Preparation of AlSi9Mg alloy semi-solid slurry by electromagnetic stirring combined with mechanical vibration
Vanluu Dao
1,2a, Shengdun Zhao
1b, Wenjie Lin
1c, Chenyang Zhang
1d1School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China
2Le Quy Don Technical University, Ha Noi, Viet Nam
a[email protected], b[email protected], c[email protected], d[email protected]
Keywords: semi-solid, AlSi9Mg alloy, electromagnetic stirring, mechanical vibration, EMSCMV
Abstract. The electromagnetic stirring combined with mechanical stirring (EMSCMV) method was developed for preparing the AlSi9Mg alloy semi-solid slurry. The experimental results demonstrate that, the preparation of semi-solid slurry by EMSCMV is sufficient to obtain fine microstructure of billet. The primary α-Al particles are small, spherical and uniform throughout the microstructure.
When increasing the stirring power and the vibrating power, the size of primary α-Al particles decreases while the shape factor increases, and its distribution is more uniform. The fine microstructure of billet prepared by EMSCMV was obtained at the stirring power of 2.0~2.4 kW and vibrating power of 0.8 kW.
Introduction
Semi-solid metal processing (SSM) is a potential forming technology which combined the advantages of conventional hot forging and casting. It can realize near-net-shape forming process with high quality in few forming steps and has been considered as one of the most promising production technologies in the 21st century [1-3]. However, preparation of specific slurry or billet with small non-dendritic spherical primary grains is a key step of SSM processing, which decided the fineness of the microstructure and thus the product quality. At present, there are many methods for preparing the aluminum alloy semi-solid slurry, such as electromagnetic stirring [4], mechanical stirring [5], twin screw rheocasting [6], new-rheocasting [7], low superheat and weak electromagnetic stirring [8-9], annulus electromagnetic stirring [10-11], ultrasonic vibration [12], low superheat pouring with a shear rate [13], strain induced melt activation (SIMA ) [14] and gas induced semi-solid [15], etc. Due to non-contact, cleanliness and high controllability, the electromagnetic stirring (EMS) was extensively used for the preparation of the aluminum alloy semi-solid and has been successfully applied in industry [3-4, 8-11, 16]. However, preparing semi-solid slurry by EMS has some drawbacks. Because of the electromagnetic induced skin effect, the magnetic force exerted in the edge is much higher than that in the inner of the slurry. Therefore, the microstructure is non-uniform. The primary grains are relatively small and slightly spherical in outer layer but are relatively larger and rosette-like or dendritic in inner of the slurry, as a result, the quality of billet is poor [10-11, 16].
In order to solve the above problems, an advanced method for semi-solid metal slurry preparation process, named the electromagnetic stirring combined with mechanical vibration (EMSCMV) was developed. Simultaneously, the effect of main process parameters such as stirring power and vibrating power on the microstructure was investigated in this study.
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Experimental procedure
The schematic diagram and experimental apparatus of preparation of aluminum alloy semi-solid slurry by EMSCMV is shown in Fig. 1, it contains crucible, heater, insulating layers, mechanical vibrator and stirrer which consists of yoke, core and coil. The yoke and core made of high permeability silicon steel, the coil made of copper wire and set up into the core, forming a three-phase two-poles stirrer. The stirrer design entails the placement of the coil around the crucible in order to generate a rotational motion along the horizontal direction. A K-type thermocouples is located at the center of the crucible. The crucible is made of stainless steel, inner diameter 63 mm, outer diameter 72 mm. Two mechanical vibration motors were symmetrically installed under the bottom plate. The electromagnetic stirrer power and each vibrator power are adjustable from 0~4.8 kW and 0~0.75 kW, respectively.
(a) (b)
Fig. 1. The schematic of SSM slurry preparation by EMSCMV (a) and experimental apparatus (b) In this work, the AlSi9Mg alloy was used, its chemical composition as follows: Si 9.23%, Mg 0.25%, Al balance, respectively. The liquidus and solidus temperature of this alloy obtained by differential scanning calorimetry (DSC) method are 595oC and 555oC, respectively.
The AlSi9Mg alloy was melted in a resistance furnace at 620~625oC. The melt was poured into the crucible, which was preheated to about 300oC before pouring. After pouring, during solidification process, the melt in the crucible was stirred and vibrated by electromagnetic force and mechanical vibration about 60s. Subsequently, the slurry was immediately cooled into water and quenched. The specimens for the metallographic examination were cut off from the middle of billet.
Each sample was roughly grinded, finely polished and etched by solution with 0.5% HF. The microstructures were observed and analyzed with optical microscope. The micrographs of the samples were analyzed by the quantitative metallographic analysis software. The equivalent particles diameter (d) and shape factor (Sf) of primary phases were calculated using as follows:
π d 4A
= (1)
2
4 P Sf A
= π (2)
where A and P are the area and perimeter of a particle, respectively. The Sf varies from 0 to 1, when the value of Sf is close to 1, the particle shape approaches to a circle. The average particles diameter (APD) and average shape factor (ASF) were based on counting equivalent particles diameter and shape factor of all primary α-Al particles in a photograph, respectively.
Results and discussion
Fig. 2 shows the microstructure at the edge and center of the billet prepared by normal EMS with stirring power 2.0 kW. It can be seen that, the microstructure is non-uniform. At the outside position, the primary α-Al particles is quite small and spherical. However, in the center of billet, there are many relatively large rosette-like α-Al particles and few spherical ones.
Fig. 2. The microstructure obtained by normal EMS, (a) at edge, (b) at center
Fig. 3 illustrates the microstructure at the edge and center of billet prepared using normal mechanical vibration with vibrating power 0.8 kW. It can be seen that, there are many relatively large dendritic primary α-Al particles with length of more than 200 µm and few large rosette-like ones throughout in the microstructure.
Fig. 3. The microstructure obtained by normal mechanical vibration, (a) at edges, (b) at center
Fig. 4 shows the microstructure at the edge and the center of the billet prepared by EMSCMV process under different vibrating power 0.4, 0.6 and 0.8 kW and with a stirring power 2.0 kW. It can be seen that, there are many small non-dendritic primary α-Al particles, which distribute uniformly throughout the microstructure. However, when the vibrating power is still low (0.4 kW), the primary α-Al particles are quite large and exist little rosette-like ones, the microstructure at the edge and at the center is different in some ways. As the vibrating power increases, all of α-Al particles evolve smaller and more spherical and uniformly distribute. As the vibrating power increases to 0.8 kW, the microstructure almost consists of very small spherical primary α-Al, there is no difference in microstructure between at the edge and at center (shown in Fig. 4 (e) and 4(f)). Compared with normal EMS or normal vibration, the microstructure obtained by EMSCMV significantly enhances.
It indicates that preparation of semi-solid slurry by EMSCMV is sufficient to obtain uniform small spherical primary α-Al particles. On the other hand, the vibrating power should not be too high otherwise it has a bad influence on thermocouple and other components of EMSCMV equipment.
Fig. 5 show the microstructure of the billet prepared by EMSCMV under different stirring power 0.4, 0.8, 1.2, 1.6, 2.0, 2.4, 2.8 and 3.2 kW with a vibrating power 0.8 kW. The effects of stirring power on the APD and ASF are shown in Fig. 6 under the above conditions.
Fig. 4. The microstructure obtained by EMSCMV under different vibrating power and position, (a) 0.4 kW, at edges, (b) 0.4 kW, at center, (c) 0.6 kW, at edge, (d) 0.6 kW, at center, (e) 0.8 kW, at
edge, (f) 0.8 kW, at center
It can be seen from Fig. 5 that, when the stirring power is low, the microstructure consists of relative large rosette-like primary α-Al particles. With the increase in stirring power, the primary α-Al particles evolve into globular and smaller. When the stirring power increases to 2.0 kW, the microstructure almost consists small spherical and distributes uniformly. When stirring power increases to 2.4 kW, the size and shape factor of primary α-Al particles slightly change. As the stirring power increases to 2.8 and 3.2 kW, though primary α-Al particles are very small spherical, however, there are many caverns throughout the microstructure (Fig. 5(g) and 5(h)). It indicates that the microstructures obtained under these conditions are not fine. Too high stirring power may lead to very high velocity of slurry, which induces high shear rate and decrease in the viscosity and turbulent flow might occur, which could arise air entrapment and oxide inclusions, as a result, many caverns emerge in the microstructure, stirring effect decreases. From Fig. 6 shows that with the increase in stirring power, the size of particles APD continuously decreases while the shape factor ASF increases continuously. When the stirring power is still low, the curves of APD and ASF are quite steep. However, as the stirring power is higher than 2.0 kW, these curves become less steep. If the stirring power is higher than 2.4 kW, the microstructure is not fine, as shown in Fig. 5(g) and 5(h). Therefore, we can conclude that the stirring power ranges of 2.0~2.4 kW is satisfactory to obtain the fine microstructure of semi-solid slurry prepared by EMSCMV, and stirring power should be limited to approximately 2.4 kW.
In summary, preparation of AlSi9Mg alloy semi-solid slurry by EMSCMV is satisfactory to obtain fine microstructure. The primary α-Al particles are small spherical and uniform throughout the microstructure. With the increase in stirring power and vibrating power, the primary α-Al particles become smaller and more spherical. The fine microstructure of semi-solid slurry prepared by EMSCMV was achieved at the stirring power of 2.0~2.4 kW and vibrating power of 0.8 kW.
Fig. 5. The microstructure obtained under different stirring power (a) 0.4 kW, (b) 0.8 kW, (c) 1.2 kW, (d) 1.6 kW, (e) 2.0 kW, (f) 2.4 kW, (g) 2.8 kW and (h) 3.2 kW (vibrating power 0.8 kW)
Fig. 6. The effect of stirring power on APD and ASF (vibrating power 0.8 kW)
Conclusions
Preparation of semi-solid slurry by EMSCMV is satisfactory to obtain small spherical and uniform primary α-Al particles throughout the microstructure. Compared to the normal EMS and normal mechanical vibration, the microstructure obtained by EMSCMV is obviously enhanced, the primary α-Al particles are distributed uniformly and are significantly smaller and more spherical.
When increasing the stirring and vibrating power, the primary α-Al particles involve smaller and more spherical, as a result, the grains size APD increases while the shape factor decreases. The best microstructure of the billet prepared by EMSCMV was obtained when the stirring and the vibrating power are between of 2.0~2.4 kW and 0.8 kW, respectively.
Acknowledgements
The authors are grateful to the National Science Foundation of China (Grant No. 5097522) for funding this project.
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Preparation of AlSi9Mg Alloy Semi-Solid Slurry by Electromagnetic Stirring Combined with Mechanical Vibration
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DOI References
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