Recovery and recrystallization of a 90% cold rolled Al-0.2%Zr alloy were investigated by means of microscopic observation and hardeness measurement. Prior to cold rolling, four types of heat treatments, solution treatment, 1400 hour and 6500 hour aging at 350°C, and 4100 hour aging at 450°C were conducted. The experimental results are summarized as follows:
The recrystallization temperature was the highest in the aged specimens with particles of about 280 Å in diameter and the lowest in the aged specimens with incoherent globules. The letter specimens were softened slightly by lower temperature annealing.
The presence of Zr solute atoms in the matrix produced the pinning of dislocations during annealing and, thus, suppressed the effective movement of dislocations. Nucleation and growth rate significantly depended on the size of coherent spherical precipitates. Rod-shaped precipitates, formed by a grain boundary reaction, did not retard the recovery and recrystallization.

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Aging characterisics of the weld metal in the commercial Al-Zn-Mg alloy were studied by means of hardeness measurements and X-ray small angle scattering methods. Isothermal aging curves of hardness, particle size and integrated intensity were obtained at room temperature, 80°C and 135°C. From these aging curves, the aging process of the weld metal in the commercial Al-Zn-Mg alloy was discussed. Changes in hardness were in good correspondence with those in particle size and integrated intensity. It was found that hardness distribution in the weld matal was uniform at any aging temperature and aging time, which corresponded to macroscopically uniform distribution of solute atoms in the weld metal. It was also shown that hardness was always lower in the weld metal than in the heat-affected zone during aging at any temperature. This was due to micro-segregation of Zn atoms in the weld metal. Aging processes of specimens, solution-treated after welding, water-quenched, and aged at 80°C and 135°C, were compared with those directly aged at the same temperatures after welding. It was found that the aging process at 135°C depended on the above heat treatments after welding while the aging occurred in a similar way at 80°C. This results was discussed in terms of thermal histories of specimens before aging.

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Diffusion rate and distribution coefficient of gases during solidification in the ingot of an Al-4%Cu alloy were studied by the unidirectional solidification technique. Gas content in the ingot was measured by a vacuum fusion technique. Results obtained are as follows:
(1) Diffusion rates of gases in the solid state, the coexistent state of the solid-liquid and the liquid state were 2.9×10^-5, 3.8×10^-5 and 6.3×10^-5cm²/sec, respectively.
(2) Gas distribution coefficient tended to reach unity as the solidification rate increased.
(3) Gas content in the ingot became nearly in equilibrium with the atmosphere as the solidification proceeded. Thus, gas distribution in the ingot had the same pattern in nearly every sample except some cases.
(4) It was found that degassing by vacuum melting and solidification was due to the diffusion rather than the bubble discharge.

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Extensive studies were carried out to develop new Al-Zn-Mg alloys with excellent extrudability, weldability, resistance to stress corrosion cracking and medium strength. Besides above properties, aging characteristic, corrosion resistance, resistance to weld cracking, strength of welded joints, fatigue strength, resistance-weldability and low temperature characteristic of several Al-Zn-Mg alloys with less Mg were investigated in detail. From these studies, the following two alloys were developed and put to practical use.
The alloy, ZK60, with 6%Zn, 0.7-0.8%Mg, 0.1-0.2%Zr and Mn had better extrudability than common alloys with 4-5%Zn and 1-2%Mg and 6061. Strength, weldability and resistance to stress corrosion cracking of this new alloy was nearly equal to those of common Al-Zn-Mg alloys, when it was T5-treated.
The alloy, ZK61, with 6%Zn, 0.3-0.4%Mg and 0.1-0.2%Zr was also developed and had remarkably good extrudability which was equivalent to that of 6063 alloy. Tensile and yield strength of the naturally aged alloy were 30 and 18kg/mm², respectively and seemed to be higher than those of 5083 alloys. Weldability and corrosion resistance were similar to those of 5083 alloy. However, the alloy had better resistance to stress corrosion cracking and low temperature characteristics.
These two alloys are currently and widely used as strength member of container, trailer, rolling-stocks, rims of auto-cycles and bicycles.

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Residual stress in 25mm thick plates of A5083-o, which to circular patches of 50-400mm in diameter had been welded with MIG-semiautomatic welding, was measured by the stress-relief method of cutting the gage section out of the plate. In all specimens, the circumferential stress, σ_θ, and the radial stress, σ_γ, were tensile within the patch. However, outside the patch, σ_θ became compressive, but σ_γ was still tensile. The maximums of σ_θ and σ_γ, σ_θmax and σ_γmax, were observed at the neighborhood of the welds. Only in the case of the patch with small diameter, σ_θmax and σ_γmax changed slightly along circumferential direction.
σ_θmax and σ_γmax depended on the patch diameter. As the patch diameter increased to 100-200mm, σ_θmax and σ_γmax increased to 16.5 and 16.0kg/mm², respectively. However, they approached to constant values in a decreasing way when the patch diameter was beyond 400mm, which was identified as the optimum size of the patch diameter. The dependence of the residual stress on the patch diameter was considered due to a heat cycle at the welding part. Although both-sides symmetrical block-welding was somewhat effective for reducing the residual stress, the effect was not so remarkable taat as in steel.

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