Evolution of Microstructure and Texture in Mg-Al-Zn Alloys ...
Microstructure evolution of semi-solid Mg Si/ A356 ...
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384 CHINA FOUNDRY Vol. 17 No. 5 September 2020 Research & Development *Yong Hu 1,2 , Sheng-qi Fu 1,2 , Long-zhi Zhao 1,2 , Da-hao Wang 1,2 , Fei Liu 1,2 1. School of Materials Science and Engineering, East China Jiaotong Uinversity, Nanchang 330013, China 2. Key Laboratory of Advanced Materials for Vehicles & Laser Additive Manufacturing of Nanchang City, East China Jiaotong University, Nanchang 330013, China A luminum matrix composites have wide application prospects in automotive and aerospace industries because of their high specific strength, specific modulus, as well as high wear resistance and excellent thermal stability [1-3] . However, the ductility of aluminum matrix composites is low, and the plastic deformation capacity is poor, which limits the application of aluminum matrix composites. Semi-solid metal forming method invented by Flemings in the 1970s has been rapidly developed [4-6] . It is a method taking advantage of the thixotropic behavior of alloys in the two-phase field between solidus and liquidus. The deformation resistance is low during semi-solid metal forming, which is suitable for forming aluminum matrix composites. Compared to traditional casting or forging, semi-solid metal forming has many advantages, such as extension of service life for forming die, capability of producing complex structural components, high dimensional accuracy, low production costs, and so on [7] . The above advantages derive from the non-dendritic structures of semi-solid metal. As an important semi-solid Abstract: The semi-solid processed Mg 2 Si/A356 composites were fabricated using a sloping plate, and the phase and morphology evolution of the semi-solid Mg 2 Si/A356 slurry during the remelting process was investigated. Results indicate that compared to as-cast microstructure, the size of primary α-Al phase and Mg 2 Si phase of semi-solid microstructure fabricated by using a sloping plate decreases and the morphology of α-Al phase becomes fine and globular. With increasing the reheating temperature and prolonging the holding time, the primary α-Al phase spheroidizes, the liquid fraction in semi-solid microstructure increases, and the Chinese script Mg 2 Si phase embedded in the primary α-Al phase cannot be observed. The Chinese script Mg 2 Si phase is distributed in the secondary α-Al phase and becomes smaller. The net-shaped eutectic phase also becomes finer after reheating. The optimum remelting parameters suitable for thixoforming in this study are remelting at 580 °C for 30 min. Key words: semi-solid; Mg 2 Si; aluminum matrix composites; remelting; microstructure CLC numbers: TG146.21 Document code: A Article ID: 1672-6421(2020)05-384-05 *Yong Hu Male, born in 1982, Ph. D., Associate Professor. His research interests mainly focus on light-weight metal materials and the semi-solid process. To date, he has published about 30 papers. E-mail: [email protected] Received: 2019-12-23; Accepted: 2020-05-25 https://doi.org/10.1007/s41230-020-9158-7 Microstructure evolution of semi-solid Mg 2 Si/ A356 composites during remelting process metal forming method, thixoforming has attracted much attention due to its technical and economic advantages. Three steps are involved in the process: preparing semi-solid billets, remelting the billets to solid-liquid temperature range, and thixoforming the final near-net shaped products [8-10] . Remelting is a key step for the thixoforming process because the reheating temperature and holding time have a great impact on the globular microstructure. A higher reheating temperature or too long holding time may result in the decrease of the sphericity of the globular microstructure. Therefore, the aim of this study is to investigate the effect of reheating temperature and holding time during the remelting process on the semi-solid microstructure evolution of Mg 2 Si/A356 slurry. Moreover, the mechanism of semi-solid microstructure evolution was discussed in detail. 1 Experimental procedure Commercial A356 aluminum alloy, pure Mg and pure Si were used in this experiment. The chemical compositions (wt.%) of A356 alloy are Si 7.1, Mg 0.35, Fe 0.15, Ti 0.1, Cu 0.05, Zn 0.01, and Al the balance. Ratio of pure Mg to pure Si is corresponded to that of stoichiometric Mg 2 Si. The content of Mg 2 Si (wt.%) in the composite is 10%. The A356 aluminum alloy was melted in a resistance furnace. The pure Mg and
Transcript of Microstructure evolution of semi-solid Mg Si/ A356 ...
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CHINA FOUNDRY Vol. 17 No. 5 September 2020Research & Development
*Yong Hu 1,2, Sheng-qi Fu 1,2, Long-zhi Zhao 1,2, Da-hao Wang 1,2, Fei Liu 1,2
1. School of Materials Science and Engineering, East China Jiaotong Uinversity, Nanchang 330013, China2. Key Laboratory of Advanced Materials for Vehicles & Laser Additive Manufacturing of Nanchang City, East China Jiaotong University,
Nanchang 330013, China
Aluminum matrix composites have wide application prospects in automotive and aerospace industries
because of their high specific strength, specific modulus, as well as high wear resistance and excellent thermal stability [1-3]. However, the ductility of aluminum matrix composites is low, and the plastic deformation capacity is poor, which limits the application of aluminum matrix composites.
Semi-solid metal forming method invented by Flemings in the 1970s has been rapidly developed [4-6]. It is a method taking advantage of the thixotropic behavior of alloys in the two-phase field between solidus and liquidus. The deformation resistance is low during semi-solid metal forming, which is suitable for forming aluminum matrix composites. Compared to traditional casting or forging, semi-solid metal forming has many advantages, such as extension of service life for forming die, capability of producing complex structural components, high dimensional accuracy, low production costs, and so on [7]. The above advantages derive from the non-dendritic structures of semi-solid metal. As an important semi-solid
Abstract: The semi-solid processed Mg2Si/A356 composites were fabricated using a sloping plate, and the phase and morphology evolution of the semi-solid Mg2Si/A356 slurry during the remelting process was investigated. Results indicate that compared to as-cast microstructure, the size of primary α-Al phase and Mg2Si phase of semi-solid microstructure fabricated by using a sloping plate decreases and the morphology of α-Al phase becomes fine and globular. With increasing the reheating temperature and prolonging the holding time, the primary α-Al phase spheroidizes, the liquid fraction in semi-solid microstructure increases, and the Chinese script Mg2Si phase embedded in the primary α-Al phase cannot be observed. The Chinese script Mg2Si phase is distributed in the secondary α-Al phase and becomes smaller. The net-shaped eutectic phase also becomes finer after reheating. The optimum remelting parameters suitable for thixoforming in this study are remelting at 580 °C for 30 min.
Key words: semi-solid; Mg2Si; aluminum matrix composites; remelting; microstructure
CLC numbers: TG146.21 Document code: A Article ID: 1672-6421(2020)05-384-05
*Yong HuMale, born in 1982, Ph. D., Associate Professor. His research interests mainly focus on light-weight metal materials and the semi-solid process. To date, he has published about 30 papers.
E-mail: [email protected]: 2019-12-23; Accepted: 2020-05-25
https://doi.org/10.1007/s41230-020-9158-7
Microstructure evolution of semi-solid Mg2Si/A356 composites during remelting process
metal forming method, thixoforming has attracted much attention due to its technical and economic advantages. Three steps are involved in the process: preparing semi-solid billets, remelting the billets to solid-liquid temperature range, and thixoforming the final near-net shaped products [8-10]. Remelting is a key step for the thixoforming process because the reheating temperature and holding time have a great impact on the globular microstructure. A higher reheating temperature or too long holding time may result in the decrease of the sphericity of the globular microstructure.
Therefore, the aim of this study is to investigate the effect of reheating temperature and holding time during the remelting process on the semi-solid microstructure evolution of Mg2Si/A356 slurry. Moreover, the mechanism of semi-solid microstructure evolution was discussed in detail.
1 Experimental procedureCommercial A356 aluminum alloy, pure Mg and pure Si were used in this experiment. The chemical compositions (wt.%) of A356 alloy are Si 7.1, Mg 0.35, Fe 0.15, Ti 0.1, Cu 0.05, Zn 0.01, and Al the balance. Ratio of pure Mg to pure Si is corresponded to that of stoichiometric Mg2Si. The content of Mg2Si (wt.%) in the composite is 10%. The A356 aluminum alloy was melted in a resistance furnace. The pure Mg and
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Fig. 1: Microstructures of Mg2Si/A356 composites: (a) as-cast; (b) semi-solid fabricated by using of a sloping plate
pure Si were added into the melt at 800 °C and held for 15 min. The melt was cleaned and degassed using CCl6 flux for 5 min to eliminate H2 and oxides in the melt. When the temperature was decreased to 640 °C, the melt was poured using a sloping plate device to obtain semi-solid slurry. For comparison, the composites were also prepared without a sloping plate. The melt was poured into a copper mold to form an ingot with dimensions of Φ25 mm × 100 mm.
A box-type resistance furnace was used for reheating. The temperature fluctuation range of the box-type resistance furnace was ±2 °C. The samples were held at different temperatures (570, 580, and 590 °C) for 30 min, and at 580 °C for different times (20, 30, 40 and 50 min), respectively, and then water quenched at room temperature. Metallographic samples were polished through a standard procedure and etched with 0.5% HF-alcohol solution, and then their microstructures were examined
using an optical microscope (OM).
2 Results2.1 As-cast and semi-solid microstructureFigure 1(a) shows the as-cast microstructure of Mg2Si/A356 composites. It can be found that the microstructure consists of the coarse dendritic α-Al phase, Chinese script Mg2Si phase and fine eutectic phase. The Chinese script Mg2Si phase is embedded in the α-Al phase. The semi-solid microstructure of Mg2Si/A356 composites fabricated using a sloping plate is shown in Fig. 1(b). It can be seen that the coarse dendritic α-Al phase turns into fine and nodular, and the average size of the α-Al phase is decreased. It can be observed that the size of the Chinese script Mg2Si phase is also decreased.
2.2 Phase and morphology evolution during remelting process
2.2.1 Effect of reheating temperature on microstructure
Figure 2 shows the microstructures of semi-solid Mg2Si/A356 composites reheated at different temperatures and held for 30 min. It can be seen that, after reheating in semi-solid state, the size of primary α-Al phase is increased, the sphericity of globular primary α-Al phase increases at first and then decreases. When the reheating temperature is 570 °C, as shown in Fig. 2(a), the volume fraction of liquid phase is low, and the polygonal primary α-Al phase is separated by the linear liquid films and liquid molten pools. In contrast to the as-cast or semi-solid microstructure in Fig. 1, the volume fraction of the Chinese script Mg2Si phase embedded in the primary α-Al phase is decreased. When the reheating temperature is increased to 580 °C, the volume fraction of liquid phase is increased and the primary α-Al phase becomes globular, as shown in Fig. 2(b). The Chinese script Mg2Si phase embedded in primary α-Al phase cannot be observed. As shown in Fig. 2(c), the primary α-Al phase turns into rose-like when the reheating temperature is 590 °C, close to the liquidus temperature, thus the solidification
rate is close to that of the as-cast alloy. The Chinese script Mg2Si phase is distributed around the rose-like α-Al phase. Therefore, the reheating temperature suitable for thixoforming is 580 °C.
2.2.2 Effect of holding time on microstructureFigure 3 shows the microstructures of semi-solid Mg2Si/A356 composites reheated at 580 °C and held for different times. It can be found that, when the holding time is 20 min, the morphology of the primary α-Al phase turns into rose-like or near globular, the Chinese script Mg2Si phase is still embedded in the primary α-Al phase, and the primary α-Al phase is separated by liquid phase, as shown in Fig. 3(a). As shown in Fig. 3(b), when the holding time is 30 min, the volume fraction of the liquid phase is increased, and the globular primary α-Al phase is distributed uniformly in the liquid phase. When the holding time is 40 min, the sphericity of the primary α-Al phase is decreased, and the size of the primary α-Al phase is increased, as shown in Fig. 3(c). On further increasing the holding time to 50 min, the size and morphology of the primary α-Al phase almost have no change comparing to those holding for 40 min, as shown in Fig. 3(d). Therefore, the holding time suitable for thixoforming is 30 min.
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Fig. 2: Microstructures of semi-solid Mg2Si/A356 composites at different reheating temperatures held for 30 min: (a) 570 °C; (b) 580 °C; (c) 590 °C
Fig. 3: Microstructures of semi-solid Mg2Si/A356 composites reheating at 580 °C for different holding times: (a) 20 min; (b) 30 min; (c) 40 min; (d) 50 min
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Fig. 4: Microstructures of Mg2Si/A356 composites: (a) as-cast; (b) semi-solid before reheating; (c) reheating at 580 °C for 30 min
2.3 Characterization of eutectic phase and Mg2Si particles
The eutectic phase of semi-solid Mg2Si/A356 composites after reheating is much finer, as shown in Fig. 4(c), compared to the as-cast composites, Fig. 4(a) and the semi-solid composites before reheating, Fig. 4(b). The Chinese script Mg2Si phase of semi-solid Mg2Si/A356 composites after reheating is embedded in the secondary α-Al phase formed in the second solidification. The secondary α-Al phase is finer than the primary α-Al phase, which solidified again when quenched in water. The size and morphology of eutectic phase and second α-Al phase of Mg2Si/A356 composites depend on the degree of under-cooling and solidification rate. The degree of under-cooling and solidification rate of semi-solid Mg2Si/A356 composites after reheating are much higher than those of both as-cast and semi-solid alloy
before reheating due to the water quenching. Therefore, the sizes of eutectic phase and secondary α-Al phase of Mg2Si/A356 composites after reheating are smaller.
It also can be found from Fig. 4 that the size of Chinese script Mg2Si phase after reheating is smaller than those of both as-cast and before reheating. The melting temperature of Mg2Si is 1,087 °C, so it is impossible to melt into a smaller size during the reheating process because the remelting temperature is only 580 °C. According to the Al-Mg2Si pseudo binary phase diagram [11], the eutectic Al+Mg2Si phase is formed in Mg2Si/A356 composites. The eutectic Al+Mg2Si phase is remelted during the reheating process, and then water quenched at room temperature. So, its solidification rate is higher than those of both as-cast and before reheating, which results in the formation of the finer Mg2Si.
3 Spheroidization mechanism of semi-solid microstructure during remelting process
The spherodization of semi-solid microstructure is a significant stage during the remelting process. It is well known that the curvature of solid particles can obviously influence its melting point. The larger the curvature, the lower the melting point. The edges and corners of solid particles, where the curvatures are larger, will lead to a decrease in melting point. Therefore,
the edges and corners of the solid particles will be melted during the remelting process, and then the morphology of the particles will gradually become spherical or near-spherical.
Furthermore, according to the Gibbs-Thompson formula [12], the larger the curvature (or the smaller the curvature radius), the higher the atom concentration. Therefore, there is a gradient of atom concentration among the different positions of the primary α-Al particles because they have an irregular morphology. The atoms will diffuse from the higher concentration to the lower concentration during remelting process, which breaks the balance of atom concentration. So the primary α-Al particles with
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larger curvature will dissolve to maintain the balance of atom concentration. As a result, the spherical or near-spherical primary α-Al particles are obtained.
Meanwhile, the coarsening of the primary α-Al phase, which is determined by the mechanisms of Ostwald ripening, was observed with prolonged holding time. The essence of Ostwald ripening is the dissolution and disappearance of small-sized particles and the growth of large-sized particles. The growth of the particles can be described by the classical Lifshitz-Slyozov-Wagner (LSW) Eq. (1) [13]:
dt3- d0
3 = kt
where t is the holding time, s; dt is the average size of solid particles at time t, µm; d0 is the average size of the initial particles, µm; and k is the coarsening rate constant, μm3·s-1.
According to Eq. (1), the solid particles will grow into larger ones with prolonged holding time. Therefore, it is necessary to adjust the holding time reasonably to control the size of the primary phase.
4 Conclusions(1) Compared to the microstructure of as-cast Mg2Si/A356
composites, the primary α-Al phase of semi-solid Mg2Si/A356 composites fabricated using a sloping plate is finer and more globular. The size of Chinese script Mg2Si phase is also decreased obviously.
(2) With the increase of the reheating temperature and prolonging of holding time during the remelting process, the sphericity of globular primary α-Al phase increases at first and then gradually decreases, the optimum remelting parameters suitable for thixoforming are 580 °C for 30 min.
(3) The Chinese script Mg2Si phase of semi-solid Mg2Si/A356 composites after reheating is embedded in the secondary α-Al phase formed in the secondary solidification and the grain of the Chinese script Mg2Si phase is more refined compared to the as-cast and semi-solid Mg2Si/A356 composites before reheating.
(4) The curvature of the solid particles can influence their morphology. The larger the curvature, the easier it is to form spherical or near-spherical primary α-Al particles. Ostwald ripening is observed with prolonged holding time.
AcknowledgementsThis work was financially supported by the National Natural Science Foundation of China (Grant No. 51865011) and the Natural Science Foundation of Jiangxi Province, China (Grant No. 20171BAB216031).
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