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Alumina-to-alumina brazed joints were formed using 96.0 and 99.7 wt.% Al2O3 ceramics in as-ground and in ground and heat treated conditions using TICUSIL® (68.8Ag-26.7Cu-4.7Ti wt.%) braze preforms of thicknesses ranging from 50 to 250 μm. Brazing was conducted in a vacuum of 1 x 10-5 mbar at 850 °C for 10 minutes. Joint strengths were evaluated using four-point bend testing and were compared to the flexural strengths of standard test bars according to ASTM C1161-13. Post-grinding heat treatment, performed at 1550 °C for 1 hour, did not affect the average surface roughness or grain size of either grade of alumina but affected their average flexural strengths, with a small increase for 96.0 wt.% Al2O3 and a small decrease for 99.7 wt.% Al2O3. Post-grinding heat treatment led to secondary phase migration, creating a fissured 96.0 wt.% Al2O3 surface. This affected the reliability of 96.0 wt.% Al2O3 brazed joints, in which braze infiltration was observed. As the TICUSIL® braze preform thickness was increased from 50 to 150 μm, the average strengths of both 96.0 and 99.7 wt.% Al2O3 brazed joints improved. This occurred due to a microstructural evolution, in both sets of joints, which was studied using SEM, TEM and nanoindentation techniques. An increase in the TICUSIL® braze preform thickness increased the amount of Ti which was available to diffuse to the joint interfaces. This led to increases in both, reaction layer and braze interlayer thicknesses. Excess Ti in joints that were made using TICUSIL® braze preforms thicker than 50 μm, led to relatively hard Cu-Ti phases in an Ag-Cu braze interlayer. Cu-Ti phase formation, which may have reinforced joint strength whilst also reducing CTE mismatch at the joint interface, also led to Ag-rich braze outflow at the joint edges. Brazed joints made using as-ground 96.0 wt.% Al2O3 consistently outperformed brazed joints made using as-ground 99.7 wt.% Al2O3, due to the formation of Ti5Si3 phases at locations where the Ti-rich reaction layer intersected with the triple pocket grain boundary regions of the as-ground 96.0 wt.% Al2O3 surface.

Brazing, as a type of joining process, is widely used in manufacturing industries to join individual components of a structure. Structural reliability of a brazed assembly is strongly dependent on the joint mechanical properties. In the present work, mechanical reliability of low carbon steel brazed joints with copper filler metal is investigated and a methodology for failure analysis of brazed joints using the cohesive zone model (CZM) is presented. Mechanical reliability of the brazed joints is characterized by strength and toughness. Uniaxial and biaxial strengths of the joints are evaluated experimentally and estimated by finite element method using the ABAQUS software. Microstructural analysis of the joint fracture surfaces reveals different failure mechanisms of dimple rupture and dendritic failure. Resistance of the brazed joints against crack propagation, evaluated by the single-parameter fracture toughness criterion, shows dependency on the specimen geometry and loading configuration. Fracture of the brazed joints and the subsequent ductile tearing process are investigated using a two-parameter CZM. The characterizing model parameters of the cohesive strength and cohesive energy are identified by a four-point bend fracture test accompanied with corresponding FE simulation. Using the characterized CZM, the joint fracture behavior under tensile loading is well estimated. Predictability of the developed cohesive zone FE model for fracture analysis of brazed joints independent of geometry and loading configuration is validated. The developed cohesive zone FE model is extended to fatigue crack growth analysis in brazed joints. A cyclic damage evolution law is implemented into the cohesive zone constitutive model to irreversibly account for the joint stiffness degradation over the number of cycles. Fatigue failure behavior of the brazed joints is characterized by performing fully reversed strain controlled cyclic tests. The damage law parameters are calibrated based on the analytical solutions and the experimental fatigue crack growth data. The characterized irreversible CZM shows applicability to fatigue crack growth life prediction of brazed joints.

Brazing, as a type of joining process, is widely used in manufacturing industries to join individual components of a structure. Structural reliability of a brazed assembly is strongly dependent on the joint mechanical properties. In the present work, mechanical reliability of low carbon steel brazed joints with copper filler metal is investigated and a methodology for failure analysis of brazed joints using the cohesive zone model (CZM) is presented. Mechanical reliability of the brazed joints is characterized by strength and toughness. Uniaxial and biaxial strengths of the joints are evaluated experimentally and estimated by finite element method using the ABAQUS software. Microstructural analysis of the joint fracture surfaces reveals different failure mechanisms of dimple rupture and dendritic failure. Resistance of the brazed joints against crack propagation, evaluated by the single-parameter fracture toughness criterion, shows dependency on the specimen geometry and loading configuration. Fracture of the brazed joints and the subsequent ductile tearing process are investigated using a two-parameter CZM. The characterizing model parameters of the cohesive strength and cohesive energy are identified by a four-point bend fracture test accompanied with corresponding FE simulation. Using the characterized CZM, the joint fracture behavior under tensile loading is well estimated. Predictability of the developed cohesive zone FE model for fracture analysis of brazed joints independent of geometry and loading configuration is validated. The developed cohesive zone FE model is extended to fatigue crack growth analysis in brazed joints. A cyclic damage evolution law is implemented into the cohesive zone constitutive model to irreversibly account for the joint stiffness degradation over the number of cycles. Fatigue failure behavior of the brazed joints is characterized by performing fully reversed strain controlled cyclic tests. The damage law parameters are calibrated based on the analytical solutions and the experimental fatigue crack growth data. The characterized irreversible CZM shows applicability to fatigue crack growth life prediction of brazed joints.

No description available.