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ZA27 composites
Materials Science and Engineering A 394 (2005) 425–434
Centrifugally cast Zn–27Al–xMg–ySi alloys and their in situ (Mg2Si + Si)/ZA27 composites Wang Qudong∗ , Chen Yongjun, Chen Wenzhou, Wei Yinhong, Zhai Chunquan, Ding Wenjiang State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200030, PR China Received 9 June 2004; accepted 22 November 2004
Abstract Effects of composition, mold temperature, rotating rate and modification on microstructure of centrifugally cast Zn–27Al–xMg–ySi alloys have been investigated. In situ composites of Zn–27Al–6.3Mg–3.7Si and Zn–27Al–9.8Mg–5.2Si alloys were fabricated by centrifugal casting using heated permanent mold. These composites consist of three layers: inner layer segregates lots of blocky primary Mg2 Si and a litter blocky primary Si, middle layer contains without primary Mg2 Si and primary Si, outer layer contains primary Mg2 Si and primary Si. The position, quantity and distribution of primary Mg2 Si and primary Si in the composites are determined jointly by alloy composition, solidification velocity under the effect of centrifugal force and their floating velocity inward. Na salt modifier can refine grain and primary Mg2 Si and make primary Mg2 Si distribute more evenly and make primary Si nodular. For centrifugally cast Zn–27Al–3.2Mg–1.8Si alloy, the microstructures of inner layer, middle layer and outer layer are almost similar, single layer materials without primary Mg2 Si and primary Si are obtained, and their grain sizes increased with the mold temperature increasing. © 2004 Elsevier B.V. All rights reserved. Keywords: Centrifugal casting; Zn–27Al–Mg–Si alloys; Mg2 Si; Si; In situ composites
1. Introduction Because of the excellent wear resistance, ZA27 alloys have successfully substituted some copper alloys used to fabricate industry parts with high wear resistance, but their wear resistance under the condition of high velocity and bigloading need to be improved further [1,2]. It was found that the wear resistance of Zn–Al based alloy can be improved by adding Si into it to form hard phase Si in its microstructure [3,4]. On the other hand, Mg2 Si is very suitable to be used as reinforcement of metal matrix composites for its excellent prosperities such as low density, high melting point, high hardness, low heat expanding coefficient and suitable elastic modulus [5]. In recent years, in situ composites of aluminum and magnesium matrix containing Mg2 Si have been fabricated [6,7]. Although primary Si and primary Mg2 Si ∗
Corresponding author. Tel.: +86 21 62933139; fax: +86 21 62932113. E-mail address: [email protected] (W. Qudong).
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can improve the wear resistance of composites, they will decrease the whole strength and ductility due to the existence of hard and fragile primary Mg2 Si and Si [8,9]. In situ surface composites in which primary Si enrich in both inner and outer layer have been fabricated by centrifugal casting hypereutectic Al–Si alloys [10,11]. Zhangjian et al. have fabricated in situ composites where primary Mg2 Si enriched in surface by centrifugal casting Al–Mg–Si alloys [12]. Therefore, centrifugal cast is a promising technology to fabricate composites which can improve the surface wear resistance and other special properties of the surface of composites but not decrease their whole strength and plasticity. Through centrifugal casting Zn–Al–Si alloy, in situ surface composites have been obtained, whose inner layer contains lots of primary Si, outer layer contains a little primary Si and the middle layer consists of fine eutectic microstructure. Both inner and outer layers of the composites have excellent wear resistance [13], and the harm of fragile Si phase to ductility is reduced [14]. In situ composites have been fabri-
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an outside diameter of 88 mm and thickness of 12 mm was obtained. Experimental conditions are shown in Table 1. The microstructure of the tube specimen was examined with an optical microscope. Thermal analyzer (Universal V2.4F) was used to record the temperature difference curve and temperature curve during heating and cooling for Zn–27Al–xMg–ySi alloys, and the heating rate was 10 ◦ C/min, XRD (X-ray diffraction) result was obtained by D/max-2cx X-ray tester.
3. Results and discussion Fig. 1. Schematic diagram of vertical centrifugal casting apparatus: (1) thermocouple, (2) pouring cup, (3) heating furnace (4) metal mould, (5) temperature controller, (6) motor, (7) inverter.
cated in which primary Mg2 Si and Si enriched in inner and outer layers by centrifugal casting Zn–27Al–6.3Mg–3.7Si alloy [15]. In this paper, the influences of composition, rotating rate, mould temperature on the microstructure of centrifugally cast Zn–27Al–xMg–ySi alloys are systematically investigated, and the solidification process of Zn–27Al–xMg–ySi alloys and the forming process of in situ composites during centrifugal casting are analyzed.
2. Experimental procedure The alloys were prepared with commercially pure Zn, Al, Mg and Si in an electronic-resistance furnace. The specimens were produced in a vertical centrifugal casting apparatus, as shown in Fig. 1. The metal mold was preheated at different temperature before pouring, and rotated at different rate. The pouring temperature was controlled at 973 K. The heating furnace was shut down immediately after pouring and the specimen was cooled in the furnace. A tube specimen with Table 1 Experimental condition and some results
3.1. Microstructure and effects of technologic parameters 3.1.1. Effects of composition Figs. 2–4 show microstructures of centrifugally cast Zn–27Al–3.2Mg–1.8Si, Zn–27Al–6.3Mg–3.7Si and Zn–27Al–9.8Mg–5.2Si alloys respectively at mould temperature of 200 ◦ C and at rotating rate of 1000 rpm. For the in situ composites of Zn–27Al–6.3Mg–3.7Si alloy (Fig. 2), both inner and outer layer contain primary Mg2 Si and primary Si and middle layer contains eutectic microstructure. It can be observed from Fig. 2 (a) that a great deal of primary Mg2 Si with average size of 30–50 m (blocky phases with deep grayness) segregated and enriched in the inner layer of composites; meanwhile, there exists a little blocky primary Si with average size of 20–40 m (blocky phases with light grayness); the thickness of whole inner layer was about 5 mm. Fig. 2(b) shows the microstructure of transitional layer, where the microstructure transited from inner layer containing lots of blocky primary Mg2 Si and primary Si to middle layer containing fine Mg2 Si and Si. Mg2 Si and Si in middle layer (Fig. 2(c)) show shapes of thin stripes and dots, exhibiting the characteristics of eutectic microstructure. The outer layer (Fig. 2(d)), whose thickness is about 0.4 mm, contains a lit-
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Fig. 2. Microstructure of Zn–27Al–6.3Mg–3.7Si alloy (Tm = 200 ◦ C, r = 1000 rpm): (a) inner layer, (b) transitional layer, (c) middle layer, (d) outer layer.
Fig. 3. Microstructure of Zn–27Al–3.2Mg–1.8Si alloy (Tm = 200 ◦ C, r = 1000 rpm) (a) inner layer, (b) middle layer, (c) outer layer.
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Fig. 4. Microstructure of Zn–27Al–9.8Mg–5.2Si alloy (Tm = 200 ◦ C, r = 1000 rpm): (a) inner layer, (b) transitional layer, (c) middle layer, (d) outer layer.
tle small blocky primary Si and primary Mg2 Si, where the primary Mg2 Si connects with each other and distributes as long bunches. Compared with that in inner layer, the primary Mg2 Si and primary Si in outer layer are finer and fewer and the distribution is more uneven. The microstructures of inner layer (Fig. 3(a)), middle layer (Fig. 3(b)) and outer layer (Fig. 3(c) in Zn–27Al–3.2Mg–1.8Si alloy are almost similar. Blocky primary Mg2 Si and primary Si phase were not found in these microstructures. All the microstructures are made up of ␣ (Al) dentrites, white strip phase (verified as MgZn2 by EDAX) and eutectic microstructure around grain boundaries. In other word, the as-cast tube specimen of Zn–27Al–3.2Mg–1.8Si alloy is a single layer material. The center of the dentrites appears white with lower Zn content, while the edge of the dentrites is gray because of higher Zn content. Grain size becomes smaller from inner layer to outer layer. For Zn–27Al–9.8Mg–5.2Si alloy (Fig. 4), the in situ composites are obtained. Both inner and outer layer contain primary Mg2 Si and primary Si and there are no primary Mg2 Si and primary Si in the middle layer. It can be observed from Fig. 4(a) that the inner layer with whole thickness of 6 mm contains a great deal of blocky primary Mg2 Si with size of 5–80 m and a little blocky primary Si with size of 20–30 m. The middle layer with thickness of 4 mm contains eutectic Mg2 Si and eutectic Si (Fig. 4(c). The outer layer with thickness of 2 mm contains lots of small primary Mg2 Si with size of 10–20 m and a little small blocky primary Si with size of 5–10 m, where most of primary Mg2 Si distributes as long
bunches (Fig. 4(d). Compared with Zn–27Al–6.3Mg–3.7Si alloy, the amount of primary Mg2 Si in both inner and outer layer of in situ Zn–27Al–9.8Mg–5.2Si composites increases obviously, but primary Si reduces a little. 3.1.2. Effects of mold temperature Similar to the condition at mould temperature of 200 ◦ C and at rotating rate of 1000 rpm, there are no primary Mg2 Si and primary Si phase in microstructures of Zn–27Al–3.2Mg–1.8Si alloy when changing the mould temperatures of 100 and 300 ◦ C respectively. At all tested temperatures the as-cast tube specimens are single layer microstructure consisted of ␣ (Al) dentrite, MgZn2 phase and eutectic microstructure, as can be seen in Fig. 3. It is found that with mould temperature increasing, grain size of inner layer, middle layer and outer layer increases apparently, especially in outer layer. 3.1.3. Effects of rotating rate Figs. 5 and 6 show the microstructure of centrifugal cast Zn–27Al–6.3Mg–3.7Si alloy at mould temperature of 200 ◦ C and at different rotating rates of 500 rpm, 1500 rpm respectively. The results show that in situ composites whose inner and outer layer both contain blocky primary Mg2 Si and blocky primary Si and middle layer contains eutectic microstructure can be obtained at all tested rotating rates. Compared with Fig. 2, with the rotating rate increasing, thickness of inner layer increases but thickness of outer layer decreases (as showed in Table 1), the size of primary Mg2 Si and pri-
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Fig. 5. Microstructure of Zn–27Al–6.3Mg–3.7Si alloy (Tm = 200 ◦ C, n = 500 rpm): (a) inner layer, (b) transitional layer, (c) middle layer, (d) outer layer.
Fig. 6. Microstructure of Zn–27Al–6.3Mg–3.7Si alloy (Tm = 200 ◦ C, n = 1500 rpm): (a) inner layer, (b) transitional layer, (c) middle layer, (d) outer layer.
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mary Si in inner and outer layer reduces. At the same time, the amount of primary Mg2 Si and primary Si in inner layer increases, but decreases in outer layer. Under the effect of centrifugal force, the movement of primary Mg2 Si and primary Si in molten alloy depends mainly on two forces: radical push force caused by centrifugal force and viscous resistance of molten metal. The moving velocity of primary Mg2 Si and primary Si under the action of centrifugal force can be described as the follows according to Stockes equation: υ=
2(ρs − ρl )R2p ω2 r 9η
primary Si become much thicker. Because the solidification velocity in outer layer is higher than the floating inward velocity of primary Mg2 Si and primary Si there, primary Mg2 Si and primary Si will be captured and stay in outer layer [15]. Consequently, with rotating rate increasing, the thickness of outer layer reduces. Besides, with rotating rate increasing, primary Mg2 Si and primary Si are enforced by much stronger centrifugal force, and are much easier to collide with each other and break up, which results in smaller size of primary Mg2 Si and primary Si in inner and outer layer.
(1)
where ρs is the density of solid grain (ref. [7] reported that ρsi = 2330 kg/m3 , ρMg2 Si = 1990 kg/m3 ), ρl is the density of molten metal (the density of ZA27 alloy is about 4900 kg/m3 [16]), ω is the rotating rate, r is distance between solid grain and rotating center, η is dynamical viscosity of molten metal, Rp is radius of solid particle, υ is moving velocity of solid particle. Based on Eq. (1), primary Mg2 Si and primary Si, whose density is much lower than ZA27 alloy, move inward under the action of centrifugal force after precipitated from molten alloy. Given other conditions unchanged, higher centrifugal rotating rate responds to higher floating inward velocity and longer moving distance. Therefore, with the rotating rate increasing, the floating inward velocity of primary Mg2 Si and primary Si become faster and moving distance become longer. As a result, inner layer containing primary Mg2 Si and
3.1.4. Effects of modification Fig. 7 shows the microstructure of centrifugal cast Zn–27Al–9.8Mg–5.2Si alloy modified by Na-salt at mould temperature of 200 ◦ C and at rotating rate of 1000 rpm. Compared with that in Fig. 4, grains are apparently refined in inner layer, middle layer and outer layer and the size of primary Mg2 Si in inner layer becomes smaller after adding Na-salt. The modification mechanism of Na-salt may be that the addition of Na-salt promotes the formation of fine Mg2 Si particles by providing the nucleation sites for Mg2 Si particles [17] and prevents the growth of primary Mg2 Si and primary Si. 3.1.5. Determination of phase and composition Fig. 8 shows X-ray diffraction in the inner and outer layer of centrifugally cast Zn–27Al–6.3Mg–3.7Si alloy at mould temperature of 200 ◦ C and Table 2 shows the results of phases analyzed by X-ray diffraction. Both inner and
Fig. 7. Microstructure of Zn–27Al–9.8Mg–5.2Si alloy + modifier (Tm = 200 ◦ C, n = 1000 rpm): (a) inner layer, (b) transitional layer, (c) middle layer, (d) outer layer.
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Table 2 Results of phase analyzed by X-ray diffraction Alloy
Position
phases
Zn–27Al–3.2Mg–1.8Si Inner layer Zn, Al, Mg2 Si, Si, MgZn2 , Mg2 Zn11 Outer layer Zn, Al,Mg2 Si, Si, MgZn2 , Mg2 Zn11 Zn–27Al–6.3Mg–3.7Si Inner layer Zn, Al, Mg2 Si, Si, MgZn2 Outer layer Zn, Al, Mg2 Si, Si, MgZn2 Zn–27Al–9.8Mg–5.2Si Inner layer Zn, Al, Mg2 Si, Si, MgZn2 Outer layer Zn, Al, Mg2 Si, Si, MgZn2
Fig. 8. X-ray diffractograms of Zn–27Al–6.3Mg–3.7Si alloy (a) inner layer (b) outer layer.
outer layer of Zn–27Al–3.2Mg–1.8Si alloy contain Zn, Al, Mg2 Si, Si, MgZn2 and Mg2 Zn11 . The inner and outer layer of Zn–27Al–6.3Mg–3.7Si alloy and Zn–27Al–9.8Mg–5.2Si alloy contain Zn, Al, Mg2 Si, Si and MgZn2 . With the content of Mg and Si increasing, the Mg–Zn compound transforms from the coexistence of MgZn2 and Mg2 Zn11 to single MgZn2 . Composition of each phase in the centrifugally cast Zn–27Al–3.2Mg–1.8Si alloy and Zn–7Al–6.3Mg–3.7Si alloy composites are further determined by SEM and EDAX.
Figs. 9 and 10 are SEM images of Zn–27Al–3.2Mg–1.8Si and Zn–27Al–6.3Mg–3.7Si alloys respectively. EDAX results of point A, B, C and D in Figs. 9 and 10 are shown in Table 3. In Fig. 9, the phase of point A distributing around grain boundaries is Mg–Zn compound (the reason that point A contain Al is that Al is solubilized into the phase or the effect of electron beam into the matrix). The atom ratio of Zn to Mg for the Mg–Zn phase is 3.7, which is between the atom ratio of Zn to Mg in MgZn2 and that in Mg2 Zn11 (MgZn2 and Mg2 Zn11 are determined by XRD, as shown in Table 2). The plate-like eutectic microstructure is the compound of Mg–Zn-␣(Al), which is formed in binary and ternary eutectic reaction. Deep gray blocky phase (point A in Fig. 10(a)) is determined as Mg2 Si, and light gray blocky phase (point B in Fig. 10(a)) can be determined as primary Si. Bright strip phase (point C in Fig. 10(a)) can be analyzed as MgZn2 phase by the atom ratio 2 of Zn to Mg (Table 3). The bright strip phase (point C in outer layer in Fig. 10(b)) is determined as MgZn2 and the deep gray strip phase (point A in Fig. 10(b)) is Mg2 Si. The light gray stick-like phase (point D in the outer layer in Fig. 10(c)) is determined as Si.
Fig. 9. SEMs of Zn–27Al–3.2Mg–1.8Si alloy.
Fig. 10. SEMs of Zn–27Al–6.3Mg–3.7Si alloy: (a) inner layer; (b) and (c) outer layer.
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Table 3 EDAX results of Zn–27Al–3.2Mg–1.8Si and Zn–27Al–6.3Mg–3.7Si alloys Element
Alloy Zn–27Al–3.2Mg–1.8Si
O Mg Al Si Zn
Zn–27Al–6.3Mg–3.7Si
Point A (at%)
Point B (at%)
Point C (at%)
Point D (at%)
Point A (at%)
Point B (at%)
Point C (at%)
Point D (at%)
– 18.46 12.63 – 68.91
– 1.89 19.49 71.37 7.24
– 52.47 3.51 28.38 15.63
– 14.16 29.93 0.60 55.31
15.87 41.47 10.97 25.48 6.22
– – – 100 –
– 36.54 – – 63.46
– – 8.52 87.31 4.17
3.2. Forming process of composites Little research result about the solidification process of Zn–Al–Mg–Si alloys has been reported. The forming process of composites is analyzed as follows. Firstly, the solidification process of Zn–27Al–xMg–ySi alloys is analyzed, based on the phase analysis results and the DTA curves (Figs. 11 and 12) of Zn–27Al–6.3Mg–3.7Si alloy and Zn–27Al–3.2Mg–1.8Si alloy and the ternary phase diagrams of Zn–Al–Si [18–19] and Zn–Al–Mg, the binary phase diagrams of Zn–Al and Zn–Mg. Secondly, the forming process of Zn–27Al–xMg–ySi composites during centrifugal casting is further investigated. It can be observed from Figs. 2 and 4 that Zn–27Al–6.3Mg–3.7Si alloy and Zn–27Al–9.8Mg–5.2Si alloy composites contain blocky Mg2 Si and blocky Si. The DTA curve of Zn–27Al–6.3Mg–3.7Si alloy (Fig. 11) indicates that following reactions would take place during the initial solidification stage of Zn–27Al–6.3Mg–3.7Si alloys: L → L + Mg2 Si and L → L + Si. The exothermal peaks at 648.87 ◦ C and 568.51 ◦ C in Fig. 11 correspond the precipitation of primary Mg2 Si and primary Si respectively. For the studied Zn–27Al–6.3Mg–3.7Si alloy, following reaction would happen during the temperature range from
400 ◦ C to 500 ◦ C according to the ternary phase diagram of Zn–Al–Mg [19]: L → L + ␣(Al). The exothermal peak at 444.67 ◦ C in Fig. 11 confirms this reaction. With temperature decreasing, Mg-Zn compound precipitates (Figs. 3–5), which corresponds with the exothermal peak at 359.09 ◦ C in the DTA curve (Fig. 6). Based on ternary phase diagram of Zn–Al–Si [19], the ternary eutectic reaction: L → ␣(Al) + Zn + Si will occur at 383 ◦ C when the contents of Si and Zn are 0.06% and 87% respectively. It can be conjectured that, when the content of the solidifying molten alloy reaches its eutectic composites, the ternary or quaternionic eutectic reaction: L → ␣(Al) + Zn + (Si) + (Mg2 Si) will take place during the final solidification stage of the quaternionic alloy Zn–27Al–6.3Mg–3.7Si, which corresponds with the exothermal peak at 325.95 ◦ C in DTA curves (Fig. 11). However, exothermal peaks of primary Mg2 Si and primary Si have not been detected in the DTA curve (Fig. 12) of Zn–27Al–3.2Mg–1.8Si alloy, which is consistent with the result that primary Mg2 Si and primary Si have not been detected in Zn–27Al–3.2Mg–1.8Si alloy. The DTA curves of Zn–27Al–3.2Mg–1.8Si alloy (Fig. 12) have gone through similar procedures as following: the reaction of L → L + ␣(Al) at 445.23 ◦ C, the precipitation of
Fig. 11. DTA curve of Zn–27Al–6.3Mg–3.7Si alloy.
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Fig. 12. DTA curve of Zn–27Al–3.2Mg–1.8Si alloy.
Mg–Zn compound at 335.06 ◦ C and the ternary or quaternionic eutectic reaction: L → ␣(Al) + Zn + (Si) + (Mg2 Si). For Zn–27Al–9.8Mg–5.2Si alloy, it can be inferred from its optical microstructure in Fig. 4 that its solidification process is similar with that of Zn–27Al–6.3Mg–3.7Si alloy, except that primary Mg2 Si precipitated more and primary Si precipitated much less. Based on the above analysis to the solidification process of Zn–27Al–xMg–ySi alloy, the forming process of the in situ composites of Zn–27Al–xMg–ySi alloy under the action of centrifugal force is further analyzed as follows. For Zn–27Al–6.3Mg–3.7Si alloy, when molten alloy is poured into the tube mould, the outer layer of composites solidifies firstly due to the rapid cooling effect of the metal mould wall, and a little primary Mg2 Si and primary Si precipitate. The density of each primary phases is much lower than that of the molten alloy, and the floating velocity of Mg2 Si and Si inward (VMg2Si , VSi ) is respectively slower than the solidification velocity (Rs ). As a result, some primary Mg2 Si and primary Si are captured and stay in outer layer. Because the cooling velocity in outer layer is very quick, the primary Mg2 Si and primary Si have no time to grow up and are very small. With the temperature of molten alloy decreasing, Mg2 Si and Si precipitate continually and move to the inner layer under centrifugal force. On the other hand, due to the cooling effect of the air in the tube mold, the inner layer of composites also begins to solidify at a lower velocity. Meanwhile, the floating direction of blocky primary Mg2 Si and blocky primary Si in the inner layer is opposite with the solidification direction. Therefore, lots of primary Mg2 Si and primary Si stay in inner layer. With solidification going on, the content of Mg and Zn in solidifying molten alloy decreases. Finally, the content of solidifying molten alloy reaches the eutectic composition;
therefore eutectic microstructure in the middle layer is obtained. For Zn–27Al–9.8Mg–5.2Si alloy composites, there are more primary Mg2 Si precipitated, therefore the inner and outer layers containing primary Mg2 Si and primary Si are much thicker than those of Zn–27Al–6.3Mg–3.7Si alloy composites. For Zn–27Al–3.2Mg–1.8Si alloy, because there are not primary Mg2 Si and primary Si precipitated during its solidification process. In situ composite, which contain primary Mg2 Si and Si in the inner and outer layers, can not be fabricated. It is demonstrated from the above analysis that composition, solidification velocity under the effect of centrifugal force and the floating velocity inward of primary Mg2 Si and primary Si determine the position, quantity and distribution of primary Mg2 Si and primary Si [15].
4. Conclusions 1. During the initial solidification stage of Zn–27Al–6.3Mg–3.7Si alloy and Zn–27Al–9.8Mg–5.2Si alloy, there are primary Mg2 Si and primary Si precipitates. However, there are no primary Mg2 Si and primary Si in Zn–27Al–3.2Mg–1.8Si alloy. 2. In situ composites of Zn–27Al–6.3Mg–3.7Si alloy and Zn–27Al–9.8Mg–5.2Si alloy have been fabricated by centrifugal casting using the heated permanent mold. This kind of composites is consisted of three layers, inner layer segregates lots of blocky primary Mg2 Si and a little blocky primary Si, middle layer contains without primary Mg2 Si and primary Si, outer layer contains some primary Mg2 Si and primary Si. The position, quantity and distribution of primary Mg2 Si and primary Si are determined by compo-
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4.
5.
6.
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sition, solidification velocity under the effect of centrifugal forces and floating velocity inward of primary Mg2 Si and primary Si. The inner and outer layer of Zn–27Al–9.8Mg–5.2Si alloy are respectively thicker than those of Zn–27Al–6.3Mg–3.7Si alloy and have more primary Mg2 Si. During the centrifugal casting of Zn–27Al–6.3Mg–3.7Si alloy, with rotating rate increasing, the thickness of the inner layer increases and the outer layer become thinner. At the same time, the size of primary Mg2 Si and primary Si in both inner and outer layer reduces. The amount of primary Mg2 Si and primary Si in inner layer increases, and the amount of that in outer layer decreases. The addition of Na-salt can refine grain and primary Mg2 Si, and make primary Mg2 Si distribute more evenly and make primary Si nodular. Only single layer materials without primary Mg2 Si and primary Si can be obtained by centrifugal cast Zn–27Al–3.2Mg–1.8Si alloy. At the same time, with the mould temperature increasing, grain sizes increase.
Acknowledgments The authors express their thanks to National Natural Science Foundation of China (Contract No. 59901007) and the Visiting Scholar Foundation of Key Lab in University for the financial support of this work.
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Centrifugally cast Zn–27Al–xMg–ySi alloys and their in situ (Mg2Si + Si)/ZA27 composites Wang Qudong∗ , Chen Yongjun, Chen Wenzhou, Wei Yinhong, Zhai Chunquan, Ding Wenjiang State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200030, PR China Received 9 June 2004; accepted 22 November 2004
Abstract Effects of composition, mold temperature, rotating rate and modification on microstructure of centrifugally cast Zn–27Al–xMg–ySi alloys have been investigated. In situ composites of Zn–27Al–6.3Mg–3.7Si and Zn–27Al–9.8Mg–5.2Si alloys were fabricated by centrifugal casting using heated permanent mold. These composites consist of three layers: inner layer segregates lots of blocky primary Mg2 Si and a litter blocky primary Si, middle layer contains without primary Mg2 Si and primary Si, outer layer contains primary Mg2 Si and primary Si. The position, quantity and distribution of primary Mg2 Si and primary Si in the composites are determined jointly by alloy composition, solidification velocity under the effect of centrifugal force and their floating velocity inward. Na salt modifier can refine grain and primary Mg2 Si and make primary Mg2 Si distribute more evenly and make primary Si nodular. For centrifugally cast Zn–27Al–3.2Mg–1.8Si alloy, the microstructures of inner layer, middle layer and outer layer are almost similar, single layer materials without primary Mg2 Si and primary Si are obtained, and their grain sizes increased with the mold temperature increasing. © 2004 Elsevier B.V. All rights reserved. Keywords: Centrifugal casting; Zn–27Al–Mg–Si alloys; Mg2 Si; Si; In situ composites
1. Introduction Because of the excellent wear resistance, ZA27 alloys have successfully substituted some copper alloys used to fabricate industry parts with high wear resistance, but their wear resistance under the condition of high velocity and bigloading need to be improved further [1,2]. It was found that the wear resistance of Zn–Al based alloy can be improved by adding Si into it to form hard phase Si in its microstructure [3,4]. On the other hand, Mg2 Si is very suitable to be used as reinforcement of metal matrix composites for its excellent prosperities such as low density, high melting point, high hardness, low heat expanding coefficient and suitable elastic modulus [5]. In recent years, in situ composites of aluminum and magnesium matrix containing Mg2 Si have been fabricated [6,7]. Although primary Si and primary Mg2 Si ∗
Corresponding author. Tel.: +86 21 62933139; fax: +86 21 62932113. E-mail address: [email protected] (W. Qudong).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.11.055
can improve the wear resistance of composites, they will decrease the whole strength and ductility due to the existence of hard and fragile primary Mg2 Si and Si [8,9]. In situ surface composites in which primary Si enrich in both inner and outer layer have been fabricated by centrifugal casting hypereutectic Al–Si alloys [10,11]. Zhangjian et al. have fabricated in situ composites where primary Mg2 Si enriched in surface by centrifugal casting Al–Mg–Si alloys [12]. Therefore, centrifugal cast is a promising technology to fabricate composites which can improve the surface wear resistance and other special properties of the surface of composites but not decrease their whole strength and plasticity. Through centrifugal casting Zn–Al–Si alloy, in situ surface composites have been obtained, whose inner layer contains lots of primary Si, outer layer contains a little primary Si and the middle layer consists of fine eutectic microstructure. Both inner and outer layers of the composites have excellent wear resistance [13], and the harm of fragile Si phase to ductility is reduced [14]. In situ composites have been fabri-
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an outside diameter of 88 mm and thickness of 12 mm was obtained. Experimental conditions are shown in Table 1. The microstructure of the tube specimen was examined with an optical microscope. Thermal analyzer (Universal V2.4F) was used to record the temperature difference curve and temperature curve during heating and cooling for Zn–27Al–xMg–ySi alloys, and the heating rate was 10 ◦ C/min, XRD (X-ray diffraction) result was obtained by D/max-2cx X-ray tester.
3. Results and discussion Fig. 1. Schematic diagram of vertical centrifugal casting apparatus: (1) thermocouple, (2) pouring cup, (3) heating furnace (4) metal mould, (5) temperature controller, (6) motor, (7) inverter.
cated in which primary Mg2 Si and Si enriched in inner and outer layers by centrifugal casting Zn–27Al–6.3Mg–3.7Si alloy [15]. In this paper, the influences of composition, rotating rate, mould temperature on the microstructure of centrifugally cast Zn–27Al–xMg–ySi alloys are systematically investigated, and the solidification process of Zn–27Al–xMg–ySi alloys and the forming process of in situ composites during centrifugal casting are analyzed.
2. Experimental procedure The alloys were prepared with commercially pure Zn, Al, Mg and Si in an electronic-resistance furnace. The specimens were produced in a vertical centrifugal casting apparatus, as shown in Fig. 1. The metal mold was preheated at different temperature before pouring, and rotated at different rate. The pouring temperature was controlled at 973 K. The heating furnace was shut down immediately after pouring and the specimen was cooled in the furnace. A tube specimen with Table 1 Experimental condition and some results
3.1. Microstructure and effects of technologic parameters 3.1.1. Effects of composition Figs. 2–4 show microstructures of centrifugally cast Zn–27Al–3.2Mg–1.8Si, Zn–27Al–6.3Mg–3.7Si and Zn–27Al–9.8Mg–5.2Si alloys respectively at mould temperature of 200 ◦ C and at rotating rate of 1000 rpm. For the in situ composites of Zn–27Al–6.3Mg–3.7Si alloy (Fig. 2), both inner and outer layer contain primary Mg2 Si and primary Si and middle layer contains eutectic microstructure. It can be observed from Fig. 2 (a) that a great deal of primary Mg2 Si with average size of 30–50 m (blocky phases with deep grayness) segregated and enriched in the inner layer of composites; meanwhile, there exists a little blocky primary Si with average size of 20–40 m (blocky phases with light grayness); the thickness of whole inner layer was about 5 mm. Fig. 2(b) shows the microstructure of transitional layer, where the microstructure transited from inner layer containing lots of blocky primary Mg2 Si and primary Si to middle layer containing fine Mg2 Si and Si. Mg2 Si and Si in middle layer (Fig. 2(c)) show shapes of thin stripes and dots, exhibiting the characteristics of eutectic microstructure. The outer layer (Fig. 2(d)), whose thickness is about 0.4 mm, contains a lit-
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Fig. 2. Microstructure of Zn–27Al–6.3Mg–3.7Si alloy (Tm = 200 ◦ C, r = 1000 rpm): (a) inner layer, (b) transitional layer, (c) middle layer, (d) outer layer.
Fig. 3. Microstructure of Zn–27Al–3.2Mg–1.8Si alloy (Tm = 200 ◦ C, r = 1000 rpm) (a) inner layer, (b) middle layer, (c) outer layer.
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Fig. 4. Microstructure of Zn–27Al–9.8Mg–5.2Si alloy (Tm = 200 ◦ C, r = 1000 rpm): (a) inner layer, (b) transitional layer, (c) middle layer, (d) outer layer.
tle small blocky primary Si and primary Mg2 Si, where the primary Mg2 Si connects with each other and distributes as long bunches. Compared with that in inner layer, the primary Mg2 Si and primary Si in outer layer are finer and fewer and the distribution is more uneven. The microstructures of inner layer (Fig. 3(a)), middle layer (Fig. 3(b)) and outer layer (Fig. 3(c) in Zn–27Al–3.2Mg–1.8Si alloy are almost similar. Blocky primary Mg2 Si and primary Si phase were not found in these microstructures. All the microstructures are made up of ␣ (Al) dentrites, white strip phase (verified as MgZn2 by EDAX) and eutectic microstructure around grain boundaries. In other word, the as-cast tube specimen of Zn–27Al–3.2Mg–1.8Si alloy is a single layer material. The center of the dentrites appears white with lower Zn content, while the edge of the dentrites is gray because of higher Zn content. Grain size becomes smaller from inner layer to outer layer. For Zn–27Al–9.8Mg–5.2Si alloy (Fig. 4), the in situ composites are obtained. Both inner and outer layer contain primary Mg2 Si and primary Si and there are no primary Mg2 Si and primary Si in the middle layer. It can be observed from Fig. 4(a) that the inner layer with whole thickness of 6 mm contains a great deal of blocky primary Mg2 Si with size of 5–80 m and a little blocky primary Si with size of 20–30 m. The middle layer with thickness of 4 mm contains eutectic Mg2 Si and eutectic Si (Fig. 4(c). The outer layer with thickness of 2 mm contains lots of small primary Mg2 Si with size of 10–20 m and a little small blocky primary Si with size of 5–10 m, where most of primary Mg2 Si distributes as long
bunches (Fig. 4(d). Compared with Zn–27Al–6.3Mg–3.7Si alloy, the amount of primary Mg2 Si in both inner and outer layer of in situ Zn–27Al–9.8Mg–5.2Si composites increases obviously, but primary Si reduces a little. 3.1.2. Effects of mold temperature Similar to the condition at mould temperature of 200 ◦ C and at rotating rate of 1000 rpm, there are no primary Mg2 Si and primary Si phase in microstructures of Zn–27Al–3.2Mg–1.8Si alloy when changing the mould temperatures of 100 and 300 ◦ C respectively. At all tested temperatures the as-cast tube specimens are single layer microstructure consisted of ␣ (Al) dentrite, MgZn2 phase and eutectic microstructure, as can be seen in Fig. 3. It is found that with mould temperature increasing, grain size of inner layer, middle layer and outer layer increases apparently, especially in outer layer. 3.1.3. Effects of rotating rate Figs. 5 and 6 show the microstructure of centrifugal cast Zn–27Al–6.3Mg–3.7Si alloy at mould temperature of 200 ◦ C and at different rotating rates of 500 rpm, 1500 rpm respectively. The results show that in situ composites whose inner and outer layer both contain blocky primary Mg2 Si and blocky primary Si and middle layer contains eutectic microstructure can be obtained at all tested rotating rates. Compared with Fig. 2, with the rotating rate increasing, thickness of inner layer increases but thickness of outer layer decreases (as showed in Table 1), the size of primary Mg2 Si and pri-
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Fig. 5. Microstructure of Zn–27Al–6.3Mg–3.7Si alloy (Tm = 200 ◦ C, n = 500 rpm): (a) inner layer, (b) transitional layer, (c) middle layer, (d) outer layer.
Fig. 6. Microstructure of Zn–27Al–6.3Mg–3.7Si alloy (Tm = 200 ◦ C, n = 1500 rpm): (a) inner layer, (b) transitional layer, (c) middle layer, (d) outer layer.
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mary Si in inner and outer layer reduces. At the same time, the amount of primary Mg2 Si and primary Si in inner layer increases, but decreases in outer layer. Under the effect of centrifugal force, the movement of primary Mg2 Si and primary Si in molten alloy depends mainly on two forces: radical push force caused by centrifugal force and viscous resistance of molten metal. The moving velocity of primary Mg2 Si and primary Si under the action of centrifugal force can be described as the follows according to Stockes equation: υ=
2(ρs − ρl )R2p ω2 r 9η
primary Si become much thicker. Because the solidification velocity in outer layer is higher than the floating inward velocity of primary Mg2 Si and primary Si there, primary Mg2 Si and primary Si will be captured and stay in outer layer [15]. Consequently, with rotating rate increasing, the thickness of outer layer reduces. Besides, with rotating rate increasing, primary Mg2 Si and primary Si are enforced by much stronger centrifugal force, and are much easier to collide with each other and break up, which results in smaller size of primary Mg2 Si and primary Si in inner and outer layer.
(1)
where ρs is the density of solid grain (ref. [7] reported that ρsi = 2330 kg/m3 , ρMg2 Si = 1990 kg/m3 ), ρl is the density of molten metal (the density of ZA27 alloy is about 4900 kg/m3 [16]), ω is the rotating rate, r is distance between solid grain and rotating center, η is dynamical viscosity of molten metal, Rp is radius of solid particle, υ is moving velocity of solid particle. Based on Eq. (1), primary Mg2 Si and primary Si, whose density is much lower than ZA27 alloy, move inward under the action of centrifugal force after precipitated from molten alloy. Given other conditions unchanged, higher centrifugal rotating rate responds to higher floating inward velocity and longer moving distance. Therefore, with the rotating rate increasing, the floating inward velocity of primary Mg2 Si and primary Si become faster and moving distance become longer. As a result, inner layer containing primary Mg2 Si and
3.1.4. Effects of modification Fig. 7 shows the microstructure of centrifugal cast Zn–27Al–9.8Mg–5.2Si alloy modified by Na-salt at mould temperature of 200 ◦ C and at rotating rate of 1000 rpm. Compared with that in Fig. 4, grains are apparently refined in inner layer, middle layer and outer layer and the size of primary Mg2 Si in inner layer becomes smaller after adding Na-salt. The modification mechanism of Na-salt may be that the addition of Na-salt promotes the formation of fine Mg2 Si particles by providing the nucleation sites for Mg2 Si particles [17] and prevents the growth of primary Mg2 Si and primary Si. 3.1.5. Determination of phase and composition Fig. 8 shows X-ray diffraction in the inner and outer layer of centrifugally cast Zn–27Al–6.3Mg–3.7Si alloy at mould temperature of 200 ◦ C and Table 2 shows the results of phases analyzed by X-ray diffraction. Both inner and
Fig. 7. Microstructure of Zn–27Al–9.8Mg–5.2Si alloy + modifier (Tm = 200 ◦ C, n = 1000 rpm): (a) inner layer, (b) transitional layer, (c) middle layer, (d) outer layer.
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Table 2 Results of phase analyzed by X-ray diffraction Alloy
Position
phases
Zn–27Al–3.2Mg–1.8Si Inner layer Zn, Al, Mg2 Si, Si, MgZn2 , Mg2 Zn11 Outer layer Zn, Al,Mg2 Si, Si, MgZn2 , Mg2 Zn11 Zn–27Al–6.3Mg–3.7Si Inner layer Zn, Al, Mg2 Si, Si, MgZn2 Outer layer Zn, Al, Mg2 Si, Si, MgZn2 Zn–27Al–9.8Mg–5.2Si Inner layer Zn, Al, Mg2 Si, Si, MgZn2 Outer layer Zn, Al, Mg2 Si, Si, MgZn2
Fig. 8. X-ray diffractograms of Zn–27Al–6.3Mg–3.7Si alloy (a) inner layer (b) outer layer.
outer layer of Zn–27Al–3.2Mg–1.8Si alloy contain Zn, Al, Mg2 Si, Si, MgZn2 and Mg2 Zn11 . The inner and outer layer of Zn–27Al–6.3Mg–3.7Si alloy and Zn–27Al–9.8Mg–5.2Si alloy contain Zn, Al, Mg2 Si, Si and MgZn2 . With the content of Mg and Si increasing, the Mg–Zn compound transforms from the coexistence of MgZn2 and Mg2 Zn11 to single MgZn2 . Composition of each phase in the centrifugally cast Zn–27Al–3.2Mg–1.8Si alloy and Zn–7Al–6.3Mg–3.7Si alloy composites are further determined by SEM and EDAX.
Figs. 9 and 10 are SEM images of Zn–27Al–3.2Mg–1.8Si and Zn–27Al–6.3Mg–3.7Si alloys respectively. EDAX results of point A, B, C and D in Figs. 9 and 10 are shown in Table 3. In Fig. 9, the phase of point A distributing around grain boundaries is Mg–Zn compound (the reason that point A contain Al is that Al is solubilized into the phase or the effect of electron beam into the matrix). The atom ratio of Zn to Mg for the Mg–Zn phase is 3.7, which is between the atom ratio of Zn to Mg in MgZn2 and that in Mg2 Zn11 (MgZn2 and Mg2 Zn11 are determined by XRD, as shown in Table 2). The plate-like eutectic microstructure is the compound of Mg–Zn-␣(Al), which is formed in binary and ternary eutectic reaction. Deep gray blocky phase (point A in Fig. 10(a)) is determined as Mg2 Si, and light gray blocky phase (point B in Fig. 10(a)) can be determined as primary Si. Bright strip phase (point C in Fig. 10(a)) can be analyzed as MgZn2 phase by the atom ratio 2 of Zn to Mg (Table 3). The bright strip phase (point C in outer layer in Fig. 10(b)) is determined as MgZn2 and the deep gray strip phase (point A in Fig. 10(b)) is Mg2 Si. The light gray stick-like phase (point D in the outer layer in Fig. 10(c)) is determined as Si.
Fig. 9. SEMs of Zn–27Al–3.2Mg–1.8Si alloy.
Fig. 10. SEMs of Zn–27Al–6.3Mg–3.7Si alloy: (a) inner layer; (b) and (c) outer layer.
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Table 3 EDAX results of Zn–27Al–3.2Mg–1.8Si and Zn–27Al–6.3Mg–3.7Si alloys Element
Alloy Zn–27Al–3.2Mg–1.8Si
O Mg Al Si Zn
Zn–27Al–6.3Mg–3.7Si
Point A (at%)
Point B (at%)
Point C (at%)
Point D (at%)
Point A (at%)
Point B (at%)
Point C (at%)
Point D (at%)
– 18.46 12.63 – 68.91
– 1.89 19.49 71.37 7.24
– 52.47 3.51 28.38 15.63
– 14.16 29.93 0.60 55.31
15.87 41.47 10.97 25.48 6.22
– – – 100 –
– 36.54 – – 63.46
– – 8.52 87.31 4.17
3.2. Forming process of composites Little research result about the solidification process of Zn–Al–Mg–Si alloys has been reported. The forming process of composites is analyzed as follows. Firstly, the solidification process of Zn–27Al–xMg–ySi alloys is analyzed, based on the phase analysis results and the DTA curves (Figs. 11 and 12) of Zn–27Al–6.3Mg–3.7Si alloy and Zn–27Al–3.2Mg–1.8Si alloy and the ternary phase diagrams of Zn–Al–Si [18–19] and Zn–Al–Mg, the binary phase diagrams of Zn–Al and Zn–Mg. Secondly, the forming process of Zn–27Al–xMg–ySi composites during centrifugal casting is further investigated. It can be observed from Figs. 2 and 4 that Zn–27Al–6.3Mg–3.7Si alloy and Zn–27Al–9.8Mg–5.2Si alloy composites contain blocky Mg2 Si and blocky Si. The DTA curve of Zn–27Al–6.3Mg–3.7Si alloy (Fig. 11) indicates that following reactions would take place during the initial solidification stage of Zn–27Al–6.3Mg–3.7Si alloys: L → L + Mg2 Si and L → L + Si. The exothermal peaks at 648.87 ◦ C and 568.51 ◦ C in Fig. 11 correspond the precipitation of primary Mg2 Si and primary Si respectively. For the studied Zn–27Al–6.3Mg–3.7Si alloy, following reaction would happen during the temperature range from
400 ◦ C to 500 ◦ C according to the ternary phase diagram of Zn–Al–Mg [19]: L → L + ␣(Al). The exothermal peak at 444.67 ◦ C in Fig. 11 confirms this reaction. With temperature decreasing, Mg-Zn compound precipitates (Figs. 3–5), which corresponds with the exothermal peak at 359.09 ◦ C in the DTA curve (Fig. 6). Based on ternary phase diagram of Zn–Al–Si [19], the ternary eutectic reaction: L → ␣(Al) + Zn + Si will occur at 383 ◦ C when the contents of Si and Zn are 0.06% and 87% respectively. It can be conjectured that, when the content of the solidifying molten alloy reaches its eutectic composites, the ternary or quaternionic eutectic reaction: L → ␣(Al) + Zn + (Si) + (Mg2 Si) will take place during the final solidification stage of the quaternionic alloy Zn–27Al–6.3Mg–3.7Si, which corresponds with the exothermal peak at 325.95 ◦ C in DTA curves (Fig. 11). However, exothermal peaks of primary Mg2 Si and primary Si have not been detected in the DTA curve (Fig. 12) of Zn–27Al–3.2Mg–1.8Si alloy, which is consistent with the result that primary Mg2 Si and primary Si have not been detected in Zn–27Al–3.2Mg–1.8Si alloy. The DTA curves of Zn–27Al–3.2Mg–1.8Si alloy (Fig. 12) have gone through similar procedures as following: the reaction of L → L + ␣(Al) at 445.23 ◦ C, the precipitation of
Fig. 11. DTA curve of Zn–27Al–6.3Mg–3.7Si alloy.
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Fig. 12. DTA curve of Zn–27Al–3.2Mg–1.8Si alloy.
Mg–Zn compound at 335.06 ◦ C and the ternary or quaternionic eutectic reaction: L → ␣(Al) + Zn + (Si) + (Mg2 Si). For Zn–27Al–9.8Mg–5.2Si alloy, it can be inferred from its optical microstructure in Fig. 4 that its solidification process is similar with that of Zn–27Al–6.3Mg–3.7Si alloy, except that primary Mg2 Si precipitated more and primary Si precipitated much less. Based on the above analysis to the solidification process of Zn–27Al–xMg–ySi alloy, the forming process of the in situ composites of Zn–27Al–xMg–ySi alloy under the action of centrifugal force is further analyzed as follows. For Zn–27Al–6.3Mg–3.7Si alloy, when molten alloy is poured into the tube mould, the outer layer of composites solidifies firstly due to the rapid cooling effect of the metal mould wall, and a little primary Mg2 Si and primary Si precipitate. The density of each primary phases is much lower than that of the molten alloy, and the floating velocity of Mg2 Si and Si inward (VMg2Si , VSi ) is respectively slower than the solidification velocity (Rs ). As a result, some primary Mg2 Si and primary Si are captured and stay in outer layer. Because the cooling velocity in outer layer is very quick, the primary Mg2 Si and primary Si have no time to grow up and are very small. With the temperature of molten alloy decreasing, Mg2 Si and Si precipitate continually and move to the inner layer under centrifugal force. On the other hand, due to the cooling effect of the air in the tube mold, the inner layer of composites also begins to solidify at a lower velocity. Meanwhile, the floating direction of blocky primary Mg2 Si and blocky primary Si in the inner layer is opposite with the solidification direction. Therefore, lots of primary Mg2 Si and primary Si stay in inner layer. With solidification going on, the content of Mg and Zn in solidifying molten alloy decreases. Finally, the content of solidifying molten alloy reaches the eutectic composition;
therefore eutectic microstructure in the middle layer is obtained. For Zn–27Al–9.8Mg–5.2Si alloy composites, there are more primary Mg2 Si precipitated, therefore the inner and outer layers containing primary Mg2 Si and primary Si are much thicker than those of Zn–27Al–6.3Mg–3.7Si alloy composites. For Zn–27Al–3.2Mg–1.8Si alloy, because there are not primary Mg2 Si and primary Si precipitated during its solidification process. In situ composite, which contain primary Mg2 Si and Si in the inner and outer layers, can not be fabricated. It is demonstrated from the above analysis that composition, solidification velocity under the effect of centrifugal force and the floating velocity inward of primary Mg2 Si and primary Si determine the position, quantity and distribution of primary Mg2 Si and primary Si [15].
4. Conclusions 1. During the initial solidification stage of Zn–27Al–6.3Mg–3.7Si alloy and Zn–27Al–9.8Mg–5.2Si alloy, there are primary Mg2 Si and primary Si precipitates. However, there are no primary Mg2 Si and primary Si in Zn–27Al–3.2Mg–1.8Si alloy. 2. In situ composites of Zn–27Al–6.3Mg–3.7Si alloy and Zn–27Al–9.8Mg–5.2Si alloy have been fabricated by centrifugal casting using the heated permanent mold. This kind of composites is consisted of three layers, inner layer segregates lots of blocky primary Mg2 Si and a little blocky primary Si, middle layer contains without primary Mg2 Si and primary Si, outer layer contains some primary Mg2 Si and primary Si. The position, quantity and distribution of primary Mg2 Si and primary Si are determined by compo-
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3.
4.
5.
6.
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sition, solidification velocity under the effect of centrifugal forces and floating velocity inward of primary Mg2 Si and primary Si. The inner and outer layer of Zn–27Al–9.8Mg–5.2Si alloy are respectively thicker than those of Zn–27Al–6.3Mg–3.7Si alloy and have more primary Mg2 Si. During the centrifugal casting of Zn–27Al–6.3Mg–3.7Si alloy, with rotating rate increasing, the thickness of the inner layer increases and the outer layer become thinner. At the same time, the size of primary Mg2 Si and primary Si in both inner and outer layer reduces. The amount of primary Mg2 Si and primary Si in inner layer increases, and the amount of that in outer layer decreases. The addition of Na-salt can refine grain and primary Mg2 Si, and make primary Mg2 Si distribute more evenly and make primary Si nodular. Only single layer materials without primary Mg2 Si and primary Si can be obtained by centrifugal cast Zn–27Al–3.2Mg–1.8Si alloy. At the same time, with the mould temperature increasing, grain sizes increase.
Acknowledgments The authors express their thanks to National Natural Science Foundation of China (Contract No. 59901007) and the Visiting Scholar Foundation of Key Lab in University for the financial support of this work.
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