PSI - Issue 74

Karel Slámečka et al. / Procedia Structural Integrity 74 (2025) 85 – 90 Karel Slámečka / Structural Integrity Procedia 00 (202 5 ) 000 – 000

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At 500 °C, the coating consists only of the DZ. For a small Δ α , residual stresses are low, being tensile in the Ti core and compressive in the DZ (nearly a biaxial stress state, out-of-plane σ 1 ~ 0 >> in-plane σ 2 ≈ σ 3 ). As Δ α increases, compression in the DZ intensifies. At 600 °C, a thin, a stiff CL forms and introduces a modest increase in tensile stress within the Ti core, and a slight reduction in compression within the DZ. The CL itself carries only mild tensile stresses. For a representative Δ α = 1.0 µstrain/°C, the peak stress in the CL is ~15 MPa. At 700 °C, the nature and magnitudes of stresses are similar to those at 600 °C. The tensile peak in the CL near the CL/DZ interface increases but remains modest, ~25 MPa for the same Δ α . A simulated uniaxial-tension test was performed for each of the five coating variants to obtain the effective Young’s modulus. Although Δ α alters the local stresses, it does not affect the global stress-strain response. The moduli for the 500 °C and 550 °C cases were essentially identical (~82.5 GPa). Once a compound layer formed, stiffness increased markedly, reaching 109 GPa, 112 GPa, and 115 GPa for nitriding temperatures of 600 °C, 650 °C, and 700 °C, respectively (Fig. 4). The present model omits the early fracture of the brittle nitride layer, which produces through thickness cracks whose propagation rate increases with coating thickness (Guo et al. (2018)). This limitation will be addressed in forthcoming fracture-mechanics-based FE simulations. 3.2. Tensile response

Fig. 4. Simulated global stress– strain curves of nitrided neck variants under uniaxial tension.

4. Conclusions A finite-element model of a single gas- nitrided sintering neck has been developed to analyse how low-temperature gas nitriding modifies the elastic response and residual-stress field in direct- ink -written microporous titanium. The framework combines experimentally measured coating thicknesses with a graded description of diffusion-zone properties derived from the theoretical nitrogen-concentration profile. Simulations indicate that the diffusion zone accommodates high compressive cooling stresses, whereas the TiN compound layer and the Ti core remain only mildly stressed. In the next phase, fracture-mechanics criteria will be included so that crack initiation and propagation across the neck can be simulated directly. Acknowledgements This work was supported by the Czech Science Foundation under the project number 23 -07879S.