Yasushi Fukuzawa1, Sigeru Nagasawa1, Yo Kojima1, Hiroshi Sakuta1 and Kazusige Yakubo1
1Department of Mechnical Engineering, Nagaoka University of Technology, Nagaoka
As metal-ceramics jointed materials are processed at elevated temperatures, thermal residual stresses are generated on the bonding interface during the cooling process. The discrepancy of thermal expansion coefficients between metals and ceramics causes a large residual stress on the ceramic side interface and the overall ceramics strength decreases at a edge of the bonding interface as a results of residual stress concentration effects. In order to make a high strength joint, the thermal peak residual stress has to be reduced and its distribution is expected to be uniform. E£ects of three types of interface shapes, flat, convex and concave on the bonding strength are investigated in this study. An electrical conductive TiB2-base ceramic is reshaped using an electrical conductive discharge machine (EDM) with a nickel electrode. The ceramic is then jointed with the electrode using a monel sheet as an insert material by the diffusion bonding heat process. To study the detailed residual stress levels and their distribution around the interface, tensile tests are conducted. A 2-dimensional finite element method (FEM) analysis based on an elasto-plastic condition of these shaped joints is also carried out. From experimental observations, the FEM analysis results imply that the crack propagation behavior is associated with the residual stress levels and their distributions. The results also show that in all cases the largest tensile stress level is present around the outside periphery of the interface, and the distribution of the principal stress is affected by the shape of the interface.
diffusion bonding, interface shape, residual stress, electrical discharge machining, electrical conductive ceramic, finite element method
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