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 Acta Metall Sin  2017, Vol. 53 Issue (9): 1091-1100    DOI: 10.11900/0412.1961.2017.00084
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A New Understanding Toward Effect of Solute Ti on Grain Refinement of Aluminum by Al-Ti-B Master Alloy: Kinetic Behaviors of TiB2 Particles and Effect of Solute Ti
Lili ZHANG1, Hongxiang JIANG1, Jiuzhou ZHAO1(), Lu LI2, Qian SUN1
1 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2 Patent Examination Cooperation Tianjin Center of the Patent Office. SIPO., Tianjin 300304, China
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Abstract

Grain refinement may not only promote the formation of a fine quiaxed grain structure, which endows the Al alloy castings with good mechanical properties, but also cause a reduction in the casting defects, such as segregation and hot tearing, which has a dominating effect on the processability of Al alloys. It is, thus, essential for both the cast and wrought Al alloys. Although many techniques, e.g. mechanical vibration, electromagnetic stirring, ultrasound vibration, etc. may be used for the grain refinement nowadays, inoculation remains the most widely applied method in the industrial production due to its simplicity and high efficiency. For most Al alloys, Al-Ti-B master alloy is used as the grain refiner. Much work has been done to investigate the solidification behaviors of the Al alloys inoculated with Al-Ti-B master alloys since the 1970 s. Models were developed to describe the microstructure formation under the effect of inoculants. These researches clearly demonstrate that the grain refining efficiency or the heterogeneous nucleation rate is closely related to the concentration of solute Ti as well as the number density and size distribution of TiB2 particles in the melt. One shortcoming of the previous research work in this field is that the kinetic behaviors of TiB2 particles during the heating or cooling processes of the melt, i.e. dissolution/growth, coarsening and precipitation of TiB2 particles, are neglected. Generally the size distribution of TiB2 particles in the Al-Ti-B master alloy was used in the modeling and simulation of the solidification of Al alloys. In this work, solidification experiments were carried out to investigate the kinetic behaviors of TiB2 particles in the melt and the effect of solute Ti. A model was developed to describe the kinetic behaviors of TiB2 particles during the whole process from the beginning of the addition of TiB2 particles to the melt until the solidification of the melt. Calculations were carried out according to the experiments conditions. The results demonstrate that TiB2 particles may dissolve and coarsen during the holding temperature period, and grow during the cooling period of the melt. The kinetic behaviors of TiB2 particles have an obvious effect on the grain refining efficiency of the master alloys. The addition of solute Ti can significantly suppress the growth/dissolution, the Ostwald ripening of TiB2 particles and thus affects the grain refining efficiency of the master alloy.

Received:  17 March 2017
 ZTFLH: TG111.4
Fund: Supported by National Natural Science Foundation of China (Nos.51501207 and 51471173), China's Manned Space Station Project (No.TGJZ800-2-RW024) and Natural Science Foundation of Liaoning Province (No.201501043)

1 The authors contributed equally to this work.

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 Table 1  Impurity element and its mass fraction c0, equilibrium partition coefficient k and liquidus slope m in the commercial-purity Al used in the present work Fig.1  SEM images of Al-3Ti-1B (a) and Al-3Ti (b) master alloys (Inset in Fig.1a shows the enlarged view of the Al-3Ti-1B master alloy) Fig.2  Radius distribution of TiB2 particles $RTiB2$ in the Al-3Ti-1B master alloy Fig.3  OM images of commercial-purity Al without inoculation (a) and inoculated with 0.48%(Al-3Ti-1B) master alloy plus 0 (b), 0.11% (c), 0.22% (d), 0.33% (e) and 0.54% (f) Al-3Ti master alloys Fig.4  Average grain sizes of the commercial-purity Al as a function of the additive amount of Al-3Ti master alloy (or solute Ti content in the Al melt) Fig.5  SEM images of the TiB2 particles in the commercial-purity Al with the addition of 12%(Al-3Ti-1B) master alloy solidified at a cooling rate of 50 K/s after holding temperature at 1123 K for 30 min (a), 60 min (b), 90 min (c) and 120 min (d) Fig.6  Changes of the average radius of TiB2 particles$R¯TiB2$in the commercial-purity Al with the addition of 12%(Al-3Ti-1B) master alloy solidified at a cooling rate of 50 K with holding temperature time t at 1123 K Fig.7  Time dependences of supersaturation of the melt ΔK and volume fraction of TiB2 particles $φTiB2$ in the Al melt with the addition of 0.48%(Al-3Ti-1B) master alloy Fig.8  Size distributions of TiB2 particles$f(RTiB2, t)$in the Al melt with the addition of 0.48%(Al-3Ti-1B) master alloy at different times (Tm—melting temperature of Al) Fig.9  Time dependence of $R¯TiB2$ in the Al melt with the addition of 0.48%(Al-3Ti-1B) master alloy Fig.10  Time dependences of the number density of TiB2 particles $NTiB2$ (a) and $R¯TiB2$ (b) in the Al melt with the addition of 0.48%(Al-3Ti-1B) master alloy and different amounts of Al-3Ti master alloy (Inset shows the enlarged view of $R¯TiB2$ in the Al melt from 899.1 s to 903.5 s) Fig.11  Experimental (solid spheres) and calculated average grain sizes of commercial-purity Al varied with $QTotal$ and $NTiB2$ by CKB (red line) and NKB (blue line) (The experimental (open spheres) and calculated average grain sizes of commercial-purity Al are illustrated in XZ-plane projection (dotted line) and in YZ-plane projection (dashed line), respectively)