Morphological Evolution of TiB and TiAl in Al-Ti-B Master Alloy Using Different Ti Adding Routes.

Three different Ti addition routes were used to prepare an Al-5Ti-B Master Alloy: the halide salt route, the Ti-sponge route, and the partial Ti-sponge route. In the halide salt route, the raw materials were Al + KBF + KTiF; KTiF was completely replaced by pure titanium for the Ti-sponge route versus the halide salt route; in the partial Ti-sponge route, KTiF was partially replaced by pure titanium. Here, 30% Ti-sponge or 60% Ti-sponge route means that 30% or 60% KTiF was replaced by pure titanium, respectively. The above Ti addition routes have a significant influence on the growth pattern and morphological evolution of TiAl and TiB, which greatly affect the refining performance of Al-Ti-B Master Alloy. When using the halide salt route, a streamlined "rich Ti, B area" exists in the aluminum melt, which is a complex compound of (Ti, Al) B. The "rich Ti, B area" is essential for the nucleation and growth of TiAl and TiB. Blocky TiAl was obtained and its average size was 4.7 μm based on the halide salt route. In the Ti-sponge route, the nucleation of TiAl mainly depends on the mutual diffusion of Al and Ti, and TiAl forms around pure Ti particles, i.e., the so-called Ti-TiAl mechanism. The average size of the blocky TiAl was 9.8 μm based on the Ti-TiAl mechanism. For the partial Ti-sponge route, the "rich Ti, B area" gradually decreases with the increase in Ti powder's contents, and large TiAl coexists with the small TiAl. Compared with the Ti-sponge route, the halide salt route can form smaller TiAl. In the Ti-sponge route, there is a small amount of "rich Ti, B area" due to the influence of the Ti-TiAl mechanism, which does not meet the requirements of TiB growth. In the halide salt route, there is sufficient "rich Ti, B area", which is conducive to the formation of TiB. Both the crystal defects and the crowded growth environment caused by the "rich Ti, B area" are fundamental reasons for the fragility and the irregular shape of the TiB. The refining effect of the Al-Ti-B Master Alloy prepared by the halide salt route is better than the Ti-sponge route. The refining effect of 30% Ti-sponge route is better than that of Ti-sponge route and worse than that of halide salt route.