Effect of Casting Parameters on the Microstructure and Mechanical Propertiesof ADC10 Alloys Using a Semisolid Die Casting and Heat Treating Process

Byung Keun Kang1, Chun Pyo Hong1, Young Soo Jang2, Byoung Hee Choi2 and Il Sohn1,+

1Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-ku, Seoul 120-749, Korea2Samgsung Electronics Co., Ltd. 129, Samsung-ro Yeongtong-gu, Suwon-si, Gyeonggi-do 443-742, Korea

The effect of casting parameters on the microstructure and mechanical properties during semisolid die casting using commercial ADC10alloys was studied. Fine and uniform globular microstructures were produced using optimized casting conditions, where low pouringtemperatures of 878K (605°C) and pre-heating of the slurry-making container temperatures up to 523K (250°C) resulted in bettermicrostructural control. To obtain the conditions for high quality slurries within a mass production system, the microstructural characteristics ofslurries produced with various cooling rates were analyzed. Cooling rates between 0.1°C/s and 0.9°C/s were found to result in comparativelygood microstructural characteristics, which corresponded to form factors of 0.75 or greater and ¡-Al particles less than 65µm in the slurries. Thehardness and tensile strength were evaluated for T6 heat-treated semisolid die cast products and compared with the properties of high-pressuredie cast specimens. Transmission electron microcopy (TEM) and electron probe micro-analysis (EMPA) were also used to identify and verify theprecipitated secondary phases and the solute distribution. [doi:10.2320/matertrans.M2015406]

(Received November 4, 2015; Accepted December 15, 2015; Published February 25, 2016)

Keywords: electro-magnetic stirring, rheo-die casting, ADC10 alloy, cooling rate, tensile strength, hardness

1. Introduction

To improve the mechanical properties of products made ofaluminum alloy through typical casting and forging process-es, heat treating including solution and aging is used. In thesolution heat treatment step, alloying elements are dissolvedinto the aluminum matrix at temperatures above 723K(450°C) and then quenched in water at room temperature. Inthe aging step, the aluminum alloy is held between 403K(130°C) and 473K (200°C), the elements are precipitatedfrom the matrix and second phase clusters are formed. Theseclusters prevent the movement of dislocations, which resultin improved mechanical properties such as higher hardnessand tensile strength.1,2) For higher productivity and economicfeasibility, high-pressure die casting (HPDC) for lightweightvehicles and electronics has been widely used.3,4) However,in general it is difficult to conduct a commercial scale heattreatment process during HPDC because of the occurrence ofinner defects, such as porosity, as gases become trappedduring the high-speed filling of the mold with molten metal.A porosity increase in the HPDC process up to 1,000 timescan be observed when solution heat treatment is applied.5)

Thus, the application of HPDC for the production of high-strength products through heat treating has been limitedbecause of blister defects and size deformations originatingfrom the increased porosity. To meet these challenges, thesemisolid die casting process utilizing a semisolid slurry hasbeen studied as a method for reducing the inherent porositydefects within the HPDC and to provide a means to inhibitblister defects and size deformation.3,6­10)

Unlike traditional HDPC processes, the semisolid diecasting process minimizes turbulence during mold filling ofthe liquid metal by maintaining uniform solute and temper-ature fields within the molten metal and refining themicrostructure by maximizing the initial nucleation of themolten metal during solidification.7) In semisolid die casting,

the amount of gas mixed during the injection of molten metalinto the mold having a laminar flow pattern is reduced alongwith the total gas content in the molten metal. Further, themolten metal has a low pouring temperature, which lowersthe thermodynamic saturated gas solubilities in the melt toensure a higher internal quality. Thus, heat treatment in thesemisolid die casting process is possible and can enhance themechanical properties of the product, which is impossiblewith traditional HPDC. This is because of the lower gasconcentrations that exist after solution heat treatments, whichinhibits gas-induced blisters.4,11) Despite this advantage, theapplication of semisolid die casting to the commercialmanufacturing scale has yet to be established and has beeninvestigated only on an experimental level.

The initial quality of the slurry plays an essential role inobtaining improved product quality through a semisoliddie casting process. A high quality slurry has a uniformdispersion of fine and globular ¡-Al particles within thesemisolid melt. The form-factor, which is the globularity of¡-Al particles, ranges from 0 to 1, and a higher value of theform-factor indicates that the particles are more globular.12,13)

In the semisolid casting of ADC10 alloy, compared to otherAl-alloys, the relatively small difference between the liquidusand solidus makes it difficult to obtain a high qualitysemisolid slurry. Past work by Hong and Kim on thesemisolid die casting of ADC10 alloys have been pre-sented,8,14) wherein the effects of lower pouring temperatureand conditions of the slurry-making container was discussed.However, the detailed casting parameters such as the coolingrate, container thickness and temperature, and temperaturedistribution along the container have yet to be fullyunderstood.

In the present study, casting parameters in a 125-tonsemisolid die casting process with heat treatment of theADC10 alloy was investigated. Casting parameters includingpouring temperature, container thickness, and pre-heatingtemperature of the slurry-making container were optimizedto control and minimize porosity defects. By incorporating a+Corresponding author, E-mail: ilsohn@yonsei.ac.kr

Materials Transactions, Vol. 57, No. 3 (2016) pp. 410 to 416©2016 The Japan Institute of Metals and Materials

modified heat treatment process, the mechanical properties ofthe specimens were evaluated and compared with those ofconventional HPDC samples.

2. Experimental Procedure

2.1 Semisolid die casting experimental apparatus andslurry formation procedure

Figure 1 is a schematic representation of the equipmentused in the present study. The setup is composed of anautomated electromagnetic (EM) stirrer installed on a 125-tondie casting machine (BD-125V4, TOYO Machinery & MetalCol, LTD., Hyogo, Japan) that is used to produce a semisolidslurry during the die casting process and to feed the slurryinto the sleeve. The chemical composition of the ADC10(JIS) alloy used in this study was Al­9mass% Si­3mass%

Cu­0.3mass% Mg, and the liquidus temperature of theADC10 alloy is 866K (593°C). The molten metal wasmaintained at 973K (700°C) for 1 h to achieve thermalequilibrium within the melt and to remove excess dissolvedgas. The slurry container was made of stainless steel(SUS304) with a diameter of 50mm and a length of70mm. The inside of the container was coated with BN toinhibit excessive sticking of the melt and to ensure ease ofmelt pouring. Tcenter in Fig. 1 denotes the temperature at thecenter of the slurry, whereas Tsurface denotes the temperaturenear the surface of the container. To calculate the relationshipof the solid fraction and temperature, the Thermo-Calcsoftware was used. The intensity of the electromagneticstirring was approximately 0.03 T, as measured withapplication of g a single-phase 220V and 10A at 60Hz.Figure 2(a) shows the process sequence of a single semisoliddie casting cycle using electromagnetic stirring, whichincludes electromagnetic stirring, transportation of the semi-solid slurry, die casting, and product removal. The total cycletime is 30 s.

2.2 Specimen preparation and testingFigure 2(b) describes the semisolid die cast specimens

created by pouring the slurry manufactured by the automatedelectromagnetic stirrer into the 125-ton die casting machine.T6 heat treatments were conducted on the prepared speci-mens using a solution heat treatment and aging treatment asshown in Fig. 2(c). The microstructures were observed byoptical microscopy (KSM-IA5, SAMWS, Seoul, Korea),electron probe micro-analysis (EPMA) (JXA-8500F, JEOLLtd., Tokyo, Japan) and transmission electron microscopy(TEM) (JEM-2100F, JEOL Ltd., Tokyo, Japan). Hardnessand tensile strength were measured and compared with thoseof normal HPDC specimens. Rockwell B hardness testingwas performed using a hardness testing machine (ARK-600,MITUTOYO, Kanagawa, Japan). Tensile samples were

Fig. 1 A schematic drawing of the automated electromagnetic stirringsystem and the temperature measurement positions.

(a)

(c)(b)

Fig. 2 (a) The sequence of the semisolid die cast process. (b) A schematic drawing of the die cast specimen. (c) The conditions of T6 theheat treatment.

Effect of Casting Parameters on the Microstructure and Mechanical Properties of ADC10 Alloys 411

machined to the specifications of the ASTM E8M and testedusing a universal testing machine (UNITECH S series, R&BCo. Ltd., Daejeon, Korea).

3. Results and Discussions

3.1 Effect of the melt pouring temperature on themicrostructure

The products were made using the slurries that weremanufactured with the cycle time of the die casting process.To ensure a sound product, it is important to fabricate a high-quality slurry that has small deviations in the temperaturedistribution and uniform dispersion of ¡-Al particles withinthe slurry. To minimize these issues that can be detrimental toa high quality product, the slurry-making conditions wereoptimized by varying the pouring temperature, the temper-ature of the slurry container, and the container thickness.

The microstructures of the slurry were analyzed at variouspouring temperatures, 878K (605°C), 898K (625°C), and918K (645°C) corresponding to Figs. 3(a), 3(b), and 3(c),respectively. The electromagnetically stirred slurry isquenched in water at room temperature when the temperatureof each slurry reaches 583°C (856K), which corresponds to asolid fraction of 0.15. When the pouring temperature waslow, the slurry uniformity and globularity of the ¡-Alparticles improved because of the large amount of recales-cence. As the pouring temperature is lowered, superheat islowered and better rapid cooling can occur throughout theentire liquid, resulting in accelerated nucleation and growthtaking place in the absence of re-melting. At lower pouringtemperatures, the ¡-Al particles become smaller andglobularity increases.8,9,13) Therefore, 873K (605°C) wasfound to be the optimal pouring temperature to ensure auniform and fine semisolid slurry for die casting. Thus,further experiments were performed to explore other castingparameters at the optimal pouring temperature of 873K(605°C).

Figure 3(c) shows a coarse dendrite appearing whenpouring is performed at the high temperature of 918K(645°C). Increased pouring temperatures have higher super-heat, resulting in increased temperature gradients between themelt and the wall of the slurry-making container, which madesome of the dendrites grow. A fine and tiny microstructureis also produced when the nuclei re-melt because of localoverheating caused by the temperature difference between themolten metal at the surface and the center of the slurry-making container at high pouring temperatures. The un-melted nuclei grow dendritically and become coarse, as the

diffusion of solute elements at higher temperatures isincreased.

3.2 Effect of temperature distribution within the slurry-making container on the microstructure

In the case of the alloy ADC10, wherein the Si content isclose to the eutectic, an inappropriate slurry-making contain-er temperature can deteriorate the slurry quality, and in somecases can result in a solidified shell on the container wallbecause of the temperature difference between the surfaceand the center of the slurry.9) Figures 4 and 5 show,respectively, the microstructures and cooling curves at thecenter and surface of the slurry at the slurry-making containertemperatures of 293K (25°C) and 573K (250°C) with a fixedpouring temperature of 878K (605°C). The container waspre-heated in a box furnace up to the target temperature for1 h. Figure 4 shows the sizes of the ¡-Al particles to becomparable irrespective of the two container temperaturesaccording to the image analyzer using the image analyzersoftware (Image-Pro, Media Cybernetics, Inc., Rockville,USA). However, significant differences in the globularitywere observed with drastically reduced globularity at lowercontainer temperatures. Because of the large differencebetween the initial container temperature and the temperatureof the molten metal, solidification occurs quickly at thecontainer surface when the container temperature is low, asshown in Fig. 5(a). The large temperature gradient betweenthe center and surface provides an increased heat flux, as theheat is extracted from the container walls. When the containertemperature is 523K (250°C), the temperature differencebetween the surface and the center of the slurry is less than5K (5°C), as shown in Fig. 5(b). The temperature differencereference point is taken when the solid fraction of 0.15 nearthe surface of the container is achieved at 856K (583°C).Because of the temperature difference between the center andsurface across the slurry, solidification at the container surfaceis completed before the center of the slurry reaches the desiredsolid fraction, which causes significant inhomogeneity in thesolidified microstructure and thus differences in the mechani-cal properties. In addition, a separated solidified shell can alsobe formed that adheres onto the container walls, makingcontinuous operation impossible when the temperaturedifference is large between the surface and the center.

3.3 Effect of cooling rate on the microstructureTo identify the optimal in situ casting conditions at a

pouring temperature of 878K (605°C), the size and formfactor of the ¡-Al particles as a function of cooling rate is

Fig. 3 The observed microstructures obtained under various pouring temperatures: (a) 605°C, (b) 625°C and (c) 645°C.

B. K. Kang, C. P. Hong, Y. S. Jang, B. H. Choi and I. Sohn412

shown in Fig. 6. The cooling path traverses the liquidustemperature of 866K (593°C) to the semisolid temperature of856K (583°C) having a solid fraction of 0.15. As the coolingrate increases, the size of the ¡-Al particles and theglobularity (form factor) of the ¡-Al particles decrease. Inthe case of excessively high cooling rates above 0.5°C/s,prevalent dendrite formation within the microstructure allowseffective removal of excess heat, which can be detrimental tothe slurry die casting process. Thus, Fig. 6 shows that slurrieswith good microstructural characteristics (i.e., form factors of0.75 or greater, and ¡-Al particle with sizes less than 65 µm)can be produced at cooling rates between 0.1°C/s and0.5°C/s. The form factor (F), which is the criterion of theglobularity of the primary Al particles, is calculated via theequation: F = p2/4³A, where A is the area of the primary Alparticles and p is their perimeter.13)

(a) (b)

Fig. 5 Temperature-time curves obtained by the cooling curve experiment during slurry making for ADC10 alloy with various containertemperature: (a) 25°C and (b) 250°C. Here, Tss is the semisolid temperature at solid fraction 0.15 and ¦Tsur-cen is the temperaturedifference between the surface and the center positions.

Fig. 6 The variation of ¡ particle size and form factor with a pouringtemperature of 605°C for various cooling rates.

Fig. 4 The observed microstructures obtained under various temperature of slurry-making container and position: (a-1) 25°C (center),(a-2) 25°C (surface), (b-1) 250°C (center) and (b-2) 250°C (surface).

Effect of Casting Parameters on the Microstructure and Mechanical Properties of ADC10 Alloys 413

The microstructures at the surface of the slurry obtainedwith various slurry-making container thicknesses at a pouringtemperature of 878K (605°C) and a container temperature of523K (250°C) are shown in Fig. 7. For a 2-mm-thick slurry-making container, the globularity of the ¡-Al particles at thesurface of the slurry was low because the cooling rate at thesurface was approximately 0.9°C/s, even though the contain-er temperature was high, as shown in Fig. 7(a). Figure 7(b)shows the microstructure of increased globularity in the ¡-Alparticles for the 1mm wall thickness, where the measuredcooling rate was 0.3°C/s. This occurs because lowering thecooling rate at the wall surface during the initial stage ofsolidification increased the globularization time of theparticles. Thus, the cooling rate at the early stage ofsolidification appears to be a very important variable in theproduction of ADC10 alloy slurries. It can be considered thatwhen the size of the slurry-making container is changed, thetemperature and thickness of the container for obtaining asemisolid slurry with uniform and globular ¡-Al particlesshould be optimized using this concept.

3.4 Effect of heat treating on the mechanical propertiesHeat treatment was conducted to improve the strength of

the product when Al alloy is used for casting. However, inthe HPDC process, if air is trapped in the product because ofthe high-speed feeding of molten metal into the cavity duringa long duration of solution heat treatment, blisters may formon the product surface and make it difficult to conduct theheat treatment. To prevent inner defects, attempts have beenmade to reduce the solution treatment time; however, becausesufficient solute diffusion does not always occur, it is difficultto achieve the required mechanical properties in theproduct.4,9) For the semisolid die casting process using anautomated electromagnetic stirrer, blister formation can beprevented by minimizing the generation of bubbles withlaminar-flow pouring into the cavity. In addition, comparedwith the coarse dendritic particles in HPDC specimens, thetest specimens have finer and more uniform primary particles.Thus, because of the shorter diffusion distance when thesolute nuclei are diffused inside the primary particles,sufficient solute diffusion can be obtained during solutionheat treatment, even when the duration of the heat treatmentis short. By preventing blister formation, the short solutionheat treatment is expected to not only achieve high-strength

and high-quality products but also produce economic benefitsby shortening the entire heat treatment process. The heattreatment conditions used in the present study are shown inTable 1. Solution heat treatment was conducted undervarious ranges of temperature and time, and the resultingblister occurrences were analyzed. Blistering occurred whenthe solution heat treatment lasted for more than 2 hours at atemperature of 773K (500°C). Blisters were not observedwhen the solution heat treatment temperature was below748K (475°C).15) The microstructures of the specimensproduced at each solution heat treatment temperature areshown in Fig. 8. The globularity of eutectic Si was greatlydecreased when the solution heat treatment was conducted at723K (450°C). Temperatures at or below 723K (450°C)appear to be insufficient for solute diffusion during solutionheat treatment. Thus, the optimal conditions for solution heattreatment were determined to be 2 hours or less at 748K(475°C).

Figure 9 shows the TEM image of the secondary phaseprecipitated within the matrix. The shape of precipitatedparticle is round with the size of about 28.1 nm and themorphology of the precipitate particles is comparable to thesemisolid die cast and conventional HPDC specimens.Precipitate particle has 28.1 nm of diameter. Figure 10(a)and (b) indicate the back scattered electron (BSE) images ofmicrostructures with the distribution of solute elements fromthe electron probe micro-analysis (EPMA). Micro-porositiesobserved in HPDC specimens expand during heat treatmentand cause blisters and deformation, resulting in decreasedmechanical properties. Contrary to HDPC specimens, the

(a) (b)

Fig. 7 The observed microstructures at the surface obtained under various thickness of slurry-making container (cooling rate): (a) 2mm(C.R = 0.9°C/s) and (b) 1mm (C.R. = 0.3°C/s).

Table 1 Conditions of the solution treatment.

Solution treatment

Blistering Inside crackTemperature(°C)

Time(hour)

5003 O O

2 O O

4753 X O

2 X X

4503 X X

2 X X

B. K. Kang, C. P. Hong, Y. S. Jang, B. H. Choi and I. Sohn414

semisolid die cast specimens showed no micro porosities.8) Inaddition, the constitutional distribution of Si has an effect onthe precipitation. Long and rectangular Si particles hinder themovement of solute diffusion for clustering, which also

reduces the mechanical properties that arise from heattreating. As shown in Table 2 and Fig. 11, the hardnessand tensile strength of electromagnetically stirred semisoliddie cast specimens were tested and compared with those ofHPDC specimens. In HPDC specimens, the hardness afterheat treatment was reduced because of defects, such asblisters, inside the product.15) In contrast, the semisolid diecast specimens had an HRB value of 71.9, which is 25.7%greater than the HRB value of the HPDC die cast specimens.The tensile strengths of the HPDC die cast specimens afterheat treatment failed to exceed 200MPa, which is indicativeof low-level material properties; in contrast, tensile strengthsexceeding 300MPa were achieved in the semisolid die castspecimens because there was sufficient solute diffusion, evenwhen the solution heat treatment was short.

4. Conclusion

In the present study, high-quality slurries were producedusing a pouring temperature of 878K (605°C) and a slurry-

Fig. 9 The transmission electron microscopy (TEM) image of precipitatedsecondary phase.

(a)

(b)

Fig. 10 BSE image of microstructure and the constitutional distribution of solute, Si and Cu after heat treatment with EPMA:(a) semisolid die cast specimen and (b) HPDC specimen.

(a) (b) (c)

Fig. 8 The observed microstructures of semisolid die cast specimens after various temperature of solution treatments for 2 h: (a) 500°C,(b) 470°C and (c) 450°C.

Effect of Casting Parameters on the Microstructure and Mechanical Properties of ADC10 Alloys 415

making container temperature of 523K (250°C). Theseconditions yielded slurries with fine and uniform globular¡-Al particles (i.e., form factors of 0.75 or greater and ¡-Alparticles of less than 65 µm). The stable feeding characteristicof the semisolid die casting process produced specimens withfew porosity defects using a 2 hours solution heat treatmentat 748K (475°C). The mechanical properties were measuredand analyzed; the hardness of the specimens exceeded that ofHPDC specimens by 20%, and their tensile strength was300MPa.

Acknowledgments

This study was supported by the Brain Korea 21 PLUS(BK21 PLUS) Project at the Division of the Eco-Human-tronics Information Materials.

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(a) (b)

Fig. 11 Comparison of the mechanical properties between HPDC and thesemisolid die cast after T6 heat treatment; (a) hardness and (b) tensilestrength.

Table 2 Mechanical properties of two processing methods obtained by various heat treatment conditions.

MethodsHeat treatment Yield Strength

(MPa)Ultimate Tensile Strength

(MPa)Elongation

(%)Tempers Conditions

HPDCF As-cast 145.1 145.1 1.09

T6 Solution (475°C/2 h); Aging (170°C/7 h) 191.1 194.1 1.25

Semisoliddie casting

F As-cast 188.2 213.2 2.02

T6 Solution (475°C/2 h); Aging (170°C/7 h) 223.3 319.3 1.16

B. K. Kang, C. P. Hong, Y. S. Jang, B. H. Choi and I. Sohn416