Fabrication and Mechanical Evaluation of Co–28Cr–6Mo–0.11N Alloy Spinal Correction Rods with a Prescribed Spinal Contour by Electron Beam Powder Bed Fusion

In spinal correction surgery, metallic rods are commonly bent intraoperatively by surgeons to conform to the patient-specific spinal curvature and are then fixed with pedicle screws. However, intraoperative rod contouring can introduce surface scratches and notches, which may reduce fatigue strength and act as initiation sites for fracture. In this study, spinal correction rods with a prescribed spinal contour were fabricated from a Co–28Cr–6Mo–0.11N alloy (hereafter referred to as the CCMN alloy) by electron beam powder bed fusion (PBF-EB) to reduce dependence on intraoperative bending. Their compressive response, cyclic compression behavior, and fracture characteristics were experimentally evaluated, and their deformation behavior was further examined by finite element analysis (FEA). The CCMN rods were subjected to one, two, or three cycles of combined aging and reverse transformation heat treatment, denoted RT, 2RT, and 3RT, respectively. Under a compressive load of 1000 N, the displacement decreased with an increasing number of reverse transformation cycles, and the 3RT specimen showed the highest apparent resistance to deformation. The loads required to produce a displacement of 5 mm were 812, 843, 865, and 896 N for the as-built, RT, 2RT, and 3RT rods, respectively; thus, the 3RT rod exhibited an increase of 84 N relative to the as-built rod. Although all specimens were fabricated using the same CAD geometry and adjusted to the same nominal dimensional profile, small specimen-specific geometric deviations could not be independently quantified. Therefore, the observed differences in load–displacement response cannot be attributed exclusively to heat-treatment-induced microstructural changes. Furthermore, increasing the maximum diameter of the middle region of the variable-diameter rod from 7 to 8 mm reduced the displacement under 1000 N from 3.3 to 2.7 mm. In a single-specimen cyclic compression screening test at 0–300 N, 1 Hz, and 1 × 106 cycles, no apparent macroscopic residual deformation or fracture was observed. This result represents runout under the specified loading condition and does not establish a fatigue limit. These results support the feasibility of using PBF-EB to fabricate CCMN rods with prescribed curved and variable-diameter geometries and demonstrate a stable mechanical response under the specific static and cyclic loading conditions examined in this study.