Microstructure characteristics and solidification behavior of wrought aluminum alloy ... ·...

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CHINA FOUNDRY 328 Vol.9 No.4 Microstructure characteristics and solidification behavior of wrought aluminum alloy 2024 rheo-diecast with self-inoculation method Male, born in 1971, Ph.D, Professor. His research interests mainly focus on Mg, Al and Zn alloys, semisolid processing, solidification process and surface modification. He has undertaken 15 projects and received 4 Gansu provincial-level awards for advancement in science and technology. He has published more than 60 papers and holds 11 invention patents of China. E-mail: [email protected] Received: 2011-12-20; Accepted: 2012-06-20 *Li Yuandong Li Yanlei 1 , *Li Yuandong 1, 2 , Li Chun 1 and Wu Huihui 1 (1. State Key Laboratory of Gansu Advanced Nonferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, China; 2. Key Laboratory of Nonferrous Metal Alloys and Processing, Ministry of Education, Lanzhou University of Technology, Lanzhou 730050, China) S emi-solid metal (SSM) processing has been developed as a near net-shape technique due to its many advantages such as less shrinkage porosity, heat-treatable, high integrity, and high mechanical properties. By using the SSM processing, the hot tearing tendency of alloys can be minimized during casting [1] . Some investigations were conducted to fabricate 2000 series wrought aluminum alloys with rheoforming and thixoforming processes. Wang et al. [2] reported a two-step reheating process, and the result showed that the grains of the semi-solid billet are finer and rounder than one-step reheating process. Xia’s [3] research showed that the near liquidus casting is able to produce thixotropic materials without employing stirring, refining and reheating. Rachmat et al. [4] reported that the EMC process Abstract: One important problem in casting wrought aluminum alloys is the high tendency to the formation of hot tears in the solidification process. By using semi-solid metal (SSM) processing, the hot tearing tendency of alloys can be minimized during casting. In the present research, the semi-solid slurry of wrought aluminum alloy 2024 was firstly prepared with a novel self-inoculation method (SIM), and then the microstructure characteristics of the semi-solid slurry and the rheo-diecastings cast with the semi-solid slurry were investigated. The results indicate that finer and more uniform globular primary α-Al particles can be obtained when the semi-solid slurry are isothermally held for a short period within the semi-solid temperature range, and the primary α -Al particles without entrapped liquid are uniformly fine, globular grains in the rheo-diecastings. The holding temperature and time affect the solid fraction, particle size, and shape factor. After the semi-solid slurry is held at 625 for 3 min and 5 min, the optimal values for the average equivalent diameter are 70.80 μm and 74.15 μm, and for the shape factor are 1.32 and 1.42, respectively. The solidification process of the rheo-diecastings is composed of the following two distinct stages: primary solidification process and secondary solidification process. The secondary solidification process consists further of the following three stages: (1) direct growth of secondary primary (α 2 ) phase from the surface of the primary α-Al phase particles without re-nucleation, (2) independent nucleation and growth of α 3 phase from the residual liquid, and (3) eutectic reaction at the end. Key words: self-inoculation method; wrought aluminum alloy 2024; solidification behavior; secondary solidification CLC numbers: TG146.21/249.2 Document code: A Article ID: 1672-6421(2012)04-328-09 could produce sound billets with fine and equiaxed grains throughout the entire structure, and the result showed the improved tensile properties of the semi-solid formed 2024 wrought aluminum alloy. Liu et al. [5] investigated a method to improve the mechanical properties of wrought aluminum alloy 2014 through heat treatment. Guo et al. [6] investigated a new rheoforming technique named as LSPSF process which can significantly reduce micro-segregation and improve the mechanical properties. Recently, many researchers have successfully studied the rheoforming process because of its simplicity without reheating unlike thixoforming. More recently the rheo-diecasting process has received significant attentions to reduce the formation of porosities because the semi-solid slurry flows into mold cavity in laminar way, and compared to the high-pressure die-casting (HPDC) process, less gas is entrapped into the slurry. The semi-solid rheo- diecasting is an innovative SSM processing technique for manufacturing components of wrought aluminum alloys. The most important consideration in rheoforming is how to obtain high-quality semi-solid slurry. In the present research, a new slurry-making method, self-

Transcript of Microstructure characteristics and solidification behavior of wrought aluminum alloy ... ·...

Page 1: Microstructure characteristics and solidification behavior of wrought aluminum alloy ... · 2012-12-10 · CHINA FOUNDRY 328 Vol.9 No.4 Microstructure characteristics and solidification

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Microstructure characteristics and solidification behavior of wrought aluminum alloy 2024 rheo-diecast with self-inoculation method

Male, born in 1971, Ph.D, Professor. His research interests mainly focus on Mg, Al and Zn alloys, semisolid processing, solidification process and surface modification. He has undertaken 15 projects and received 4 Gansu provincial-level awards for advancement in science and technology. He has published more than 60 papers and holds 11 invention patents of China. E-mail: [email protected]: 2011-12-20; Accepted: 2012-06-20

*Li Yuandong

Li Yanlei1, *Li Yuandong1, 2, Li Chun1 and Wu Huihui1

(1. State Key Laboratory of Gansu Advanced Nonferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050,

China; 2. Key Laboratory of Nonferrous Metal Alloys and Processing, Ministry of Education, Lanzhou University of Technology,

Lanzhou 730050, China)

Semi-solid metal (SSM) processing has been developed as a near net-shape technique due to its many advantages such

as less shrinkage porosity, heat-treatable, high integrity, and high mechanical properties. By using the SSM processing, the hot tearing tendency of alloys can be minimized during casting [1]. Some investigations were conducted to fabricate 2000 series wrought aluminum alloys with rheoforming and thixoforming processes. Wang et al. [2] reported a two-step reheating process, and the result showed that the grains of the semi-solid billet are finer and rounder than one-step reheating process. Xia’s [3] research showed that the near liquidus casting is able to produce thixotropic materials without employing stirring, refining and reheating. Rachmat et al. [4] reported that the EMC process

Abstract: One important problem in casting wrought aluminum alloys is the high tendency to the formation of hot tears in the solidification process. By using semi-solid metal (SSM) processing, the hot tearing tendency of alloys can be minimized during casting. In the present research, the semi-solid slurry of wrought aluminum alloy 2024 was firstly prepared with a novel self-inoculation method (SIM), and then the microstructure characteristics of the semi-solid slurry and the rheo-diecastings cast with the semi-solid slurry were investigated. The results indicate that finer and more uniform globular primary α-Al particles can be obtained when the semi-solid slurry are isothermally held for a short period within the semi-solid temperature range, and the primary α-Al particles without entrapped liquid are uniformly fine, globular grains in the rheo-diecastings. The holding temperature and time affect the solid fraction, particle size, and shape factor. After the semi-solid slurry is held at 625 ℃ for 3 min and 5 min, the optimal values for the average equivalent diameter are 70.80 μm and 74.15 μm, and for the shape factor are 1.32 and 1.42, respectively. The solidification process of the rheo-diecastings is composed of the following two distinct stages: primary solidification process and secondary solidification process. The secondary solidification process consists further of the following three stages: (1) direct growth of secondary primary (α2) phase from the surface of the primary α-Al phase particles without re-nucleation, (2) independent nucleation and growth of α3 phase from the residual liquid, and (3) eutectic reaction at the end.

Key words: self-inoculation method; wrought aluminum alloy 2024; solidification behavior; secondary solidificationCLC numbers: TG146.21/249.2 Document code: A Article ID: 1672-6421(2012)04-328-09

could produce sound billets with fine and equiaxed grains throughout the entire structure, and the result showed the improved tensile properties of the semi-solid formed 2024 wrought aluminum alloy. Liu et al. [5] investigated a method to improve the mechanical properties of wrought aluminum alloy 2014 through heat treatment. Guo et al. [6] investigated a new rheoforming technique named as LSPSF process which can significantly reduce micro-segregation and improve the mechanical properties. Recently, many researchers have successfully studied the rheoforming process because of its simplicity without reheating unlike thixoforming. More recently the rheo-diecasting process has received significant attentions to reduce the formation of porosities because the semi-solid slurry flows into mold cavity in laminar way, and compared to the high-pressure die-casting (HPDC) process, less gas is entrapped into the slurry. The semi-solid rheo-diecasting is an innovative SSM processing technique for manufacturing components of wrought aluminum alloys. The most important consideration in rheoforming is how to obtain high-quality semi-solid slurry.

In the present research, a new slurry-making method, self-

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inoculation method (SIM) [7], was presented to fabricate high-quality semi-solid slurry with fine and uniform globular microstructures. The SIM process integrated with high-pressure die-casting machine to prepare a sound semi-solid slurry of wrought aluminum alloy 2024. Then the microstructural characteristics of cold water quenched semi-solid slurry and rheo-diecasting products were investigated. The discussions focused on microstructure evolution and solidification behavior of the semi-solid slurry of wrought aluminum alloy 2024 in the SIM rheo-diecasting process.

1 Experimental detailThe experimental material used in this research was wrought aluminum alloy 2024 prepared by melting pure aluminum (99.95%), pure magnesium (99.99%), and Al-50%Cu master alloy. The chemical composition of the aluminum alloy 2024 was 4.53%Cu, 1.57%Mg, 0.5%Mn, 0.12%Si, 0.25%Fe, 0.02%Cr, 0.01%Ti, 0.08%Zn, and balance Al (by weight). The differential scanning calorimetric analysis (DSC) was performed on the aluminum alloy 2024 within its semi-solid temperature range with PE7 thermal analysis instrumentation. Figure 1 is the DSC curve of wrought aluminum alloy 2024, which can be used to measure the solidus and liquidus temperatures of the alloy by means of extrapolation. Through extrapolation, the solidus and liquidus temperatures were determined to be 490.5 ℃ and 636.2 ℃, respectively.

The SIM process is illustrated schematically in Fig. 2. There are two basic functional systems, a semi-solid slurry supply equipment system and a standard cold chamber HPDC machine. The SIM slurry supply system consists of a graphite crucible, a multi-stream mixing cooling channel, and an isothermal holding steel crucible. The cooling channel was specially designed for its length control, slope control, and water cooling control. It can be used to transfer the heat of the melt and conflux the four streams of liquid metal into one. The self-inoculant particles had the same composition as the melt and their sizes were about 5 mm × 5 mm × 5 mm. The semi-solid Fig. 2: Schematic diagram of SIM process

Fig. 1: DSC curve of wrought aluminum alloy 2024

slurry was provided by the SIM supply system, and collected in an accumulator when the slurry reached the given pouring temperature, and then the slurry was poured into the die cavity to achieve the final shape of the component.

The experimental alloy was melted in a crucible by using a 10-Kg SG2-75-10 electrical resistance furnace. A K-type thermocouple was inserted into the melt to measure its temperature, 1% C2Cl6 was used to make refinement when the melt reached 750 ℃, and the melt was quiescently cooled down to 720 ℃ (the melt treatment temperature). Then the self-inoculant was added to the melt, and the melt was stirred with a steel bar. Finally, the melt alloy was poured into an accumulator through multi-stream mixing cooling channel to get sound semi-solid slurry. After brushing ZnO coating on the surfaces of cooling channel and accumulator, a high-sensitivity 16-channel temperature acquisition device was used to record the temperature changes of the cooling channel. The temperature acquisition interval was 200 ms. Figure 3 shows the inlet and outlet temperatures of the cooling channel ranging 670 ℃ to 690 ℃ and 615 ℃ to 635 ℃, respectively. It has been shown by the result of foregoing work that when the temperature of melt treatment was 720 ℃, the inclination angle of cooling channel was 45°, the length of cooling channel was 600 mm, and the mass-fraction of added self-inoculant was 5%, sound semi-solid slurry could be obtained. The semi-solid slurry flew into the accumulator, which was set to a desired temperature between liquidus and solidus, and the slurry was isothermally held for 0-10 min. The morphology of primary solid phase of the semi-solid slurry was coarsened and spheroidized in the accumulator during the isothermal holding process in semi-solid state. It is shown by the DSC curve (Fig. 1) and the outlet temperature curve (Fig. 3) that the isothermal holding temperature of rheo-diecasting was determined as 635 ℃, 625 ℃, and 615 ℃, when the slurry was isothermally held for 0 min (no holding in holding furnace), 3 min, 5 min, and 10 min at the above temperatures, respectively. In the slurry accumulator, protective argon was used to avoid oxidation in the holding furnace. The accumulator was quenched in the cold water to keep up the microstructure characteristics obtained and the evolution process in semi-solid state. In the rheo-diecasting (RDC) process, the semi-solid slurry was held isothermally

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2 Results and discussion2.1 Microstructure evolution during

isothermal holding treatmentFigure 4 shows the microstructure of the alloy 2024 produced by permanent mold casting with a pouring temperature of 700 ℃, in which the white phase is composed of primary α-Al, and the black continuous matrix is intermetallic compounds. Figure 5 shows the morphologies of primary solid phase of the semi-solid slurry quenched in cold water after isothermal holding at 615 ℃, 625 ℃, and 635 ℃ for 0 min, 3 min, 5 min, and 10 min, respectively. In this work, the semi-solid slurry was prepared with self-inoculation method at the melt treatment temperature of 720 ℃. Its microstructure is composed of globular and nearly spherical primary α-Al particles homogeneously distributed in the liquid matrix, and most of the solid particles are isolated among non-entrapped liquid instead of the dendrites of well-developed primary α-Al, as shown in Fig. 4.

Fig. 3: Temperature variation curves of alloy melt during the pouring process into the cooling channel

Fig. 4: As-cast microstructure of alloy 2024 produced with permanent mold casting

in accumulator for 3-5 min, and immediately transferred to the shot chamber of the HPDC machine. The rheo-diecasting parameters were as follows: shot chamber temperature at 450 ℃, pressurization at 150 to 180 MPa, injection speed at 2 to 5 m·s-1, and mould cavity temperature at 250 ℃.

Specimens for the metallographic examination were polished and etched with a reagent containing 1vol.% HF, 1.5vol.% HCl, 2.5vol.% HNO3, and 95vol.% H2O. The microstructure of the specimens was examined using a MEF-3 optical microscope (OM), D/Max-2400 X-ray diffractometer (XRD), and JSM-6700F scanning electron microscopy (SEM) equipped with energy dispersive spectroscope (EDS) and backscattered electron imaging (BSE). The average equivalent diameter (D) and shape factor (F) were analyzed by using image analysis software Image-Pro Plus 5.0. The average equivalent diameter (D) and the shape factor (F) of primary α -Al particle were calculated using the average area (A), i.e. D = (4A/π)1/2, and the equation: F = 4πA/P2, respectively, where P is the average perimeter of a particle. The solid fraction was measured by means of self-made SSM analysis software.

(a)

(d)

(b)

(e)

(c)

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Figure 6(a) shows the effect of isothermal holding time on particle size and solid fraction at different holding temperatures. When the holding time was 0 min and the isothermal holding temperature was 635 ℃, the size of primary α -Al particles was 32.43 μm and the solid fraction was 25.14%. When the isothermal holding temperature was 625 ℃, the particle size of primary α -Al particles was about 37.68 μm and the solid fraction is 45.43%. As the isothermal holding temperature reached 615 ℃, the primary α-Al particles' diameter was 40.48 μm and the solid fraction was 51.17%. When the isothermal holding time reached 10 min, the final solid fractions were 30.93%, 44.81%, and 48.56% with the primary α-Al particle size being 114.12 μm, 131.37 μm, and 133.46 μm, respectively, at corresponding isothermal holding temperature of 635 ℃, 625 ℃, and 615 ℃. With the decrease of holding temperature, the size of primary α-Al particles became larger and the solid fraction got higher. At same holding temperature, increasing

holding time did not appear to change the solid fraction greatly, but the primary α-Al particles grew to a significantly larger size, as shown in Fig. 6(a). The relationship between the holding time and shape factor of the samples is illustrated in Fig. 6(b). The semi-solid slurry had a shape factor of 1.86, 1.71, 1.56, and 1.25 when holding at 635 ℃ for 0, 3, 5, and 10 min, respectively, indicating that the isothermal holding would lead to globular primary α-Al particles in the liquid matrix, as shown in Fig. 5 and Fig. 6(b). With increase of holding time, general trend of the morphology of primary α -Al particles would become spheriodized. With holding at temperature 615 ℃ and 625 ℃, the shape factor of the primary α-Al particles after being held for 10 min was greater than that with holding time of 5 min and 3 min, respectively. Because agglomeration of solid particles occurring in the slurry forms non-spherical clumps, and with the increase of holding time, the chance to form particles with irregular shape becomes greater [8]. The

Fig. 5: Microstructure evolution of water quenched semi-solid slurry of primary α -Al particles during isothermal holding treatment at: (a) 615 oC, (b) 625 oC, (c) 635 oC for 0 min; at: (d) 615 oC, (e) 625 oC, (f) 635 oC for 3 min; at: (g) 615 oC, (h) 625 oC, (i) 635 oC for 5 min; and at: (j) 615 oC, (k) 625 oC, (l) 635 oC for 10 min

Fig. 6: Effect of isothermal holding time on particle size and solid fraction (a) and on shape factor (b)

(g)

(j)

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ideal solid fraction for rheo-casting process should be between 30% and 50%. However, for holding the semi-solid slurry at temperature 615 ℃ and 635 ℃ for different time periods, the average solid fraction was 50% and 25%, respectively. Based on the experimental results, the optimal parameters after holding at 625 ℃ for 3 min and 5 min, appear to be 1.32 and 1.42 for the shape factor being, and 70.80 μm and 74.15 μm for the average equivalent diameter, respectively.

2.2 Solidification behavior of rheo-diecasting process

The solidification process of rheo-diecasting with self-inoculation method is composed of two distinct stages: the primary solidification and the secondary solidification. The former includes three key steps as follows: (1) add self-inoculant into the melt; (2) pour the melt through multi-stream mixing cooling channel; and (3) hold isothermally semi-solid slurry within semi-solid temperature range in the preheated crucible. In order to analyze the primary solidification process, two aspects are concerned in this paper: nucleation and its growth.

2.2.1 Nucleation and nuclei survivalOn account of the existence of some high-melting point phases, e.g. Al3Ti in alloy 2024, the melt temperature is high in the self-inoculation process before the self-inoculant is added into the melt. These high-melting point phases in the melt may act as a heterogeneous matrix of the globular primary α -Al particles in the subsequent solidification process. The added solid self-inoculant with small size into the liquid metal will be melted and distributed in the high temperature melt diffusively and become the matrix of heterogeneous nucleation, leading to more nuclei formation and grain refinement. Furthermore, changing the melt internal temperature field, the inoculant particles may be taken as internal chill points; they play a role in heat absorption from the melt and make the melt fall in undercooling state to reach suitable pouring temperature range for preparation of semi-solid slurry. The chill effect of inoculant causes local undercooling of the melt, and gives birth to a large number of nuclei around the self-inoculant particles,

Fig. 7: Rheo-diecasting microstructure of alloy 2024 with isothermal holding at 625 oC for holding time of 3 min (a) and 5 min (b)

(a) (b)

Figure 7 shows the rheo-diecasting microstructure of the alloy 2024 after being held at 625 ℃ for 3 min and 5 min, respectively. It can be seen from the Fig.7(a) and (b) that the uniformly distributed globular primary α-Al particles disperse separately in the liquid matrix. No coarse dendrite was found in any samples produced by SIM-RDC. The average equivalent diameter was 66.70 μm and 70.12 μm, and the shape factor is 1.36 and 1.34, respectively.

and then decreases the melt temperature gradient of the melt. Thus, the formation of atomic clusters will be stimulated and the nucleation rate increases [9]. The atomic clusters serve as the nucleation matrix and lead to rapid heterogeneous nucleation in the under-cooling state. Pouring process of the melt through multi-stream mixing cooling channel is composed of pouring superheated melt through inclined cooling channel and multi-stream mixing cooling channel into the preheated crucible. As the melt contacts with the cooling slope wall, the heat diffusion takes place on the channel surface, so that the cooling channel surface can provide remarkable undercooling, and a large number of nuclei are formed on the surface of the wall. The heterogeneous nucleation takes place immediately when the liquid flows on the surface of the wall or near the slope wall as the matrix of heterogeneous nucleation. When the melt flows on the surface of the multi-stream mixing cooling channel, the melt can be divided into four strands and converged on the bottom of the cooling channel. The fluid shear force and turbulence will make the nuclei dispersed in the entire melt. Meanwhile, the multi-stream mixing cooling channel can provide homogenous temperature and composition fields, promoting the survival of the nuclei.

The continuous heat extraction and melt convection near the cooling channel surfaces can boost the latent heat release and achieve eruptive nucleation. The nucleation rate and nuclei density are influenced by the degree of undercooling and the melt temperature. In the SIM process, the melt temperature decrease leads to the increase of nuclei density. Most small dendrites grow on the cooling sloping wall; some of those are fragmented due to the effect of fluid shear force, and then those fragments are transported into the melt. The quenched

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microstructure shows those fragments are widely distributed in the melt, as shown in Fig. 5(a-c). Lots of nuclei survive and disperse in the whole melt, which will lead to nucleus multiplication [10]. Preparation of high-quality semi-solid slurry with self-inoculation method makes the feature of effective and continuous nucleation and survival of the nuclei. The self-inoculant and cooling channel wall increase grain density and inhibit grain growth. The semi-solid slurry holds isothermally within semi-solid temperature range in preheated accumulator. A lot of atoms clusters are developed into nuclei inside the alloy melt at liquidus temperature [11]. Temperature and composition fields in the liquid become uniform, providing a relatively stable solid/liquid interface. These nuclei are favorable to the globular growth of the grains before their agglomeration [12]. The observable change in average particle size and shape factor is shown in Fig.6, the quenched microstructure grows in the semi-solid state, showing that a mixture of globular, equiaxed, and few dendrite particles have evolved into fine and spherical primary particles and have been dispersed in the slurry. The primary particles have reached the predetermined solid fraction via the control of temperature and time for rheo-diecasting.

2.2.2 Growth and microstructure evolutionIn the SIM process, the primary particles grow fast after formation of a large amount of nuclei on the multi–stream mixing cooling channel wall. The cooling channel makes the melt flow mixed and turbulent while the shear flow and turbulent flow make the primary particles spherical. The turbulent and mixing flow increase the forced convection and cooling rate; the forced convection will release the latent heat of crystallization, increase the solute diffusion rate, decrease the temperature gradient and concentration gradient near liquid-solid interface, and decrease interfacial turbulence, so that the stability of liquid-solid interface is maintained. At the same time, the solid particles may be washed away from the surface of cooling channel wall by shear flow, causing particle rotation and decreasing the degree of undercooling. The exit uniform temperature and concentration fields enforce the solid particles to grow uniformly in all direction and inhibit its preferential growth, so that the morphology of solid particles will evolve into spherical particles rapidly. The spherical microstructure in the SIM process is shown in Fig. 5(a). Another primary particles growing way is the dendrite growth depressed by the shear flow and turbulent flow. The dendrite structure is formed on the surface of the cooling channel. The plastic deformation of dendrite arm is developed by the shear force in the melt flow over the cooling channel wall. The low melting point solute aggregates at the root of dendrite arm as the forced convection is induced by the temperature and composition fluctuations. Fragmented dendrite arm is separated by its remelting at the root of secondary arm. The melt flow scouring and acceleration of gravity will strengthen fragment of the dendrite arms, and enhance the grain multiplication. The coarsened and fragmented grinding particles evolve into globular grains, which finally lead to the formation of fine and

spherical microstructures in the quenched samples. The morphology of primary α -Al particles will evolve

during isothermal holding of the semi-solid slurry in the semi-solid state. The grain growth is composed of two parts: agglomeration or coalescence and coarsening. The grains cohere to each other due to the decrease of relative movement between melt and grains. The primary α -Al particles agglomeration is likely to form an irregular-shaped non-spherical clump. With the decrease of holding temperature, the solid fraction will increase, therefore, increasing the chance of particles agglomeration [8].

The semi-solid slurry is transferred into the accumulator and held at constant temperature. The growing kinetics of primary α-Al particles in the semi-solid state is similar to a diffusion-controlled coarsening [13-14], and the isothermal coarsening of fine and spherical particles obeys Ostwald ripening. In the theory of Ostwald ripening, the particles size (d) increases with time (t) as expressed by the classical LSW relationship:

d n- (d0)n =Kt

where d0 is the initial particle size, d is the particle size at time t, n is the coarsening exponent, and K is the coarsening rate constant. The semi-solid slurry maintains in a statically stable condition and the grain growth is controlled by diffusion in the liquid. Based on our experiment conditions, by decreasing the holding time t, a fine, spherical, and homogeneous microstructure can be obtained. In the diffusion-controlled coarsening process, the primary α-Al particles will grow, the solute atoms move from the solid surface into the liquid, and the low melting point solute atoms aggregate at the root of dendrites arm. The fusing of dendrite arm root will take place due to the local low melting point. Figures 8 and 9 show this fragment mechanism obtained from SEM element analysis. The fragmentation of dendrite arm leads to the formation of fine and spherical grains.

Fig. 8: Example of dendrite arm root necking

2.3 Secondary solidification processThe secondary solidification starts when the semi-solid slurry in accumulator reaches the isothermal holding temperature and then transfers into the shot chamber of the HPDC machine

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nucleation. Furthermore, according to the classical solidification theory [15], a higher solidification speed leads to constitutional supercooling at the growing front of solid/liquid interface and a perturbation on the supercooled solid/liquid interface. The perturbation caused by primary particle tips of α-Al will reject the solute, and the concave parts of the interface will accumulate the solute and inhibit the growth of particles. As shown in Fig. 11(a), the EDS result reveals the difference in Cu and Mg concentrations in these two types of α-Al particles, α and α 2. The average concentration of Cu was measured to be 2.05 wt.% and 2.54wt.% in α and α 2, respectively, and 1.70wt.% and 2.14wt.% for Mg. The concentrations of Cu and Mg in the α 2 particles are higher than those in α -Al phase, showing that the enrichment of the solute element in the solid/liquid interface can inhibit the growth of particles. Finally, the particle tips generated by perturbation will evolve into irregular toe-shaped particles. After the formation of secondary primary (α 2) phase on the surface of the primary α-Al phase particles, the heterogeneous nucleation occurs in the residual liquid and all the nuclei will survive and grow because the residual liquid still has uniform temperature and composition field. The morphology of independent nucleation and growth (α 3) from residual liquid will evolve into fine equiaxed grains, whose particle size range is from 5 to 15 μm. The last process in the solidification of the residual liquid is the eutectic reaction [16]:

L →α-Al + θ-Al2Cu + S-Al2CuMg.

As shown in Fig.12, the XRD results prove that the as-cast structure is composed of α-Al, S-Al2CuMg and θ-Al2Cu phases. Software Pandat was employed to carry out the thermodynamic simulation of the solidification process of alloy

Fig. 9: Composition distribution in alloy 2024 microstructure with dendrite arm root

Fig. 10: SEM micrograph of secondary solidification microstructure

immediately. A higher cooling rate in HPDC leads to the formation of finer microstructure. As shown in Fig. 10, the big globular grey particles are primary α -Al and the toe-shaped α 2-Al particles are on its surface, while the small grey ones are fine α 3-Al particles. The white net phase forms the eutectic structure along primary phase boundaries. The secondary solidification process consists of three stages: (1) the secondary primary (α 2-Al) phase directly grows from the surface of the primary α -Al phase particles without re-nucleation, (2) independent nucleation and growth of α 3-Al from residual liquid, (3) eutectic reaction at the end.

The secondary primary (α 2) phase has the crystal structure similar to that of the primary α -Al phase, functioning as the heterogeneous matrix of toe-shaped α 2-Al particles.Therefore, the secondary primary (α 2) directly grows based on the primary α-Al phase which the process doesn’t need re-

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2024, and the similar result was obtained. Figure 13 (a) shows the eutectic distribution along the grain boundary with an irregular net-like morphology. The eutectics result from non-equilibrium solidification, which leads to the enrichment of the Cu and Mg solute along the grain boundary. The S-Al2CuMg and θ-Al2Cu phases are Cu-rich ones [17]. Figure 13 (c) shows the back-scattered electron imaging of SEM micrographs. It can be seen that the Cu-rich phase around the primary particles [18] is in bright area. The EDS result as illustrated in Fig. 11(b), shows that the atomic percentages of Cu and Mg are 32.09% and 66.94%, respectively, demonstrating that the Cu-rich phase is mainly the θ-Al2Cu phase. Under high cooling rate, eutectic solidification is developed by coupled growth. The eutectic α -Al attaches to the θ -Al2Cu phase and grows with bridge growth mechanism. The α-Al and θ -Al2Cu phases precipitate alternately after each other, and α -Al + θ -Al2Cu phase will take a laminar eutectic structure, as shown in Fig.13 (b).

3 Conclusions(1) When the melt fluid flows over the multi-stream

mixing cooling channel wall, a large degree of heterogeneous nucleation will occur on the wall surface due to the uniform composition and temperature fields. Under the gravitational action, the shear flow and turbulent flow lead to nucleus

Fig. 11: EDS analysis of different areas of alloy 2024 with rheo-diecasting

Fig.12: XRD analysis of different phases in rheo-diecasting alloy 2024

Fig.13: SEM/EBS micrographs showing the morphology of eutectic phase: (a) eutectics at the grain boundary, (b) laminar eutectic structure, and (c) Al2CuMg/Al2Cu eutectic phase

(a) (b)

(a)

(b)

(c)

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This work was financially supported by the National Natural Science Foundation of China (No.50964010), and the Basic Scientific Research Fund for the Universities in Gansu Province (No.1201ZTC056).

multiplication. The entire nuclei will survive and promote the formation of fine and globular primary α -Al phase. Particle coarsening through Ostwald ripening will occur during isothermal holding for a short time, and spherical and uniform microstructure will form.

(2) The SIM process can be used to fabricate high-quality semi-solid slurry with fine and uniform primary α -Al grains in rheo-diecasting. The primary α-Al phase particles are those with non-entrapped liquid. The average equivalent diameter is 70.80 μm and 74.15 μm, the shape factor is 1.32, and 1.42 after the semi-solid slurry isothermal holding at 625 ℃ for 3 min and 5 min, respectively.

(3) The secondary solidification process begins with direct growth of the secondary primary (α 2) phase from the surface of the primary α -Al phase particles without re-nucleation. Subsequently, the α 3–Al phase will independently nucleate from residual liquid and grow into fine equiaxed morphology. Finally, α -Al + θ -Al2Cu phase will form a laminar eutectic structure distributed along the boundary of grains with an irregular net-like morphology through eutectic reaction.

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