Magnesium alloy AZ31 was friction welded using a brake type friction welding machine under argon shielding. Microstructures and mechanical properties of the friction welded joints were investigated. The fine grain structure resulting from disappearance of fibrous one is observed closely near the weld interface. The hardness of both the weld interface and heat affected zone are nearly equal to those of the base metal. The tensile strength of welded joints are improved with increasing friction time, but it is lower than that of the base metals. In case of friction pressure of 50MPa and friction time of 6 seconds the elongation of the joint shows the highest value, but it is inferior to that of the base metal. All the impact specimens with the notch into the weld interface showed impact strength inferior to that of the base metals, but impact strength increases with increasing notch distance from the weld interface. There is no trouble in flaming during the welding process.

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Long time brazeability of a 3004 aluminum alloy was improved by the heat treatment at about 270°C for 20 to 168 h. The improvement was mainly explained by the coarsening of recrystallized grain size after brazing in a 3004 base metal. In finer grains, molten filler metal spread widely on the base metal surface and the erosion depth was also larger than that in coarse grains. Therefore, the fillet once formed disappeared. Both the wide spread of the filler metal and the deep erosion in the base metal were the main reason for the fillet size reduction. Base metals appropriately heat treated maintained large fillet after brazing at 600°C for 60min.

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Diffusion bonding of superplastic aluminum alloys has been generally carried out with insert materials or in vacuum, because the oxidized layer is too firm for diffusion bonding to take place. Diffusion bonding behavior of the superplastic 7475 aluminum alloy sheets was investigated without an insert material in air taking into account of the manufacturing process being easier by changing initial surface roughness, strain-rate, bonding pressure and time. Bonding was successfully performed at the thickness strain larger than 15%. Bonding strength proportionally increased with increasing thickness strain. The value of bonding strength was maximized by adjusting the strain rate during forming to that of superplastic condition (ε_??_1.1×10^-4s^-1) and with proper preparation of the initial surface roughness of the sheets. The maximum value obtained in this study was 40% of the original sheet.

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Rapidly solidified powder metallurgy Al-Si-3%Cu-1%Mg alloy (20, 25, 30%Si) were jointed by friction welding. Regardless of the silicon content, the tensile strength of welded joints were lower than those of the base metals. Elongation of welded joints of Al-30%Si-3%Cu-1%Mg alloy was nearly equal to that of the base metal. On the other hand, those of Al-20%Si-3%Cu-1%Mg and Al-25%Si-3%Cu-1%Mg alloys were slightly lower. All the friction welded joints were fractured at the weld interface. As for the heat affected zone, the hardness increased in a narrower range than that of the base metal while in the position 4mm distance from the weld interface. The original fibrous structures were pressed forward to the outside along the flux of burr on the heat affected zone by the heat and axial pressure at the welding process, while they disappeared near the weld interface, and welded layer formed at the weld interface.

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Vacuum brazeability of various aluminum alloys was investigated to examine the effect of Mg content and the origin of poor brazeability was proved. Brazeability of aluminum alloys containing Mg more than 1wt% remarkably decreased. The poor brazeability of higher Mg containing aluminum alloys was derived from the lack of filler alloy both by the spreading on the surface and by the erosion through grain boundary of the alloy during brazing.

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SPF/DB (Superplastic forming/Diffusion bonding) joints of 7475 aluminum alloy were produced, where various surface finishes and use of insert materials for removing surface oxide film were attemped. SPF/DB using a cryogenic rolling sheet of 7475 aluminum alloy brought better results of bonding and fatigue strength than that using a conventional cold rolling sheet. Surface finish was required to obtain high-quality bond, but Ag insert material was enable SPF/DB without surface finish for thin and weak oxide films. 5052 aluminum alloy was not effective as the insert material. From the results of fatigue life prediction based on the fracture mechanics analysis, it was considered that the fatigue strength of the cryogenic rolling sheet bonded specimen was almost equivalent to that of the bulk specimen.

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With an increase in the automotive applications of Al-Mg alloy sheets, the fatigue strength of spot welded joints has become an important subject. In this paper, the influence of nugget diameter, successive spot welding and oxide film on the base metal surface on the fatigue property of spot welded joints were examined for the Al-4.500Mg-Cu alloy sheets. The tensile shear fatigue strength of the spot welded joints increased in proportion to the nugget diameter. An acid cleaning of the base metal was fairly useful to prevent degradation of the mechanical property of the joints successively spotwelded. The cleaning significantly lowered the electric contact resistance of an Al-4.5Mg-Cu alloy and resulted in the improvement of the nugget quality.

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The optimum welding condition for a 5056 aluminum alloy was investigated using a responce surface method. The optimum welding condition to yield maximum tensile strength at the weld was attained through a steepest ascent path. A second-order equation predicting the weld strength accurately was established. The successful weld showed 89.2% joint efficiency in tensile strength. The optimum welding condition was obtained using a comparatively small up-set burn off. Therefore, the economical consideration such as a loss of material was not needed to be taken into account. The observation of the weld showed that the friction layer formed at the friction interface disappeared in the successful weld.

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In joining of four kinds of ceramics, Si₃N₄, SiC, Al₂O₃ and PS-ZrO₂ (partially stabilized), to aluminum with fusible inserts of Al-Mg and Al-Si alloys, mechanical metallizing with aluminum was attemped and its effects were investigated in connection with an insert and joining temperature. As to the metallizing treatment, aluminum was rubbed over the joining surface of ceramics. The metallizing treatment increased bond strength remarkably for Al-Mg alloy insert, but did not for Al-Si insert. The increase in bond strength using Al-Mg alloy insert was due to relief of high thermal stress, which was caused by the metallized aluminum. A large decrease in the joining temperature was not obtained to assure the bond strength higher than the tensile strength of the base aluminum.

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