The effects of post weld heat treatment (PWHT) on microstructures and mechanical properties of friction stir welded AZ31B magnesium alloy were investigated. Average grain sizes at the upper region and at the bottom region in the stir zone (SZ) of as welded joint were 9.5 μm and 14.4 μm, respectively. When heat treatment temperature at 673 K, a few relatively large grains were formed at the retreating side in upper region, subsequently the region extended in the stir zone as heat treatment time increased. It is believed that the abnormal grain growth is caused by inhomogeneous distribution of dislocation. The elongation to failure of joint was improved by the heat treatment at 573 K for 7.2 ks. However, the formation of abnormal large grains caused degrading elongation. The yield stress at upper side in SZ of as welded joint was larger than that at bottom side and was decreased gradually as the joint was annealed. Because the additional strength at the upper side was due to the work hardning that was added by the shoulder of tool.

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Attempts have been made to improve corrosion resistance of Al–Mg–Si alloy 6063 by an electrochemical treatment. We have found that the intermetallic compounds exposured on the 6063 alloy specimen have been preferentially and selectively removed by cathodic current of −2 kA·m⁻² for 300 s in a 20 mass% H₂SO₄ solution. Above treatment leads to experimental results that the 6063 alloy specimens, which have had free intermetallic compounds and anodized in a 20% H₂SO₄ solution, show excellent corrosion resistance in a NaCl solution.

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By using hydrogen microprint technique (HMPT) , effect of microstructure on hydrogen permeation behavior in 7075 and 7475 aluminum alloys exposed to gaseous hydrogen has been investigated. The important findings are as follows. The amount of permeated hydrogen is strongly dependent on crystalline structure, temper and impurity content. Emission sites of hydrogen are constituent particles such as Al₇Cu₂Fe. The results indicate that subgrain boundaries and misfit dislocations present at the interface between stable η precipitate and the matrix can act as trapping sites for hydrogen. Invasion sites of hydrogen are presumed to be the constituent particles. From quantitative investigations on the amount of emitted hydrogen and on lattice diffusion of hydrogen, as well as from the above result that the subgrain boundaries act as trapping sites, migration of hydrogen is deduced to take place by grain boundary diffusion, not by lattice diffusion, nor by pipe diffusion along dislocations.

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For the purpose of the development of a cheap, excellent in formability AZ31 alloy sheet, the influence of rolling and heat-treatment on grain refinement was examined using the cast strip processed by the twin-roll casting as a sample. As a result, it became clear that the almost uniform, fine grain structure of average grain size 4.7 μm is obtained by performing appropriate rolling, the intermediate and final annealing for 1 h at 300°C, respectively, after homogenization for 8 h at 450°C. In addition, finest average grain size of 3.2 μm was obtained by the model experiment to imitate flash annealing. 0.2% proof stress increased with decreasing in the grain size, and the relation of Hall-Petch was almost approved between them.

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Carbon fiber reinforced aluminum alloy composites (CF/Al composites) are expected in aerospace and electric power cable industries due to superior specific strength and specific modulus. But, it is known that CF/Al composites form aluminum carbide (Al₄C₃) at the interface between carbon fiber and aluminum alloy when CF/Al composites are fabricated. However, effects of type of carbon fiber (PAN, pitch) on growth mechanism of Al₄C₃ and tensile strength of CF/Al composites have not been clarified. In this study, at first, CF/Al composites are fabricated with ultrasonic infiltration method. Secondary, effects of type of carbon fiber and fabricating time on quantity and size of Al₄C₃ were investigated. Thirdly, effects of quantity and size of Al₄C₃ on tensile strength of CF/Al composites were examined. The length of Al₄C₃ increased with increase in fabricating time for PAN-based composites. It was suggested that the numbers of nucleating sites of Al₄C₃ increased with an increase in fabricating time for pitch-based composites. As the result, as to the PAN-based composites, it should be controlled less than 100 nm of the length of Al₄C₃ to inhibit degradation of tensile strength. As to the pitch-based composites, fabricating time should be shorter.

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[in Japanese]

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