2022 Volume 63 Issue 12 Pages 1657-1661
The amount of the primary phase crystallized in a eutectic alloy at a set cooling rate is determined by the alloy composition. Only a small amount of the primary phase can be crystallized in a near-eutectic composition alloy. Here we show an interesting phenomenon in which the microstructure is changed by the application of mechanical vibration to a near-eutectic Al–Si alloy during cooling from the molten metal to the semi-solid state. The amount of primary α-Al phase can be obtained with the application of mechanical vibration is more than equilibrium solidification. The amount of non-equilibrium α-Al phases, which is more than the amount with equilibrium solidification, increases with the velocity amplitude of the vibration. The non-equilibrium α-Al phases had uniform shape that consisted of fine spherical particles. These non-equilibrium α-Al phase crystals had the same composition as the primary α-Al phase crystals solidified without vibration. Primary Si particles could also be observed by the application of mechanical vibration.
Solidification is widely used for industrial processes such as casting, refining, and recycling. Eutectic solidification is a widespread phenomenon observed in industrial alloys, such as Fe–C, Al–Si, and Sn–Pb. In particular, the Al–Si based alloy is widely used as a casting aluminum alloy. The microstructure of the alloy influences the properties of products, particularly the mechanical properties.1,2) Therefore, control of the microstructure of the eutectic structure is a critical issue for industrial processes. The solidification of the primary α-Al phase is also used in the fractional crystallization method for the refining3,4) and recycling process5–7) of aluminum. The yield of this process is determined by the amount of primary α-Al phase formed during solidification.
We have examined the solidification phenomenon under the application of mechanical vibration using hypoeutectic Al–Si alloy.8–10) The aim of this study was to prepare a semi-solid slurry to use for a semi-solid high-pressure die-casting (HPDC) process. Mechanical vibration was applied to the aluminum alloy during cooling from a fully molten state to solid-liquid mixed phase. In this semi-solid process, the shape and size of the solid phases and fraction of solid have a strong effect on the formability. In hypoeutectic Al–Si alloys, such as the ASTM A386 alloy, the semi-solid material consists of primary α-Al particles and a residual liquid phase. The fraction of solid in the semi-solid slurry is determined by the amount of primary α-Al phase. However, only a small amount of primary α-Al phase can be present in the near-eutectic alloy.
We have previously reported that more of the primary α-Al phase could be obtained than that for equilibrium solidification by the application of mechanical vibration in the near-eutectic alloy JIS ADC12.10) If the amount of primary α-Al phase can be increased during the solidification process, then not only would control of the solid fraction in the semi-solid process be more easily achieved, but also the yield in the fractional crystallization method would be expected to increase. Mechanical vibration is known to affect solidification11–19) and this has some practical applications in casting processes.20–24) A non-equilibrium α-Al phase can be crystallized by the application of ultrasonic vibration, a process known as sono-solidification.23–26) On the other hand, mechanical vibration can be easily applied to liquid metal over a wide area. Vibrations with frequencies lower than ultrasonic vibration, such as mechanical vibrations, are known to affect the refinement of the microstructure; however, they have not been expected to affect the amount of α-Al phase.11,16)
In this study, we applied the mechanical vibration method9) to JIS ADC12 alloy. This alloy has a near-eutectic Al–Si content (Al–10.7 mass% Si–1.9 mass% Cu) and is commonly used in HPDC. The mechanical vibration parameters include the vibration frequency f (Hz), displacement amplitude d (mm), velocity amplitude V (mm/s), and acceleration amplitude A (mm/s2). These parameters have the following relationships:8)
| \begin{equation} d = V/2\pi f = A/(2\pi f)^{2}, \end{equation} | (1) |
| \begin{equation} V = d \cdot 2\pi f = A/2\pi f, \end{equation} | (2) |
| \begin{equation} A = d\cdot (2\pi f)^{2} = V\cdot 2\pi f. \end{equation} | (3) |
We investigated the effects of these parameters on the microstructure of the ADC12 aluminum alloy in a semi-solid state.
The method for preparation of a semi-solid ADC12 aluminum alloy slurry was similar to that previously reported,9) and a schematic of the experimental apparatus is shown in Fig. 1. Approximately 220 g or 1 kg of molten ADC12 alloy was poured into a vibrating stainless-steel container at 650°C. Mechanical vibration with a sinusoidal wave profile was applied in the horizontal direction for 20 s to produce a semi-solid slurry. The cooling rate at the temperature just above the liquidus line was approximately 3.0 K/s. The slurry was cooled rapidly in water after standing for 15 s to examine the microstructure of the semi-solid slurry. A stainless steel container with an inner diameter of 38.7 mm, 90.0 mm high, and 1.2 mm thick was used for the 220 g sample, while a stainless steel container with an inner diameter of 78.5 mm, 100.0 mm high, and 2.6 mm thick was used for the 1 kg sample of ADC12 alloy. The thickness of the crucible was varied according to the balance of the amount of heat with the molten metal. A BN release agent was sprayed on the internal surface of the container to prevent reaction between the molten metal and the container. The microstructures of the specimens were observed under an optical microscope; the specimens were set in epoxy resin at room temperature, ground with SiC paper, polished with diamond paste, and then etched in 0.5% HF solution.
Schematic illustration of the experimental apparatus.
Figure 2 shows the microstructures of the specimens. These micrographs show that the microstructure consists of the primary α-Al phase and a matrix composed of the eutectic α-Al and Si phases. Under conditions without vibration (Fig. 2(a)), the primary α-Al phase particles have a dendritic shape, and a few Si crystals were observed. When mechanical vibration is applied (Fig. 2(b)), the primary α-Al phase particles become finer globular particles that were distributed within the eutectic matrix, and the amount of Si crystals was increased. Figure 3 shows the microstructures of specimens prepared under each vibration condition. In these micrographs, the amount of primary α-Al phase particles and Si crystals appeared to increase with the velocity amplitude. The microstructures showed this same tendency for both the 220 g and 1 kg casting weights.
Microstructures of ADC12 alloy slurry prepared (a) without vibration and (b) with vibration at approximately f = 50 Hz, a = 245.0 m/s2, and v = 0.78 m/s.
Microstructures of ADC12 alloy slurry prepared by application of various mechanical vibration conditions.
Figure 4 shows the relationships between the areal fraction of primary α-Al particles and the various vibration parameters. The areal fraction was measured by image analysis. The correlation coefficient r between the areal fraction and the vibration parameters was calculated from these results (Fig. 3), and that between the areal fraction of α-Al particles and velocity amplitude was 0.91. These results indicate that the velocity amplitude has the highest correlation with the areal fraction of primary α-Al particles. The correlation coefficient between the areal fraction of α-Al particles and the vibration frequency was 0.15. Therefore, no correlation with the vibration frequency was evident. The correlation coefficients between the areal fraction of α-Al particles and the acceleration amplitude, and that between the areal fraction of α-Al particles and the displacement amplitude were 0.77 and 0.70, respectively, which indicate a weak correlation. It is noted that both the acceleration amplitude and displacement amplitude increased with the velocity amplitude at the same frequency, which is described as eqs. (1) and (2). The same trends were observed for both the 220 g and 1 kg casting weights. The primary α-Al phase in ADC12 alloy can crystallize at a volume fraction of less than 0.2.27) The areal fraction of α-Al particles was higher than that reported in Ref. 27), even under the condition without mechanical vibration, and it was increased with the velocity amplitude of the mechanical vibration. The fraction of primary α-Al phase was increased to 0.41 at most by the application of mechanical vibration. These results thus show that the non-equilibrium α-Al phase, which is more than the amount with equilibrium solidification, can be obtained by application of mechanical vibration. The energy of mass point m in the sinusoidal vibration is obtained as a sum of the potential energy U and kinetic energy K:
| \begin{equation} U = \frac{1}{2} m\omega^{2}x(t)^{2} = \frac{1}{2}m \omega^{2}d^{2} \sin^{2}\omega t, \end{equation} | (4) |
| \begin{equation} K = \frac{1}{2}m \omega^{2}v (t)^{2} = \frac{1}{2}m \omega^{2}d^{2} \cos^{2}\omega t. \end{equation} | (5) |
Effect of vibration parameters on the areal fraction of primary α-Al particles.
Therefore, the total energy E can be described as
| \begin{equation} E = U + K = \frac{1}{2}m \omega^{2}d^{2} = \frac{1}{2}mV^{2}. \end{equation} | (6) |
These equations indicate that the energy of mechanical vibration is proportional to the square of the velocity amplitude. The energy of the mechanical vibration is increased by the increase in the velocity amplitude at the same frequency. Therefore, these results suggest that the non-equilibrium α-Al phase can be obtained by application of high energy mechanical vibration.
Figure 5(a) shows a scanning electron microscopy (SEM) image and energy dispersive X-ray spectroscopy (EDX) distribution maps of the Al and Si elements in the samples prepared with and without mechanical vibration. Both samples consist of α-Al particles and the eutectic phase, and Si crystals. Figure 5(b) shows EDX quantitative analysis results for the eutectic phase and primary α-Al phase in the samples. The constituent concentration of the primary α-Al phase was almost the same between the samples prepared with and without mechanical vibration. On the other hand, Cu and Fe were concentrated in the eutectic phase. The concentration of Si in the eutectic phase was not so different between the samples prepared with and without mechanical vibration. When mechanical vibration was applied, the Si concentration in the eutectic phase did not change, despite the increase in the amount of α-Al phase particles. This is considered to be due to the increase in the amount of Si crystals with increase areal fraction of α-Al particles by mechanical vibration.
(a) SEM images and EDX distribution maps of Al and Si elements of samples prepared with and without mechanical vibration. (b) EDX quantitative analysis results of the eutectic phase and primary α-Al phase in the samples.
Tsunekawa and colleagues reported that crystallization of the non-equilibrium α-Al phase can be achieved by sono-solidification.23,25) This result is similar to our results. They also reported that the Si content in the primary α-Al phase and the eutectic temperature were increased by ultrasonic vibrations. This is due to the generation of high pressures of over 1 GPa by ultrasonic vibration by the collapse of cavitation bubbles. This leads to a change of the phase diagram; the liquidus temperature of α-Al increases at high pressure and the silicon content at the eutectic point also increases. This is considered to be the cause of the crystallization of the non-equilibrium α-Al phase by sono-solidification. On the other hand, when mechanical vibration is applied, the pressure is considered to be affected by the acceleration amplitude. However, the acceleration amplitude had a marginal effect on the amount of α-Al phase in the present experiments. The concentration of Si in the primary α-Al phase is also increased if the eutectic point is moved. Tsunekawa et al. reported that the Si content in the primary α-Al phase was increased by application of ultrasonic vibration.23) However, in the present study, the constituent concentration of the primary α-Al phase was almost the same for the samples prepared with and without mechanical vibration. Therefore, a different mechanism is considered to occur in the case of mechanical vibration.
Miwa et al. reported that when an Al–11%Si alloy is stirred mechanically during eutectic solidification, free Si crystals form in the residual liquid and the eutectic structures turn into α-rich phases.28) In this study, the α-Al phase exhibited two shapes: spherical dendrite fragments, which were considered to have broken down from the primary α-phase dendrites by stirring, and fine dendritic agglomerates, which were considered to be generated by eutectic phase decomposition. The increase in the amount of α-Al phase was thus similar to the present study. However, the shape of the α-Al phase in the present study was different, namely there was no fine dendritic agglomerated α-Al phase observed, and the size of the α-Al phase was coarser than in the present study. The reasons for these differences are considered as follows. The velocity amplitude in the previous study by Miwa et al. was 1.1 m/s, which close to that in the present experiments; however, the flow was in one direction. Flow by vibration has more effect on the diffusion of alloying elements than flow in one direction. Therefore, compositional supercooling is suppressed by diffusion of the alloying elements, so that growth of the α-Al phase crystals was achieved by the application of mechanical vibration.
These results indicated that a semi-solid slurry of near-eutectic aluminum alloy ADC12 can be obtained by the application of high-velocity amplitude vibration. The results also suggest that mechanical vibrations with high-velocity amplitude have the effect of refining the microstructure, similar to sono-solidification.23,24) This is corroborated by previous studies of solidification in hypo-eutectic Al–Si alloy,21,22) which have indicated that the microstructure was refined by mechanical vibration.
Mechanical vibration was applied to the solidification of a near-eutectic Al–Si alloy from a liquid state to a semi-solid state. The effect of the parameters of mechanical vibration on the microstructure, especially for the primary α-Al phase, was investigated.
The amount of primary α-Al phase was increased by the application of mechanical vibration during the solidification process. The amount of primary α-Al phase was increased with the frequency, displacement amplitude, velocity amplitude, and acceleration amplitude; however, the velocity amplitude had the highest correlation. The Si content in the primary α-Al phase was not varied by the application of mechanical vibration; therefore, the equilibrium diagram was not varied by the mechanical vibration.
