Hot torsion tests were carried out on the specimens taken out of the semi-continuous cast ingots of Al-Mg-Si alloys for studying the effects of homogenizing heat treatment on their hot workability. Ingot structures after homogenizing heat treatment under various conditions were also examined by metallography and electrical resistivity measurements by the eddy current method. It was shown that appreciable improvements in hot workability were attained by applying homogenizing heat treatment in the following way; (1) to solutionize the as cast ingots at high temperatures above 550°C in order to remove solute segregation due to rapid solidification and to dissolve the second phase particles, (2), subsequently, to cause precipitation of Mg₂Si by heating at 400-450°C. The coarse precipitates of Mg₂Si thus obtained, which are uniformly distributed, show little dispersion hardening effect. Solid solution strengthening effect of the matrix is also reduced because of decrease in solute concentration during precipitation. Thus, appreciable reduction in flow stress in deformation under the hot working conditions was attained. It was also shown that the curse Mg₂Si particles uniformly distributed considerably improved the high temperature ductility of ingots.
For the extrusion products to be used under T5 conditions, care must be taken to allow complete solutionizing to take place during deformation. In this case, too coarse Mg₂Si particles, produced by slow cooing after high temperature solutionizing of ingots such as by furnace cooling, may not be desirable for the products to attain the optimum mechanical properties.

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In this paper, there are investigated the notch toughness and fracture characteristics of Al-4%Zn-2%Mg alloy welds made with filler metals of Al-5%Mg, Al-4%Mg-2%Zn and Al-4%Zn-2%Mg alloys.
The notch toughness was evaluated by the ratio of tensile strengths between notched and unnotched specimens and the Charpy impact values.
The decreasing order of notch toughness of base metals by tensile tests was as follows, according to treatments of specimens :T6>T7>T4. Whereas, the reverse tendency was observed in Charpy impact tests. In any case, however, notch toughness was decreased, and notch sensitivity was increased with the drop of testing temperature.
The decreasing order of notch toughness of welds by both of tensile and Charpy impact tests was as follows, according to filler metals: Al-5%Mg>Al-4%Mg-2%Zn>Al-4%Zn-2%Mg. However, the toughness was also decreased with the ageing after welding. The above tendencies were more remarkable in Charpy impact tests than in tensile tests. The Charpy impact values of welds were hyperbolically decreased with the increase of hardness, and they tended to approach approximately constant values in the lower limit.
Uniform granular ruggedness was observed in the fracture of welds with excellent notch toughness, but lamellar bands peculiar to weld metals were likely to appear in that with poor notch toughness. By electron fractography, the former showed dimple patterns associated with ductile rupture and the latter showed cleavage patterns associated with brittle rupture.
The notch toughness of welds was considerably improved by decreasing the heat input per one pass and increasing the number of passes. The reason of the above behavior would be due to the refining of dendrite cells and distribution of eutectic compounds following the "mass transport theory", and the dissolution of the compounds into the matrix by the subsequent weld heat cycles.
It was considered that the notch toughness and fracture characteristics of welds macroscopically depended upon the mutual relation between strength and ductility. However, they would also microscopically depend upon the state of precipitates and distribution of dendrite cells and eutectic compounds.

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The effects of pre-heat treatment for cold rolled Al-Zr alloy sheets containing Si and Sn at 200350°C on retardation of their recrystallization at 500°C were studied by means of Vickers hardness measurements and transmission electron microscopy.
The results obtained were summarized as follows:
(1) The time of pre-heat treatment required for retarding the recrystallization was longer with the increase of Si addition, but was shorter by addition of a small amount of Sn.
(2) Reversion (Rückbildung) was not very clear in aging process of the alloys. The incubation time of age hardening curve in Al-Zr-Si alloys was extended by addition of Sn, which would be owing to the promotion of nucleation.
(3) It was observed in thin foils that proper pre-heat treatment was effective in maintaining the subgrains at 500°C. Then, the mechanism of elevating recrystallization temperature of Al-Zr alloys would be the pinning effect of precipitates in the subrain boundaries.

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Effects of Cu on the impact strength and structures of Al-Si alloys were examined by means of Charpy impact test and metallographical method. The following three types of specimens were used:
a) As cast.
b) Solution heat treated at 500°C for 240min. for the above specimens.
c) Artificially age hardened at 200°C for 300min. after the above solution heat treatment.
(1) The impact strength of as cast alloys tended to be decreased with the increase in Si content and Cu addition. When Si content was low (approximately below 4%), the strength was markedly decreased with the addition of Cu. However, the strength was gradually decreased with increasing Si content and no effects of Cu were seen on the strength at about 15% of Si content.
(2) By the addition of a small amount of Cu to the hypo-eutectic alloy of nearly eutectic composition with no primary Si crystals, a marked decrease of the impact strength was seen on the specimens solution heat treated at 500°C. The above decrease resulted from the formation of primary Si crystals due to the addition of Cu. The configuration of these primary Si crystals was the cause of stress concentration as reported in the previous paper, ¹⁾ and it was kept unchanged by holding at 500°C.
(3) The impact strength of the alloys, which had been artificially age hardened at 200°C after solution heat treatment at 500°C, tended to be decreased with the increase in Si content and Cu additioon. Especially, the effect of Cu addition was greater than that of Si content.

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Studies were made on the relation between the three dimensional morphology of eutectic silicon in Al-13%Si alloy and the mechanical properties of the alloy.
The examination under a scanning electron microscope or by the successive polishing method revealed that the eutectic silicon crystals, which were apparently "acircular"on polished surfaces (by observation under on optical microscope), were flaky or stick-like and interconnected with one another over a wide range. The specimens aged at 400550°C for 45hrs. or less were sounded and then showed the neckings of silicon flakes in several places, which would be led to division of crystals. The silicon crystals of aged specimens were larger in thickness, narrower in width, and advanced in granulation as compared with the as cast ones. The shape of the eutectic silicon crystals in sodium treated specimens was "fibrous" and they were interconnected with one another over a wide range.
The change in interconnection of crystals and the change of shape toward granulation by aging at high temperatures resulted in slight decrease in yield and tensile strengths (at 0.2% in proof stress and ultimate tensile strength), except for the drop of strengths at the initial stage of aging. However, the duitility (total elongation and reduction in crosssectional area) was remarkably improved in proportion to the aging time.

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