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Microstructure and properties of rheo-diecasting wrought aluminum alloy with Sc additions
Materials Letters 173 (2016) 22–25
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microstructure and properties of rheo-diecasting wrought aluminum alloy with Sc additions Junwen Zhao a,b,n, Chao Xu a, Guangze Dai a, Shusen Wu b, Jing Han a a School of materials science and engineering, Key Lab of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, China b State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 October 2015 Received in revised form 14 January 2016 Accepted 27 February 2016 Available online 2 March 2016
In this paper semisolid slurry of Al-6.3Zn-1.9Mg-1.8Cu-0.15Zr alloy with different levels of Sc was prepared, and the effect of Sc content on the morphology of microstructure and mechanical properties of the rheo-diecasting samples was investigated. The results indicate that the excellent semisolid slurry of the wrought Al alloy could be obtained with the addition of 0.22%Sc. Sc additions not only influenced the morphology of the primary α-Al particles in the slurry, but also promoted the formation of fine α-Al globules in the solidification process of the remaining liquid, which contributed to the considerable improvement in tensile strength and elongation. The tensile strength and elongation of the rheo-diecasting samples with 0.45%Sc after T6 heat treatment increased by 82% and 147% respectively compared to the die-casting samples of the base alloy. & 2016 Elsevier B.V. All rights reserved.
Keywords: Aluminum alloy Rheocasting Scandium Particles Microstructure Solidification
1. Introduction
2. Materials and methods
Wrought aluminum alloys such as Al‒Zn‒Mg‒Cu alloy (7xxx) are widely used in aerospace and transportation fields because of its high strength and ductility [1,2]. The semisolid casting, including thixocasting and rheocasting for these alloys offers opportunity to manufacture components with less investment, more complicated shape and similar mechanical properties to those obtained by conventional plastic forming. Among them the rheocasting has drawn increasing attention due to its lower cost and higher productivity [3–5]. Addition of Sc has the potential to be a simple and effective process to produce semisolid slurry of wrought aluminum alloys. Previous study revealed that the presence of Sc can refine as-cast microstructure and inhibit recrystallization of Al alloys during aging [6–8]. However, semisolid slurry preparation of wrought aluminum alloys with Sc additions is a relatively new method in semisolid processing. In this work, semisolid slurry of Al‒Zn‒Mg‒ Cu‒Zr alloy was prepared by Sc additions and the effect of Sc additions on the microstructure and mechanical properties of rheodiecasting samples was investigated.
The main chemical compositions of the studied base wrought Al alloy are Zn 6.32%, Mg 1.9%, Cu 1.8%, Zr 0.15% and Al balance (mass fraction), which is a modified alloy based on 7A04 (a Chinese Al alloy code). The solidus and liquidus points of this alloy determined by DSC are 478 °C and 637 °C, respectively. The base alloy was melted in a graphite crucible using an electric resistance furnace. Once molten, Sc was added to increase the Sc content to the desired levels. After degassed at 750 °C for 15 min, about 500 g of the melt was poured into a metal container preheated to 500 °C in a furnace. The temperature of the poured melt would reach about 650 °C in 30 s through the thermal balance between the melt and then cooled at a rate of 10 °C/min. After cooled to the temperature of 630 °C, some slurry was extracted out by a quartz tube with an inner diameter of 6 mm and quenched in water immediately, and the remaining slurry was shaped by a cold-chamber high pressure die-casting machine to produce tensile test samples with a diameter of 6.4 mm. For comparison, the die-casting samples of the Sc-free alloy were also made under the same processing conditions. The tensile tests were performed using a universal materials testing machine at a crosshead speed of 1 mm/min. Part of the quenched rods and the ends of tensile test samples were cut to be used as specimens for the metallographic examination. Micrographs of the samples were analyzed using a quantitative metallographic analysis software, where the average
n Corresponding author at: School of materials science and engineering, Key Lab of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, China E-mail address: [email protected] (J. Zhao).
http://dx.doi.org/10.1016/j.matlet.2016.02.143 0167-577X/& 2016 Elsevier B.V. All rights reserved.
J. Zhao et al. / Materials Letters 173 (2016) 22–25
23
particles diameter(APD) and shape coefficient (ASC) were used to characterize the size and shape of the α-Al particles and defined as:
D = 2(A/π )1/2
(1)
F = 4π A / P 2
(2)
wherein A is the sectional area of a particle in a micrograph, and P is the circumference. The APD and ASC were calculated out by the software, based on counting all the primary particles in a photograph.
3. Results and discussion Fig. 1 shows the quenched microstructure of the semisolid slurry with different levels of Sc. It can be seen that the morphology of primary α-Al particles are greatly influenced by Sc additions. With the increase in Sc content, more α-Al rosettes were changed to fine globular particles. Fig. 2 presents the relationship between APD and ASC of the primary α-Al particles and Sc content. The results indicate that APD decreased and ASC increased with increasing Sc content. It can be concluded that very good semisolid slurry with fine and spherical solid particles can be obtained with the addition of only 0.22%Sc, and the APD and ASC of the primary α-Al particles were 72 mm and 0.58, respectively. Fig. 3 shows the microstructures of the rheo-diecasting samples with Sc levels of 0.11%, 0.22%, 0.34%, 0.45%, respectively. When increasing Sc content from 0% to 0.45%, the primary α-Al particles
Fig. 2. Variation of APD and ASC of the primary α-Al particles against Sc content.
are uniformly distributed in the matrix and the APD decreases from 90 to 40 mm while the ASC increases from 0.7 to 0.9. It is also clear that a great number of fine α-Al globules with average diameter of below 10 mm and shape coefficient of above 0.9 are observed in the samples when Sc content increases to 0.34%. Increasing Sc to 0.45%, more fine α-Al particles are found in the samples. In fact, in the rheo-diecasting process, the primary solid phase in slurry is usually below 30 vol%, hence the solidification of the remaining liquid of the slurry plays an important role in determining the final mechanical properties [9]. When the Sc containing aluminum alloy melt is poured into the crucible and reaches the temperature near the liquidus point, Al3Sc particles will be formed throughout the entire volume of the
Fig. 1. Quenched microstructures of semisolid slurry of wrought aluminum alloys with different Sc levels: (a) 0.1%Sc, (b) 0.22%Sc, (c) 0.34%Sc and (d) 0.45%Sc.
24
J. Zhao et al. / Materials Letters 173 (2016) 22–25
Fig. 3. Microstructures of the rheo-diecasting samples with different levels of Sc: (a) 0.1%Sc, (b) 0.22%Sc, (c) 0.34%Sc and (d) 0.45%Sc.
melt. As Al3Sc particles are excellent heterogeneous nucleants for α-Al grains owing to their good match of lattice type and lattice constant [10,11], these effective nucleation sites lead to the formation of primary α-Al grains. Furthermore, the grain growth with Sc additions is restricted as a result of constitutional supercooling, where the growing primary α-Al particles reject Sc into the solid– liquid interface as they grow. As the concentration of Sc increases at the interface, it may reach the level where new Al3Sc nucleants could form. The presence of Al3Sc particles encourages the formation of new α-Al nuclei within the interface layer. Such mechanism ensures the formation of fine and non-dendriticα-Al particles. Therefore, finer and rounder α-Al particles can be obtained with more amount of Sc. On the other hand, with increasing Sc amount, it is more likely to form agglomeration of the Al3Sc particles because of smaller spacing between each other and segregation originated from solute redistribution during solidification, which degrades the potency of Al3Sc particles acting as nucleation sites. This may account for the small variation in particle size and shape factor as the 0.22% level of Sc is surpassed. The finer and globular microstructure of the rheo-diecasting samples with Sc additions could be attributed to homogeneous dispersion of numerous Al3Sc particles in the remaining liquid as a result of intensive shearing of the slurry when passing through the narrow gate during its mould filling. The mechanical properties of rheo-diecasting samples with different levels of Sc are shown in Table 1. It can be known that the addition of only 0.11%Sc caused a sharp rise in the tensile strength and elongation of the rheo-diecasting samples, which was increased by 32.6% and 44% respectively compared to those of Scfree alloy. Higher Sc content resulted in both higher tensile strength and elongation of the samples, but the former increased
Table 1 Mechanical properties of the samples with different levels of Sc. Sc content (wt%)
0 0.11 0.22 0.34 0.45
Tensile strength(MPa)
Elongation(%)
As-cast
T6
As-cast
T6
181 240 292 310 311
303 407 502 543 552
2.5 3.6 4.8 5.7 6.1
2.1 3.0 4.1 4.6 5.2
slightly whereas the latter still increased significantly when the Sc reached 0.22%. However, higher concentration of Sc seems unable to obtain further marked increase in both tensile strength and elongation. After T6 heat treatment, the tensile strength of all the studied alloys was increased by from 67.4% to 77.5% while the elongation decreased by 14–17% as the amount of Sc increased from 0 to 0.45%. It is noted that there are some pores in the microstructure, which may account for the relatively low mechanical properties of rheo-diecasting samples with respect to those processed by plastic forming [12,13]. For the same cause, the mechanical properties of samples could not be improved notably with over 0.45% Sc. Higher mechanical properties of rheo-diecasting samples with Sc additions mainly benefit from the finer and globular microstructure. The significant increase of elongation for samples with Sc levels above 0.22%Sc might be mainly attributed to the large quantities of fine α-Al globules formed during the solidification of the remaining liquid. The precipitation hardening of Al3Sc particles results in the great increase of the tensile strength of samples after T6 heat treatment.
J. Zhao et al. / Materials Letters 173 (2016) 22–25
4. Conclusion Good semisolid slurry of wrought Al alloy could be obtained with the addition of 0.22%Sc, and the average diameter and shape coefficient of primary α-Al particles were 72 mm and 0.58 respectively. Sc additions not only influenced the morphology of the primary α-Al in the slurry, but also promoted the formation of fine α-Al globules in the solidification process of the remaining liquid, which contributed to the considerable improvement in tensile strength and elongation. Samples with the level of 0.45%Sc had the maximum tensile strength and elongation after T6 heat treatment, which were increased by 82% and 147% respectively compared to the die-casting samples of the base alloy.
25
State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (P2015-10), China.
References [1] [2] [3] [4] [5] [6] [7] [8]
Acknowledgements
[9] [10] [11]
This research was financially supported by the National Key Technology Support Program of China (2015BAG12B01) and the
[12] [13]
M. Nakai, T. Eto, Mater. Sci. Eng. A 285 (2000) 62–68. G. Xie, D.J. Thompson, C.J.C. Jones, J. Sound Vib. 293 (2007) 921–932. U.A. Curle, Trans. Nonferrous Met. Soc. 20 (2010) 1719–1724. N. Mahathaninwong, T. Plookphol, J. Wannasin, S. Wisutmethangoon, Mater. Sci. Eng. A (2012) 91–99. S. Lü, S. Wu, C. Lin, Z. Hu, P. An, Mater. Sci. Eng. A 528 (2011) 8635–8640. E.L. Huskins, B. Cao, K.T. Ramesh, Mater. Sci. Eng. A 527 (2010) 1292–1298. G. Du, J. Deng, Y. Wang, D. Yan, L. Rong, Scr. Mater. 61 (2009) 532–535. M.F. Kilicaslan, W.R. Lee, T.H. Lee, Y. Sohn, S.J. Hong, Mater. Lett. 71 (2012) 164–167. M. Hitchcock, Y. Wang, Z. Fan, Acta Mater. 55 (2007) 1589–1598. K. Yu, W. Li, S. Li, J. Zhao, Mater. Sci. Eng. A 368 (2004) 88–93. O.N. Senkov, S.V. Senkova, M.R. Shagiev, Met. Mater. Trans. A. 39 (2008) 1034–1053. R.C. Dorward, D.J. Beerntsen, Met. Mater. Trans. A. 26 (1995) 2481–2484. E. Salamci, Turk. J. Eng. Environ. Sci. 26 (2002) 345–352.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microstructure and properties of rheo-diecasting wrought aluminum alloy with Sc additions Junwen Zhao a,b,n, Chao Xu a, Guangze Dai a, Shusen Wu b, Jing Han a a School of materials science and engineering, Key Lab of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, China b State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 October 2015 Received in revised form 14 January 2016 Accepted 27 February 2016 Available online 2 March 2016
In this paper semisolid slurry of Al-6.3Zn-1.9Mg-1.8Cu-0.15Zr alloy with different levels of Sc was prepared, and the effect of Sc content on the morphology of microstructure and mechanical properties of the rheo-diecasting samples was investigated. The results indicate that the excellent semisolid slurry of the wrought Al alloy could be obtained with the addition of 0.22%Sc. Sc additions not only influenced the morphology of the primary α-Al particles in the slurry, but also promoted the formation of fine α-Al globules in the solidification process of the remaining liquid, which contributed to the considerable improvement in tensile strength and elongation. The tensile strength and elongation of the rheo-diecasting samples with 0.45%Sc after T6 heat treatment increased by 82% and 147% respectively compared to the die-casting samples of the base alloy. & 2016 Elsevier B.V. All rights reserved.
Keywords: Aluminum alloy Rheocasting Scandium Particles Microstructure Solidification
1. Introduction
2. Materials and methods
Wrought aluminum alloys such as Al‒Zn‒Mg‒Cu alloy (7xxx) are widely used in aerospace and transportation fields because of its high strength and ductility [1,2]. The semisolid casting, including thixocasting and rheocasting for these alloys offers opportunity to manufacture components with less investment, more complicated shape and similar mechanical properties to those obtained by conventional plastic forming. Among them the rheocasting has drawn increasing attention due to its lower cost and higher productivity [3–5]. Addition of Sc has the potential to be a simple and effective process to produce semisolid slurry of wrought aluminum alloys. Previous study revealed that the presence of Sc can refine as-cast microstructure and inhibit recrystallization of Al alloys during aging [6–8]. However, semisolid slurry preparation of wrought aluminum alloys with Sc additions is a relatively new method in semisolid processing. In this work, semisolid slurry of Al‒Zn‒Mg‒ Cu‒Zr alloy was prepared by Sc additions and the effect of Sc additions on the microstructure and mechanical properties of rheodiecasting samples was investigated.
The main chemical compositions of the studied base wrought Al alloy are Zn 6.32%, Mg 1.9%, Cu 1.8%, Zr 0.15% and Al balance (mass fraction), which is a modified alloy based on 7A04 (a Chinese Al alloy code). The solidus and liquidus points of this alloy determined by DSC are 478 °C and 637 °C, respectively. The base alloy was melted in a graphite crucible using an electric resistance furnace. Once molten, Sc was added to increase the Sc content to the desired levels. After degassed at 750 °C for 15 min, about 500 g of the melt was poured into a metal container preheated to 500 °C in a furnace. The temperature of the poured melt would reach about 650 °C in 30 s through the thermal balance between the melt and then cooled at a rate of 10 °C/min. After cooled to the temperature of 630 °C, some slurry was extracted out by a quartz tube with an inner diameter of 6 mm and quenched in water immediately, and the remaining slurry was shaped by a cold-chamber high pressure die-casting machine to produce tensile test samples with a diameter of 6.4 mm. For comparison, the die-casting samples of the Sc-free alloy were also made under the same processing conditions. The tensile tests were performed using a universal materials testing machine at a crosshead speed of 1 mm/min. Part of the quenched rods and the ends of tensile test samples were cut to be used as specimens for the metallographic examination. Micrographs of the samples were analyzed using a quantitative metallographic analysis software, where the average
n Corresponding author at: School of materials science and engineering, Key Lab of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, China E-mail address: [email protected] (J. Zhao).
http://dx.doi.org/10.1016/j.matlet.2016.02.143 0167-577X/& 2016 Elsevier B.V. All rights reserved.
J. Zhao et al. / Materials Letters 173 (2016) 22–25
23
particles diameter(APD) and shape coefficient (ASC) were used to characterize the size and shape of the α-Al particles and defined as:
D = 2(A/π )1/2
(1)
F = 4π A / P 2
(2)
wherein A is the sectional area of a particle in a micrograph, and P is the circumference. The APD and ASC were calculated out by the software, based on counting all the primary particles in a photograph.
3. Results and discussion Fig. 1 shows the quenched microstructure of the semisolid slurry with different levels of Sc. It can be seen that the morphology of primary α-Al particles are greatly influenced by Sc additions. With the increase in Sc content, more α-Al rosettes were changed to fine globular particles. Fig. 2 presents the relationship between APD and ASC of the primary α-Al particles and Sc content. The results indicate that APD decreased and ASC increased with increasing Sc content. It can be concluded that very good semisolid slurry with fine and spherical solid particles can be obtained with the addition of only 0.22%Sc, and the APD and ASC of the primary α-Al particles were 72 mm and 0.58, respectively. Fig. 3 shows the microstructures of the rheo-diecasting samples with Sc levels of 0.11%, 0.22%, 0.34%, 0.45%, respectively. When increasing Sc content from 0% to 0.45%, the primary α-Al particles
Fig. 2. Variation of APD and ASC of the primary α-Al particles against Sc content.
are uniformly distributed in the matrix and the APD decreases from 90 to 40 mm while the ASC increases from 0.7 to 0.9. It is also clear that a great number of fine α-Al globules with average diameter of below 10 mm and shape coefficient of above 0.9 are observed in the samples when Sc content increases to 0.34%. Increasing Sc to 0.45%, more fine α-Al particles are found in the samples. In fact, in the rheo-diecasting process, the primary solid phase in slurry is usually below 30 vol%, hence the solidification of the remaining liquid of the slurry plays an important role in determining the final mechanical properties [9]. When the Sc containing aluminum alloy melt is poured into the crucible and reaches the temperature near the liquidus point, Al3Sc particles will be formed throughout the entire volume of the
Fig. 1. Quenched microstructures of semisolid slurry of wrought aluminum alloys with different Sc levels: (a) 0.1%Sc, (b) 0.22%Sc, (c) 0.34%Sc and (d) 0.45%Sc.
24
J. Zhao et al. / Materials Letters 173 (2016) 22–25
Fig. 3. Microstructures of the rheo-diecasting samples with different levels of Sc: (a) 0.1%Sc, (b) 0.22%Sc, (c) 0.34%Sc and (d) 0.45%Sc.
melt. As Al3Sc particles are excellent heterogeneous nucleants for α-Al grains owing to their good match of lattice type and lattice constant [10,11], these effective nucleation sites lead to the formation of primary α-Al grains. Furthermore, the grain growth with Sc additions is restricted as a result of constitutional supercooling, where the growing primary α-Al particles reject Sc into the solid– liquid interface as they grow. As the concentration of Sc increases at the interface, it may reach the level where new Al3Sc nucleants could form. The presence of Al3Sc particles encourages the formation of new α-Al nuclei within the interface layer. Such mechanism ensures the formation of fine and non-dendriticα-Al particles. Therefore, finer and rounder α-Al particles can be obtained with more amount of Sc. On the other hand, with increasing Sc amount, it is more likely to form agglomeration of the Al3Sc particles because of smaller spacing between each other and segregation originated from solute redistribution during solidification, which degrades the potency of Al3Sc particles acting as nucleation sites. This may account for the small variation in particle size and shape factor as the 0.22% level of Sc is surpassed. The finer and globular microstructure of the rheo-diecasting samples with Sc additions could be attributed to homogeneous dispersion of numerous Al3Sc particles in the remaining liquid as a result of intensive shearing of the slurry when passing through the narrow gate during its mould filling. The mechanical properties of rheo-diecasting samples with different levels of Sc are shown in Table 1. It can be known that the addition of only 0.11%Sc caused a sharp rise in the tensile strength and elongation of the rheo-diecasting samples, which was increased by 32.6% and 44% respectively compared to those of Scfree alloy. Higher Sc content resulted in both higher tensile strength and elongation of the samples, but the former increased
Table 1 Mechanical properties of the samples with different levels of Sc. Sc content (wt%)
0 0.11 0.22 0.34 0.45
Tensile strength(MPa)
Elongation(%)
As-cast
T6
As-cast
T6
181 240 292 310 311
303 407 502 543 552
2.5 3.6 4.8 5.7 6.1
2.1 3.0 4.1 4.6 5.2
slightly whereas the latter still increased significantly when the Sc reached 0.22%. However, higher concentration of Sc seems unable to obtain further marked increase in both tensile strength and elongation. After T6 heat treatment, the tensile strength of all the studied alloys was increased by from 67.4% to 77.5% while the elongation decreased by 14–17% as the amount of Sc increased from 0 to 0.45%. It is noted that there are some pores in the microstructure, which may account for the relatively low mechanical properties of rheo-diecasting samples with respect to those processed by plastic forming [12,13]. For the same cause, the mechanical properties of samples could not be improved notably with over 0.45% Sc. Higher mechanical properties of rheo-diecasting samples with Sc additions mainly benefit from the finer and globular microstructure. The significant increase of elongation for samples with Sc levels above 0.22%Sc might be mainly attributed to the large quantities of fine α-Al globules formed during the solidification of the remaining liquid. The precipitation hardening of Al3Sc particles results in the great increase of the tensile strength of samples after T6 heat treatment.
J. Zhao et al. / Materials Letters 173 (2016) 22–25
4. Conclusion Good semisolid slurry of wrought Al alloy could be obtained with the addition of 0.22%Sc, and the average diameter and shape coefficient of primary α-Al particles were 72 mm and 0.58 respectively. Sc additions not only influenced the morphology of the primary α-Al in the slurry, but also promoted the formation of fine α-Al globules in the solidification process of the remaining liquid, which contributed to the considerable improvement in tensile strength and elongation. Samples with the level of 0.45%Sc had the maximum tensile strength and elongation after T6 heat treatment, which were increased by 82% and 147% respectively compared to the die-casting samples of the base alloy.
25
State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (P2015-10), China.
References [1] [2] [3] [4] [5] [6] [7] [8]
Acknowledgements
[9] [10] [11]
This research was financially supported by the National Key Technology Support Program of China (2015BAG12B01) and the
[12] [13]
M. Nakai, T. Eto, Mater. Sci. Eng. A 285 (2000) 62–68. G. Xie, D.J. Thompson, C.J.C. Jones, J. Sound Vib. 293 (2007) 921–932. U.A. Curle, Trans. Nonferrous Met. Soc. 20 (2010) 1719–1724. N. Mahathaninwong, T. Plookphol, J. Wannasin, S. Wisutmethangoon, Mater. Sci. Eng. A (2012) 91–99. S. Lü, S. Wu, C. Lin, Z. Hu, P. An, Mater. Sci. Eng. A 528 (2011) 8635–8640. E.L. Huskins, B. Cao, K.T. Ramesh, Mater. Sci. Eng. A 527 (2010) 1292–1298. G. Du, J. Deng, Y. Wang, D. Yan, L. Rong, Scr. Mater. 61 (2009) 532–535. M.F. Kilicaslan, W.R. Lee, T.H. Lee, Y. Sohn, S.J. Hong, Mater. Lett. 71 (2012) 164–167. M. Hitchcock, Y. Wang, Z. Fan, Acta Mater. 55 (2007) 1589–1598. K. Yu, W. Li, S. Li, J. Zhao, Mater. Sci. Eng. A 368 (2004) 88–93. O.N. Senkov, S.V. Senkova, M.R. Shagiev, Met. Mater. Trans. A. 39 (2008) 1034–1053. R.C. Dorward, D.J. Beerntsen, Met. Mater. Trans. A. 26 (1995) 2481–2484. E. Salamci, Turk. J. Eng. Environ. Sci. 26 (2002) 345–352.