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

Abstract

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-5Ti-0.62C-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-5Ti-0.62C-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.

Keywords
commercial pure aluminum; Al-5Ti-0.62C-0.2Nd grain refiner; grain refinement; mechanical properties

1. 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 ^3-5. However, there are many problems in the application of Zr/Cr alloys and boride agglomeration ^6-8. As a result, some alternative choices were studied, such as Al-Ti-C and Al-Ti-C-B grain refiners ^8-11. 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 ^13-15 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 Ce₂O₃ on the thermodynamics of Al-Ti-C-RE prepared by the fluorine salt method. The results show that Ce₂O₃ 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 Nd₂O₃ in the thermal explosion reaction of pure molten aluminum. The grain refinement and its influence on mechanical capacities of commercial pure Al were investigated.

2. Experimental Procedures

The main raw materials for preparation of Al-5Ti-0.62C and Al-5Ti-0.62C-0.2Nd grain 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), Nd₂O₃ 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 Nd₂O₃ is 2 wt.%. Second, the pure aluminum ingot was melted in a resistance furnace at 800ºC, 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 C₂C1₆, the melt was finally cast into a steel mould (Φ 50× 30 mm).

The grain-refinement test was carried out by adding Al-5Ti-0.62C and Al-5Ti-0.62C-0.2Nd grain 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 C₂C1₆, 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.

The phase composition of Al-5Ti-0.62C and Al-5Ti-0.62C-0.2Nd grain refiners was identified using a Rigaku D/max-A X-ray diffractometer (XRD, PW 3040/60, PANalytical, Rotterdam, The Netherlands) with a range of 0.02º for each step, 2θ, and 20º-90º for Cu K radiation, and an image plate detector. The composition of the grain refiner was measured using inductively coupled plasma atomic-emission spectrometry (ICP-AES, HK-8100, Beijing Huake Yi Tong Analytical Instrument Co. Ltd.) and an infrared carbon apparatus (CS-320C, Chongqing Research Rui Instrument Co. Ltd.). The microstructure of the samples was characterized by large optical microscope (OM, MEF3, Leica, Inc., Vienna, Austria) and a JSM-7500 scanning electron microscope (SEM, SSX-550 fitted with energy-dispersive spectroscopy (EDS) equipment, Shimadzu Corp., Kyoto, Japan) after rough grinding, finishing of the grinding, and electrolytic polishing (10% HClO₃ + 90% absolute alcohol, electrolyte composition in volume fraction, 20 V voltage). The refined samples were corroded by a specific reagent (60% HCl + 30% HNO₃+ 5% HF +5% H₂O, in volume fraction), and the grain size was determined by the linear intercept method.

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.

3. Results and Discussion

3.1. 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-0.62C and Al-5Ti-0.62C-0.2Nd grain refiners. It can be seen that, compared with Al-5Ti-0.62C, the Al-5Ti-0.62C-0.2Nd grain refiner not only contains α-Al, TiAl₃, and TiC, but also contains Ti₂Al₂₀Nd phase. In addition, the diffraction peaks of TiAl₃ and TiC in Al-5Ti-0.62C-0.2Nd are obviously higher than those of the Al-5Ti-0.62C grain refiner, indicating that the addition of Nd₂O₃ can promote the synthesis of TiAl₃ and TiC.

Figure 1
XRD patterns of (a) Al-5Ti-0.62C and (b) Al-5Ti-0.62C-0.2Nd grain refiners.

Figure 2 shows the optical microstructure of Al-5Ti-0.62C and Al-5Ti-0.62C-0.2Nd grain refiners. They are mainly composed of block-like particles and granular particles in the aluminum matrix, while the particles in Al-5Ti-0.62C-0.2Nd are obviously greater in number than those in the Al-5Ti-0.62C grain refiner. Figure 3 shows the magnification inverse scattering SEM images of Al-5Ti-0.62C and Al-5Ti-0.62C-0.2Nd grain refiners. From Fig. 3(a), it can be seen that there are a large number of block-like TiAl₃ particles and granular TiC particles on the aluminum matrix of the Al-5Ti-0.62C grain refiner. In Fig. 3(b), a considerable number of block-like 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 TiAl₃ and the granular particles were TiC, while the bright-white block-like particles were Ti₂A1₂₀Nd.

Figure 2
Optical microstructures of (a) Al-5Ti-0.62C and (b) Al-5Ti-0.62C-0.2Nd grain refiners.

Figure 3
Magnification images of (a) Al-5Ti-0.62C and (b) Al-5Ti-0.62C-0.2Nd grain refiners.

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

Compared with the Al-5Ti-0.62C grain refiner, there is a new phase named Ti₂Al₂₀Nd that appeared in the Al-5Ti-0.62C-0.2Nd grain refiner. The reaction mechanism might be as follows: In the reaction process, the addition of Nd₂O₃ improved the wettability of Al powder, Ti powder, C powder, and aluminum melt. Firstly, the TiAl₃ phase was formed by the reaction of Al with Ti at high temperature. Then, under the energy supply of aluminothermic reaction, a very high temperature can be produced in the local high temperature microregion of melt, which causes the carbothermal reaction (2) between Nd₂O₃ and C at high temperature, and produces the gas CO, which makes the melt appear to be boiling intensively. As long as T>1920K (1647 ºC), the reaction can occur spontaneously. Once the reaction (2) begins, reaction (3) can easily start and produce TiC particles because of △G₃≦0, and [Nd] produced by the reaction has higher chemical activity and is a very strong surfactant. It is mainly adsorbed on TiAl₃ phase to form a new rare-earth composite phase Ti₂Al₂₀Nd. In addition, the heat produced by the continuous reaction of Al with Ti and the active [Nd] produced by reaction (3) are also beneficial to the occurrence of reaction (4) and the formation of TiC particles.

(1) $\begin{array}{c}\mathit{Ti}+3\mathit{Al}={\mathit{TiAl}}_3,\hfill \\ \Delta {\mathit{G}}_1=0.03432\mathit{T}-152.445\left(\mathit{J}/\mathit{mol}\right)\hfill \end{array}$

(2) $\begin{array}{c}{\mathit{Nd}}_2{\mathit{O}}_3\left(\mathit{s}\right)+7\mathit{C}\left(\mathit{s}\right)=2{\mathit{NdC}}_2\left(\mathit{s}\right)+3\mathit{CO}\left(\mathit{g}\right)\uparrow ,\hfill \\ \Delta {\mathit{G}}_2=-0.80324\mathit{T}+1323.858\left(\mathit{J}/\mathit{mol}\right)\hfill \end{array}$

(3) $\begin{array}{c}{\mathit{NdC}}_2\left(\mathit{s}\right)+2\mathit{Ti}\left(\mathit{s}\right)=2\mathit{TiC}+\left[\mathit{Nd}\right],\hfill \\ \Delta {\mathit{G}}_3=0.1507\mathit{T}-294.859\left(\mathit{J}/\mathit{mol}\right)\hfill \end{array}$

(4) $\begin{array}{c}\mathit{C}\left(\mathit{s}\right)+\mathit{Ti}\left(\mathit{s}\right)=\mathit{TiC}.\hfill \\ \Delta {\mathit{G}}_4=0.01091\mathit{T}-184.058\left(\mathit{J}/\mathit{mol}\right)\hfill \end{array}$

From the above analysis, we can see that Nd₂O₃ 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 Ti₂Al₂₀Nd during in situ reaction synthesis of the Al-5Ti-0.62C grain refiner by adding Nd₂O₃ 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.

3.2 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-0.62C and Al-5Ti-0.62C-0.2Nd grain 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-0.62C grain 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-0.62C grain refiner, when the addition of the Al-5Ti-0.62C-0.2Nd grain 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-0.2Nd grain refiner is better than that of the Al-5Ti-0.62C grain refiner for the same addition amount.

Figure 5
Macrostructures of commercially pure Al refined by different grain refiners (holding for 5 min): (a) unrefined; (b) 0.15-wt.%-Al-5Ti-0.62C; (c) 0.2-wt.%-Al-5Ti-0.62C; (d) 0.15-wt.%-Al-5Ti-0.62C-0.2Nd; (e) 0.2-wt.%-Al-5Ti-0.62C-0.2Nd

To further investigate the resistance to grain-refinement fading of the Al-5Ti-0.62C and Al-5Ti-0.62C-0.2Nd grain 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-0.2Nd grain refiner does not decrease obviously, even if the holding time is 60 min. However, the refining effect of the Al-5Ti-0.62C grain refiner attenuates obviously with increasing refinement time, as shown in Figs. 6(d)-6(f).

Figure 6
Effects of holding time on refinement effect of pure Al refined by different grain refiners (adding 0.2 wt.%): (a) Al-5Ti-0.62C-0.2Nd, 10 min; (b) Al-5Ti-0.62C-0.2Nd, 30 min; (c) Al-5Ti-0.62C-0.2Nd, 60 min; (d) Al-5Ti-0.62C, 10 min; (e) Al-5Ti-0.62C, 30 min; (f) Al-5Ti-0.62C, 60 min.

Although there are still some differences in the grain-refinement 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-0.62C grain 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-0.62C grain 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 TiAl₃ (i.e., the formation of the Ti-rich layer ^23,24).

Figure 7
Relation curve between holding time and average size of solidified commercial pure Al samples refined by 0.2-wt.%-Al-TiC, Al-5Ti-0.62C and Al-5Ti-0.62C-0.2Nd grain refiners.

Compared with Al-5Ti-0.62C, the Al-5Ti-0.62C-0.2Nd grain refiner not only contains more TiC and TiAl₃ particles, but also contains Ti₂A1₂₀Nd. When the Al-5Ti-0.62C-0.2Nd grain refiner is added to the aluminum melt, the Ti₂A1₂₀Nd will dissolve preferentially into TiAl₃ as the temperature of the melt is prolonged ¹³. The dissolution of Ti₂A1₂₀Nd 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 Al₄C₃^14,25, 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 grain-refinement efficiency of Al-5Ti-0.62C-0.2Nd is better than that of the Al-5Ti-0.62C grain 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-TiC and Al-5Ti-0.62C grain 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-0.62C grain refiner, the tensile strength and elongation of the refined samples with the 0.2-wt.%-Al-5Ti-0.62C-0.2Nd grain 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.

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

Following the Hall-Petch type equation :

${\mathit{\sigma }}_{\mathit{s}}={\mathit{\sigma }}_0+{\mathit{kd}}^{-\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$

where 𝜎_s is the yield stress, 𝜎₀ 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-0.62C grain refiner. Therefore, a new method of preparing Al-5Ti-0.62C-0.2Nd grain refiner by adding Nd₂O₃ in the thermal explosion reaction of pure aluminum is proposed, which provides the possibility for industrial application.

4. Conclusions

A novel Al-5Ti-0.62C-0.2Nd grain 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-0.2Nd grain refiner containing A1, TiAl₃, TiC and Ti₂A1₂₀Nd phase was successfully prepared by adding Nd₂O₃ in the thermal explosion reaction of pure molten aluminum.

2. Al-5Ti-0.62C-0.2Nd grain refiner has better refining performance compared with an Al-5Ti-0.62C grain refiner. With addition of 0.2-wt.%-Al-5Ti-0.62C-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.

3. When 0.2-wt.%-Al-5Ti-0.62C-0.2Nd grain refiner was added, the tensile strength and elongation reached 65.9 MPa and 44.6%, an increase of 18.3% and 83.5%, respectively.

5. Acknowledgments

We would like to thank LetPub (www.letpub.com) for providing linguistic assistance during the preparation of this manuscript. This research was financially supported by the National Natural Science Foundation of China (No. 51661021; 51665033). The authors would like to acknowledge the financial support of the Natural Science Foundation of Gansu Province in China (No. 1606RJZA161) and Gansu key research and development program (18YF1GA061).

Conflicts of Interest: The authors declare no conflict of interest.

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Publication Dates

• Publication in this collection
  03 Dec 2018
• Date of issue
  2019

History

• Received
  25 Apr 2018
• Reviewed
  19 Aug 2018
• Accepted
  25 Oct 2018