Grain Refinement of Al-5Ti-0.62C-0.2Nd Grain Refiner for Commercial Pure Aluminum and Its Effect on Mechanical Properties

It is well known that the mechanical properties of commercial pure Al are influenced by size of α-Al dendrites. In the present work, Al-5Ti-0.62C-0.2Nd grain refiner was prepared by a pure molten aluminum thermal explosion reaction, and its effect on grain refinement and mechanical properties of commercial pure Al was investigated. Microstructure and phase composition show that Al-5Ti0.62C-0.2Nd grain refiner consists of α-Al, granular TiC, lump-like TiAl3, and block-like Ti2Al20Nd. Grain-refining tests on commercial pure Al show that an Al-5Ti-0.62C-0.2Nd grain refiner has better refining performance compared with Al-5Ti-0.62C grain refiner. With addition of 0.2-wt.%-Al-5Ti0.62C-0.2Nd grain refiner, the average grain size of commercial pure Al can be refined from roughly 2800 to 155±5 μm effectively, and it has higher resistance to grain-refinement fading. On account of the grain refinement, the tensile strength and elongation are increased by approximately 18.3% and 83.5%, respectively.


Introduction
Grain refinement of aluminum and its alloys can improve their mechanical properties, casting properties, deformation treatment properties, and surface quality 1,2 .Therefore, grain refinement is an important research topic in the modern aluminum processing industry.In the past two decades, aluminum grain refinement is mainly accomplished by a Al-Ti-B grain refiner [3][4][5] .However, there are many problems in the application of Zr/Cr alloys and boride agglomeration [6][7][8] .As a result, some alternative choices were studied, such as Al-Ti-C and Al-Ti-C-B grain refiners [8][9][10][11] .However, due to the poor wettability between liquid aluminum and graphite, Al-Ti-C grain refiner production is still a key problem in the aluminum industry 12,13 .It is reported 13 that rare-earth (RE) elements have a grain effect as well.In the process of preparing an Al-Ti-C grain refiner, adding REs can facilitate the formation of tiny TiC particles and improve the grain-refining performance.Therefore, different Al-Ti-C-RE [13][14][15] grain refiners were prepared by the fluorine salt and doping methods.However, current preparation methods still have the problems of complex production steps and high production cost.
RE oxides have been widely used as reaction promoters in the preparation of composite materials 16,17 , but there are few studies on the application of RE oxides in the synthesis of Al-Ti-C grain refiners.Wang et al. 18 have studied the effect of Ce 2 O 3 on the thermodynamics of Al-Ti-C-RE prepared by the fluorine salt method.The results show that Ce 2 O 3 not only reduces the reaction temperature, but also improves the wettability of C and Al melts, and promotes the formation of TiC particles.In the present work, Al-Ti-C-Nd grain refiner was synthesized by adding Nd 2 O 3 in the thermal explosion reaction of pure molten aluminum.The grain refinement and its influence on mechanical capacities of commercial pure Al were investigated.

Experimental Procedures
The main raw materials for preparation of Al-5Ti-0.62Cand Al-5Ti-0.62C-0.2Ndgrain refiners include Al powder (99.6%, 61-74 µm in size), Ti powder (99.3%, 38-44 µm in size), C powder (99.8%, 11-30 µm in size), Nd 2 O 3 powder (99.9%, 40-50 nm in size), and commercial pure Al (99.7%).First, major start materials are converted into precast blocks (Φ 25 mm×50 mm) by ball mixing and cold pressing under a pressure of 50-60 MPa.The molar ratio of Al, Ti, and C powder is 5:2:1 in the prefabricated blocks, while the content of Nd 2 O 3 is 2 wt.%.Second, the pure aluminum ingot was melted in a resistance furnace at 800 ºC, and then the prefabricated blocks were added.About 3~5 min later, the melt was stirred by a graphite rod, and the melt temperature was again kept at 800ºC for 5 min.After purified and deslagged with C 2 C1 6 , the melt was finally cast into a steel mould (Φ 50× 30 mm).
The grain-refinement test was carried out by adding Al-5Ti-0.62Cand Al-5Ti-0.62C-0.2Ndgrain refiners into commercial pure Al melt separately.First, the commercial pure Al was melted and heated to 730ºC.After that, a different amount of grain refiner (0.15 wt.%, 0.2 wt.%) was added to the molten aluminum, stirring thoroughly and maintaining the temperaturefor 5 min to ensure the homogeneity of the composition.After purified and deslagged with C 2 C1 6 , the alloy melt was cast into a steel mold (Φ 50 mm×30 mm).To investigate the resistance to grain-refinement fading, different grain refiners were added to the melted aluminum and held for different time (10, 30, 60, 90, and 120 min) at 730ºC.
According to GB/T 228-2002, the tensile test bars with 40 mm length and 8 mm diameter were processed from the cast round bars to evaluate the mechanical properties of the samples.Tensile tests were carried out under the condition of room temperature and strain rate of 0.5mm/min by using an MTS810 machine (MTS System Company, Eden Prairie, MN, USA).The tensile strength and elongation data of each alloy reported below are average values of three tensile specimens.

Microstructure of Al-5Ti-0.62C and Al-5Ti-0.62C-0.2Nd grain refiners
Figure 1 shows XRD patterns of the prepared Al-5Ti-0.62Cand Al-5Ti-0.62C-0.2Ndgrain refiners.It can be seen that, compared with Al-5Ti-0.62C,the Al-5Ti-0.62C-0.2Ndgrain refiner not only contains α-Al, TiAl 3 , and TiC, but also contains Ti 2 Al 20 Nd phase.In addition, the diffraction peaks of TiAl 3 and TiC in Al-5Ti-0.62C-0.2Ndare obviously higher than those of the Al-5Ti-0.62Cgrain refiner, indicating that the addition of Nd 2 O 3 can promote the synthesis of TiAl 3 and TiC.
Figure 2 shows the optical microstructure of Al-5Ti-0.62Cand Al-5Ti-0.62C-0.2Ndgrain refiners.They are mainly composed of block-like particles and granular particles in the aluminum matrix, while the particles in Al-5Ti-0.62C-0.2Ndare obviously greater in number than those in the Al-5Ti-0.62Cgrain refiner.Figure 3 shows the magnification inverse scattering SEM images of Al-5Ti-0.62Cand Al-5Ti-0.62C-0.2Ndgrain refiners.From Fig. 3(a), it can be seen that there are a large number of block-like TiAl 3 particles and granular TiC particles on the aluminum matrix of the Al-5Ti-0.62Cgrain refiner.In Fig. 3(b), a considerable number of blocklike particles are distributed on the aluminum substrate, and most of the block-like particles are gray, while some of them are light white.In order to identify the phases, EDS analyses were performed on the different particles.Based on the energy-spectrum analysis of Figs.4(a)-4(c)and analyzing the XRD pattern, the gray lump-like particles were determined to be TiAl 3 and the granular particles were TiC, while the bright-white block-like particles were Ti 2 A1 20 Nd.
Compared with the Al-5Ti-0.62Cgrain refiner, there is a new phase named Ti 2 Al 20 Nd that appeared in the Al-5Ti-0.62C-0.2Ndgrain refiner.The reaction mechanism might be as follows: In the reaction process, the addition of Nd (1) From the above analysis, we can see that Nd 2 O 3 is the promoter of reactants and reactions.However, these theories are preliminary speculation, which is consistent with that reported by Wang et al. 18 .Since there are few published reports on the formation mechanism of Ti 2 Al 20 Nd during in situ reaction synthesis of the Al-5Ti-0.62Cgrain refiner by adding Nd 2 O 3 to the thermal explosion reaction of pure molten aluminum, further in-depth study and analysis of its thermodynamics and dynamics are needed in the future.

Grain refinement of Al-5Ti-0.62C and Al-5Ti-0.62C-0.2Nd grain refiners on commercial pure Al
Figure 5 shows the macroscopic structure of commercial pure Al after adding Al-5Ti-0.62Cand Al-5Ti-0.62C-0.2Ndgrain refiners for 5 min separately.It can be observed from Fig. 5(a) , the macrostructure of unrefined Al is composed of outer columnar grains and coarse equiaxed central grains, with an average grain size of approximately 2800±5 µm.After adding a 0.15-wt.%-Al-5Ti-0.62Cgrain refiner, the macrocrystalline grains were obviously refined, and the columnar and coarse equiaxed grains were replaced by the equiaxed grains, as shown in Fig. 5(b).Upon increasing the addition level to 0.2 wt.%, an obvious change was found on the finer equiaxed grains, and the average grain sizes were approximately 190±5 µm, as shown in Fig. 5(c).Compared with the Al-5Ti-0.62Cgrain refiner, when the addition of the Al-5Ti-0.62C-0.2Ndgrain refiner was increased from 0.15 to 0.2 wt.%, the average grain size of pure Al decreased from 215±5 to 155±5 µm, as shown in Figs.5(d) and 5(e).This proves that the grain-refinement performance of the Al-5Ti-0.62C-0.2Ndgrain refiner is better than that of the Al-5Ti-0.62Cgrain refiner for the same addition amount.
To further investigate the resistance to grain-refinement fading of the Al-5Ti-0.62Cand Al-5Ti-0.62C-0.2Ndgrain refiners, 0.2-wt.%grain refiners were added to commercially pure Al melt and held for different time.Figure 6 shows the macrostructures of refined samples obtained after different heat preservation.It can be seen from Figs. 6(a)-6(c) that the refining effect of the Al-5Ti-0.62C-0.2Ndgrain refiner does not decrease obviously, even if the holding time is 60 min.However, the refining effect of the Al-5Ti-0.62Cgrain refiner attenuates obviously with increasing refinement time, as shown in Figs.6(d)-6(f).
Although there are still some differences in the grainrefinement mechanism of the Al-Ti-C grain refiner 19,20 , it has been recognized that TiC can serve as a good nucleating agent for α-Al because of its low lattice mismatch with α-Al 21,22 .As shown in Fig. 7, after adding the 0.2-wt.%Al-TiC grain refiner and holding for 5 min, the average grain size of commercial pure aluminum was approximately 325±5 µm.However, the refining effect of the Al-TiC grain refiner is obviously weakened with the prolongation of holding time.The reason may be that when TiC itself is used as a nucleus most of the TiC particles are pushed by the dendrites into the grain boundary, and the nucleation is limited 23 .The quantity and activity of TiC particles have an important effect on the refining effect of the grain refiner.The Al-5Ti-0.62Cgrain refiner shows a better grain-refinement effect than the Al-TiC refiner, as shown in Fig. 7, the main reason being that the Al-5Ti-0.62Cgrain refiner not only contains a large number of fine and evenly distributed TiC particles, but also that these TiC particles are active and stable during the refining process due to the assistance of TiAl 3 (i.e., the formation of the Ti-rich layer 23,24 ).
Compared with Al-5Ti-0.62C,the Al-5Ti-0.62C-0.2Ndgrain refiner not only contains more TiC and TiAl 3 particles, but also contains Ti 2 A1 20 Nd.When the Al-5Ti-0.62C-0.2Ndgrain refiner is added to the aluminum melt, the Ti 2 A1 20 Nd will dissolve preferentially into TiAl 3 as the temperature of the melt is prolonged 13 .The dissolution of Ti 2 A1 20 Nd can release the free Ti atoms early and provide a large amount of free Nd atoms for the aluminum melt.Owing to the high activity of Ti atoms and RE Nd atoms, they will form a composite protective layer of Ti and RE Nd atoms near TiC particles.The protective layer can promote the adaptability of TiC and α-Al structures 24 , protect the TiC transformation into Al 4 C 3 14,25 , enhance their wettability, and improve the heterogeneous nuclear potential of TiC particles.On the other side, the layer guarantees that even tiny TiC particles cannot easily precipitate together.The uniform distribution of TiC in the aluminum melt in a suspended state not only can fully undertake the role of the nucleation core, but can also provide a good grain-refinement recession and guarantee a long-term refining effect 26,27 .
In addition, the released Nd atoms combine with dissociative atoms to form binary or multi-component compounds with high melting point and low density in aluminum melt,  providing a heterogeneous nucleus for α-Al 28 .Furthermore, the accumulation of RE elements at grain boundaries can effectively inhibit α-Al columnar crystal growth and promote the formation of finer equiaxed grains 29 .Therefore, the grainrefinement efficiency of Al-5Ti-0.62C-0.2Nd is better than that of the Al-5Ti-0.62Cgrain refiner, as shown in Fig. 7.

3.3.Effects of different grain refiners on mechanical properties of commercial pure Al
Figure 8 shows the mechanical properties of commercial pure aluminum refined by different grain refiners at room temperature after holding for 5 min.As shown in Fig. 8, the tensile strength and elongation of unrefined pure aluminum are 55.7 MPa and 24.3%, respectively.After addition of 0.2-wt.%-Al-TiCand Al-5Ti-0.62Cgrain refiners, the tensile strength and elongation reached 58.1 and 62.2 MPa, and 36.6% and 41.5%, respectively.Compared with the Al-5Ti-0.62Cgrain refiner, the tensile strength and elongation of the refined samples with the 0.2-wt.%-Al-5Ti-0.62C-0.2Ndgrain refiner were increased by 5.9% and 7.5%, respectively, and reached 65.9 MPa and 44.6%, respectively, which were 18.3% and 83.5% higher than those of unrefined pure aluminum, respectively.
Following the Hall-Petch type equation : where  s is the yield stress,  0 is the force needed to move a single dislocation to overcome the lattice friction, k is a constant, and d is the average grain diameter.It can be seen that the yield strength of the material is proportional to the square root of the reciprocal of the grain size.Therefore, grain refinement can not only improve the strength of the material, but also improve the plasticity of the material.In addition, the Nd and Ti combine with dissociative atoms to form binary or multi-component compounds with high melting point and low density in aluminum melt, can lead to Orowan strengthening and thermal mismatch strengthening if they are distributed in α-Al grain boundaries and fine grain strengthening if they are distributed at α-Al grain boundaries.
The results show that Al-5Ti-0.62C-0.2Nd is an excellent grain refiner, which can refine grain and improve tensile strength and elongation of commercial pure Al.In addition, the grain refinement efficiency of Al-5Ti-0.62C-0.2Nd is better than that of Al-5Ti-0.62Cgrain refiner.Therefore, a new method of preparing Al-5Ti-0.62C-0.2Ndgrain refiner by adding Nd 2 O 3 in the thermal explosion reaction of pure aluminum is proposed, which provides the possibility for industrial application.

Conclusions
A novel Al-5Ti-0.62C-0.2Ndgrain refiner was prepared, and its effect on the grain refinement and mechanical properties of commercial pure Al was studied.The following conclusions were drawn: 1. Al-5Ti-0.62C-0.2Ndgrain refiner containing A1, TiAl 3 , TiC and Ti 2 A1 20 Nd phase was successfully prepared by adding Nd 2 O 3 in the thermal explosion reaction of pure molten aluminum.2. Al-5Ti-0.62C-0.2Ndgrain refiner has better refining performance compared with an Al-5Ti-0.62Cgrain refiner.With addition of 0.2-wt.%-Al-5Ti-0.62C-0.2Ndgrain refiner, the average grain size of commercial pure Al can be refined from roughly 2800 to 155±5 µm effectively, and it has higher resistance to grain-refinement fading.3. When 0.2-wt.%-Al-5Ti-0.62C-0.2Ndgrain refiner was added, the tensile strength and elongation reached 65.9 MPa and 44.6%, an increase of 18.3% and 83.5%, respectively.kd s 0

a
State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou 730050, Gansu, China b School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, Gansu, China

Figure 4 .
Figure 4. EDS composition analysis of points A, B and C in Fig 3(b): (a) point A; (b) point B; (c) point C.

Figure 8 .
Figure 8. Mechanical properties of pure Al refined by different grain refiners at room temperature