In order to realize light weight structure with multi-materials, this research applied ultrasonic joining method for dissimilar joining. As the joining members, aluminum alloy and CFRTP, CFRP using thermoplastic as base resin are selected because of their specific strength. By using ultrasonic joining system which generates maximum stress amplitude on joining plane, this stress fluctuation by ultrasonic application promote plastic flow in aluminum alloy. As a result, the plastic flow at interface remove resin of the CFRTP matrix, and the exposed carbon fibers are embedded in the Al alloy. Those mechanical interlocking at the interface enabled this dissimilar joining. Focusing on those effects caused by ultrasonic, we examined conditions that show high joining strength. By preparing specimens under various conditions, discuss relationship between joining conditions and joining strength by a series of cross tensile strength test. Moreover, the series of experiments with changing ultrasonic application time revealed that this joining method has the most suitable processing time, 0.8 s, and excess processing decrease its joining strength. Finally mechanism of this joining method has been discussed with the results of joining interface observation. And it was confirmed that carbon fibers embedded into Al alloy by plastic deformation form mechanical interlocking between carbon fiber and Al alloy.

Weld-joint has been attracting attention to assemble structures of dissimilar metals, particularly in automobile industry. However, the application of weld-joint to steel and aluminum plates is still limited due to uncertainty of the fatigue strength. Fatigue strength of weld-joint is rather complicated to evaluate because both effects of stress concentration at the edge and formation of intermetallic compounds along the interface should be considered carefully. This study focused on finding the key factor that determines the fatigue strength of steel-aluminum brazing joint by considering the variation of strength along interface. The welded part of single lap joint was partially removed by a wire-cut electric discharge machine to investigate the influence of length and location of interface on fatigue strength. When the applied load was relatively low it was found that the numbers of cycles to failure were found to be similar, even though the lengths of interface were different due to partial removal. Furthermore, observation on the fracture surface indicated that early stage of crack propagation showed similar topography regardless of the partial removal of weld part. These results clarified that the number of cycles to failure was mainly consumed at the early stage. Finite element analysis was then conducted to investigate the stress component affecting the fatigue strength. As a result, principal stresses were maximum at the fracture initiation site in all the specimens. Therefore, we concluded that fatigue life of weld-joint is dominant in the early stage of crack propagation, which is characterized by the principal stress.

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