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Acta Metall Sin  2018, Vol. 54 Issue (5): 824-830    DOI: 10.11900/0412.1961.2017.00312
Special Issue for the Solidification of Metallic Materials Current Issue | Archive | Adv Search |
Rapid Solidification of Ti-6Al-4V Alloy Micro-Droplets Under Free Fall Condition
Bin ZHAI, Kai ZHOU, Peng Lü, Haipeng WANG()
Department of Applied Physics, Northwestern Polytechnical University, Xi'an 710072, China
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Especially in the past decades, Ti-6Al-4V alloy has received much attention, not only due to its high melting temperature, good corrosion resistance, low density and high hardness, but also because of the diverse and complicated microstructures formed under different conditions. This makes Ti-6Al-4V a potential candidate in both aerospace industries and fundamental research. It is well known that the solidified microstructures of alloy have a great influence on their mechanical properties. Therefore, it is crucial to investigate the mechanical properties of Ti-6Al-4V solidified under different conditions, in particular in the undercooling conditions. However, it is noted that most research on the solidification of Ti-6Al-4V alloy was carried out under equilibrium condition. With respect to Ti-6Al-4V alloy solidified under substantial undercooling conditions, few studies could be found. Thus, it is interesting to study two points: (1) the feature of the microstructure of Ti-6Al-4V alloy solidified under highly undercooled conditions and large cooling rate, (2) the influence of undercooling and cooling rate on the mechanical property of Ti-6Al-4V alloy. To address these two problems, Ti-6Al-4V alloy was rapidly solidified in a drop tube. The main results are summarized as follows. The microstructure of the Ti-6Al-4V alloy solidified under free fall condition displays "lamellar α+βα dendrites→basket-weave α'+β→ needle-like α'→ needle-like α'+ anomalous β " transformation with decreasing the droplets diameter. And the needle-like α' phase in the original boundaries of equiaxed β grains is transformed into a continuous distribution and anomalous structure of β phase when the droplet size is less than about 400 μm. The microhardness of this alloy ranges from 506 kg/mm2 to 785 kg/mm2 when the droplet diameter decreases from 1420 μm to 88 μm, which is much higher than that of the master alloy. For "lamellar structure of α+β phases", "needle-like α' phase" and "needle-like α' phase+ anomalous β phase", the microhardness increases with the decrease of droplet diameter. But for 'basket-weave' microstructure, the microhardness diminishes with the decrease of droplet diameter.

Key words:  titanium alloy      high undercooling      rapid solidification      microstructure      microhardness     
Received:  25 July 2017     
ZTFLH:  TG113.12  
Fund: Supported by National Natural Science Foundation of China (Nos.51474175, 51522102, 51506182 and 51734008) and Aeronautical Science Foundation (No.2016ZF53060)

Cite this article: 

Bin ZHAI, Kai ZHOU, Peng Lü, Haipeng WANG. Rapid Solidification of Ti-6Al-4V Alloy Micro-Droplets Under Free Fall Condition. Acta Metall Sin, 2018, 54(5): 824-830.

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Fig.1  XRD pattern (a) and DSC curves (b) of Ti-6Al-4V master alloy (ΔH—solid phase transition enthalpy)
Fig.2  Calculated undercooling (ΔT) (a) and cooling rate (Rc) (b) of Ti-6Al-4V alloy vs droplet diameter (D) in drop tube
Fig.3  Solidification microstructures of Ti-6Al-4V alloy
(The α phase is lighter and the β phase is darker)(a) master alloy (b) D=1330 μm droplets, ΔT=30 K, Rc =2.1×103 K/s
Fig.4  Solidification microstructures of Ti-6Al-4V alloy droplets (The insets in the Figs.4e and f indicate that the droplets are spherical)
(a) D=790 μm, ΔT=65 K, Rc=5.0×103 K/s (b) D=700 μm, ΔT=94 K, Rc=5.7×103 K/s
(c) D=600 μm, ΔT=110 K, Rc=9.3×103 K/s (d) D=515 μm, ΔT=124 K, Rc=9.0×103 K/s
(e) D=330 μm, ΔT=174 K, Rc=2.4×104 K/s (f) D=88 μm, ΔT=424 K, Rc=3.9×105 K/s
Fig.5  The volume fraction of β phase of Ti-6Al-4V alloy vs droplet diameter
Fig.6  Microhardness of Ti-6Al-4V alloy vs droplet diameter (Insets show microstructures of Ti-6Al-4V alloy with different diameters)
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