1995 年 45 巻 6 号 p. 339-345
To investigate the grain refining mechanism of pure aluminum ingots by addition of an Al-Ti-B master alloy, unidirectional solidification and ordinary casting techniques were used. Equiaxed crystals appeared in unidirectionally solidified ingots which fluid motion did not occur during casting. It is thought that the residual TiB2 in the molten aluminum, which originated from the master alloy, acted as a nucleation site in the same manner as a peritectic reaction. An ingot consisting of only columnar structure obtained by unidirectional solidification was remelted, and then cast into a chilled mold. Each cast structure showed the equiaxed crystal zone, although no TiB2 existed in the melt. These equiaxed crystals nucleated on the mold wall and separated from the wall when the melt was cast. It is concluded that the two grain refining mechanisms during casting are important; one is due to the residual Ti compounds in the molten aluminum acting as nucleation sites, and the other is due to crystal separation from the mold wall caused by fluid motion introduced to the melt. After the Ti compounds have dissolved into the melt, only the latter mechanism may be effective.
