1. Clarification of interface control and formation mechanism of friction welding (friction stir welding (FSW), rotary friction welding, and linear friction welding)

The FSW of medium/high carbon steels, nuclear vessel steels, ODS steels, TRIP steels, Ni steels, Cr steels, 1600 MPa high tensile strength steels, high-strength Ti alloys, flame-resistant Mg alloys, various kinds of Cu alloys, ultrafine-grained Al alloys, thick-plate Al alloys, etc., is being conducted. Dissimilar joining of Al/Mg, Mg/steel, ODS steel/steel, metal/ceramic, etc., is also being conducted.

The FSW technique for high-melting-point metals has been developed. Based on this technique, the material, which currently has the highest melting point (2620℃) in the world, was successfully friction stir welded. A low-temperature welding method for carbon steels below the Al point (723℃) was also developed. This low-temperature welding technique for carbon steels regardless of the carbon content has been attracting much attention from lots of industrial fields. Furthermore, the laser-combined hybrid FSW technique and double-acting FSW technique in which the shoulder and the probe can be separately driven have been developed, thus the FSW technique, by using which no hole is left at the end of the weld seam, was successfully created.

A low-temperature FSW technique in which the heat affected zone (HAZ) is not softened was also developed for Al alloys and ultra-high tensile strength steels. The development of a novel tool material and tool shape and the design of porous materials by using the friction stir processing (FSP) technique is also being conducted.

2. The development of novel joining technique and surface modification processing

In order to solve the issue of the FSW tool life, the tool-free linear friction welding and the friction welding, in which the weld interface is severely plastic deformed, were developed. A low-temperature FSW technique in which the HAZ is not softened was also developed for Al alloys and ultra-high tensile strength steels. The development of a novel tool material and tool shape and the design of porous materials using the FSP technique is also being conducted.

Furthermore, the inverse FSW technique, which is an innovative joining method used for hollow structures such as pipes, was successfully developed. In the normal FSW, the high-speed rotating tool is inserted into the workpieces from the top surface, then the workpieces are joined. However, in the inverse FSW, the tool is inverted, and inserted into the workpieces from the bottom surface, which facilitates the joining of the structures from the inside.

The friction stir powder processing (FSPP) technique, in which the alloying powder is added to the FSP in order to modify the microstructure of the designated part of the structure, was successfully developed. Based on this novel technique, surface modification is being conducted on steel using Cr powder.

3. Clarification of the formation mechanism of the fusion weld interface and molten pool

The microstructural evolution and the formation process of the solidification cracks in the TIG welding process were clarified by in-situ observation utilizing high-brilliant X-rays available in the large synchrotron radiation facility (SPring-8).

The respective effect of the tool shoulder and tool probe on the size of the stir zone and the microstructural evolution in the stir zone during the FSW was elucidated using a tool with a stationary shoulder. Moreover, the mechanism of the stir zone formation and the defect formation in the stir zone as well as the influence of the laser preheat on the motor torque and the defect formation during the FSW were investigated using a 3-D imaging system.

The AA-TIG (Advanced A-TIG) welding method has been developed in our laboratory based on the accurately measured surface tension data of the Fe-O system. This novel welding method has been granted patents in 8 countries around the world and a new approach for further enlarging the application range is currently underway.

4. Weld interface structure analysis

FSW was performed on various FCC metals such as pure Cu, brass, pure Al etc., The microstructure formed during the high-temperature plastic deformation of the FSW was frozen by a stop action technique using liquid CO2 cooling. In combination with post-annealing, the microstructural evolution during the high-temperature plastic deformation of the FSW and the annealing effect during the cooling process after the tool pass can be separately studied, so that the microstructural evolution during the FSW process can be systematically clarified. It is found that static recrystallization and grain growth occur during the cooling process after the tool pass in the FSW. Furthermore, the strain and strain rate in specific locations around the tool in the FSW process were estimated using the tracer method.

Double-side asymmetric FSW was performed on Mg alloys having the HCP structure and the texture formation mechanism was clarified. On the other hand, a high pressure torsion was performed on the biomedical Co-Cr-Mo alloy in order to induce a microstructural refinement.

5. Control of solid/liquid interface formation

Wettability is a physical property between a solid and liquid. In the most commonly used wettability measurement method, called the sessile drop method, the advancing contact angle is obtained, which is generally larger than the true value. In order to solve this issue, a new wettability measurement method was developed, in which the advancing and receding contact angles can be simultaneously measured in the same system and environment based on a new system utilizing an ultrasonic transducer.

Based on this modified sessile drop method having an improved measurement accuracy, the physical properties of various molten metals were measured, which assists in clarifying various industrial processes. For example, the wettability of various hard coatings with molten pure Fe, SUS304, pure Cu, and pure Al was measured. Moreover, the cutting resistance of the workpieces and the torque of the motor, which controls the FSW tool, were also measured. Based on these measurements, it is found that not only the properties of the cutting tools, but also those of the FSW tools have a close correlation with the wettability. Since the research on the wettability can provide a guidance for developing the next-generation FSW tools, it has attracted much attention in various industrial fields.