The effect of the holding process to primary aluminum ingot has long been studied on. When the holding is more too long, the molten metal gets gradually contaminated and the surface of ingot made of such metal is coloured in yellow, grey or violet. It is learnt from the above fact that the holding time must be limited, and the appropriate holding time is dependent upon the ratio between the surface area and quantity of the molten metal. The higher the ratio is, the sooner the ingot is stained. Similar phenomenon is observed on sheet ingots and residual metal in crucibles.
Regarding the mechanical properties, any difference is seen between thus stained ingot and normal one. But it has been found that such stained ingot absorbs more gas than normal one.

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It has already been found out by us that if the primary aluminium is kept in a holding furnace for too long time, it is contaminated and the surface of ingots thus produced is stained.
This report is on our study to look for the causes of this phanomenon. Examination was made on Na and Al₄C₃ contained in molten aluminium immediately after it was tapped out from a reduction furnace. Al₂O₃ contained in ingotwas also analysed. It was considered, however, these components are not the causes of such contamination or stain of primary metal.
It is imagined that the stain phenomenon of the surface of primary ingot is due to the small particles of metal oxide which are produced during holding operation and mixed and diffused into molten metal. Under the limited range of the present analysis system, it is impossible to find out the shape and structure of such metal oxides.

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It has so far been difficult to find the structural changes of aluminium alloys in aging process, because the changes can not be recognized by optical microscope, but can only be distinguished slightly by the X-ray analysis. With the development of electron microscope, however, it has become usual that, this kind of microscope is used in the field of research on the changes of the metal structure in aging.
This is to report on our recent study about the structural change in the aging of three sorts of Al-Cu alloys by means of the two-steps reprica in the filmy-replica type acetilcellulose method. By this usually applied method, the specimen can be prepared in camparative ease, though it is generally known that the electron microscopic specimen making is rather difficult. Through this experiment, our attempt was attained to some extent.

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Most of aluminium alloys normally used in Japanese rolling mills belong to Al-Mg-Si alloy system. It may be because of the fact that the alloys have good resistance to corrosion and suitable strength for working. A51S alloy of this system is for rolling and forging and its mechanical properties are improved very effectively by heat-treatment.
An experiment was made by authors to determine the operating conditions for this alloy such as melting, casting, rolling and heat-treatment.
The findings are as follows.
1. Casting conditions (casting temperature, cooling speed, decending speed. sump-height and pouring technique) must carefully determined, because the range of liquidus and solidus of this alloy is very wide.
2. Suitable temperature for quenching and tempering are 520° and 170° respectively.
3. Grain size of recrystallized specimen of 20%-reduced is particularly larger than that of other recrystallized and reduced specimens.

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Experiments have been made to examine the mechanism of recrystallization temperature rise of aluminium when zirconium is added to it.
The following results obtained:
(1) Zr in the aluminium is easily retained in solid solution by cooling rate in solidification and ZrAl₃ precipitated from solid solution by preheating is far finer than in solidification, and its precipitation from solid solution is comparatively rapid.
(2) Effect of Zr on recrystallization temperature rise of aluminium appears to be by finer ZrAl₃ precipitated from solid solution. This fine ZrAl₃ delays very remarkably recrystallization of aluminium, while coarse ZrAl₃ precipitated in solidification or solid solution structure has a minor effect on the delay and temperature rise.

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When aluminium sheets are finished by cold-rolling, it is rolled in the same direction of hot-rolling or rolled crosswise. Sometimes it is rolled in both directions each other in the process of cold-rolling. This is to report on the experiment to find out the relation between the above-mentioned cases of rolling methods and re-crystallization of sheet. It was found out that those sheets do not only show the similar softeninig tendency throngh annealing and the same grade of grain size after re-crystallization but also show the remarkably different behaviours for the secondary re-crystallization phenomenon.

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The effects on the properties of 1S and 2S aluminium, which appear when 0.5% Mg is added to it, was studied through experiments.
Findings are summed up as follows:
(1) Hardness of cold-rolled or annealed sheets gets higher by addition of 0.5% Mg while the softening time becames longer.
(2) Corrosion resistance against NaCl 6%+H₂O₂ 0.3% aqueous solution is improved.
(3) Producing velocity of oxide film by anodic oxidation process is promoted and the film thus produced is thicker than that produced on 1S or 2S under same conditions. This gives the alloy good corrosion and erosion resistances.
(4) Through preheating at 420°C of slab ingot, the casting structure gets finer and the compounds are precipitated. This phenomenon takes place more remarkably in supercooled ingot.
(5) When preheated, the recrystallized grains become coarser, the hardness after annealing becomes lower and the velocity of softening becomes faster.
(6) In rapidly cooled ingot, the recrystallized grains becomes finer and the softening velocity becomes faster. In continuously cast slab ingot, the grains in the part of granular structure become finer than those in the part of columnar structure. This phenomenon is also concerned to the condition of casting structure.

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