Semi-solid metal (SSM) processing is the forming

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1 Research & Development CHINA FOUNDRY Rheological behavior of semi-solid 7075 aluminum alloy at steady state *Li Yageng, Mao Weimin, Zhu Wenzhi, and Yang Bin University of Science & Technology Beijing, Beijing , China Abstract: The further application of semi-solid processing lies in the in-depth fundamental study like rheological behavior. In this research, the apparent viscosity of the semi-solid slurry of 7075 alloy was measured using a Couette type viscometer. The effects of solid fraction and shearing rate on the apparent viscosity of this alloy were investigated under different processing conditions. It can be seen that the apparent viscosity increases with an increase in the solid fraction from 10% to 50% (temperature 620 ºC to 630 ºC) at steady state. When the solid fraction was fixed, the apparent viscosity can be decreased by altering the shearing rate from s -1 to s -1 at steady state. An empirical equation that shows the effects of solid fraction and shearing rate on the apparent viscosity is fitted:. The microstructure of quenched samples was examined to understand the alloy s rheological behavior. Key words: apparent viscosity; semi-solid; 7075 aluminum alloy; steady state CLC numbers: TG Document code: A Article ID: (2014) *Li Yageng Semi-solid metal (SSM) processing is the forming of metals at a temperature between the solidus and liquidus temperatures, which was found by Flemings and his fellow workers in the 1970s. This technology has already been applied in automobile, electric, communication and aerospace industries [1, 2]. The semisolid forming technology mainly includes thixocasting and rheocasting. The process of thixocasting offers a number of advantages such as improved mechanical properties, good surface finish and near net shape; but this process needs special feedstock and re-melting. A more expensive premium billet has to be paid for when thixocasting [3]. Rheocasting can eliminate this additional specialized casting step, as the product can be cast into a near net shaped part directly from the molten metal state [4, 5]. Therefore, rheocasting has the advantage from an energy and cost saving point of view when compared to thixocasting. The application of semi-solid rheocasting relies on further learning from fundamental research. SSM slurries with a volume fraction of solid less than 60% usually exhibit unique rheological properties [6]. Joly and Mehrabian [7] found pseudo-plasticity and thixotropic Male, born in 1989, Master degree candidate. His present research interest is in semi-solid metal processing. liyageng@163.com Received: Accepted: behavior in research on a semi-solid Sn-15Pb alloy. Kattamis et al. [8] investigated the rheological behavior of semi-solid Al-4.5Cu-1.5Mg alloy slurry in continuous cooling experiments. Shear rate thickening behavior was found by Kumar et al. [9] during the transient circumstances. Semi-solid processing technology relies on one or all of these properties in the same process. Therefore, it was important to study the rheological behavior of the semi-solid state alloy in order to improve the quality in semi-solid rheocasting operations. Many investigators have studied the characteristics of the metal alloys such as Al-Si alloys and Sn-Pb alloys [10], but few have focused on wrought aluminum alloys. The aluminum alloy 7075 investigated in this study has high mechanical properties, with strength comparable to many steels, and has good fatigue strength. It would be more widely useable if the casting defects of 7075 could be solved. Adriana Neaga et al. [11] investigated the flow behavior of 7075 during backward extrusion. However, they only examined the flow behavior of 7075 with high solid fraction, so more data is required when the solid fraction is between 10% and 50% if some complex shaped parts are going to be produced by rheocasting. Kim et al. [12] studied the rheological behavior of wrought aluminum alloys with varying processing parameters, such as liquid fraction, strain rate, shear rate and materials. In spite of these efforts, there is still a great need to build an empirical equation for the viscosity in order to provide a model for the 79

2 CHINA FOUNDRY Research & Development simulation of semi-solid 7075 aluminum alloy processing. It is to this end that the present fundamental study was undertaken. The Couette viscometer was used to examine the apparent viscosity at different temperatures (with different solid fraction) and shearing rates. 1 Experimental method 1.1 Materials The chemical composition of the experimental 7075 aluminum alloy used in this study is shown in Table 1. Table 1: Chemical compositions of experimental 7075 alloy Element Zn Mg Cu Fe Mn Si Al Content (wt.%) Bal. 1.2 Apparatus The rheological behavior was detected using a Couette viscometer of our own design, as shown in Fig. 1. The molten alloy was sheared in the annular gap between the graphite cup and bob. The cup was connected to a servo-motor which has a speed range of rpm, while the bob was linked to a torque transducer that transfers the torque signal to the collection system. The bob was grooved to prevent slipping of the alloy. where η a is the apparent viscosity; M is the torque on the bob; R is the inner radius of the cup and r is the radius of the bob; h is the immersed depth of the bob into the metal liquid; n is the revolutions per minute; w is the angular velocity; 4 g is the average of shear rate. 1.3 Procedures The volume solid fraction of 7075 aluminum alloy corresponding to a given temperature was calculated by differential scanning calorimetry (DSC) [13]. The DSC analyses were carried out in an argon atmosphere with a scanning rate of 5 min -1. The heat flow vs. temperature curve obtained from the solidification scan was used to calculate the solid fraction vs. temperature data. The DSC analysis was designed to better approximate the cooling routines employed in the present study. The aluminum alloy was superheated to 50 above the liquidus temperature. After being held at this temperature for 30 min, shearing was conducted continuously during cooling from above the liquidus temperature. The material was cooled at 5 min -1 to a pre-determined temperature, at which it was then held isothermally. Stirring was continued until the torque attained a constant steady state value. This experiment was repeated for variable shearing velocity and isothermal temperature. 2 Results and discussion 2.1 Thermal analysis of 7075 alloy The curve of solid fraction vs. temperature for 7075 alloy is shown in Fig. 2. In the present study, the isothermal temperatures were pre-determined at five different temperatures which correspond to five different solid fractions in Table 2, according to Fig lifting nut, 2 - torque transducer, 3 - cooling water jacket, 4 - insulation material, 5 - heating furnace, 6 - argon shield, 7 - thermal couple, 8 - bob, 9 - liquid metal, 10 - cup, 11 - circulating water, 12 - servo motor Fig. 1: Schematic diagram of viscometer T Fig. 2: Solid fraction of 7075 alloy vs. temperature The apparent viscosity of the given alloy can be calculated as: (1) (2) Table 2: The chosen experimental conditions Temp. (ºC) Solid fraction (%)

3 Research & Development CHINA FOUNDRY 2.2 Influence of solid fraction on apparent viscosity Figure 3 shows the apparent viscosity of semi-solid 7075 alloy at steady state as a function of solid fraction. It can be seen that the viscosity increases with an increase in solid fraction. The viscosity increases slightly at the early stage, while it increases dramatically with higher solid fractions. This law is the same as other alloys studied by earlier researchers [14, 15]. Fig. 5: Apparent viscosity of 7075 alloy vs. shearing rate f s Fig. 3: Apparent viscosity of 7075 alloy vs. solid fraction When the solid fraction is low (in the range of f s from 0.1 to 0.2), the increase in solid fraction has a little influence on the viscosity. As the solid fraction increases, the friction and collision between the solid particles become more and more frequent, which leads to a sharper increase in the viscosity. At high shearing rates (> s -1 ), the influence of the solid fraction on the viscosity is low even at high solid fractions f s >0.4. Finally, when the solid fraction reaches such an extent, as f s >0.4, that the solid particles almost touch each other, even a tiny decrease of temperature can lead to a huge increase of viscosity, as shown in Fig. 4. Sn-Pb alloys [1, 19-20]. This behavior can be explained by qualitative models. The average particle size decreases with increasing shear rate. At the same time, the stronger the shearing force, the more nondendritic and globular the solid particles. The liquid trapped in the solid particles is also reduced when the shearing rate is high, because the increased rate of shear reduces the amount of entrapped liquid in individual particles; this can lead to a smaller effective solid fraction. A schematic of the change of particles morphology for different shearing rates is shown in Fig. 6. All of the above lead to a decrease in the apparent viscosity. Fig. 6: Schematic of microstructural evolution in annular gap 2.4 Anomalous behavior Fig. 4: Schematic of microstructural evolution in annular gap 2.3 Influence of shearing rate on apparent viscosity According to Fig. 5, the shearing rate also has a large effect on the apparent viscosity of semi-solid 7075 aluminum alloy. The viscosity decreases with an increase in the shearing rate. The result agrees with the steady state rheological behavior of semisolid slurry recorded in other literature with aluminum [15-18] and When the solid fraction is higher than 50%, the torque fluctuates dramatically or begins to drop down. The first condition is attributed to the formation of a particulate bridge between the cup and the bob, which has been found by other researchers, such as Turng and Wang [14], and Brady and Bossis [21]. In this work, the anomalous behavior is dependent on the shearing rate. When the shearing rate is higher, this phenomenon will occur in higher solid fraction, which can be attributed to less entrapped liquid in the solid particles; this leads to a smaller effective solid fraction, as discussed before. The reason for the latter anomalous condition is the shrinkage of the liquid in higher solid fraction. The semisolid alloy adheres to the cup and bob, which may produce a gap in the liquid. As a result, friction and collisions between the solid particles are reduced. 81

4 CHINA FOUNDRY Research & Development 2.5 Model of apparent viscosity at steady state From the analysis above, the solid fraction and the shearing rate are the two important factors that impact the apparent viscosity. Figure 3 shows that the relationship between apparent viscosity and solid fraction conforms to the exponential growth model. Therefore the apparent viscosity at a certain shearing rate can be described as follows: (3) B is almost independent of the shearing rate, while A decreases with the increase in shearing rate. So B is considered a constant value which is the average of these five values. The relationship between value A and shearing rate can be determined by a power function model. The result is shown in Fig. 7. f s Fig. 8: Experimental results and model for apparent viscosity as a function of solid fraction The influence of solid fraction and shearing rate on apparent viscosity can be described as: Table 3 shows the outcomes of model fitting. Figures 8 and 9 indicate that the determined result coincides with the experimental points. (4) (5) Table 3: Outcomes of model fitting Shearing rate (s -1 ) η a = A exp (B f s ) A = , B = A = , B = A = , B = A = , B = A = , B = 3.23 Fig. 7: A-value vs. shearing rate 2.6 Microstructure characteristics of semisolid alloy The rheological behavior of semi-solid 7075 aluminum above is a reflection of the microscopic structure characteristics of the alloy. Therefore the microstructural characteristics of the Fig. 9: Experimental results and model for apparent viscosity as a function of shearing rate alloy were also investigated. The alloy was completely melted at 700, then the liquid alloy was cooled down continuously with a cooling rate of 5 min -1 under constant shearing rate ( s -1 ) until 630 was reached. During the following isothermal holding, the shearing time was 5 min, 10 min and 20 min, respectively. The quenched microstructure of the semi-solid alloy for each shearing time is shown in Fig. 10. In Fig. 10, the bright colored part in the photograph is the primary α-al phase and the dark part is the quenched phase. It can be seen that the amount of primary α-al phase in the sample without stirring is greater than that of the sample with different shearing times. There is also a morphology evolution from the dendritic to non-dendritic because the shearing force leads to shrinkage in the root of the dendrite arms that make them finally fall off the trunk, and some of them bend and turn into spheres by collision and friction during the shearing process. The longer the shearing time, the stronger the potential of the globularization procedure. The influence of shearing rate on the semi-solid 7075 alloy was also studied at 630. As shown in Fig. 11, when the shearing rate is low, the primary phase is coarse dendrite. 82

5 Research & Development CHINA FOUNDRY (a) No stirring (b) 5 min (c) 10 min (d) 20 min Fig. 10: Microstructure after different shearing times (a) shearing rate s-1 (c) shearing rate s-1 Fig. 11: Microstructure at different shearing rates (b) shearing rate s-1 With an increase in the shearing rate, the morphology of this primary phase turns gradually into non-dendrite, and some even becomes spherical. Moreover, the grain size of this primary phase decreases with the increase in shearing rate. This is mainly because the stirring force on the slurry increases with the high shearing rate, which favors the non-dendritic morphology of the primary grains. Thus, the microstructural evolution can explain the decrease in apparent viscosity with the increase in shearing rate. The shearing rate influences the morphology of the solid particles, while this morphology of solid particles then affects the apparent viscosity. 83

6 CHINA FOUNDRY Research & Development 3 Conclusions (1) The apparent viscosity of semi-solid 7075 aluminum alloy increases with an increase in solid fraction. It rises slowly at low solid fraction but a sudden increase occurs at higher solid fraction especially at low shearing rates. (2) The apparent viscosity of semi-solid 7075 aluminum alloy decreases with an increase in shearing rate, i.e. there is a shearing thinning phenomenon, which could be attributed to less liquid being entrapped in the solid particles. (3) An anomalous behavior of the torque was observed at solid fractions above 60%. The fluctuation of torque which appeared with solid fractions above 50% can be attributed to the formation of a bridge of particulates between the cup and the bob. The decrease of the torque is attributed to the shrinkage of the semi-solid alloy. (4) An empirical equation has been formulated to describe the apparent viscosity model:. (5) The grain size of the α-al primary phase decreases with the increase in both shearing time and shearing rate. The morphology of the α-al primary phase becomes nearly spherical during this process and this in turn decreases the apparent viscosity of the semi-solid 7075 aluminium alloy. References [1] Spencer D B, Mehrabian R, and Flemings M C. Rheological Behavior of Sn-15Pb in the Crystallization Range. Metall. Trans. A, 1972, 3A: [2] Mehrabian R and Flemings M C. Diecasting of partially solidified alloys. AFS Trans., 1972, 80: [3] Haga T and Kapranos P. Simple rheocasting processes. J. Mater. Process. Tech., 2002, : [4] Eisen P and Young K. Diecasting system for semi-liquid and semisolid metal casting-applications. In: Proc. 6th Int. Conf. on Semisolid Processing of Alloys and Composites, Turin, Italy, 2000: [5] Liu Zheng, Liu Xiaomei and Mao Weimin. Semi-solid A356 alloy slurry for rheocasting prepared by a new process. China Foundry, 2013, 3: [6] Chen J Y, Fan Z, and Ju L P. Rheological characteristics of Al-6.5Si semi-solid metal slurries. In: Proc. 7th Int. Con. on Semisolid Processing of Alloys and Composites, Tsukuba, Japan, 2002: [7] Joly P A. The rheology of a partially solid alloy. J. Mater. Sci., 1976, 11: [8] Kattamis T Z and Piccone T J. Rheology of semisolid Al-4.5Cu- 1.5Mg alloy. Mater. Sci. Eng. A, 1991, 131(2): [9] Kumar P, Martin C L, and Brown S. Shear rate thickening flow behavior of semi-solid slurries. Metall. Trans., 1993, 24A(5): [10] Xiong Shoumei, Zhang Lizhong, Du Yunhui, et al. Rheologic behaviors of A356 aluminum alloy billet produced by semisolid continuous casting process. China Foundry, 2004, 1: [11] Neag A, Favier V, Bigot R, et al. Microstructure and flow behavior during backward extrusion of semi-solid 7075 aluminium alloy. J. Mater. Process. Tech., 2012, 212: [12] Kim W Y, Kang C G and Lee S M. Effect of viscosity on microstructure characteristic in rheological behavior of wrought aluminium alloys by compression and stirring process. Mater. Sci. Tech-Lond., 2010, 26: [13] Birol Y. Solid fraction analysis with DSC in semi-solid metal processing. J. Alloy Compd., 2009, 486: [14] Turng L S and Wang K K. Rheological behavior and modelling of semi-solid Sn-15Pb alloy. J. Mater. Sci., 1991, 26: [15] Quaak C J, Horsten M G, and Kook W H. Rheological behavior of partially solidified aluminum matrix composites. Mater. Sci. Eng. A, 1994, 183A(1-2): [16] Blanco A A, Azpilgain Z, and Lozares J. Rheological characterization of A201 aluminum alloy. T. Nonferr. Metal. Soc., 2010, 20: [17] Ferrante M and Freitas E. Rheology and microstructural development of a Al-4Cu alloy in the semi-solid state. Mater. Sci. Eng. A, 1999, 271(1-2): [18] Yurko J and Flemings M C. Rheology and microstructure of semi-solid aluminum alloys compressed in the drop-forge viscometer. Metall. Trans. A, 2002, 33: [19] Koke J and Modigell M. Flow behavior of semi-solid metal alloys. J Non-newton Fluid, 2003, 112: [20] Mclelland A R A, Henderson N G, Atkinson H V, et al. Anomalous rheological behavior of semi-solid alloy slurries at low shear rates. Mater. Sci. Eng. A, 1997, 232(1-2): [21] Brady F J and Bossis G. Stokesian Dynamics. Ann. Rev. Fluid Mec., 1988, 20: This research was financially supported by the National Basic Research Program of China (No. 2011CB ) and the National Natural Science Foundation of China (No ). 84