract Scope |
Hybrid additive+subtractive manufacturing offers a route to control not only component geometry but also near-surface microstructure. This study examines the influence of machining conditions on microstructure evolution and hardness in additively manufactured P91, 316L, and compositionally graded P91+316L deposits. Following deposition, surfaces were machined using varied spindle speeds, coolant conditions, and part-temperature conditions to determine how machining-induced deformation modifies local microstructure and property response across ferritic/martensitic, austenitic, and intermediate compositions. Microstructural characterization was performed using optical microscopy, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and microhardness mapping. Emphasis was placed on the machined surface and subsurface region, where deformation-induced lattice misorientation, quantified using kernel average misorientation (KAM), and hardness gradients were evaluated as functions of alloy composition and machining condition. The results show that machining produces a localized deformation layer whose severity depends strongly on spindle speed, cooling condition, part temperature during machining, and composition. In general, higher spindle speed and reduced cooling increased KAM-inferred stored deformation near the machined surface and produced higher hardness, suggesting important implications for post-fabrication heat-treatment response and recrystallization behavior. Deposit composition also influenced the degree of stored deformation. Ferrite-/martensite-forming compositions generally exhibited lower KAM-inferred stored deformation after machining than austenite-forming compositions. These findings highlight machining as an active microstructure-control step in hybrid manufacturing rather than only a finishing operation. By tailoring machining conditions, localized hardness, deformation gradients, and heat-treatment-responsive near-surface microstructures can be engineered in additively manufactured P91+316L graded materials for high-temperature nuclear and fossil energy components. |