TiB2 is not a good substrate for aluminum to nucleate on because of the somewhat large misfit between Al and TiB2. So TiB2 particles are in the grain refining addition (they do not form later in the melt), so that TiAl3 first forms on them. TiAl3 has a very low misfit with solid aluminum therefore is an excellent substrate. So TiB2 is a substrate for TiAl3 to nucleate on, on which solid aluminum nucleates. That is why excess Ti is needed.

@ Murat Tiryakioglu , thanks for the reply