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

Naoki Date,Shizuka Nakano,Hisashi Sato,Naoko Sato,Yoshimi Watanabe,Shunya Yamamoto,Shinsuke Suzuki

doi:10.1007/s11661-021-06333-2

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).

Highlights

THE titanium alloy Ti6Al4V is the most widely used alloy in the medical,[1] aircraft, and advanced aerospace industries[2] because of its high specific strength and excellent corrosion resistance[3] It is well known that metals and alloys such as Ti6Al4V usually solidify with coarse columnar grain structures under normal casting conditions
When measuring the temperature of the melt pool using a two-color pyrometer, a vapor deposition layer was observed on the inner surface of the glass tube at all solidification rates
As the solidification rate decreases, the shadow becomes darker because a thicker vapor deposition layer is formed, and this vapor deposition disrupts the measured temperature distribution in the melt pool

Summary

Introduction

THE titanium alloy Ti6Al4V is the most widely used alloy in the medical,[1] aircraft, and advanced aerospace industries[2] because of its high specific strength and excellent corrosion resistance[3] It is well known that metals and alloys such as Ti6Al4V usually solidify with coarse columnar grain structures under normal casting conditions. These columnar grains are generally regarded as unfavorable because their presence can impart solidification defects and anisotropic mechanical. A fine-equiaxed microstructure is desired in additive manufacturing

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