The formation of a porous TiO 2 layer by immersing Ti in 5 M NaOH at 303 K was investigated using electrochemical measurements, a scanning electron microscope, and grazing incident X-ray diffraction. The porous layer was readily obtained in the case of the presence of titanium hydride (TiH 2 ) on the surface before immersion. The crystalline structure of the porous layer is composed of a mixture of the rutile and anatase type TiO 2 . The porous layer is hard to produce without the presence of TiH 2 . The TiH 2 is directly changed to TiO 2 by a dissolution reaction in alkaline solution. The presence of TiH 2 on the surface is an important factor for the preparation of the porous TiO 2 layer.

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The friction stir welding (FSW) process is a solid-state joining process and the joining temperature is lower than that used in the fusion welding processes. Therefore, for dissimilar metal welding, FSW is considered to offer several advantages over fusion welding. The present work investigated the weldability of duralumin and titanium alloys using friction stir welding. The aluminum plates used in this work were 2024-T3 and 7075-T651, and the titanium plates used were pure titanium and Ti-6Al-4V. The average tensile strength of the Ti/2024 FSW joints was 311 MPa, and the tensile strength of the Ti/2024 joint was higher than that of the Ti/7075 FSW joint when the joining conditions were the same. A mixed region of Ti alloy and Al alloy was observed at the joint interface, and the joints mainly fractured at this region, where there was an intermetallic compound layer. In this region, a TiAl 3 intermetallic compound was detected by XRD. Therefore, it can be understood that this TiAl 3 intermetallic compound affects the tensile strength of butt joints.

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