Microstructures and mechanical properties of dissimilar welding joint between Al alloy and Mg alloy by Friction Stir Welding (FSW) were investigated in comparison with laser welding of the same combination. Dissimilar joint of Al and Mg alloy by laser welding was very brittle because of building up Mg 17 Al 12 inter metallic compounds in fusion zone. On the other hand, FSW is anticipated to welding dissimilar alloys with enough joint strength because it is a solid-state process without melting. In this paper, FSW was carried out to make dissimilar butt joints of Al alloy and AZ31 magnesium alloy with various tool rotational speed and welding speed. These joints showed higher hardness in their stir zones than that of parent AZ31 alloy because of Mg-Al inter metallic compound formation. However, the hardness of stir zone was lower than that of fusion zone of laser welding, and was changed with the welding parameters of tool rotational speed and welding speed (i.e. heat input ratio of FSW). The optimum welding conditions of Mg and Al dissimilar FSW joint and the influence of inter metallic compound distribution with mixing of materials in stir zone were discussed.
A hollow-cone Foucault (HCF) imaging method using Lorentz microscopy was developed. Hollow-cone illumination was realized by using deflectors above the specimen and an inclined electron beam circulating with respect to the optical axis. The advantage of the HCF method, having the bright and dark-field modes, is that it can simultaneously visualize both magnetic domains and magnetic domain walls under the in-focus condition. Furthermore, schlieren images, obtained under the specific inclination angle of the illumination beam by adjusting the angle between the bright-field and dark-field modes, can qualitatively visualize the electromagnetic fields in spaces around the specimen.
In conventional transmission electron microscopy (TEM), observation of magnetization structures and dielectric polarization structures are difficult to observe because these structures weakly interact with electron beams. Lorentz microscopy was developed to observe these structures effectively and practically. In Lorentz microscopy, however, Fresnel and Foucault imaging methods have the following disadvantages: in the Fresnel imaging method no additional contrast can be generated on the domains because of just defocusing for images and in the Foucault imaging method only the filtered out information can be obtained from selected domains. To overcome these difficulties, hollow-cone Foucault (HCF) imaging method was developed [1], where an incident electron beam on the specimen was tilted with respect to the optical axis and was circulated in all azimuths around the optical axis. Both magnetic domains and domain walls were simultaneously visualized with sufficient contrasts under the infocus condition. Furthermore, it was confirmed that schlieren imaging method [2], known as a highspeed imaging method applicable to low refractive index media, was realized. Figure 1 shows a schematic diagram of the optical system for the HCF imaging. The parallel electron beams having less than 10 -6 rad diffusion angle are irradiated on the specimen with the inclination angles in X and Y directions controlled by using the beam deflector system placed above the specimen. The circulating electron beam is illuminated in all azimuthal directions around the optical axis. This special condition leads to realization of small-angle hollow-cone beams with an inclination angle as small as 10 -4 rad. The experiment was performed using a 200-kV thermal field-emission TEM (JEM-2100F) and the HCF images were recorded with a 2k × 2k pixel charge-coupled device camera (Ultrascan camera). Each image was recorded through 12 turns in the illumination azimuthal rotation in 7.5 seconds, corresponding to 1.6 Hz. An Fe0.88Ga0.12 alloy, which has large magnetostriction at room temperature, was thinned to 250 nm thickness by focused ion beam instrument (NB-5000). Figures 2(a) and (b)show the bright-field and dark-field HCF images for different inclination angle conditions. The selected area aperture with opening size of 100 µm in diameter corresponding to the 1.30×10 -3 rad was utilized as the angle-limiting tool. Figure 2(a) is a bright-field HCF image, where domain walls are observed as white lines and several domains have slight dark contrast. Figure 2(b) is a dark-field HCF image, where domain walls are observed with black lines and several domains also have slight bright contrast. The contrast in the dark-field HCF image in Fig. 2(b) is reversed with respect to the bright-field HCF image in Fig. 2(a).In addition, schlieren imaging mode, obtained under specific inclination angle of the illumination beam between the bright-field and dark-field modes, can qualitatively show magnetic field leaking from the specimen over a wide range and to a far distance....
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