Finite element simulations and experimental validation of LENS-deposited IN625 yield surface identification under sequential tension/compression–torsion probing using a novel anisotropic UMAT framework

Anisotropic yield criteria and kinematic hardening laws have been independently validated for additively manufactured alloys. However, their specific combination has not been implemented in a finite element user subroutine and validated against 360° sequential biaxial probing data for LENS-deposited nickel-base alloys. Present work delivers the first validated implementation of this coupling for LENS-deposited Inconel 625, combining the Szczepiński quadratic yield criterion with Armstrong–Frederick kinematic hardening. The implemented return-mapping algorithm accumulates the plastic multiplier across Newton–Raphson iterations before updating the plastic and equivalent plastic strain state variables. This distinction is significant for the non-radially-symmetric Szczepiński surface where the flow direction rotates at each iteration unlike the fixed radial direction of isotropic return mapping. A two-step iterative boundary-condition correction scheme ensures each probing direction converges within ±1.5° of the experimental stress angle, achieved for 16 of 17 sequential paths. Path P1, the virgin specimen opening probe retains a residual angular deviation of −2.97° due to absent accumulated back-stress at the start of the sequential cycle. The validated UMAT reproduces all 17 directional yield points with a mean absolute angular error of 0.86°, mean absolute axial and shear stress deviations of 11.4 MPa and 12.5 MPa, respectively. Path-level deviations reach a maximum of 32 MPa in axial stress at P10 and 41.4 MPa in shear stress at P5, attributed to angular misalignment between back-stress and probing direction in the compressive–shear and near-pure-torsion sectors, respectively. This establishes the framework as a computational tool for characterising multiaxial yield surface under complex loading.