Nihon Kikai Gakkai ronbunshu (Aug 2024)
Effect of Nb interlayer on residual and bending stress relaxation in Si3N4/Nb/Ti-6Al-4V joints
Abstract
In dissimilar joints between Si3N4 and Ti-6Al-4V, insertion of a ductile Nb interlayer is effective for stress relaxation under bonding and loading. In this study, elastoplastic analysis using the finite element method (FEM) was applied to determine the optimum thickness of the Nb interlayer in Si3N4/Nb/Ti-6Al-4V joints needed to maximize the bending strength, and the analytical results were compared to experimental results to validate the proposed method. The joining process involved transient liquid phase bonding using Ni and Cu fillers on the Nb/Ti-6Al-4V side to suppress brittle intermetallic compounds, and brazing with Ag-Cu-Ti filler on the Si3N4/Nb side to promote wettability and interfacial bonding with Si3N4. The average bending strength was maximized at 366 MPa when the cross-sectional area was 3 × 4 mm2 and the thickness of Nb interlayer was 0.25 mm. Joints fractured near the Si3N4/Nb interface brazed with Ag-Cu-Ti filler. According to FEM analysis, stress singularity fields were found near the interfaces at free surfaces after cooling and bending owing to the discontinuity of stiffness in each material. The stress singularity field in Si3N4 after bending was minimum when the cross-sectional area was 3 × 4 mm2, and the thickness of Nb interlayer was 0.25 mm because the stress singularity fields near Si3N4/Nb and Nb/Ti-6Al-4V interfaces separated and the deformation constraint of Nb from Si3N4 and Ti-6Al-4V was relatively effective. Therefore, the results from experiments and the proposed analytical method were in qualitative agreement. The optimum thickness of the Nb interlayer in Si3N4/Nb/Ti-6Al-4V joint was 0.25 mm for a cross-sectional area of 3 × 4 mm2 and 0.75 mm for a cross-sectional area of 10 × 10 mm2; therefore, the optimum interlayer-thickness-to-joint-height ratio was approximately 0.08.
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