Papers by Author: Satoshi Hirano

Abstract: The authors have developed a new friction-stir welding (FSW) tool that enables to weld high-softening-temperature materials (HSTMs), such as steels, titanium and zirconium alloys. The new tool is made of a Co-based heat-resistant alloy strengthened by precipitating intermetallics, Co₃(Al,W), with a L1₂ structure at high temperatures. The Co-based alloy tool exhibits yield strengths higher than 500 MPa at 1000 deg C, so it might have a great potential as a tool material for FSW of HSTMs. In this study, the feasibility of using the Co-based alloy tool with various HSTMs was examined. Changes in the tool shape during FSW and the weld appearances produced with the Co-based alloy tool will be briefly shown.

Abstract: Bobbin FSW technique is assessed in terms of process productivity and robustness considering the metallurgical and mechanical qualities of the joint. Several different designed bobbin tools were applied to aluminium A6xxx alloy extrusions, at feed rate of 400-1000mm/min with tool rotation speed of 600-1000rpm. In case of A6068-T6, a joint efficiency of 68% was achieved at 600mm/min, which is almost equivalent to the productivity and quality of the conventional FSWed joints, hence the potential of bobbin FSW technique was suggested. The bobbin tool design was further optimized considering the practical process robustness to the part fit issue, and defect free welds have been achieved for the joints with a set gap up to 1mm. However, the oxide remnant, so called “lazy S” was observed in a joint made with an inadequate designed tool, which significantly deteriorates the mechanical properties.

Abstract: The characteristics of microstructures in friction stir (FS) weld of 304 austenitic stainless steel were examined. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallized and recovered microstructures, respectively. The hardness of the SZ was higher than that of the base material and the maximum hardness was located in the TMAZ. The higher hardness in TMAZ was attributed to high density of dislocations and sub-boundaries. Electron microscopic observations revealed that ferrite and sigma phases were formed in austenite matrix in the SZ during friction stir welding (FSW).

Abstract: Microstructural characterization and temperature analysis have been performed in friction stir welding (FSW) of A383 and 5052 dissimilar aluminum alloys. Marked difference in microstructure was observed between joints with different arrangements of materials. The temperature at four points on each side of the joint line was measured during FSW in various conditions. In addition, an analytical model assumed that the work generated by the rotation of the tool led to the work for stirring materials and heat generation of the material and the tool. The temperature of the retreating side (RS) for the joint of the advancing side (AS):A383/RS:5052 was about 50K higher than that of AS, while the temperatures of AS and RS for the joint of AS:5052/RS:A383 were almost the same. The experimental temperature could be calculated reasonably by using the model with assumption of the work for stirring the material.

Abstract: Friction stir welding (FSW) was applied to a 0.53% nitrogen stainless steel. The nitrogen content change and the microstructural evolution in the weld were investigated. The nitrogen content analysis revealed that the stir zone (SZ) showed roughly the same nitrogen content as the base material (BM). This result suggests that FSW is an effective welding process for keeping up nitrogen content of high-nitrogen steel weld. The microstructural observation showed that the weld had the BM, the SZ, the partially recrystallized zone (PRZ) and the heat affected zone (HAZ). The coarse grain structure of the BM changed to relatively fine microstructure of equiaxially recrystallized austenite grain structure in the SZ during FSW. The PRZ contained both the fine and coarse grain structures. The detailed TEM observations showed that the particles with about 11m in size were present in the advancing side of the SZ both along grain boundaries and grain interiors. The Cr nitride less than 100 nm was formed in the HAZ along grain boundaries, which indicates that a slight sensitization occurred in the HAZ during FSW.

Abstract: Friction Stir Welding (FSW) has been applied to ultra fine grained (UFG) steel which is the plane carbon steel with the average grain size of 1mm. The plates of 5mm thickness was successfully welded using pcBN tool (polycrystalline cubic boron nitride) and the metallurgical and mechanical properties of the joint were evaluated. In the stir zone, Bainite has been formed which shows the highest harness in the joint. On the other hand, grain growth is observed in the heat affected zone. The joint effect of 88% has been obtained for UFG steel by FSW process.