Surface free energy of various carbon fibers was determined by tensiometric method in three different procedures. The dispersive ys a and the nondispersive y~ components were separately determined by contact angle measurements in two-phase system, formamide-n-alkanes. The yw increased by oxidation and decreased by hydrogen reduction, while the ys a remained almost constant. The y~ value determined from contact angle of water/ethyleneglycol solution tended to become higher. The critical surface flee energy was in harmony with the total free energy determined from formamide-n-alkanes system as long as liquids were properly selected. The formation of oxygen-containing functional groups was evidenced by ESCA, and the surface [O]/[C] ratio was related linearly to the surface polarity defined by yw a + y~]. Oxidation in liquid phase resulted in the formation of both hydroxyls and carboxyls, while only hydroxyls were formed in gas phase. The interracial shear strength of the fiber-epoxy resin could be closely correlated with thermodynamical work of adhesion calculated on the basis of surface free energy and its component.

Dissimilar friction stir welding of aluminum alloy and steel is attractive method to fabricate light structure for transportation vehicle. The welding conditions, such as heat input (tool rotation speed and welding speed) and welding tool shape influence on the weld material and the material flow during welding. Control of material flow is important for fabrication of sound weld without defect. Thus, the material flow in the weld was investigated by using X-ray CT technique after welding. Chip of steel from weld interface was used as marker material for 3D visualization of material flow in aluminum alloy. From the observation, the heat input and probe shape affected on the material flow and formation of weld defect. Screw probe effectively stirred aluminum, induced downward and circumferential material flow around the probe and fabricated defect free weld with high weld strength.

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