Morphological Evolution of TiB₂ and TiAl₃ in Al–Ti–B Master Alloy Using Different Ti Adding Routes

Abstract

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 + KBF4 + K2TiF6; K2TiF6 was completely replaced by pure titanium for the Ti-sponge route versus the halide salt route; in the partial Ti-sponge route, K2TiF6 was partially replaced by pure titanium. Here, 30% Ti-sponge or 60% Ti-sponge route means that 30% or 60% K2TiF6 was replaced by pure titanium, respectively. The above Ti addition routes have a significant influence on the growth pattern and morphological evolution of TiAl3 and TiB2, 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 (Tix, Al1−x) By. The “rich Ti, B area” is essential for the nucleation and growth of TiAl3 and TiB2. Blocky TiAl3 was obtained and its average size was 4.7 μm based on the halide salt route. In the Ti-sponge route, the nucleation of TiAl3 mainly depends on the mutual diffusion of Al and Ti, and TiAlx forms around pure Ti particles, i.e., the so-called Ti–TiAlx mechanism. The average size of the blocky TiAl3 was 9.8 μm based on the Ti–TiAlx mechanism. For the partial Ti-sponge route, the “rich Ti, B area” gradually decreases with the increase in Ti powder’s contents, and large TiAl3 coexists with the small TiAl3. Compared with the Ti-sponge route, the halide salt route can form smaller TiAl3. In the Ti-sponge route, there is a small amount of “rich Ti, B area” due to the influence of the Ti–TiAlx mechanism, which does not meet the requirements of TiB2 growth. In the halide salt route, there is sufficient “rich Ti, B area”, which is conducive to the formation of TiB2. 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 TiB2. 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.

Keywords

• Al–Ti–B Master Alloy
• TiB₂
• TiAl₃
• halide salt route
• “Ti-sponge” route
• “partial Ti-sponge” route