Effects of Solidification Conditions on Grain Refinement Capacity of TiC in Directionally Solidified Ti6Al4V Alloy

Abstract

In this study, the effects of solidification conditions on the grain refinement capacity of heterogeneous nuclei TiC in directionally solidified Ti6Al4V alloy were investigated using experimental and numerical approaches. Ti6Al4V powder with and without TiC particles in a Ti6Al4V sheath was melted and directionally solidified at various solidification rates via the floating zone melting method. In addition, by using the phase field method, the microstructural evolution of directionally solidified Ti6Al4V was simulated by varying the temperature gradient G and solidification rate V. As the solidification rate increased, the increment of the prior β grain number by TiC addition also increased. There are two reasons for this: first, the amount of residual potent heterogeneous nuclei TiC is larger. Second, the amount of TiC particles that can nucleate becomes larger. This is because increasing the constitutional undercooling ΔTc leads to the activation of a smaller radius of heterogeneous nuclei and a higher nucleation probability from each radius. At a cooling rate R higher than that in the floating zone melting experiment (R = 3 to 1000 K/s), the maximum degree of constitutional undercooling ΔTc,Max has a peak value, which suggests that constitutional undercooling ΔTc has a smaller contribution at higher cooling rates, such as those that occur during electron beam melting (EBM), including laser powder bed fusion (LPBF).

Microstructure and Strength of Ti-6Al-4V Samples Additively Manufactured with TiC Heterogeneous Nucleation Site Particles

“…On the other hand, the Ti-6Al-4V alloy powder with TiC particles seems to start to melt at 1655 • C during heating, and almost corresponding reactions start at 1700 • C during cooling. The difference in the melting point between the Ti-6Al-4V alloy powders with and without TiC particles of about 35 • C is consistent with the difference between the melting point of pure Ti and the eutectic temperature of the Ti-TiC reaction of about 20 • C. This may suggest that carbon in the TiC particles diffused into and reacted with neighboring Ti-6Al-4V alloy [32] powder during the heating process in DTA before reaching the eutectic melting temperature since the heating rate is very slow (0.17 • C/s), resulting in the occurrence of eutectic melting at 1655 • C during heating.…”