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This is the stenographic report of above entitled two lectures, held on July, 1954, in the Hall of Mining Society, by the present writer.In the first lecture, the cause of occurence of "hard spot" in the aluminium alloy die-castings as well as the compositions of the hard spot itself have been explained discussed in details, quoting the metallographic observations carried out by the preset writer. Even if there are no contaminations in the melt of aluminium alloys, the "hard spot" may be generated as the primary crystals of hard intermetallic compounds in the melting-pot, if the temperature of the melt is lowered. As to the sorts of the intermetallic compounds which may be found as the hard spot in the die-eastings, the ternary compounds; (Al Fe Si) α and (Al Mn Si) α respectively, as well as the quaternary compound; (Al Fe Si Mn) have been pointed out.In the latter lecture, the difference in the structure of the aluminium alloy die-castings temperatures has been shown: When the casting temperature is fairly high; that is the complete melt is poured into the die, the structure of the castings is of course fairly homogeneous. On the contrally, if the half-melt is poured or pressed into the die, the primary aluminium-crystals, which are already generated in the melting-pot, may be detected to be gathered in some part of the die-casting, thus resulting the structure to be heterogeneous.

ON THE SO-CALLED "HARD-SPOT" WHICH APPEARS IN THE ALUMINUIM ALLOY DIE-CASTINGS