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Guanglong Li
, Yaohua Yang
Corresponding authors:
Received: 2019-02-5
Revised: 2019-05-16
Accepted: 2019-09-5
Online: 2020-03-01
Copyright: 2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology
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Abstract
In order to improve the strength of short carbon fibers reinforced aluminum matrix (Csf/Al) composite, the dispersion of short carbon fibers with multi-orientation was controlled with a square crucible by mechanical stirring. The three-dimensional flow field models of liquid aluminum melt in the square/round crucibles were established and calculated, and the results were compared. The calculated results show that turbulent flow could be induced both in the square and round crucible, while the non-axisymmetric structure of the square crucible results in higher turbulent kinetic energy in the melt. Therefore, the uniformity and multi-orientation dispersion of the short fibers can be improved by the intensive turbulent flow in the square crucible, which will be increased by increasing the rotational velocity. The distribution of the short carbon fibers in the aluminum matrix prepared under different rotation velocities in square crucible was experimentally investigated. With the increase of stirring velocity, the multi-orientation dispersion of the short fibers in the composites increased gradually. The experimental results are consistent with the calculation results. The tensile testing results show that the strength of the Csf/Al composite can reach 172 MPa when the rotational velocity is 1000 rpm, and it is 48.3% higher than that prepared by the round crucible under the same conditions, which results from the improved multi-orientation dispersion of short carbon fibers in aluminum matrix.
Keywords：
In recent years, carbon fibers have attracted wide attention for its excellent properties as reinforcements of aluminum matrix composites [1]. Carbon fiber exhibits ultrahigh thermal conductivity and low coefficient of thermal expansion along the fiber axis, which has a good application prospect for heat dissipation devices [2]. Carbon fiber reinforced aluminum matrix composite is also a promising structure material because of its high specific modulus and specific strength, as well as the elasticity modulus can reach 230-415 GPa [[3], [4], [5]]. Since the non-random short fiber orientation will result in anisotropic properties of the composite, the randomly oriented short carbon fibers reinforced aluminum matrix (Csf/Al) composite has better properties under multiaxial loading conditions than the continue carbon fibers reinforced aluminum matrix composite [6,7]. Therefore, the multi-orientation distribution of carbon fibers in the matrix is an important problem to be solved in the preparation process of composites. At present, compared with other technologies of preparing Csf/Al composites, such as powder metallurgy and semi-solid process, the liquid method has the advantages of higher production efficiency, lower cost, and the potential to produce complex parts or components [[8], [9], [10]].
Carbon fiber is a flexible whisker reinforcement material. Compared with SiC, Al2O3 and TiC, it has a higher aspect ratio (l/d), a different density from the matrix, and a poor wettability [11,12]. Therefore, the wettability and dispersion seriously restrict the development of Csf/Al composites. Ni or Cu plating technology on the carbon fibers surface can obviously improve the wettability and inhibit the undesired reaction of Al4C3 formation [13,14]. The dispersion of the short fibers will seriously affect the mechanical properties of Csf/Al composites, and the carbon fibers randomly distributed in the matrix can exhibit a more balanced property under multiaxial loading conditions [15,16]. Yang et al. [17] reported that the stirring during melting leads to effectively solute transfer behaviors and promotes the uniformity of solute in the melt. Yang et al. [18] used a square electromagnetic cold crucible to directional solidification. The results show that the use of square crucible will cause complex circulation in the molten pool, and strong radial convection will occur at the front of solid-liquid interface. The design of the baffle system in round crucible during the stirring process is an effective way to promote particle dispersion. Escamilla-Ruíz et al. [19] showed that the flow field in a square crucible is similar to that in the round crucible with baffle system, which would involve the reduction of manufacturing cost for the baffled crucible. The effects of round and square crucible on the separation of alumina particles in the aluminum melt were compared by Shu et al. [20,21], and the results show that the secondary flow in the square crucible greatly shortens the separation time of particles.
Therefore, the shape of crucible will seriously affect the flow field distribution in the process of preparing Csf/Al composites by mechanical stirring, but this problem has been neglected in previous studies. In this paper, three-dimensional (3-D) models of different shape crucibles were established according to the actual structure of crucible and impeller. Then the flow field under different crucible shapes and different stirring velocities were calculated. Moreover, the Csf/Al composites were prepared in different crucibles with mechanical stirring and under different rotational velocities in the square crucible, and the dispersion of the short carbon fibers and their mechanical properties were tested.
The schematic diagram of experiment process is shown in Fig. 1. The diameter of stirring paddle is 36 mm. In order to keep the momentum consistent in the different crucible, the volume of aluminum melt is the same. When the depth of the graphite crucible is 75 mm, the side length of the square crucible is 48.7 mm and the diameter of the round crucible is 55 mm. Al-Si (93, 7 wt%) alloy was used as the matrix, as it prevents adverse reaction between carbon fibers and aluminum melt [22]. Polyacrylonitrile-based carbon fibers (T-300) produced by the Institute of Coal Chemistry at the Chinese Academy of Sciences were selected as the reinforcement material. The diameter of the fiber is 8 μm, and the carbon fibers were chopped into the length of the aspect ratio is 800. In order to make the carbon fibers fully stirred and dispersed in the aluminum melt with good mobility, the experimental temperature is 800 °C. During mechanical stirring, the carbon fibers were gradually added to the aluminum melt. Finally, the melt was quenched into cold water to rapidly cool the specimen.
Fig. 1. Schematic diagram of the experiment process.
The distribution of the short fibers in the Csf/Al composites were investigated through the TM3030 scanning electron microscopy. The room-temperature tensile testing was carried out on 5569 Instron testing machine at the loading rate of 0.5 mm/min. The thickness of the test samples was 2 mm, and the dimensions of the test samples are shown in Fig. 2. At least three separate tensile tests were conducted for each specimen, and the mean tensile strength values were reported.
Fig. 2. Shape of the sample for tensile test (unit: mm).
The 3-D numerical model was established according to the experimental equipment, and the flow field inside melt during mechanical stirring process was calculated using the program ANSYS (distributed by ANSYS HIT). Standard k-epsilon turbulence model was selected for the aluminum melt flow. The density and viscosity of the molten aluminum, and the main parameters for flow field analysis are presented in Table 1. The geometry and mesh of the model are presented in Fig. 3
Table 1 Parameters applied in simulations.
| Parts | Parameters |
|---|---|
| Rotation velocity of stirring paddle (rpm) | 1200, 1000, 800, 600, 400 |
| Density (kg/m3) | 2300 |
| Dynamic viscosity (Pa s) | 0.0012 |
| Impeller distance from the bottom (mm) | 25 |
Fig. 3. 3D geometry of mechanical stirrer and mesh of the mechanical stirrer for flow field calculation: (a, b) round crucible, (c, d) square crucible.
The Navier-Stokes (N-S) equation is applied to the calculating domain, which can precisely characterize the 3-D flow and the distribution of pressure in the melt:
The flow in melt is described by the Navier-Stocks equations can be written as follows:
Continuity equation:
Momentum equation:
where ρ, v and p are the density, dynamical viscosity and pressure, respectively.
The velocity (v) on free surface of melt satisfy the non-slip conditions:
νx=vy=vz(3)
The computational domain was discretized using tetrahedron and hexahedron elements. There were 462304 elements in the whole domain of the round crucible for solving the flow field by Fluent and 382257 elements in the square crucible solution region. The k-ε two-equation model was used to solve the turbulent flow; for each simulation, convergence to steady state was usually accomplished in 1000 iterations.
As shown in Fig. 4, the flow field in the different shape crucibles under the rotating velocity of 1200 rpm were calculated, and the pressure distribution on the X-Y plane and the turbulence kinetic energy near the crucible walls were also analyzed. As shown in Fig. 4(a), it can be seen that the surrounding melt is driven by the rotating region due to the effect of the viscosity coefficient, and therefore only one circular flow on the X-Y plane in the round crucible, as well as the pressure distribution was analyzed. The centrifugal force of the melt and the radial pressure shown gradient near the crucible wall result in the Ekman pumping effects, which induces the secondary flow [23]. The high speed secondary flow will significantly increase the intensity of turbulence, which is beneficial to the uniform mixing of solutes [24,25]. But after stirring for a period of time, the steady-state flow field will have a spin-up effect, and therefore, a uniform circulating flow field is formed inside the round crucible, which inhibits the effect of the secondary flow. Hence there are fewer secondary flow on the X-Z plane and the turbulent kinetic energy is weak near the crucible wall as shown in Fig. 4(b). This flow behavior under mechanical stirring in the round crucible matches well with the results of Wang et al [26].
Fig. 4. Effect of different shape crucibles on flow field and fibers dispersion under the rotating velocity of 1200 rpm: (a, c) flow field and pressure distribution on the X-Y plane, (b, d) flow field and turbulent kinetic energy on the X-Z plane.
The flow field on the X-Y plane in the square crucible were calculated and the velocity of flow field are shown in Fig. 4(c). As mentioned above, the flow field in the rotating region of the square crucible is similar with that observed in the cylindrical container. But the round crucible is axisymmetric structure while the square crucible is non-axisymmetric structure. The Taylor numbers in the non-axisymmetric structure under stirring is higher than that of the axisymmetric structure on the finite container, which is favorable for the formation of secondary flow close to the wall [23]. In the square crucible, the secondary flow is induced not only by the Ekman pumping effect, but also the inertial forces, which play an important role. The melt impacts violently with the crucible walls near the corner area due to the non-axisymmetric structure of the square crucible. This is why the melt velocity in the square crucible is lower, but the internal pressure distribution is higher than that in the round crucible as shown in Fig. 4(a). The movement of the melt after high-speed impact on the crucible wall becomes very complicated, which is favorable for the formation of secondary flow close to the wall. In this case, the number of secondary flows in the square crucible X-Z plane is significantly higher than that in the round crucible, which is advantage to the enhancement of turbulence intensity in the melt. The calculated results of the turbulent kinetic energy near the crucible walls in the round crucible and the square crucible are displayed in Fig. 4(b) and (d). It can be seen that the higher turbulence intensity appears close to the vertical crucible walls in the square crucible, due to the non-axisymmetric structure of the square crucible. In summary, mechanical stirring in a square crucible can lead to the appearance of a complex three-dimensional flow.
The mechanical stirring is an efficient means of achieving a good homogeneity of the melt, which is characterized by a high turbulent intensity [27]. Umbrashko et al. [28] reveal that the velocity plays a main role in convective heat and mass transfer when flow structure contains two or more vortexes. In order to investigate the effect of rotation velocity on melt flow field in a crucible, and to find a reasonable rotation velocity for fiber dispersion, the calculations of the mechanical stirring were carried out under different velocities of 400 rpm, 600 rpm, 800 rpm and 1000 rpm, respectively.
The calculated flow field and pressure distribution in the round crucible at steady state are shown in Fig. 5. The distribution of the flow field is the same as the rotating velocity of 1200 rpm as shown in Fig. 4(a), which results in a stable circulating flow due to the spin-up effect in the melt. With the decrease of rotating velocity, the pressure caused by the centrifugal force and the friction between the melt and the crucible walls decreased significantly. As mentioned above, this is not conducive to the generation of secondary flow, thereby inhibiting the dispersion of the solute. The distribution of the flow field and pressure in the square crucible on the X-Y plane are shown in Fig. 6. It can be seen that there is an obvious complex flow near the corne. The maximum velocity of melt flow increases from 0.27 m/s to 0.51 m/s when the rotational velocity increases from 400 rpm to 1000 rpm. When the stirring velocity is low, the impact and friction between the melt and the crucible walls are weakened, and the pressure in the melt decreases significantly. Meanwhile, the Taylor numbers in the melt is small, and the spin-up flow behavior of the melt in the square crucible is similar to that in the round crucible [23]. Therefore, a higher flow velocity in a square crucible contributes to the increase of the turbulent kinetic energy of the melt.
Fig. 5. Flow fields in the round crucible under different rotation velocities: (a) 400 rpm, (b) 600 rpm, (c) 800 rpm, (d) 1000 rpm.
Fig. 6. Flow fields in the square crucible under different rotation velocity: (a) 400 rpm, (b) 600 rpm, (c) 800 rpm, (d) 1000 rpm.
David et al. [29] reported that a complex three-dimensional flow is suitable for obtaining homogenized melt. The distribution of the short carbon fibers in the Csf/Al composites prepared by mechanical stirring in the round crucible is shown in Fig. 7(a). It can be seen that lots of cavity defects appear in the composite, and the magnified figure reveals that a large number of un-infiltrated short fibers can be found in the cavity, which causes the result that the agglomerated carbon fibers cannot be completely dispersed by the stirring action of the stable circulating flow within the melt. Therefore, lots of un-infiltrated defects are present in composite prepared in the round crucible, which severely affects the properties of the Csf/Al composites. Even though some of the fibers are dispersed, the property advantage of the composite still cannot be achieved due to the same fibers orientation.
Fig. 7. Distribution of short carbon fibers in the composites: (a) prepared with round crucible, (b) prepared with square crucible.
As shown in Fig. 7(b), no obvious cavity defect is found on the sample prepared in the square crucible. It also can be seen that the short carbon fibers are randomly oriented and uniformly dispersed in the aluminum matrix through the magnified figure. Because of the inertia force, the short fibers in the melt will continuously impact and rub the end wall in the non-axisymmetric crucible, which results in the initial dispersion of the carbon fibers and the appearance of a large amount of secondary flow in the melt. Then, the short fibers are dispersed due to the convection between mainstream and the secondary flow, as well as the shearing effect caused by the velocity difference between the turbulent streams. This is an ideal way for short carbon fiber reinforcements to exist in the matrix, which is favor for stress transferring from the matrix to the carbon fibers [30]. Thus, the complex three-dimensional flow created by mechanical stirring in a square crucible facilitates the dispersion of the short fibers. However, there are still some un-infiltrated short fibers in the corner as shown in magnified figure, due to the existence of stirred dead zone in the right angle structure.
The microstructures of the composites prepared by square crucible and round crucible are compared, as shown in Fig. 8. It can be seen that there are a large number of cavity defects in the matrix due to fiber agglomeration and un-infiltration, as shown in Fig. 8(a). The presence of a large amount of fiber agglomerations in the composite results in a significant reduction in fiber content in the matrix. As shown in Fig. 8(b), only a small amount of dispersed fibers are present in the matrix, which is difficult to strengthen the materials. As shown in Fig. 8(c) and (d), the fibers can be dispersed in the matrix of composites prepared by square crucible, thus greatly reducing the existence of fiber agglomeration defects. Not only can the content of the fiber in the matrix be ensured, but also the fibers can be evenly distributed in the matrix. Therefore, the dispersion of the carbon fibers is improved in the square crucible than that in the round crucible due to the complex three-dimensional flow.
Fig. 8. Comparison of microstructure of the composites prepared by square crucible and round crucible: (a, b) round crucible; (c, d) square crucible.
The distribution of the short carbon fibers in the Csf/Al composites prepared under different rotational velocities in the round crucible are shown in Fig. 9. The un-infiltrated defects caused by fiber agglomeration obviously exist in aluminum matrix, and the agglomeration phenomenon is more serious at low rotational velocity. This is because the spin-up effect of the melt is not conducive to the dispersion of the fibers in the round crucible, and the weak turbulence intensity is further reduced with the decrease of rotating velocity. When the rotating velocity is lower than 600 rmp, the fibers cannot enter the aluminum melt by centrifugal force.
Fig. 9. Shorts carbon fibers distributions in the composites prepared with round crucible under different rotation velocities: (a) 600 rpm, (b) 800 rpm, (c) 1000 rpm.
In the square crucible, there are also significant un-infiltrated defects in the sample prepared with low rotating velocity, as shown in Fig. 10(a) and (b). It also can be seen from Fig. 7(a) and (b) that the melt has a uniform circulation in the crucible, so there are not enough turbulence intensity to disperse the short fibers in this case. With the rotational velocity increases to 800 rpm, a large number of carbon fibers distribute in the matrix and few un-infiltrated defects appear as shown in Fig. 10(c). However, the orientation of the fibers is uniform, which easily causes multi-region mechanical anisotropy within the sample. When the rotational velocity reaches 1000 rpm, almost all short fibers uniformly distributed in the matrix. Compared with Fig. 10(c), the carbon fibers show a random orientation distribution in Fig. 10(d), which will have better comprehensive performance [3]. Therefore, the higher rotational velocity and stronger impact and friction promote the dispersion of fibers, which coincides with the results of simulated calculation and analysis in Fig. 6.
Fig. 10. Shorts carbon fibers distributions in the composites with the square crucible under different rotation velocities: (a) 400 rpm, (b) 600 rpm, (c) 800 rpm, (d) 1000 rpm.
Based on the results above, the change of the crucible shape and rotational velocity will greatly affect the distribution of the short carbon fibers in the Csf/Al composites prepared by mechanical stirring. Thus, the tensile strength of the Csf/Al composites under different crucible shapes and velocities were tested and analyzed, as shown in Fig. 11. With the increase of rotational velocity, the tensile strength of the composites first increases and then tends to be constant. As mentioned above, when the rotational velocity is 400 rpm and 600 rpm respectively, there are a large amount of un-infiltrated defects in the sample. In this case, the un-infiltrated short carbon fibers not only fail to play the role of reinforcement but also decrease the properties of the Csf/Al composites. The tensile strength is only 84 MPa and 86 MPa at the velocity of 400 and 600 rpm, respectively.
Fig. 11. Tensile strength of the matrix and the composites prepared with different rotational velocities.
When the impeller velocity reaches 800 rpm, the tensile strength of the Csf/Al composites is observably improved due to the reduced carbon fibers agglomeration, and the fibers can withstand the main load from the matrix. When the rotating velocity is 1000 rpm, the fibers are fully stirred in the melt, and the fibers evenly disperses in multiple orientations. At this time, the uniformly dispersed fibers will bear all the load of the specimen and greatly improve the tensile strength. As shown in Fig. 11, the highest tensile strength can reach 172 MPa, which is 53.6% higher than that of the matrix prepared under the same conditions.
When the rotational velocity reaches 1200 rpm, the tensile strength of the Csf/Al composites does not continue to improve as the fiber has been evenly mixed at 1000 rpm, but it is 48.3% higher than that prepared by the round crucible under the same conditions, as shown in Fig. 11. Due to the restriction of crucible shape to the fibers dispersion, the dispersion of fibers in the round crucible is still heterogeneous. As shown in Fig. 7, the un-infiltrated fiber defects in the matrix will be the source of fracture during the tensile process, which seriously damages the properties of the composites. Even though some of the fibers are dispersed, a very small amount of fiber is difficult to play a significant reinforcing effect. The uniformity and multi-orientation dispersion of the fibers can be achieved due to the complex melt flow in the square crucible. In this case, the more load of the sample can be transferred to the carbon fibers. Based on the above discussion, the mechanism, that the strength of composites prepared by square crucible is higher than that of circular crucible is summarized as shown in Fig. 12. Therefore, considering the energy consumption and cost in the actual production process in the future, it is an ideal way to prepare Csf/Al composites by mechanical stirring in a square crucible under the rotational velocity of 1000 rpm.
Fig. 12. Illustration of the change of microstructure and strength of Csf/Al composites prepared by square crucible and round crucible.
The effects of crucible shape and rotational velocity with square crucible on short carbon fibers distribution in the Csf/Al composites prepared by mechanical stirring and its properties were studied by numerical calculation and experiments. The conclusions are as follows:
(1) Compared with the steady circular flow of the melt in the round crucible, the melt will continuously impact and friction the end walls through mechanical stirring, as well as the convection and shear interaction between two or more vortexes, which result in a complex three-dimensional flow and higher turbulent kinetic energy inside the non-axisymmetric square crucible. High turbulent kinetic energy is beneficial to the uniform distribution of solute, which will be increased by increasing of the rotational velocity in the square crucible.
(2) The distribution of the short carbon fibers in the aluminum matrix prepared in square crucible is better than that in round crucible due to the non-axisymmetric structure. Because of the collision of the fibers with the crucible walls and the convection and shear between the secondary flows, the multi-orientation distribution of the fibers is promoted in the square crucible.
(3) The tensile strength of the Csf/Al composites prepared under the rotation velocity of 1000 rpm in the square crucible can be up to 172 MPa, which is 48.3% higher than that prepared by the round crucible under the same conditions. This is due to the uniformity and multi-orientation dispersion of the short fibers in the composites contributes to the improvement of the tensile strength.
This work was supported financially by the Innovation Team Project of Liaoning Province (No. LT2015020) and the Special Professor Project in Liaoning Province.
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