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Influence of Al–5Ti–1B master alloy addition on the grain size of AZ91 alloy
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ScienceDirect Journal of Magnesium and Alloys 5 (2017) 313–319 www.elsevier.com/journals/journal-of-magnesium-and-alloys/2213-9567
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Influence of Al–5Ti–1B master alloy addition on the grain size of AZ91 alloy Andrey Koltygin, Viacheslav Bazhenov *, Uralboy Mahmadiyorov National University of Science and Technology “MISiS”, Leninskiy pr. 4, Moscow 119049, Russia Received 1 February 2017; revised 27 July 2017; accepted 14 August 2017 Available online 4 September 2017
Abstract The mechanical properties of castings depend on the grain size. There is evidence that titanium and boron (Al–5Ti–1B master alloy) affect the grain size of magnesium alloys. Here, the influence of the addition of 0–1 wt. % of Al–5Ti–1B master alloy on the grain size of AZ91 magnesium alloy was investigated. Melting of the alloy was performed in steel and corundum crucibles. To study the effect of cooling rate on grain size, cylindrical samples were cast in steel and fireclay molds. The Al–5Ti–1B master alloy addition did not change the phase composition of the AZ91 alloy. This study demonstrates that the addition of Al–5Ti–1B did not contribute to the grain refinement of the AZ91 alloy, but rather led to its coarsening for samples cast in both the steel and fireclay molds. Increasing the holding time after the addition of the Al–5Ti–1B master alloy from 15 to 110 minutes also did not lead to significant grain coarsening. The mechanical properties of the AZ91 alloy samples slightly improved after Al–5Ti–1B addition. © 2017 Production and hosting by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: AZ91 magnesium alloy; Grain refinement; Al–5Ti–1B master alloy; Ti addition; Growth restriction factor
1. Introduction Magnesium alloys are widely used in industries where there are strict weight limitations on the parts and equipment, particularly the aircraft and spacecraft industries. The requirements for the mechanical properties of the materials used in such applications are very high [1]. However, magnesium alloys do not always meet these specifications because of their insufficient strength [2]. One way of improving the mechanical properties of the castings is grain refinement [3], which is usually achieved by the addition of other elements to ensure grain refinement in the as-cast condition. Typically, magnesium alloys can be divided into two groups: those that contain aluminum and those that do not [3]. Various alloying elements may be added depending on whether the magnesium alloy belongs to the first or the second group. Typically, the addition of carbon is used for grain refinement in the
* Corresponding author. National University of Science and Technology “MISiS”, Leninskiy pr. 4, Moscow 119049, Russia. E-mail address: [email protected] (V. Bazhenov).
alloys containing aluminum. Grain refinement is then based on the formation of Al4C3 particles in the alloy to serve as nucleation sites [4,5]. Zirconium is often used for grain refinement in the aluminum-free alloys [6,7]. Other elements are also added less commonly for the grain refining of magnesium alloys. For example, the addition of titanium or titanium in combination with boron to decrease the grain size of magnesium alloys is well known [8–11]. However, StJohn et al. have noted that the grain refinement effect is not always predictable and its mechanism is not fully understood [12]. For example, some research papers have shown that the alloy grains are not refined upon the addition of titanium [13,14]. Other studies have shown that decreasing the grain size by the addition of master alloys that contain titanium and boron is related to the availability of TiB2 particles, which can act as nucleation sites for the formation of the magnesium solid solution grains during the solidification process [15,16]. This can be achieved by the addition of 0.1 wt. % of Al–5Ti–1B master alloy or 1 wt. % of Al–1Ti–3B master alloy. This mechanism of grain refinement is widely known for application to aluminumbased alloys [17–19], for which Al–Ti–B master alloys are mainly produced.
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The influence of titanium on the grain size of magnesium alloys can also be explained by the growth restriction factor (GRF) as previously described [20]. Moreover, titanium is one of the most effective elements for restricting the growth of magnesium grains during the solidification process [21–23]. However, the solubility of titanium in solid or liquid magnesium is low [24], rendering its addition difficult. Many authors have studied the influence of titanium on the grain size of magnesium alloys but have paid insufficient attention to the experimental conditions. For instance, crucibles made from graphite [13,16,25] or steel [22,26] have been used when melting magnesium alloys, whereas the crucible material was not specified in other studies [11,23,27], and this factor may have a very strong influence on the experimental results. Iron is known to be one of the most effective elements for restricting grain growth in magnesium alloys. The presence of carbon in aluminum-containing alloys contributes to the formation of the Al4C3 particles that promote grain refinement [3–5,28,29]. Therefore, the contact of iron and carbon with AZ91 magnesium alloy during the melting process and while holding the molten metal may itself promote grain refinement. The aims of this paper are (i) to examine the ability of Al–5Ti–1B master alloy to promote the grain refinement of AZ91 alloy when it is melted in steel and corundum crucibles, (ii) to investigate the influence of both cooling rate and Al–5Ti–1B master alloy addition using steel and fireclay molds for sample casting, and (iii) to determine the influence of holding time after the addition of the Al–5Ti–1B master alloy on grain refinement. 2. Materials and methods As the raw materials, the AZ91 industrial alloy made by Avialit (Russia), and aluminum (99.99 wt. %) and Al–5Ti–1B (Al with 5 wt. % Ti and 1 wt. % B) TiBor 5/1 master alloy rod made by Kawecki-Billiton Metaalindustrie (Netherlands) were used. Magnesium alloys are typically melted in steel crucibles, although iron can promote grain refinement of the AZ91 alloy [3], and a method has been reported for grain refinement of the
AZ91 alloy by melting in a steel crucible with overheating [3,5]. As this grain refinement effect of iron could conceal the influence of Al–5Ti–1B master alloy on the grain size of the AZ91 alloy, in this study melting was performed in a high-frequency induction furnace in either steel or corundum crucibles. The charge for each melt was 300 g for the steel crucible and 50 g for the corundum crucible. The melting was carried out using carnallite (KCl·MgCl2) flux. The Al–5Ti–1B master alloy was added to the melt in the amount of 0–1.0 wt. %. After the addition of the master alloy, the melt was held for 15 minutes and then poured into the mold at 740 °C. However, addition of the Al–5Ti–1B master alloy leads to an increase in the aluminum content of the AZ91 alloy. Thus, to obtain the same aluminum content for all of the alloys, aluminum was added to the AZ91 alloy in an amount depending on the added quantity of Al–5Ti–1B master alloy. The melts prepared in steel crucibles were poured into a steel mold. The melts prepared in corundum crucibles were poured into a fireclay mold. After addition of the Al–Ti–B master alloy to the melt, a period of time may be required for the intermetallic Al3Ti and TiB2 particles to spread throughout the melt volume. This may influence the effectiveness of the grain refinement. Moreover, during the melting process under industrial conditions, sufficient time can pass between the addition of the master alloy to the melt and alloy solidification. To study the influence of the Al–5Ti–1B master alloy addition and the holding time of the melt after the master alloy addition on the grain size of the AZ91 alloy, 0.3 wt. % of Al–5Ti–1B master alloy was added to the melt and sample extraction was carried out at regular time intervals. The samples were poured into the steel mold at 740 °C. Samples of 80 mm height, 15 mm bottom diameter, and 30 mm top diameter were made in the steel mold. The draft angle was needed to prevent shrinkage cavity formation deep inside the samples. Samples of 50 mm height and 25 mm diameter were obtained from the fireclay mold. Metallographic sections were cut from these samples at a distance of 15 mm from the bottom. The chemical compositions of the alloys are presented in Table 1. The chemical compositions were determined by X-ray
Table 1 Chemical compositions of the AZ91 alloy samples. Sample
1SC* 2SC 3SC 4SC 1CC** 2CC 3CC 4CC 1IH (0 min)*** 2IH (45 min) 3IH (80 min)
Addition of Al–5Ti–1B master alloy, wt. %
Al
Content of each component, wt. % Mn
Zn
Crucible material
Mold material
0 0.2 0.5 1.0 0 0.3 0.55 1.0 0.3 0.3 0.3
8.29 ± 0.10 8.21 ± 0.10 8.61 ± 0.15 9.37 ± 0.01 9.03 ± 0.51 8.61 ± 0.40 8.86 ± 0.47 9.00 ± 0.68 8.83 ± 0.07 8.49 ± 0.39 9.17 ± 0.29
0.35 ± 0.01 0.34 ± 0.02 0.34 ± 0.05 0.38 ± 0.01 0.17 ± 0.06 0.16 ± 0.02 0.14 ± 0.03 0.17 ± 0.05 0.36 ± 0.01 0.31 ± 0.04 0.38 ± 0.05
0.78 ± 0.04 0.60 ± 0.04 0.70 ± 0.01 0.53 ± 0.04 0.53 ± 0.02 0.52 ± 0.03 0.50 ± 0.03 0.53 ± 0.08 0.60 ± 0.03 0.60 ± 0.03 0.64 ± 0.01
steel steel steel steel corundum corundum corundum corundum steel steel steel
steel steel steel steel fireclay fireclay fireclay fireclay steel steel steel
* SC ═ The samples were obtained using the melt prepared in a steel crucible. ** CC ═ The samples were obtained using the melt prepared in a corundum crucible. *** IH ═ The samples were obtained using the melt after isothermal holding (the holding time is indicated in parentheses).
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Polythermal sections of the phase diagrams were calculated using the Thermo-Calc Software [31] with the TTMG3 magnesium-based alloys database version 3 [32]. 3. Results and discussion
Fig. 1. Polythermal section of the ternary phase diagram for Al–5 wt. % Ti–(0– 3.0) wt. % B.
energy-dispersive spectroscopy (EDS) of areas with the dimensions of 1 × 1 mm. The alloy microstructure was investigated using a Carl Zeiss Axio Observer D1m optical microscope (OM) and a Tescan Vega SBH3 scanning electron microscope (SEM) equipped with an EDS analysis system (Oxford Instruments AZtecEnergy). The etchant used for revealing the grain boundaries was a 5% aqueous solution of tartaric acid [30]. As-cast samples were used for microstructure observation and heat-treated samples were used for grain size measurement. To reveal the grain boundaries, the samples were heat treated for four hours at 420–430 °C and then air cooled. After this heat treatment, the grain boundaries could be determined accurately, but their shift had not occurred yet. Grain size was measured by the linear intercept method. To measure the mechanical properties, cylindrical samples with a diameter of 5 mm, machined from the cast samples after T4 heat treatment (annealing at 420 °C for 12 hours and then air cooled), were used. Tensile tests were performed on the Instron 5569 universal testing machine.
The phase composition of the Al–5Ti–1B master alloy was investigated. Fig. 1 shows a polythermal section of the ternary Al–Ti–B phase diagram in the region corresponding to the chemical composition of the Al–5Ti–1B master alloy. According to the phase diagram, the phase composition of the Al–5Ti–1B master alloy at room temperature should be an aluminum solid solution containing particles of TiB2 and Al3Ti. This is consistent with the data for the chemical composition of the master alloy with the same composition [12,33]. Fig. 2 shows the microstructure of the Al–5Ti–1B master alloy (rod cross section). A solid solution (Al) is visible in the alloy microstructure, in which the particles of the intermetallic phase TiB2 and Al3Ti with sizes <1 µm are evenly distributed. Furthermore, large Al3Ti crystals with sizes of up to 100 µm can be seen in the structure. The phase identification was carried out by EDS. The phase composition of the AZ91 alloy samples did not change upon the addition of the Al–5Ti–1B master alloy in amounts up to 1%. The structure of these samples was found to be a magnesium solid solution surrounded by an intermetallic Mg17Al12 phase. Moreover, an Al8Mn5 phase was present in the structure (Fig. 3). The microstructure study revealed the presence of Al3Ti particles in the center of some dendritic grains. Thus, the Al3Ti particles were the nucleation sites for the magnesium solid solution. However, most of the Al3Ti particles were almost entirely covered with a layer of the Al8Mn5 phase (Fig. 4). Therefore, it would appear that not all of the Al3Ti particles could act as nucleation sites for the magnesium solid solution when the alloy contained manganese. Moreover, the Al8Mn5 particles in the AZ91 alloy possessed a needle or compact shape (Fig. 5a). Upon addition of the Al–5Ti–1B master alloy, the Al8Mn5 phase particles became more compact (Fig. 5b). Hence, when increasing the master alloy addition up to 1 wt. %, Al8Mn5 particles and Al3Ti
Fig. 2. Microstructure of the Al–5Ti–1B master alloy, as observed by SEM.
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Fig. 3. The microstructures of the as-cast samples, as observed by SEM: (a) AZ91, (b) AZ91 + 1 wt. % Al–5Ti–1B master alloy. The alloys were melted in a steel crucible (SC samples).
particles could be observed in the alloy microstructure. Together these formed confluences of Al8Mn5 and Al3Ti particles up to 20 µm (Fig. 5c). The average grain sizes of the AZ91 alloy samples containing various amounts of the Al–5Ti–1B master alloy were determined (Fig. 6). These samples were formed from the alloys melted in a steel crucible. The samples were heat treated for four hours at 420–430 °C to reveal the grains. The grain size was found to increase with increasing amount of master alloy in the AZ91 alloy. The addition of 1 wt. % of the Al–5Ti–1B master alloy increased the grain size by 35 µm or 49% (Table 2). Fig. 7 shows the microstructures of the AZ91 alloy samples containing various amounts of the Al–5Ti–1B master alloy. The effect of the Al–5Ti–1B master alloy addition on the grain size of the alloy melted in a corundum crucible was also assessed. Melting in a corundum crucible was performed to eliminate the influence of iron on the grain size (Fig. 8), and afterwards the samples were poured into a fireclay mold. The grain size was found to increase by approximately three times relative to the samples formed in the steel crucible and mold. The grain size was also observed to increase upon the addition of the Al–5Ti–1B master alloy. The addition of 1 wt. % of Al–5Ti–1B master alloy increased the grain size of the AZ91 alloy by 64 µm or 26% (Table 2). The grain coarsening observed upon adding the Al–5Ti–1B master alloy does not correspond to the results obtained in other investigations [12]. For example, grain refinement was previously observed after the addition of the Al–Ti–B master alloy to the AZ91 alloy [13]. However, another master alloy is also known that contains more TiB2 particles. This has also been mentioned in another article, where the authors recommended the use of a master alloy with a high content of boron to obtain a stable refinement effect of magnesium solid solution grains [19]. The grain sizes of the samples of the AZ91 alloy after the addition of 0.3 wt. % of Al–5Ti–1B with different holding times of the liquid melt are shown in Fig. 9. It can be seen that the grain size of the alloy remained almost constant. Thus, changing the melt holding time prior to casting had no effect on the grain refinement of the AZ91 alloy.
Fig. 4. (a) An Al3Ti particle covered by an Al8Mn5 layer in the center of the magnesium solid solution dendrite and (b) the distribution of elements across the indicated line for the 1IH sample (Table 1).
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Fig. 5. The Al8Mn5 phase in the as-cast samples of (a) AZ91, (b) AZ91 + 0.5 wt. % Al–5Ti–1B, (c) AZ91+ 1 wt. % Al–5Ti–1B, as observed by SEM. The samples were obtained using alloys melted in a steel crucible (SC samples).
Coarser grains were observed for the AZ91 alloy in all of the experiments after the addition of the Al–5Ti–1B master alloy. The most likely reason for this phenomenon is the high ability of titanium to form intermetallic phases with the main impurities in the magnesium alloy [13]. Fe increases the GRF and has one of the highest abilities to promote grain refinement of magnesium alloys [3]. After Ti addition, Fe forms
compounds with Ti and the amount of Fe in the magnesium solid solution becomes lower. The same effect can also be observed for other impurities. The mechanical properties of the heat-treated (T4) samples are presented in Table 3. It can be seen that the addition of the Al–5Ti–1B master alloy to the AZ91 alloy had almost no effect on the mechanical properties, although the yield strength
Table 2 Grain sizes of the AZ91 alloy samples.
Fig. 6. The influence of the amount of Al–5Ti–1B master alloy on the grain size of the AZ91 alloy melted in a steel crucible (SC samples).
Sample
Addition of Al–5Ti–1B master alloy, wt. %
Grain size d, µm
1SC* 2SC 3SC 4SC 1CC** 2CC 3CC 4CC 1IH (0 min)*** 2IH (45 min) 3IH (80 min)
0 0.2 0.5 1.0 0 0.3 0.55 1.0 0.3 0.3 0.3
72 ± 8 92 ± 15 81 ± 11 107 ± 15 247 ± 10 299 ± 11 287 ± 10 311 ± 13 73 ± 7 75 ± 7 75 ± 7
* SC ═ The samples were obtained using the melt prepared in a steel crucible. ** CC ═ The samples were obtained using the melt prepared in a corundum crucible. *** IH ═ The samples were obtained using the melt after isothermal holding (the holding time is indicated in parentheses).
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Fig. 7. The microstructures of samples of the AZ91 alloy melted in a steel crucible, as observed by OM: (a) 0 wt. % Al–5Ti–1B, (b) 0.2 wt. % Al–5Ti–1B, (c) 0.5 wt. % Al–5Ti–1B, (d) 1 wt. % Al–5Ti–1B. The samples were heat treated for four hours at 420–430 °C to reveal the grains.
slightly increased. This can probably be ascribed to the purification ability of titanium in the AZ91 alloy. The purification effect of titanium in the AZ91 alloy is due to the formation of a new phase containing Ti and the trace elements Fe and Si [13]. The addition of the Al–5Ti–1B master alloy into the AZ91 alloy should lead to a similar effect.
4. Conclusions
Fig. 8. Influence of the addition of Al–5Ti–1B master alloy on the grain size of the AZ91 alloy melted in a corundum crucible (CC samples).
Fig. 9. Influence of the melt holding time after the addition of 0.3 wt. % Al–5Ti–1B master alloy on the grain size of the AZ91 alloy (IH samples).
The results of this study can be summarized as follows: (1) The grain size of the AZ91 magnesium alloy increased upon the addition of up to 1 wt. % of the Al–5Ti–1B master alloy.
A. Koltygin et al. / Journal of Magnesium and Alloys 5 (2017) 313–319 Table 3 Mechanical properties of the AZ91 alloy and the AZ91 alloy containing 0.3 wt. % of Al–5Ti–1B master alloy.
AZ91 AZ91 + 0.3 wt. % Al–5Ti–1B
Yield strength (σ0.2), MPa
Ultimate strength (σU), MPa
Elongation (δ), %
128 ± 1 143 ± 13
233 ± 30 232 ± 13
9.3 ± 2.8 9.6 ± 1.5
(2) The addition of the Al–5Ti–1B master alloy to the AZ91 alloy led to the formation of Al3Ti particles, which grew in conjunction with the Al8Mn5 phase. (3) Holding of the AZ91 melt in the steel crucible at a temperature of 740 °C for 120 minutes after the addition of 0.3 wt. % of the Al–5Ti–1B master alloy resulted in minor changes to the grain size. (4) The addition of the Al–5Ti–1B master alloy to the AZ91 alloy provided a purification effect due to intermetallic compound formation. (5) The addition of 0.3 wt. % of the Al–5Ti–1B master alloy had no major effect on the mechanical properties of the AZ91 alloy, although it slightly increased the yield strength. References [1] I.J. Polmear, Light Alloys, fourth ed., Butterworth-Heinemann, Oxford, 2005. [2] W.J. Kim, Y.G. Lee, Mater. Sci. Eng. A Struct. Mater. 528 (2011) 2062–2066. [3] D.H. StJohn, M. Qian, M.A. Easton, P. Cao, Z. Hildebrand, Metall. Mater. Trans. A 36 (2005) 1669–1679. [4] M. Qian, P. Cao, Scr. Mater. 52 (2005) 415–419. [5] P. Cao, M. Qian, D.H. StJohn, Scr. Mater. 53 (2005) 841–844. [6] W. Yang, L. Liu, J. Zhang, S. Ji, Z. Fan, Mater. Lett. 160 (2015) 263–267. [7] A. Luo, M.O. Pekguleryuz, J. Mater. Sci. 29 (1994) 5259–5271. [8] J. Chen, Y. Sun, J. Zhang, W. Cheng, X. Niu, C. Xu, J. Magnes. Alloys 3 (2015) 121–126. [9] X. Ma, X. Wang, X. Li, L. Yang, Trans. Nonferrous Met. Soc. China 20 (2010) 397–401.
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H O S T E D BY
ScienceDirect Journal of Magnesium and Alloys 5 (2017) 313–319 www.elsevier.com/journals/journal-of-magnesium-and-alloys/2213-9567
Full Length Article
Influence of Al–5Ti–1B master alloy addition on the grain size of AZ91 alloy Andrey Koltygin, Viacheslav Bazhenov *, Uralboy Mahmadiyorov National University of Science and Technology “MISiS”, Leninskiy pr. 4, Moscow 119049, Russia Received 1 February 2017; revised 27 July 2017; accepted 14 August 2017 Available online 4 September 2017
Abstract The mechanical properties of castings depend on the grain size. There is evidence that titanium and boron (Al–5Ti–1B master alloy) affect the grain size of magnesium alloys. Here, the influence of the addition of 0–1 wt. % of Al–5Ti–1B master alloy on the grain size of AZ91 magnesium alloy was investigated. Melting of the alloy was performed in steel and corundum crucibles. To study the effect of cooling rate on grain size, cylindrical samples were cast in steel and fireclay molds. The Al–5Ti–1B master alloy addition did not change the phase composition of the AZ91 alloy. This study demonstrates that the addition of Al–5Ti–1B did not contribute to the grain refinement of the AZ91 alloy, but rather led to its coarsening for samples cast in both the steel and fireclay molds. Increasing the holding time after the addition of the Al–5Ti–1B master alloy from 15 to 110 minutes also did not lead to significant grain coarsening. The mechanical properties of the AZ91 alloy samples slightly improved after Al–5Ti–1B addition. © 2017 Production and hosting by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: AZ91 magnesium alloy; Grain refinement; Al–5Ti–1B master alloy; Ti addition; Growth restriction factor
1. Introduction Magnesium alloys are widely used in industries where there are strict weight limitations on the parts and equipment, particularly the aircraft and spacecraft industries. The requirements for the mechanical properties of the materials used in such applications are very high [1]. However, magnesium alloys do not always meet these specifications because of their insufficient strength [2]. One way of improving the mechanical properties of the castings is grain refinement [3], which is usually achieved by the addition of other elements to ensure grain refinement in the as-cast condition. Typically, magnesium alloys can be divided into two groups: those that contain aluminum and those that do not [3]. Various alloying elements may be added depending on whether the magnesium alloy belongs to the first or the second group. Typically, the addition of carbon is used for grain refinement in the
* Corresponding author. National University of Science and Technology “MISiS”, Leninskiy pr. 4, Moscow 119049, Russia. E-mail address: [email protected] (V. Bazhenov).
alloys containing aluminum. Grain refinement is then based on the formation of Al4C3 particles in the alloy to serve as nucleation sites [4,5]. Zirconium is often used for grain refinement in the aluminum-free alloys [6,7]. Other elements are also added less commonly for the grain refining of magnesium alloys. For example, the addition of titanium or titanium in combination with boron to decrease the grain size of magnesium alloys is well known [8–11]. However, StJohn et al. have noted that the grain refinement effect is not always predictable and its mechanism is not fully understood [12]. For example, some research papers have shown that the alloy grains are not refined upon the addition of titanium [13,14]. Other studies have shown that decreasing the grain size by the addition of master alloys that contain titanium and boron is related to the availability of TiB2 particles, which can act as nucleation sites for the formation of the magnesium solid solution grains during the solidification process [15,16]. This can be achieved by the addition of 0.1 wt. % of Al–5Ti–1B master alloy or 1 wt. % of Al–1Ti–3B master alloy. This mechanism of grain refinement is widely known for application to aluminumbased alloys [17–19], for which Al–Ti–B master alloys are mainly produced.
https://doi.org/10.1016/j.jma.2017.08.002 2213-9567/© 2017 Production and hosting by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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The influence of titanium on the grain size of magnesium alloys can also be explained by the growth restriction factor (GRF) as previously described [20]. Moreover, titanium is one of the most effective elements for restricting the growth of magnesium grains during the solidification process [21–23]. However, the solubility of titanium in solid or liquid magnesium is low [24], rendering its addition difficult. Many authors have studied the influence of titanium on the grain size of magnesium alloys but have paid insufficient attention to the experimental conditions. For instance, crucibles made from graphite [13,16,25] or steel [22,26] have been used when melting magnesium alloys, whereas the crucible material was not specified in other studies [11,23,27], and this factor may have a very strong influence on the experimental results. Iron is known to be one of the most effective elements for restricting grain growth in magnesium alloys. The presence of carbon in aluminum-containing alloys contributes to the formation of the Al4C3 particles that promote grain refinement [3–5,28,29]. Therefore, the contact of iron and carbon with AZ91 magnesium alloy during the melting process and while holding the molten metal may itself promote grain refinement. The aims of this paper are (i) to examine the ability of Al–5Ti–1B master alloy to promote the grain refinement of AZ91 alloy when it is melted in steel and corundum crucibles, (ii) to investigate the influence of both cooling rate and Al–5Ti–1B master alloy addition using steel and fireclay molds for sample casting, and (iii) to determine the influence of holding time after the addition of the Al–5Ti–1B master alloy on grain refinement. 2. Materials and methods As the raw materials, the AZ91 industrial alloy made by Avialit (Russia), and aluminum (99.99 wt. %) and Al–5Ti–1B (Al with 5 wt. % Ti and 1 wt. % B) TiBor 5/1 master alloy rod made by Kawecki-Billiton Metaalindustrie (Netherlands) were used. Magnesium alloys are typically melted in steel crucibles, although iron can promote grain refinement of the AZ91 alloy [3], and a method has been reported for grain refinement of the
AZ91 alloy by melting in a steel crucible with overheating [3,5]. As this grain refinement effect of iron could conceal the influence of Al–5Ti–1B master alloy on the grain size of the AZ91 alloy, in this study melting was performed in a high-frequency induction furnace in either steel or corundum crucibles. The charge for each melt was 300 g for the steel crucible and 50 g for the corundum crucible. The melting was carried out using carnallite (KCl·MgCl2) flux. The Al–5Ti–1B master alloy was added to the melt in the amount of 0–1.0 wt. %. After the addition of the master alloy, the melt was held for 15 minutes and then poured into the mold at 740 °C. However, addition of the Al–5Ti–1B master alloy leads to an increase in the aluminum content of the AZ91 alloy. Thus, to obtain the same aluminum content for all of the alloys, aluminum was added to the AZ91 alloy in an amount depending on the added quantity of Al–5Ti–1B master alloy. The melts prepared in steel crucibles were poured into a steel mold. The melts prepared in corundum crucibles were poured into a fireclay mold. After addition of the Al–Ti–B master alloy to the melt, a period of time may be required for the intermetallic Al3Ti and TiB2 particles to spread throughout the melt volume. This may influence the effectiveness of the grain refinement. Moreover, during the melting process under industrial conditions, sufficient time can pass between the addition of the master alloy to the melt and alloy solidification. To study the influence of the Al–5Ti–1B master alloy addition and the holding time of the melt after the master alloy addition on the grain size of the AZ91 alloy, 0.3 wt. % of Al–5Ti–1B master alloy was added to the melt and sample extraction was carried out at regular time intervals. The samples were poured into the steel mold at 740 °C. Samples of 80 mm height, 15 mm bottom diameter, and 30 mm top diameter were made in the steel mold. The draft angle was needed to prevent shrinkage cavity formation deep inside the samples. Samples of 50 mm height and 25 mm diameter were obtained from the fireclay mold. Metallographic sections were cut from these samples at a distance of 15 mm from the bottom. The chemical compositions of the alloys are presented in Table 1. The chemical compositions were determined by X-ray
Table 1 Chemical compositions of the AZ91 alloy samples. Sample
1SC* 2SC 3SC 4SC 1CC** 2CC 3CC 4CC 1IH (0 min)*** 2IH (45 min) 3IH (80 min)
Addition of Al–5Ti–1B master alloy, wt. %
Al
Content of each component, wt. % Mn
Zn
Crucible material
Mold material
0 0.2 0.5 1.0 0 0.3 0.55 1.0 0.3 0.3 0.3
8.29 ± 0.10 8.21 ± 0.10 8.61 ± 0.15 9.37 ± 0.01 9.03 ± 0.51 8.61 ± 0.40 8.86 ± 0.47 9.00 ± 0.68 8.83 ± 0.07 8.49 ± 0.39 9.17 ± 0.29
0.35 ± 0.01 0.34 ± 0.02 0.34 ± 0.05 0.38 ± 0.01 0.17 ± 0.06 0.16 ± 0.02 0.14 ± 0.03 0.17 ± 0.05 0.36 ± 0.01 0.31 ± 0.04 0.38 ± 0.05
0.78 ± 0.04 0.60 ± 0.04 0.70 ± 0.01 0.53 ± 0.04 0.53 ± 0.02 0.52 ± 0.03 0.50 ± 0.03 0.53 ± 0.08 0.60 ± 0.03 0.60 ± 0.03 0.64 ± 0.01
steel steel steel steel corundum corundum corundum corundum steel steel steel
steel steel steel steel fireclay fireclay fireclay fireclay steel steel steel
* SC ═ The samples were obtained using the melt prepared in a steel crucible. ** CC ═ The samples were obtained using the melt prepared in a corundum crucible. *** IH ═ The samples were obtained using the melt after isothermal holding (the holding time is indicated in parentheses).
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Polythermal sections of the phase diagrams were calculated using the Thermo-Calc Software [31] with the TTMG3 magnesium-based alloys database version 3 [32]. 3. Results and discussion
Fig. 1. Polythermal section of the ternary phase diagram for Al–5 wt. % Ti–(0– 3.0) wt. % B.
energy-dispersive spectroscopy (EDS) of areas with the dimensions of 1 × 1 mm. The alloy microstructure was investigated using a Carl Zeiss Axio Observer D1m optical microscope (OM) and a Tescan Vega SBH3 scanning electron microscope (SEM) equipped with an EDS analysis system (Oxford Instruments AZtecEnergy). The etchant used for revealing the grain boundaries was a 5% aqueous solution of tartaric acid [30]. As-cast samples were used for microstructure observation and heat-treated samples were used for grain size measurement. To reveal the grain boundaries, the samples were heat treated for four hours at 420–430 °C and then air cooled. After this heat treatment, the grain boundaries could be determined accurately, but their shift had not occurred yet. Grain size was measured by the linear intercept method. To measure the mechanical properties, cylindrical samples with a diameter of 5 mm, machined from the cast samples after T4 heat treatment (annealing at 420 °C for 12 hours and then air cooled), were used. Tensile tests were performed on the Instron 5569 universal testing machine.
The phase composition of the Al–5Ti–1B master alloy was investigated. Fig. 1 shows a polythermal section of the ternary Al–Ti–B phase diagram in the region corresponding to the chemical composition of the Al–5Ti–1B master alloy. According to the phase diagram, the phase composition of the Al–5Ti–1B master alloy at room temperature should be an aluminum solid solution containing particles of TiB2 and Al3Ti. This is consistent with the data for the chemical composition of the master alloy with the same composition [12,33]. Fig. 2 shows the microstructure of the Al–5Ti–1B master alloy (rod cross section). A solid solution (Al) is visible in the alloy microstructure, in which the particles of the intermetallic phase TiB2 and Al3Ti with sizes <1 µm are evenly distributed. Furthermore, large Al3Ti crystals with sizes of up to 100 µm can be seen in the structure. The phase identification was carried out by EDS. The phase composition of the AZ91 alloy samples did not change upon the addition of the Al–5Ti–1B master alloy in amounts up to 1%. The structure of these samples was found to be a magnesium solid solution surrounded by an intermetallic Mg17Al12 phase. Moreover, an Al8Mn5 phase was present in the structure (Fig. 3). The microstructure study revealed the presence of Al3Ti particles in the center of some dendritic grains. Thus, the Al3Ti particles were the nucleation sites for the magnesium solid solution. However, most of the Al3Ti particles were almost entirely covered with a layer of the Al8Mn5 phase (Fig. 4). Therefore, it would appear that not all of the Al3Ti particles could act as nucleation sites for the magnesium solid solution when the alloy contained manganese. Moreover, the Al8Mn5 particles in the AZ91 alloy possessed a needle or compact shape (Fig. 5a). Upon addition of the Al–5Ti–1B master alloy, the Al8Mn5 phase particles became more compact (Fig. 5b). Hence, when increasing the master alloy addition up to 1 wt. %, Al8Mn5 particles and Al3Ti
Fig. 2. Microstructure of the Al–5Ti–1B master alloy, as observed by SEM.
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Fig. 3. The microstructures of the as-cast samples, as observed by SEM: (a) AZ91, (b) AZ91 + 1 wt. % Al–5Ti–1B master alloy. The alloys were melted in a steel crucible (SC samples).
particles could be observed in the alloy microstructure. Together these formed confluences of Al8Mn5 and Al3Ti particles up to 20 µm (Fig. 5c). The average grain sizes of the AZ91 alloy samples containing various amounts of the Al–5Ti–1B master alloy were determined (Fig. 6). These samples were formed from the alloys melted in a steel crucible. The samples were heat treated for four hours at 420–430 °C to reveal the grains. The grain size was found to increase with increasing amount of master alloy in the AZ91 alloy. The addition of 1 wt. % of the Al–5Ti–1B master alloy increased the grain size by 35 µm or 49% (Table 2). Fig. 7 shows the microstructures of the AZ91 alloy samples containing various amounts of the Al–5Ti–1B master alloy. The effect of the Al–5Ti–1B master alloy addition on the grain size of the alloy melted in a corundum crucible was also assessed. Melting in a corundum crucible was performed to eliminate the influence of iron on the grain size (Fig. 8), and afterwards the samples were poured into a fireclay mold. The grain size was found to increase by approximately three times relative to the samples formed in the steel crucible and mold. The grain size was also observed to increase upon the addition of the Al–5Ti–1B master alloy. The addition of 1 wt. % of Al–5Ti–1B master alloy increased the grain size of the AZ91 alloy by 64 µm or 26% (Table 2). The grain coarsening observed upon adding the Al–5Ti–1B master alloy does not correspond to the results obtained in other investigations [12]. For example, grain refinement was previously observed after the addition of the Al–Ti–B master alloy to the AZ91 alloy [13]. However, another master alloy is also known that contains more TiB2 particles. This has also been mentioned in another article, where the authors recommended the use of a master alloy with a high content of boron to obtain a stable refinement effect of magnesium solid solution grains [19]. The grain sizes of the samples of the AZ91 alloy after the addition of 0.3 wt. % of Al–5Ti–1B with different holding times of the liquid melt are shown in Fig. 9. It can be seen that the grain size of the alloy remained almost constant. Thus, changing the melt holding time prior to casting had no effect on the grain refinement of the AZ91 alloy.
Fig. 4. (a) An Al3Ti particle covered by an Al8Mn5 layer in the center of the magnesium solid solution dendrite and (b) the distribution of elements across the indicated line for the 1IH sample (Table 1).
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Fig. 5. The Al8Mn5 phase in the as-cast samples of (a) AZ91, (b) AZ91 + 0.5 wt. % Al–5Ti–1B, (c) AZ91+ 1 wt. % Al–5Ti–1B, as observed by SEM. The samples were obtained using alloys melted in a steel crucible (SC samples).
Coarser grains were observed for the AZ91 alloy in all of the experiments after the addition of the Al–5Ti–1B master alloy. The most likely reason for this phenomenon is the high ability of titanium to form intermetallic phases with the main impurities in the magnesium alloy [13]. Fe increases the GRF and has one of the highest abilities to promote grain refinement of magnesium alloys [3]. After Ti addition, Fe forms
compounds with Ti and the amount of Fe in the magnesium solid solution becomes lower. The same effect can also be observed for other impurities. The mechanical properties of the heat-treated (T4) samples are presented in Table 3. It can be seen that the addition of the Al–5Ti–1B master alloy to the AZ91 alloy had almost no effect on the mechanical properties, although the yield strength
Table 2 Grain sizes of the AZ91 alloy samples.
Fig. 6. The influence of the amount of Al–5Ti–1B master alloy on the grain size of the AZ91 alloy melted in a steel crucible (SC samples).
Sample
Addition of Al–5Ti–1B master alloy, wt. %
Grain size d, µm
1SC* 2SC 3SC 4SC 1CC** 2CC 3CC 4CC 1IH (0 min)*** 2IH (45 min) 3IH (80 min)
0 0.2 0.5 1.0 0 0.3 0.55 1.0 0.3 0.3 0.3
72 ± 8 92 ± 15 81 ± 11 107 ± 15 247 ± 10 299 ± 11 287 ± 10 311 ± 13 73 ± 7 75 ± 7 75 ± 7
* SC ═ The samples were obtained using the melt prepared in a steel crucible. ** CC ═ The samples were obtained using the melt prepared in a corundum crucible. *** IH ═ The samples were obtained using the melt after isothermal holding (the holding time is indicated in parentheses).
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Fig. 7. The microstructures of samples of the AZ91 alloy melted in a steel crucible, as observed by OM: (a) 0 wt. % Al–5Ti–1B, (b) 0.2 wt. % Al–5Ti–1B, (c) 0.5 wt. % Al–5Ti–1B, (d) 1 wt. % Al–5Ti–1B. The samples were heat treated for four hours at 420–430 °C to reveal the grains.
slightly increased. This can probably be ascribed to the purification ability of titanium in the AZ91 alloy. The purification effect of titanium in the AZ91 alloy is due to the formation of a new phase containing Ti and the trace elements Fe and Si [13]. The addition of the Al–5Ti–1B master alloy into the AZ91 alloy should lead to a similar effect.
4. Conclusions
Fig. 8. Influence of the addition of Al–5Ti–1B master alloy on the grain size of the AZ91 alloy melted in a corundum crucible (CC samples).
Fig. 9. Influence of the melt holding time after the addition of 0.3 wt. % Al–5Ti–1B master alloy on the grain size of the AZ91 alloy (IH samples).
The results of this study can be summarized as follows: (1) The grain size of the AZ91 magnesium alloy increased upon the addition of up to 1 wt. % of the Al–5Ti–1B master alloy.
A. Koltygin et al. / Journal of Magnesium and Alloys 5 (2017) 313–319 Table 3 Mechanical properties of the AZ91 alloy and the AZ91 alloy containing 0.3 wt. % of Al–5Ti–1B master alloy.
AZ91 AZ91 + 0.3 wt. % Al–5Ti–1B
Yield strength (σ0.2), MPa
Ultimate strength (σU), MPa
Elongation (δ), %
128 ± 1 143 ± 13
233 ± 30 232 ± 13
9.3 ± 2.8 9.6 ± 1.5
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