JIS ADC12 Aluminum Alloy Semi-Solid Slurry Preparation Technology by Applying Mechanical Vibration

Abstract

Semi-solid high pressure die casting is known as a process capable of forming high quality products. JIS ADC12 aluminum alloy is widely used for high pressure die casting, but this alloy has a narrow semi-solid temperature range, making it difficult to apply the semi-solid process. In this study, the preparation of ADC12 aluminum alloy slurry by applying mechanical vibration was attempted. Mechanical vibration was applied during the solidification of ADC12 alloy from the liquid to semi-solid state, and the effects of parameters of mechanical vibration and pouring temperature on the morphology of slurry have been investigated.

The application of mechanical vibration transformed the solid phase in the slurry from dendritic shape to fine spherical shape with increasing acceleration amplitude and velocity amplitude. As a result, slurry with solid particles dispersed in the liquid phase could be obtained. Moreover, it was found that high frequencies and displacement amplitudes above a certain value are required to obtain slurry with fine spherical solid particles. Fraction solid of slurry was affected by both the pouring temperature and vibration applied time, increasing with decreasing pouring temperature and increasing vibration time. Consequently, slurry with sufficiently fine spherical particles could be obtained by applying mechanical vibration with the frequency of 50 Hz and acceleration amplitude and velocity amplitude higher than 49.0 m/s² and 0.19 m/s, respectively. In this way, the fraction solid of ADC12 alloy slurry could be controlled by controlling the pouring temperature and vibrations time.

 

This Paper was Originally Published in Japanese in J. JFS 95 (2023) 16–22. Figures 8 and 9 were slightly changed.

1. Introduction

Semi-solid high pressure die casting (SS-HPDC) is a forming method in which metal in solid-liquid coexisting state is injected into a mold at high velocity. HPDC typically uses metal in a completely liquid state, which is injected into the mold at high velocity. Although this process results in the highly efficient production of metals with complex shapes, the metals are prone to defects caused by air entrainment. Contrarily, as semi-solid metals exhibit high viscosities, the SS-HDPC process can reduce the defects caused by air entrainment during injection. Additionally, the smaller solidification shrinkage can reduce the shrinkage defects, improve dimensional accuracy, and increase the life of the mold by lowering the process temperatures [1–6]. This process is very environmentally friendly as it requires less energy compared with conventional HPDC [7]. However, the SS-HPDC process results in lower formability than conventional HPDC. In order to increase fluidity in the mold, a semi-solid slurry in which the solid particles are dispersed in a liquid state must be utilized. Additionally, finer and more spherical solid particles are desirable to improve general fluidity.

JIS ADC12 Al-Si-Cu-based aluminum alloy accounts for more than 90% of the materials employed for HPDC in Japan [8]. This alloy exhibits an Al-Si composition close to eutectic, which ensures a small range of solid-liquid coexistence temperatures and skin-forming solidification [7]. Although this feature benefits HPDC regarding good fluidity due to the low melting point and ease of pressurization, it causes complications in the SS-HPDC process. In order to reduce porosity, which favors the SS-HPDC process, it is important to use a slurry with 0.3–0.5 fraction solid [9], but this temperature range is very narrow for ADC12 alloy, being less than 5 deg [10]. Consequently, ADC12 alloys are generally unsuitable for the semi-solid process [4], and only a few reports of forming methods, such as the gas-induced semi-solid method, under low fraction solid condition less than 0.15 [11]. This limited choice of materials is among the reasons SS-HPDC has not been widely adopted.

We previously developed a method for preparing semi-solid slurries by applying mechanical vibration [12]. We evaluated the method and conditions for applying vibration to obtain a slurry with finely dispersed spherical solid-phase particles and confirmed that sufficient flowability can be achieved by applying shear stress during forming [13, 14]. Mechanical vibration was also applied to ADC12 alloys, obtaining slurries with a fraction solid of about 0.5 [15]. Conversely, the vibrational conditions for preparing slurries with fine and spherical solid phases, as well as for obtaining stable fraction solids, have not been clarified.

The mechanical vibration parameters that affect particle shapes include the vibration frequency, f (Hz); displacement amplitude, d (mm); velocity amplitude, V (mm/s); and acceleration amplitude, A (mm/s²). In a sinusoidal wave vibration with d, the displacement, x(t); vibration, v(t); and acceleration, a(t), at a time, t, are described by the angular frequency, ω (rad/s) or f, as follows [12]:

  

\begin{equation} x(t) = d\sin\omega t = d\sin 2\pi ft \end{equation} (1)

  

\begin{equation} v(t) = d\omega\cos\omega t = d\cdot 2\pi f\cos 2\pi ft \end{equation} (2)

  

\begin{equation} a(t) = -d\omega^{2}\sin\omega t = -d\cdot(2\pi f)^{2}\sin 2\pi ft \end{equation} (3)

Therefore, d, V and A are described, as follows.

  

\begin{equation} d = V/\omega = V/2\pi f = A/(2\pi f)^{2} \end{equation} (4)

  

\begin{equation} V = d\omega = d\cdot 2\pi f = A/2\pi f \end{equation} (5)

  

\begin{equation} A = d\omega^{2} = d\cdot (2\pi f)^{2} = V\cdot 2\pi f \end{equation} (6)

In this study, the slurry was prepared by applying sinusoidal mechanical vibration during cooling to reach the semi-solid temperature. The effect of the vibration conditions on the morphology of solid-phase particles in a slurry was investigated in the preparation of an ADC12 alloy semi-solid slurry. Furthermore, the effects of the pouring temperature, T and vibration time, t, on the fraction solid, as well as the conditions required for the preparation of a stable slurry, were examined.

2. Experimental Procedure

2.1 Preparation of the semi-solid slurry by applying mechanical vibration

The slurry was prepared, following the method in the literature [12]. Figure 1 shows the schematic of the procedure. Briefly, a stainless steel container with an inner diameter, height, and thickness of 38.7, 90.0, and 1.2 mm, respectively, was mechanically pre-vibrated horizontally at room temperature. The inner surface of the container was coated with a boron nitride spray to prevent reactions with the molten metal and ensure the removal of the slurry from the container. Approximately 250 g of molten ADC12 alloy, which had been heated to a fixed temperature, was poured into the container, after which a semi-solid slurry was obtained by applying vibration for a fixed duration. To examine the microstructure of the slurry prepared using the above method, the slurry with the stainless steel container was water cooled after standing for 15 s, which is the time required for the HPDC process. Thereafter, the solid-liquid coexisting phase slurry was quenched. Table 1 presents the composition of ADC12 alloy employed in this experiment.

Fig. 1

Schematic representation of experimental apparatus.

Table 1 Chemical composition of JIS ADC12 aluminum alloy used in this study.

The samples were cut vertically, and the microstructures of their centers were observed under an optical microscope. The samples were set in epoxy resin at room temperature, ground with a silicon carbide abrasive paper, and polished with diamond paste. Afterward, the samples were etched in a 0.5% hydrofluoric acid solution.

2.2 Effects of the vibration parameters on the solid-phase shape

In order to investigate the effects of the vibration parameters on the size and shape of the solid particles in the slurry, experiments were conducted under the following fixed conditions: T = 923 K and t = 20 s. At first, to estimate the effect of A, samples were prepared at a fixed frequency (50 Hz) and varying A values of 29.4, 58.8 and 88.2 m/s² (corresponding to 3.0, 6.0 and 9.0 G, respectively); the corresponding V values were 0.09, 0.19 and 0.28 m/s, respectively, and the d values were 0.30, 0.60 and 0.89 mm, respectively. For comparison, samples were also prepared without vibration and allowed to stand for 35 s before water cooling. Next, to investigate the effects of the varying frequency, f and a(t), samples were prepared with V values of 0.19 and 0.28 m/s at f values of 12.5, 25, 50 and 100 Hz. Table 2 presents a summary of the experimental conditions. For the same V, as shown in eq. (6), higher f values produced larger A, whereas d decreased compared with eq. (4).

Table 2 Experimental conditions for investigating effects of mechanical vibration on solid particle shape.

2.3 Effects of the pouring temperature and vibration time on the fraction solid and shape of the solid particles

In order to investigate the effects of T and t on the fraction solid and shape of the solid particles, slurries were prepared under the following fixed vibration conditions: f = 50 Hz, A = 88.2 m/s², V = 0.28 m/s and d = 0.89 mm. T was set at three levels (903, 923 and 953 K), and t was varied at five levels (10, 15, 20, 25 and 30 s).

3. Results

3.1 Effect of the vibration conditions on the morphology of the solid particles

Figure 2 shows the microstructure of the center part of the samples prepared without mechanical vibration at f = 50 Hz and varying A, V and d. The white areas in the microstructure denote the primary α-Al phase, and the gray matrix denotes the eutectic structure of α-Al and Si. In the solid-liquid coexisting state before rapid cooling, the white α-Al phases represented the solid phase, and the eutectic matrix represented the liquid phase. Without vibration (Fig. 2(a)), the α-Al phase grew into a dendrite-like structure. Conversely, at lower A (29.4 m/s²) and V (0.06 m/s) (Fig. 2(b)), a rosette-like α-Al phase was dispersed in the eutectic phase, gradually transforming into a more spherical α-Al phase as A and V increased. The α-Al phases became progressively more finely spherical as A and V increased. Consequently, the application of A and V above a certain level produced semi-solid slurries with finely spherical solid phases.

Fig. 2

Microstructures of ADC12 alloy slurry. Without vibration and applied mechanical vibration with fixed frequency f = 50 Hz.

Figure 3 shows the microstructures when V was fixed at 0.19 or 0.28 m/s while f, A and d varied. At V = 0.19 m/s, the α-Al phase displayed dendrite-like growth at f = 12.5 Hz (Fig. 3(a)); however, it displayed a particle-like morphology at f = 25 Hz (Fig. 3(b)) and 50 Hz (Fig. 3(c)). Conversely, the α-Al phase tended to coarsen when f was set at 100 Hz (Fig. 3(d)). Additionally, a particulate α-Al phase was observed in both cases when V was set at 0.28 m/s.

Fig. 3

Microstructures of ADC12 alloy slurry vibrated at same velocity amplitude, but different frequencies and acceleration amplitudes.

3.2 Effects of the pouring temperature and vibration time on the fraction solid and shape of the solid particles

Figure 4 shows the results of microstructural observations of the samples that were prepared by varying T to three levels (903, 923 and 953 K) and varying t to five levels (10, 15, 20, 25 and 30 s). When t was short (10 s), the higher T, the lower the amount of α-Al phase, which was considered to be the solid phase before rapid cooling, and the amount of α-Al phase tended to increase as T decreased. At the same T, the amount of α-Al phase increased as t increased from 15 to 20 s. However, no significant changes were observed in the amount of α-Al phase at t = 25 and 30 s.

Fig. 4

Microstructures of ADC12 alloy slurry made at various pouring temperatures and vibration times. Vibration frequency and acceleration amplitude 50 Hz, 78.4 m/s².

4. Discussion

4.1 Effects of the vibration parameters on the shape of the solid particles

To evaluate the size and shape of the solid particles, image analysis (Image-Pro Plus 7.0, Media Cybernetics) was performed to measure the micrograph-obtained particle diameter d_p and particle roundness R. The white solid-phase particles were extracted from the micrographs, and the perimeter L (µm) and area A_p (µm²) of each particle were measured. R of each particle was calculated by the following equation [15]:

  

\begin{equation} R = L^{2}/(4\pi A_{p}) \end{equation} (7)

The particle was a perfect circle when R = 1; the value of R increased as the particle shape deviated from a perfect circle. Subsequently, d_p, which was considered the equivalent circle diameter, was calculated by the following equation:

  

\begin{equation} d_{p} = 2\sqrt{A_{p}/\pi} \end{equation} (8)

The mean d_p and mean R under each condition were evaluated using the area-weighted values. The area-weighted mean d_p, d_ps and area-weighted mean R, R_s, were calculated by weighing A_p, A_pi, with respect to d_p, d_pi, and R, R_i, as follows:

  

\begin{equation} d_{ps} = \Sigma d_{pi}A_{pi}/\Sigma A_{pi} \end{equation} (9)

  

\begin{equation} R_{s} = \Sigma R_{i}A_{pi}/\Sigma \mathrm{A}_{\text{pi}}. \end{equation} (10)

Figure 5 shows the calculated results of d_ps and R_s when f was set to 50 Hz while varying d_ps and V, as shown in Fig. 2. Further, d_ps and R_s decreased with the increasing A and V, forming fine and spherical particles [12].

Fig. 5

Effects of vibration conditions on mean particle diameter and roundness of α-Al particles at frequency of 50 Hz.

The relationship between acceleration amplitude A and mean particle diameter d_ps, mean roundness R_s is shown in Fig. 6, and the relationship between displacement amplitude d and mean particle diameter d_ps, mean roundness R_s is shown in Fig. 7, when the velocity amplitude V was fixed as 0.19 and 0.28 m/s (as shown in Fig. 3), respectively. Under conditions where the velocity amplitude V is constant, the mean particle diameter d_ps and mean roundness R_s decrease as the acceleration amplitude A increases up to 90 m/s². However, when the acceleration amplitude A reaches 120 m/s², the particles coarsen (Fig. 6). This occurs when the displacement amplitude d is considerably small, approximately 0.3 mm (Fig. 7). The smallest mean particle size d_ps and mean roundness R_s are observed when the displacement amplitude d ranges between 0.5 and 1.0 mm. Conversely, particles are coarser when the displacement amplitude d is higher than 2.0 mm. These large displacement amplitudes d conditions are at lower vibration frequencies f, such as 12.5 Hz, as shown in Table 2. Consequently, slurries with fine and spherical solid phases can be obtained when velocity and acceleration amplitudes are high, and the frequency and displacement amplitude are maintained above a certain level. These results have the same tendency as those obtained from a previous study using AC4CH alloy [12].

Fig. 6

Effects of acceleration amplitude on mean particle diameter and roundness of Al particles at same velocity amplitude.

Fig. 7

Effects of displacement amplitude on mean particle diameter and roundness of Al particles at same velocity amplitude.

In this method, ADC12 alloy, which exhibits skin-forming solidification, is processed by applying mechanical vibration during solidification, and the solid phase that solidifies near the container wall is released into the liquid phase to produce a slurry. When vibration with f and d higher than a certain level is applied, the solid phase near the wall surface of the container is released and dispersed into the liquid phase, resulting in the dispersion of fine spherical solid phases. At lower f values, the solid phase grows because of the short time before the release. Conversely, when d is smaller, the solid phase cannot flow sufficiently into the center of the container, leading to coarsening. This method facilitates the production of slurries even in skin-forming solidification-type alloys, which are typically challenging to prepare as semi-solid slurries, by applying appropriate mechanical vibration.

4.2 Effects of the pouring temperature and vibration time on the fraction solid and shape of the solid particles

A_p of the α-Al phase was measured from the micrographs by image analysis, and the fraction solid was calculated as the area ratio of the total area. Figure 8 shows the effects of T and t on the fraction solid. The fraction solid increased with time when t was between 10 and 20 s; however, it became constant at t = 20–25 s. The fraction solid tended to decrease with the increasing T, indicating that it could be controlled by controlling T. In this method, heat transfer proceeds when the heat of the molten metal is transferred to the stainless steel container and dissipated to the outside air. Here, the heat-transfer coefficients between the stainless steel container and molten metal and between the stainless steel container and outside air were considered. The heat-transfer coefficient between the mold and molten aluminum exceeded 1000 W/m²K when ceramic coating was present on the surface [16]. Conversely, the heat-transfer coefficient between the stainless steel container and outside air was about 7.7 W/m²K when the representative length was 41.1 mm outside the diameter of the stainless steel container and the air velocity was equal to V of the vibration (0.28 m/s) [17]. The relatively large heat-transfer coefficient between the stainless steel container and molten metal allows gradual heat transfer from the molten metal to the stainless steel container until about 20 s after pouring, thus decreasing the temperature of the molten metal and increasing the fraction solid. However, the small heat-transfer coefficient between the stainless steel container and outside air limits the amount of heat that is transferred to the outside air, stabilizing the heat quantity of the molten-metal system and stainless steel container at 20–25 s after pouring. Therefore, it is assumed that the fraction solid did not increase after 25 s of pouring. The challenge of the SS-HPDC formation of ADC12 alloys is attributed to the narrow temperature range of solid-liquid coexistence and skin-forming solidification. The suitable fraction solid for SS-HPDC forming is 0.3–0.5 [18]; however, for ADC12 alloys, this range is equivalent to a temperature of less than 5 K [10]. Consequently, controlling the fraction solid using the temperature becomes challenging. Figure 9 shows the relationship between the fraction solid, temperature, and enthalpy, as calculated by the CALPHAD method. The fraction solid increased rapidly at temperature of less than 842 K but increased almost linearly with the decreasing enthalpy. Applying mechanical vibration, as in this study, a semi-solid slurry was prepared by pouring a predetermined temperature of molten metal into a stainless steel container at room temperature. Therefore, similar to the cup-cast method [19], the fraction solid can be controlled, as the heat content of the slurry after pouring remains constant. Thus, a slurry with a predetermined fraction solid can be produced by controlling the amount of heat, even for ADC12 alloys. Additionally, the applied mechanical vibration affects the process, i.e., the solid phase that solidifies from the wall surface of the container is released, even for ADC12 alloy, which solidifies in a skin-forming manner, thereby preparing the slurry.

Fig. 8

Effects of pouring temperature and vibration time on area fraction of α-Al particles.

Fig. 9

Relation between fraction solid and temperature or molar enthalpy of ADC12 alloy.

5. Conclusion

By applying sinusoidal mechanical vibration during cooling, semi-solid slurry of ADC12 aluminum alloy was obtained, after which the effects of the vibration parameters, such as the vibration frequency, f, acceleration amplitude, A, velocity amplitude, V, and displacement amplitude, d, on the shape of the solid-phase particles in the slurry were investigated. Additionally, the effect of pouring temperature T on the fraction solid was examined, and the following were observed:

1. (1)    At constant f or A, fine and spherical solid-phase particles emerged in the slurry as V increased.
2. (2)    At fixed V, f and d affected the shape of the solid-phase particles in the slurry. Fine and spherical solid-phase particles were observed when f and d exceeded certain levels.
3. (3)    Thus, a slurry with a desirable solid-phase shape can be obtained by applying vibration at f = 50 Hz, A = 49.0 m/s² and V ≥ 0.19 m/s.

REFERENCES

• 1)   M.C.  Flemings: Solidification processing, Metall. Trans. 5 (1974) 2121–2134. doi:10.1007/BF02643923
• 2)  M.C. Flemings, J. Yurko and R. Martinez: Solidification Processes and Microstructures, (2004) pp. 3–14.
• 3)   H.V.  Atkinson: Semisolid processing of metallic materials, Mater. Sci. Technol. 26 (2010) 1401–1413. doi:10.1179/026708310X12815992418012
• 4)   T.  Haga and  S.  Nishida: Up Until Now and Future of Semi-Solid Processing, PLASTOS 1 (2018) 39–43. doi:10.32277/plastos.1.1_39
• 5)   H.  Fuse: Technological Trend of Semi-Solid Die Casting, PLASTOS 1 (2018) 714–718. doi:10.32277/plastos.1.10_714
• 6)   P.  Kapranos: Current State of Semi-Solid Net-Shape Die Casting, Metals 9 (2019) 1301. doi:10.3390/met9121301
• 7)   S.  Okano: Energy Consumption of Some Manufacture Processes for Aluminum Alloy Parts —Environmental Adaptability of Semi-Solid Processing 1—, J. JSTP 48 (2007) 96–101. doi:10.9773/sosei.48.96
• 8)  “Whats diecast”, Japan Die Casting Association, https://diecasting.or.jp/diecast/pdf/19-20.pdf, (accessed 2024 01-18).
• 9)   Y.  Murakami,  K.  Miwa,  N.  Omura and  S.  Tada: Effects of Injection Speed and Fraction Solid on Tensile Strength in Semisolid Injection Molding of AZ91D Magnesium Alloy, Mater. Trans. 53 (2012) 1775–1781. doi:10.2320/matertrans.F-M2012822
• 10)   Z.  Guo,  N.  Saunders,  A.P.  Miodownik and  J.P.  Schillé: Modelling of materials properties and behaviour critical to casting simulation, Mater. Sci. Eng. A 413–414 (2005) 465–469. doi:10.1016/j.msea.2005.09.036
• 11)   S.  Janudom,  T.  Rattanochaikul,  R.  Burapa,  S.  Wisutmethangoon and  J.  Wannasin: Feasibility of semi-solid die casting of ADC12 aluminum alloy, Trans. Nonferrous Met. Soc. China 20 (2010) 1756–1762. doi:10.1016/S1003-6326(09)60370-8
• 12)   Y.  Murakami,  K.  Miwa,  M.  Kito,  T.  Honda,  N.  Kanetake and  S.  Tada: Effects of Mechanical Vibration Factors on Size and Shape of Solid Particles in JIS AC4CH Aluminum Alloy Semi-Solid Slurry, Mater. Trans. 57 (2016) 163–167. doi:10.2320/matertrans.F-M2015837
• 13)   Y.  Murakami,  K.  Miwa,  M.  Kito,  T.  Honda,  N.  Kanetake and  S.  Tada: Effects of Injection Conditions in the Semi-Solid Injection Process on the Fluidity of JIS AC4CH Aluminum Alloy, Mater. Trans. 54 (2013) 1788–1794. doi:10.2320/matertrans.F-M2013812
• 14)   Y.  Murakami,  K.  Miwa,  M.  Kito,  T.  Honda,  N.  Kanetake and  S.  Tada: Fluidity Evaluation of the AC4CH (A356) Aluminum Alloy Semi-Solid Slurry Made by Mechanical Vibration Method, Mater. Trans. 57 (2016) 168–173. doi:10.2320/matertrans.F-M2015838
• 15)   Y.  Murakami,  K.  Miwa,  M.  Kito,  T.  Honda and  N.  Omura: Development of Slurry Preparation Method by Applying Mechanical Vibration, Solid State Phenom. 285 (2019) 333–338. doi:10.4028/www.scientific.net/SSP.285.333
• 16)   H.S.  Kim,  I.S.  Cho,  J.S.  Shin,  S.M.  Lee and  B.M.  Moon: Solidification Parameters Dependent on Interfacial Heat Transfer Coefficient between Aluminum Casting and Copper Mold, ISIJ Int. 45 (2005) 192–198. doi:10.2355/isijinternational.45.192
• 17)   A.  Žukauskas: Heat Transfer from Tubes in Crossflow, Adv. Heat Transf. 8 (1972) 93–160. doi:10.1016/S0065-2717(08)70038-8
• 18)   Y.  Murakami,  K.  Miwa,  N.  Omura and  S.  Tada: Effects of Injection Speed and Fraction Solid on Tensile Strength in Semisolid Injection Molding of AZ91D Magnesium Alloy, Mater. Trans. 53 (2012) 1775–1781. doi:10.2320/matertrans.F-M2012822
• 19)   N.  Hirata,  Y.M.  Zulaida,  M.  Itamura and  K.  Anzai: Microstructure of Al Alloy Semi-Solid Slurry Made in Metallic Vessel Using Vibrating Plate, Solid State Phenom. 192–193 (2013) 386–391. doi:10.4028/www.scientific.net/SSP.192-193.386