This paper describes the effects of addition of Zr or Ti+B on weld cracking and cast cracking of Al -Zn-Mg alloys. The suseptibility to hot cracking was evaluated by Fisco type weld cracking test, I type weld cracking test and ring cast cracking test. The results obtained were as follows. (1) The addition of Zr or Ti+B to base or filler alloy had remarkable effects on reducing the susceptibility to hot cracking of AI-Zn-Mg alloys (2) Among the factors controlling weld cracking, the grain size

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... weld metal was remarkably affected by the addition of Ti+B. The size was markedly refined by its addition and stable refining was still maintained after remelting. Then, the mechanism of preventing hot cracking by Ti+B addition would primarily be caused by the refining of grain size. (3) The addition of Zr had great effects on shapes and distribution of low melting compounds or eutectic the grain boundary at nearly the end of solidification. The dihedral angle was greatly increased with the addition of Zr. Then, the mechanism of preventing cracks by Zr addition would primarily be caused by the effects on shapes and distribution of liquid composed of low melting compounds and eutectic