YAN Chunlei, MA Yonglin, HE Shuai, XING Shuqing, BAO Xinyu

(School of Materials and Metallurgy (School of Rare Earths), Inner Mongolia University of Science and Technology, Baotou 014010, China)

Abstract: The influence of the electromagnetic energy on the microstructure of Al-5Ti-B grain refiner was discussed. In this study, the electromagnetic energy was applied above the liquid phase line temperature. Compared with Al-5Ti-B without electromagnetic energy applied, the experimental results show that the size of secondary particles is reduced and its size distribution becomes more uniform. Simultaneously, the secondary phase particles are uniformly spread in the matrix response to the electromagnetic energy. Moreover, when adding Al-5Ti-B with electromagnetic energy to the pure aluminum melt, it is clear that the electromagnetic energy has a significantly impact on refining properties of Al-5Ti-B. The mean size of pure aluminum is reduced by 27.6% in maximum with more uniform size distribution. The change in the microstructure is attributed to the electromagnetic energy changes the melt structure. With the electromagnetic energy entry into the system, the electromagnetic energy reduces the size of atomic clusters and increases the number of atomic clusters, thus the number of nuclei increases.

Key words: rectangle pulsed electromagnetic field; Al-5Ti-B; electromagnetic energy; liquid phase line

1 Introduction

Fine grain size can effectively reduce shrinkage and segregation, which results in aluminum alloy with excellent properties. Currently, adding grain refiners to the melts is still one of the most commonly adopted ways to fine the grain size. The grain refiners can control grain size and morphology, which is attributed to promote heterogeneous nucleation. Therefore, there is an important relevance to find a new technology that can significantly contribute to improve the refining ability of grain refiners. Wang

et al

suggested that the principal manner to improve the refining ability of grain refiners was to improve its microstructure. To date, researchers have employed several ways to modify the microstructure of Al-5Ti-B. Han

et al

adopted the cavitation effect generated by ultrasonic to modify the size and distribution of secondary phase particles in Al-5Ti-B. Yu

et al

manufactured an Al-5Ti-B with stable refinement properties using equal channel angular pressing (ECAP). Researchers have developed many grain refining methods in recent years, among which the pulsed electromagnetic field is one of the research hotspots of grain refinement methods in recent years. It is available for a variety of metals, and possesses several advantages such as convenient operation, meeting metals cleanliness requirements, and saving energy.However, the mechanism of grain refinement with the pulsed electromagnetic field is still uncertainty. Currently, there are several theories about the mechanism of refinement of the pulsed electromagnetic field. Gao

et al

pointed out that the pulsed electromagnetic field refined the solidification structure due to crystal core removed from the mold wall with the pulsed electromagnetic field. Wang

et al

suggested that the dendritic arms were broken in response to the pulsed electromagnetic field, and the broken dendritic crystals serve as nucleation cores inside the melt. However, previous studies regarding concerning the refinement mechanism of the pulsed electromagnetic field have rarely involved the effect of electromagnetic energy on the nucleation processes. The energy fluctuations in the system were necessary for the formation of nuclei. The electromagnetic energy is transferred contactless to the atomic scale of matter in the form of electromagnetic energy under the pulsed electromagnetic field.To date, few papers have adapted the pulsed electromagnetic field to refine Al-5Ti-B. Li

et al

found that the refinement effect of Al-Cu alloy was not obvious when a low voltage pulsed electromagnetic field was applied to Al-Cu alloy during the liquid stage. In this study, a rectangle pulsed electromagnetic field was imposed on Al-5Ti-B above the liquid-phase line temperature. The influence of the electromagnetic energy on the microstructure of Al-5Ti-B at different temperatures was discussed.

2 Experimental

Al-5Ti-B alloy is a grain refiner utilized to control the solidification structure of aluminum alloys. The electromagnetic energy treatment equipment which was mainly containing a pulse power cabinet, induction heating furnace, electromagnetic energy generator (pulse induction coil and core), and resistance heater, graphite mold, as shown in Fig.1(a). The electromagnetic energy generator was placed directly above the melt surface, which weakened the influence of Joule heating and the electromagnetic force on the experimental results. Al-5Ti-B alloy was heated to 800 ℃, and was held at 800 ℃ for about 5 min, and then the melt was poured into the graphite mold preheated to peak intensity of the rectangle pulsed electromagnetic field was 100 A, frequency 20 Hz, and duty cycle 20%, as shown in Fig.1(b). After solidification by air cooling, the ingots were rolled from 47 mm thickness to 10 mm at 500 ℃ with two passes. The first pass reduction was 20 mm, and the second pass reduction was 17 mm. An X-ray diffractometer (D8 ADVANCE) was utilized to analyze the phase composition of the samples, and microstructure observation of samples using field emission scanning electron microscope (GAIA 3 XMN).

Fig.1 (a) The schematic diagram of electromagnetic energy treatment equipment:(1) electromagnetic energy generator, (2) paperless temperature recorder (3) induction heating furnace, (4) resistance heater, (5) graphite mold, (6) K-type thermocouple, (7) melt; (b) Schematic diagram of rectangular current for experiment[31]

The experimental material for grain refinement performance was pure aluminum, and the grain refinement processed involved melting the pure aluminum in induction heating furnaces at 750 ℃ and then adding rolled Al-5Ti-B to the melt. The melt was held at 750 ℃for 5 min and then poured melt into a graphite mold. After solidification, the ingots were removed from the graphite mold, and the ingots were radially cut and sampled. The solidified structure of the samples was observed by optical microscope.600 ℃. The melt in the graphite crucible solidified into ingots directly by air cooling without electromagnetic energy and was employed as the control sample. In this experiment, a K-type thermocouple was submerged into the melt of the graphite mold. When the other melt temperature in the graphite crucible decreased to 660 and 700 ℃, respectively, a rectangular pulsed electromagnetic field was applied to the melt for 30 s. The

3 Results and discussion

3.1 Microstructure of as cast Al-5Ti-B

Fig.2 exhibits the microstructure of as cast Al-5Ti-B with electromagnetic energy at different temperatures, which reveals the morphology and distribution of the secondary phase particles (TiB) in Al-5Ti-B. It is found that the electromagnetic energy causes a change in the morphology and distribution of the Al-Ti-B. In absence of electromagnetic energy, the secondary phase particles are aggregated and with large dimensions, as shown in Fig.2(a). However, when Al-5Ti-B was treated with electromagnetic energy applied at 660 ℃, the size of the secondary phase particles decreases and the number increases, as shown in Fig.2(b). When the temperature applied electromagnetic energy is 700 ℃, the size of secondary phase particles is distinctly less than that of Al-5Ti-B with electromagnetic energy at 660 ℃, while the secondary phase particles are more uniformly distributed as shown in Fig.2(c).

Fig.2 Microstructures of as cast Al-5Ti-B with electromagnetic energy at different temperatures: (a) Without electromagnetic energy; (b) With electromagnetic energy at 660 ℃; (c) With electromagnetic energy at 700 ℃

3.2 Microstructure of rolled Al-5Ti-B

Fig.3 presents the XRD scans of Al-5Ti-B with electromagnetic energy at different temperatures. The phase composition of Al-5Ti-B mainly composes of

α

-Al, TiAl, and TiB. Fig.4 shows the microstructures of rolled Al-5Ti-B with electromagnetic energy at different temperatures with its secondary phase particles morphology are blocky, which is distributed in the α-Al matrix. The size distribution of TiAlparticles in Al-5Ti-B is illustrated in Fig.5. Without electromagnetic energy, the mean size of TiAlparticles is 9.37 μm and the size range is from 3 to 21 μm, and the size distribution of TiAlparticles in Al-5Ti-B is unified. At the same time, the TiAlparticles are obviously agglomerated together (in Fig.4(a)). With electromagnetic energy at 660 ℃, The mean size of TiAlparticles is reduced to 8.85 μm with more uniform size distribution, and its distribution is more uniform in the α-Al matrix(in Fig.4(b)). With electromagnetic energy at 700 ℃, the mean size of TiAlparticles is 7.85 μm, and TiAlparticles range from 1-21 μm and the TiAlparticles have a relatively uniform size distribution, while TiAlparticles are evenly spread in the

α

-Al matrix (in Fig.4(c)). Obviously, the TiBparticles in Al-5Ti-B vary considerably in size and stick to each other without electromagnetic energy (in Fig.4(d)). When the electromagnetic energy is applied at 660 ℃, the distribution state of TiBparticles is changed apparently. However, the modification of TiBparticles size was not obvious compared with Al-5Ti- B without electromagnetic energy (in Fig.4(e)). When the electromagnetic energy was applied at 700 ℃, the size of TiBparticle reduces and TiBparticle are uniformly distributed in the α-Al matrix (in Fig.4(f)).

Fig.3 XRD patterns of Al-5Ti-B with electromagnetic energy at different temperatures: (a) Without electromagnetic energy; (b) With electromagnetic energy at 660 ℃; (c) With electromagnetic energy at 700 ℃

Fig.4 Microstructures of rolled Al-5Ti-B with electromagnetic energy at different temperatures: (a) and (d) Without electromagnetic energy; (b) and (e) With electromagnetic energy at 660 ℃; (c) and (f) With electromagnetic energy at 700 ℃

Fig.5 Size distribution of TiAl3 particles in Al-5Ti-B: (a) Without electromagnetic energy; (b) With electromagnetic energy at 660 ℃; (c) With electromagnetic energy at 700 ℃

In short, the size of secondary phase particles decreases and the TiAlparticles have a relatively uniform size distribution, while the secondary phase particles are equally spread in the

α

-Al matrix in response to the electromagnetic energy.

3.3 Cooling curves of Al-5Ti-B

Fig.6 depicts the typical cooling curves of in the central zone of Al-5Ti-B. It is clear that the initiation solidification temperature increases, and the solidification time are extended due to the electromagnetic energy. The changes in initiation solidification temperature and solidification time, as shown in Fig.7. Without electromagnetic energy, the initiation solidification temperature is 646.9 ℃ and the solidification time is 32 s. However, when Al-5Ti-B with electromagnetic energy at 660 ℃, the initiation solidification temperature is 648.3 ℃ and solidification time is 42 s. When the electromagnetic energy was imposed on Al-5Ti-B at 700 ℃, the initiation solidification temperature increases to 660.4 ℃ and the solidification time is 98 s. When the electromagnetic energy is utilized to the melt, the atomic cluster obtained energy. Gao

et al

found that the electromagnetic energy affected the nucleation of metal and changed the phase transition temperature. In this case, with the entry of the electromagnetic energy into the melt leads to an increase in the nucleation initiation temperature, along with a longer solidification time

Fig.6 Typical cooling curve of in the central zone of Al-5Ti-B with electromagnetic energy at different temperatures: (a) Without electromagnetic energy; (b) With electromagnetic energy at 660 ℃; (c) With electromagnetic energy at 700 ℃

Fig.7 Initiation solidification temperature and solidification time without and with electromagnetic energy at different temperatures: (a) Without electromagnetic energy; (b) With electromagnetic energy at 660 ℃; (c) With electromagnetic energy at 700 ℃

3.4 Refinement ability comparison of Al-5Ti-B

As in Fig.8, the solidified structure of pure aluminum refined by different Al-5Ti-B. The application of the electromagnetic energy has a remarkable impact on the grain size and size distribution of pure aluminum. The mean grain size of pure aluminum is 186.59, 149.48, and 135.09 μm, respectively. Grain size distribution of pure aluminum is illustrated in Fig.9. The grain size distribution of pure aluminum is not uniform in the range of 50 to 310 μm when grain refiners without electromagnetic energy treatment are added to the melt. When adding Al-5Ti-B with electromagnetic energy at 660 ℃ to the melt, the number of small grains increases, so the mean size of pure aluminum decreases. When adding Al-5Ti-B with electromagnetic energy at 700 ℃ to the melt, the grain size distribution of pure aluminum is relatively more uniform, and the maximum grain size reduces to 210 μm. Therefore, the refinement ability comparisons of Al-5Ti-B results indicate that the refinement effect is enhanced.

Fig.8 The solidified structure of pure aluminum refined by Al-5Ti-B: (a) Without electromagnetic energy; (b) With electromagnetic energy at 660 ℃; (c) With electromagnetic energy at 700 ℃

Fig.9 Grain size distribution of pure aluminum refined by Al-5Ti-B: (a) Without electromagnetic energy; (b) With electromagnetic energy at 660℃; (c) With electromagnetic energy at 700℃

3.5 Influence of the electromagnetic energy on nucleation

The changes in the microstructure could be accounted for as summarized below. It is well known that energy fluctuations are one of the necessary conditions for nucleation. With electromagnetic energy, Gibbs free energy can be expressed as:

where,

ΔG

is the difference of Gibbs free energy between liquid and solid phases,

μ

the vacuum permeability, and Δ

χ

the volume permeability difference between the solid phase and the liquid phase, and

H

the magnetic field strength.

The total free energy change is:

where,

r

is the radius of the crystal core, and

σ

is the interface energy.

Under electromagnetic energy, the activation energy can be expressed as:

where,

f

(

θ

) is the catalytic factor, and

θ

is the wetting angle. The electromagnetic energy density can be expressed as:

with

where,

B

is the magnetic induction strength,

μ

is the relative permeability, and

w

is the electromagnetic energy density.

The activation energy with electromagnetic energy can be expressed as:

The law of evolution of atomic groups in a certain temperature range above the liquid phase line:

where,

R(T)

is the mean radius of the atomic cluster at

T

temperature,

α

is the geometry factor, 0＜

α

≤1 and

Z

is the coordination number before melting,

r

is the distance between atoms in the atomic cluster,

k

is the Boltzmann’s constant,

C

is the relative concentration of activated atoms at the melting point,

T

is the melting point,

T

is the critical nucleus temperature, and

T

is the temperature of the melt from the liquid to the gas state, the temperature at which the first atomic cluster destabilization occurs during the transition. The electromagnetic energy decreases the activation energy and the atomic clusters size, and more significant increases in the number of atomic clusters. After the application of the electromagnetic energy, the increase in energy within the system causes a change in the melt structure. An induced current is generated inside the melt and the atomic clusters combined with the surrounding atoms, the atomic clusters of the melt to gather in large numbers and form an electromagnetic energy treatment area on the surface of the alloy melt. After cessation of a rectangle pulsed electromagnetic field, the energy in the system starts to decrease and the atoms in the clusters begin to separate, which results in a decrease in the size, as well as an increment in the number of clusters. The atomic clusters of the melt collide and combine with each other, resulting in a more uniform distribution of atomic clusters. On the other hand, the electromagnetic energy can reduce the critical nucleation radius, and reduce the nucleation barrier in the system, thus promote the nucleation processes. Consequently, the size of secondary phase particles decreases, and the distribution of secondary phase particles is changed obviously.

4 Conclusions

a) Without electromagnetic energy, the mean size of TiAlparticles is 9.37 μm and the distribution and size distribution of TiAlparticles in Al-5Ti-B is unified. With electromagnetic energy at 660 ℃, The mean size of TiAlparticles is reduced to 8.85 μm with more uniform size distribution, and distribution of TiAlparticles is more uniform. With electromagnetic energy at 700 ℃, the mean size of TiAlparticles is 7.85 μm, and it has a relatively uniform size distribution, while the distribution of TiAlparticles is more homogeneous.

b) Under electromagnetic energy, the TiBparticles are equally spread in the α-Al matrix. The electromagnetic energy is applied to Al-5Ti-B results in a longer solidification time, as well as an increase in the initiation solidification temperature.

c) The refining properties of Al-5Ti-B are improved by the effect of the electromagnetic energy. The electromagnetic energy reduces the activation energy and leads to structural changes in the melt.