Multiscale residual stress analysis and microstructure characterization of Ti-grade 2 implant fabricated by adaptive tool path-driven SPIF process

Abstract

Single Point Incremental Forming of titanium alloys for biomedical implants presents a unique challenge in balancing geometrical accuracy with the control of residual stresses. The proposed methodology introduces a novel curvature-driven adaptive toolpath for incremental forming, overcoming the limitations of conventional constant depth spiral and existing adaptive strategies. Unlike STL-based adaptive methods that rely on volumetric error correction by adding slices between consecutive layers, this approach optimizes the toolpath by removing redundant slices. By adjusting slice, the process assigns density values according to local curvature fluctuations thus creating more efficient forming while reducing forming time. Electron Backscatter Diffraction is utilized to measure the evolution of microstructure through an evaluation of misorientation distribution, deformation twinning and geometrically necessary dislocation density. X-ray diffraction technology and micro-scale residual stress measurement techniques are used to measure macro and micro residual stress fields in the produced implants. The present work correlates the tool path strategies with the observed residual stress distribution along with microstructural characteristics which uncovered the underlying deformation mechanism in implants formed by SPIF. Results highlight that adaptive tool path-driven SPIF process led to decreased amounts of residual stress while creating more uniform stress patterns within Ti-Grade 2 implants. The implant formed with adaptive tool path resulted in higher homogeneity in stress distribution with lower localized strain concentrations in comparison to those formed with conventional tool paths. In addition, microstructural characteristics denoted more uniform plastic deformation across the formed implant. The study demonstrates that the modifications in SPIF tool path bring superior results in product quality. Achieving desired residual stress states and microstructural characteristics becomes possible through SPIF which delivers improved dimensional accuracy and reliability of the formed Ti-Grade 2 implants.