Graduate School of Engineering
Department of Materials Processing

Interface Science and Engineering of Joining (Prof. Sato)

Research Topics

Grain boundary engineering of austenitic alloys

Intergranular corrosion of austenitic stainless steels and Ni alloys is a conventional and momentous problem during welding and high temperature use. One of the major reasons for such intergranular corrosion is so-called sensitization, i.e. chromium depletion due to chromium carbide precipitation at grain boundaries. Conventional methods for preventing sensitization of austenitic stainless steels include reduction of carbon content in the material, stabilization of carbon atoms as non-chromium carbides, local solution-heat-treatment by laser beam, etc. These methods, however, are not without drawbacks. Recent grain boundary structure studies have demonstrated that grain boundary phenomena strongly depend on the crystallographic nature and atomic structure of the grain boundary. Grain boundaries with low-Σ coincidence site lattices (CSLs) are immune to intergranular corrosion. The concept of grain boundary engineering (GBE), has been recently developed, involves a desirable grain boundary character distribution (GBCD), including high frequency of CSL boundaries. Our research group is trying to make a favorable GBCD by optimization of thermo-mechanical treatment to achieve strong intergranular corrosion resistance in austenitic alloys.

Microstructural studies on friction stir welding and friction stir processing

Friction stir welding is a solid-state joining process that was invented at The Welding Institute (TWI) in 1991. The process reduces manufacturing costs due to the elimination of some defects, filler materials, shielding gases, and costly weld preparation. Furthermore, friction stir welds usually exhibit better mechanical properties than fusion welds. Taking advantages of these of these positive factors, this process already been applied to the construction of some structures, such as external fuel tank of rockets, rolling stock of railways, high speed vessels, bridges, etc. Optimized welding tool designs and welding parameters have been determined since inception. However, microstructural feature and evolution in the weld, which are strongly related with weld properties, have not been fully understood. Our group is examining evolution of microstructural distribution associated with local properties during friction stir welding in aluminum alloys, magnesium alloys and steels to clarify friction stir welding phenomena. Moreover, friction stir processing, which is a unique technique for microstructure control on the basis of principle of friction stir welding, is also examined for production of metallic materials having new properties.

Interfacial phenomena in ultrasonic welding of metals

Ultrasonic welding (USW) is a solid state process which can produce a joint between thin metallic materials through ultrasonic vibrating energy and normal clamping force. The USW technique is characterized by a lower energy input, shorter welding time and thinner workpieces than the other spot welding techniques. The manufacturing technology utilizing ultrasonic metal welding technique has also received attention in recent years. Ultrasonic additive manufacturing (UAM) is a solid state welding process to build complex 3-D components directly from metallic tapes. A thin metallic tape is initially bonded ultrasonically on a base plate, with additional tapes being subsequently bonded on the previous tape. Thus USW technique has a great deal of potential as an environmentally-friendly and cost-effective tool in many industrial fields. However, there is currently little academic understanding on the ultrasonic welding mechanism despite the engineering significance and past research efforts. Our works on USW technique aim to understand the welding mechanism based on systematic examinations of the microstructural evolution and mechanical properties of the ultrasonic welds.

Fig. 1:Friction stir welding tool made of Co-based alloy for high-softening-temperature materials.

Fig. 1:
Friction stir welding tool made of Co-based alloy for high-softening-temperature materials.

Fig. 2:Microstructural characterization of ultrasonic welds in dissimilar metals

Fig. 2:
Microstructural characterization of ultrasonic welds in dissimilar metals