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Acta Metall Sin  2017, Vol. 53 Issue (9): 1140-1152    DOI: 10.11900/0412.1961.2016.00579
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Numerical Simulation of Microstructure Evolution During the Solid Phase Transformation of Ti-6Al-4V Alloy in Investment Casting
Heng SHAO1, Yan LI2, Hai NAN2, Qingyan XU1()
1 Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2 Beijing Institute of Aeronautical Materials, Beijing 100095, China。
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Investment casting is widely used in producting complex thin-wall titanium alloy components. In this process, the βα phase transformation decides the final microstructures of these components. However most of present studies on phase transformation of titanium alloys focus on the microstructure evolution in heat treatment process or after deformation rather than in casting process now. It is a main reason only this work aims at the solid phase transformation of Ti-6Al-4V alloy in investment casting. In this work, the growth model of edge of α phase plates based on multi component Zener-Hiller model, and the growth model of broad face of α phase plates based on diffusion and conservation of multi components were established. The growth competition of different colonies, which consist of α phase plates in same orientation, was simulated and the microstructures and their evolution with temperature were obtained. The comparison between simulated microstructures and their evolution with temperature and experimental data indicated that the proportion of undercooling degree caused by impurities in the alloy is about 45% of the total undercooling degree in broad face of α phase plates and a much smaller portion in edge of α phase plates. The comparison also showed that the enthalpy change of solid phase transformation of titanium alloy is about 70 kJ/kg. The simulated and experimental morphologies look like similar and the simulated growth rate is also in good accordance with experiment inferred growth rate.

Key words:  titanium alloy      solid phase transformation      numerical simulation     
Received:  28 December 2016     
ZTFLH:  TG249.5  
Fund: Supported by China-EU (European Union) Science & Technology Cooperation in Aviation, Horizon 2020 Framework Programme for Research and Innovation (2014-2020) of EU, National Basic Research Program of China (No.2011CB706801), National Natural Science Foundation of China (Nos.51171089 and 51374137), National Science and Technology Major Project (No.2012ZX04012011) and High Technology Research and Development Program of China (No.2007AA04Z141)

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Heng SHAO, Yan LI, Hai NAN, Qingyan XU. Numerical Simulation of Microstructure Evolution During the Solid Phase Transformation of Ti-6Al-4V Alloy in Investment Casting. Acta Metall Sin, 2017, 53(9): 1140-1152.

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Fig.1  Schematic of temperature and microstructure evolution of Ti64 alloy casting during investment casting
Fig.2  Schematic of diffusion-controlled growth of edge of α phase plate (r—the curvature radius of edge of α phase plate, v—the growth rate of edge of α phase plate)
Fig.3  Schematics of α/β phase interface on broad face of α phase plate (u—the growth velocity of step face, vn—the normal growth velocity of broad face)
Fig.4  Schematics of capturing rules for growth of α phase plates (n1, n2—the prior growth directions of α phase plate)
Fig.5  Curves of mass fraction vs temperature of α /β phase in Ti-6Al-4V (a) and Ti-6Al-4V-0.18Fe-0.18O-0.02C-0.01N (b) from JMatPro software
Fig.6  Flow chart of numerical simulation of solid phase transformation in investment casting of Ti64 alloy
Fig.7  Simulated competitive growths of α phase plates in same orientation(a) concentration distribution of Al (CAl) (b) concentration distribution of V (CV)
Fig.8  Simulated α phase plates in the growth directions of 90° (a), 60° (b) and 30° (c) with basement
Fig.9  Curves of temperature (a) and cooling rate (b) of Ti64 casting during investment casting
Fig.10  OM image (a) and macrostructure (b) on the section of Ti64 casting (Red lines indiate prior β grain boundaries)
Fig.11  Simulated growths of α phase plates colonies in which the proportions (k) of undercooling degree k=0.3 (a), k=0.45 (b) and k=0.6 (c), caused by impurities in them in the total undercooling degree
Fig.12  Schematic domain for temperature and microstructure coupling simulation (RG—the average radius of β phase grains, h—the height of section in a wedge with a curve side, l—the distance between section and right vertex of the wedge)
Fig.13  Cooling rate curves with different latent heats (ΔH) and k(a) k=0.45, different ΔH (b) ΔH=70 kJ/kg, different k
Fig.14  Distribution of Al in a simulated multi-colony region
Fig.15  Distribution of Al on sections near or far from α phase basement in a simulated multi-colony region(a) section 20 μm away from an α phase grain boundary(b) section 180 μm away from an α phase grain boundary
Fig.16  OM images of Ti64 investment casting(a) a corner among three β grains (b) a boundary between two β grains
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