The friction stir welding (FSW) of various kinds of ultrafine grained materials, such as steel, aluminum and copper, was performed. Ultrafine grained IF (interstitial free) steel, pure aluminum (A1050) and oxygen-free copper were produced by the accumulative roll bonding (ARB) process and the ultrafine grained low carbon steel (SM490) was produced by the Martensite process. The tensile strengths of the joints for all the materials are much higher than those of the TIG welds and increase with the increasing revolutionary pitch due to the decrease in the grain growth. High strength joints are obtained for the ultrafine grained steel and aluminum under the optimal welding conditions, although the strength of the ultrafine grained copper joints significantly decreases compared to that of the base metal due to the remarkable grain growth in the heat affected zone (HAZ).
Significant attempts have recently been made to improve the mechanical properties of materials by controlling their microstructure. Grain refinement is one of the useful methods for enhancing mechanical properties (Mishra, 2000; Takaki, 2001). According to the Hall-Petch relationship (Hall, 1951; Petch, 1953), the strength can be increased several times by producing ultrafine grains which are less than 1μm. However, the welding or joining of small parts is an essential processing step to make large or complex structures because it is difficult to directly manufacture a large or complex structure with ultrafine grains.
Friction stir welding (FSW) can restrict the grain growth in such materials during welding due to the lower heat input (Fujii, 2004, Hirano, 2004; Sato, 2004, Liu, 2003), while fusion welding easily causes a significant grain growth and decrease in the strength of the ultrafine grained materials. In this study, FSW was performed for ultrafine grained IF steel and carbon steel using a newly developed tool.
