  In this paper, the effects of Al, Nb main elements, Fe, Mo, W, Co, B, Si and
their contents on the fluidity of Ti-22Al-25Nb alloy were investigated. The
composition that was beneficial to improve the fluidity was screened through
the thermodynamic software calculating thermophysical parameters affecting the
fluidity of Ti$_2$AlNb alloy, the numerical simulation test of its fluidity and
the verification test of the fluidity of optimized alloys. Finally, the
improvement mechanism of the alloy fluidity was discussed. Results showed that
the appropriate reduction of Nb element was better than Al element for the
improvement of fluidity. The addition of trace Fe, B and Si elements were
beneficial to the improvement of fluidity, the improvement effect of B element
was best, while the addition of trace Mo, W, Co were not conducive to the
improvement of fluidity. The cessation mechanism of Ti$_2$AlNb alloy is the
cessation mechanism of the alloy with a wide crystallization temperature range.
The composition which was most beneficial to improve the fluidity was
Ti-22Al-24Nb-0.1B. The main reasons for the improvement of the fluidity had two
sides: on the one hand, the reduction of 1at% Nb and the addition of 0.1at% B
not only increased the superheat and crystallization latent heat of the alloy,
but also reduced the melt viscosity and thermal conductivity, thus improving
the fluidity. On the other hand, the TiB phase refined the grains, the fine
grains prevented the dendrite from growing into developed dendrite networks,
inhibited the adverse effect of the increase in the width of the solidification
zone on the fluidity, reduced the flow resistance of the molten metal, and
further improved the fluidity of the alloy.
