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 Journal of Materials Processing Technology224 (2015) 156–168

Contents lists available at ScienceDirect

 Journal of Materials Processing Technology

 journal homepage: www.elsevier .com/ locate / jmatprotec

Die design method for thin plates by indirect rheo-casting process and

effect of die cavity friction and punch speed on microstructures and

mechanical properties

Chul Kyu Jina, Chang Hyun Janga, Chung Gil Kangb,∗

a PrecisionManufacturing SystemDivision, Graduate School, Pusan National University, San30 Chang Jun-dong, Geum Jung-Gu,

Busan609-735, SouthKoreab Engineering Research Center forNet Shape andDieManufacturing, School of Mechanical Engineering, Pusan National University, San30 Chang Jun-dong,

Geum Jung-Gu, Busan 609-735, South Korea

a r t i c l e i n f o

 Article history:

Received 12 November 2014

Received in revised form 28 April 2015

Accepted 1 May2015

Available online12May2015

Keywords:

Semi-solid slurry

Indirect rheo-casting

Electromagnetic stirring

Thinplate

Filling simulation

A356 alloy

a b s t r a c t

Thin plates with a thickness of  1.2mm are fabricated from semi-solid A356 alloy through an indirect

rheo-casting process both with and without an electromagnetic stirrer (EMS). The thin die cavity for

forming is designedwith the fluid analysis software MAGMA. A semi-solid slurry with a solid fraction of 

40% is prepared and then injected into the die of a 200 t hydraulic press. Forming tests are performed on

the thin plates at two punch speeds (30 and 300mm/s) and two cavity friction conditions (m f = 0.4 and

m f =0.9). The formability, mechanical properties, andmicrostructure are then evaluated. The semi-solid

slurry obtained with an EMS contains fine and globular solid particles; the semi-solid slurry produced

without an EMS reveals rosette particles and coarser globular solid particles. At high friction (m f = 0.9),

the cavity is mainly filled with the liquid phase. At a higher punch rate, the thin plates show better

formability and a microstructure with fine and even solid particles. The tensile strength and elongation

of the thin plate formedwith a punch speed of 300mm/s in the cavitywith graphite lubrication (m f =0.9)

are 216MPa and 10%, respectively. These values are 57MPa and 5.5% higher, respectively, than those of 

the thin plate formed at a punch speed of 30mm/s.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The die casting process for aluminium involves the high-speed

injection ofmoltenmetal, which leads to internal defects because

of remaining gas or air in the molten metal, which in turn deteri-

orates the mechanical properties. Niu et al. (2000) f ound that the

volumeof gas porosityand the pore sizes in the castings are signif-

icantly reduced through the use of a vacuum during die casting.

This markedly improves the density and mechanical properties,

particularly the tensile strength and ductility. The forging pro-

cess has limited formability of near-net shapes and reduces the

after-treatmentproductivityand die life,whichmakeseco-friendly

production impossible. Squeeze casting is a metal forming process

wheremoltenmetal is solidifiedunder a relatively high pressure to

reduce gas or shrinkage porosity. However, this process produces

a rosette anddendrite structure (YueandChadwick, 1996) andhas

∗ Corresponding author. Tel.: +8251 5101455; fax: +8251 5181456.

E-mail address: [email protected] (C.G. Kang).

thedisadvantages ofa shorteneddielife, limited shapecomplexity,

difficultywithproducingthinparts,and limitedmaximumsize and

weight (Ghomashchi andVikhrov, 2000).

Flemings et al. (1976) developed a rheological (semi-solid)

material and the rheocasting process as an alternative to metal

forming processes such as die casting and forging. Their process

produces a highlyfluid slurryof solid spheroidsdispersed in liquid.

 Joly and Mehrabian (1976) showed that the viscosity of the slurry

at a given volume fraction of solids decreases with a decreasing

cooling rate and increasing shear rate. The rheo-forming method,

which is performed on a material in the semi-solid state (i.e., the

temperature is above the solid line but below the liquid one), is

a solution to the problems of casting and forming processes. In

the rheo-formingprocess, moltenaluminium is stirred as the tem-

perature is decreased in order to create semi-solid slurry with a

controlled grain size, which is then injected into a die and formed

with a press. Kapranos et al. (2000) described the process of pro-

ducingandassessinga high-quality thixoformed componentusing

an aluminium alloy and showed that thixoforming clearly has

near net-shape capability.  Ji et al. (2001) developed a twin-screw

http://dx.doi.org/10.1016/j.jmatprotec.2015.05.002

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C.K. Jin et al./ Journal of Materials Processing Technology 224(2015) 156–168 157

rheo-moulding process, and Fan et al. (2005) presented a rheo-

diecasting (RDC) process which directly uses liquid Al alloys.

Their results indicated that the RDC samples had close to zero

porosity and a fine and uniform microstructure throughout the

entire sampleunder theas-cast condition. Atkinson (2005) summ-

arised routes to spheroidal microstructures, types of semi-solid

processing, the advantages and disadvantages of these routes, the

background rheology, mathematical theories of thixotropy, the

transientbehaviourof semi-solidalloy slurries, and computational

modelling.

The one major drawback of rheo-forming or thixoforming for

processing semi-solid metals is controlling the liquid or solid seg-

regation (i.e., separation of the solid and liquid phases or uneven

distribution of the solid phase). When a semi-solid slurry fills the

die, the material comes in contact with the cavity’s wall. This

causes uneven flows leading to segregation of the solid phase

(primary -Al particles) from the liquid phase. Chen and Tsao

(1997) proposed semi-solid deformation mechanisms and pre-

dicted the segregation phenomenon based on deformation of a

phenomenologicalmodel. Kang et al. (2007) investigatedtheeffect

of changing the injection velocity on the globular microstructure

and mechanical properties of a product from semi-solid die cast-

ing. They found that the difference in the solid fraction between

samples with and without liquid segregation was approximately

15–20%. For thin plates, segregation of the solid and liquid phases

can be more severe, which makes their mechanical properties

uneven at different locations. Because of the problems associated

with segregation and the low initial forming temperatures for

semi-solid metal processing, no research has been carried out so

far on the fabrication of thin plates with the rheo-forming pro-

cess.

In this study, an indirect rheo-casting process was applied to

compensate for the disadvantages of indirect squeeze casting and

produce aluminium thin plates for electric and automobile parts

and fuel cell bipolar plates. The indirect rheo-casting process for

thin plates involves a low pouring temperature in a thin die cav-

ity. This makes it highly likely that the material will fail to fill the

cavity entirely and solidify starting from the centre, which willresult in incomplete forming. Therefore, the aim of this study was

to find ways to design a die for indirect rheo-casting which are

appropriate for the thin plate’s shape. A gate shape and overflow

adequate for rheological behaviour were designed with the soft-

ware MAGMA to allow the semi-solid slurry to fill the cavity. The

A356 alloy with a wide solid-liquid coexistent region was used as

the semi-solid slurry. A semi-solid slurry with fine and globular

solid particles was fabricated through the use of an electromag-

netic stirrer (EMS) to control the grain size of the A356 alloy.

The semi-solid slurry was injected into a die installed in a 200-

t hydraulic press to form a thin plate. Experiments for forming

thin plates were performed at two punch speeds and two cavity

friction conditions, and the effect of the punch speed and fric-

tionon the formability,microstructure, andmechanical propertieswas analysed. The microstructure and mechanical properties of 

the formed thin plate samples were measured at different loca-

tions.

2. Experimental procedure

 2.1. Semi-solid flowmodel

The fluid model of semi-solid materials shows that the viscos-

ity depends on the shear rate. Semi-solid materials lose viscosity

drastically as theshear rate increasesbuthave nearly constant vis-

cosityat very lowshearrates. Inorder toexpress thedependenceof 

viscosity on the shear rate in a high shear rate region, an empirical

formula called a power law is used (KimandKang, 2000; Atkinson,

2005):

 = k n−1 (1)

where  istheshear stress,  is theshearrate, k is thepower lawfac-

tor, andn is the power law index.Whenn=1, thematerialbecomes

a Newtonianfluid whose viscosity isthesame ask. Kim and Kang

(2000) set n=1 for the Newtonian fluidmodel. For the Ostwald–de

Waelefluidmodel,whichis forthesemi-solid state,theexperimen-tally obtained n value was −0.48 to 0.45 (shear rate= 3–2500s−1).

Thiswasapplied inMAGMASOFTforcomparisonof thefillinganal-

ysis results within the die. Their results demonstrated that the

Ostwald–deWaele fluid model is consistent with the experimen-

tal results. The viscous behaviour model of MAGMASOFT uses the

Ostwald–de Waele model, which expresses the non-Newtonian

aspect of semi-solid materials through a power law:

n = m n−1 (2)

where is the apparent dynamic viscosity, m is the Ostwald–de

Waele coefficient, n is the Ostwald–de Waele exponent, and is

the density.

Thegoverningequationsof MAGMASOFTare thecontrolvolume

finitedifferencemethod, continuityequation,Navier–Stokes equa-tion, energyequation,andvolumeoffluid (VOF)method. These are

the same used for liquids.

 2.2. Simulation preparation for die design

When a semi-solid slurryis compressed, theliquid phasemoves

towards the surface of the slurry, which leads to surface cracks.

The void content and segregation of the solid and liquid phases

becomemore severe at the side of the compressed specimen. Seo

et al. (2002) conducted compression experiments to investigate

the deformation behaviour of a semi-solid material with vary-

ing processing parameters, such as the test specimen size and

strain-rate.They suggestedthat therheo-forgingdie for thin plates

must be designed as an indirect type of structure. To form thin

platesusing a semi-solid slurry, a die for indirect rheo-castingwas

designedwherefilling is carried outbycompressing thesemi-solid

slurry in closed upper and bottomdies with a punch. Because this

structure is similar to thediecasting process, thedesignof thegate

systemand overflow is a critical variable.Analysis of thebehaviour

of the semi-solid slurryas it goes through the gateand fills the cav-

ity is a major factor for the design of thin plates. Seo et al. (2007)

analysed how the gate shape affects the liquid segregation of a

semi-solid slurry and concluded that a wider gate makes filling

more likely to be done in order and liquid segregation less likely

to form.

In order to examine the filling behaviour of a semi-

solid slurry according to the gate shape, a thin plate cavity

(150mm×150mm×1.2mm) was simulated with different gate

shapes. The simulations were performed by using the A356 thixo-module (Ostwald–de Waele model) of MAGMA. Table 1 lists the

 Table 1

Simulation parameters.

Parameters Values

Molten metal   Material A356

Liquidus temperature (T L) 617 ◦C

Solidus temperature (T S ) 547 ◦C

Initial temperature (T M ) 596 ◦C

Latentheat (Q ) 430kJ/kg

Die   Material SKD 61

Initial temperature (T D) 300 ◦C

Heat transfer coefficient   Material and die 7000W/m2 K

Die and die 1000W/m2 K

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158 C.K. Jin et al. / Journal of Materials Processing Technology 224 (2015) 156–168

 Table 2

Chemical composition of A356 alloy (wt%).

Si Mg Ti Fe Ni Mn Zn Pb Al

7.08 0.35 0.17 0.08 0.07 0.01 0.01 0.01 Bal.

conditions and heat transfer coefficient values used in the simula-

tion.

 2.3. Fabrication of semi-solid slurry

A356 alloy was used for the semi-solid slurry, and an EMS was

used to control the size of the solid particles. Because A356 alloy

hasgreatfluidityat thetwo-phasemushyzoneandcanenhance the

mechanical strength with heat treatment, it is used in automobile

parts such as knuckles, arms, and housing that require reliability.

In particular, themechanical properties of A356 are closely related

tothesize of theprimaryparticles, secondarydendritearm spacing

(SDAS), and Si particle shape and distribution within the eutectic

matrix. Table 2 lists thechemical compositionsof A356 alloy.Fig. 1

shows the solid fraction versus temperature of theA356 alloy. The

solid phase fraction of A356 alloy at different temperatures was

determined by using the obtained differential scanning calorime-

try (DSC) curve. The liquidus and solidus temperatures for A356

alloy were 617 and 547 ◦C, respectively, and the solid fraction for a

temperature of 596◦C was 40%.

Fig. 2(a) shows a photo of the EMSused in this study. The EMS

consisted of three phases (P, R, S) and three poles witha coil placed

Fig. 1. Solid fraction versus temperature of A356 alloy.

Fig. 2. Electromagnetic stirrer: (a) real picture and (b) schematic diagram.

Fig. 3. Variations in magnetic induction density at three positions as function of 

stirring current.

vertical to the core. The core for fixing the coil positionwas fabri-

cated by piling up several 0.35mm thick plates. The core consisted

of240unit laminationsof Si–Znalloyplates,andthecoilwaswound

around the core. Each phase was placed in a cylindrical direction,

as shown inFig. 2(b), tolet the currentmovealongside the coil, and

an electromagnetic force was generated in the cylindrical direc-

tion to stir the moltenmetal. The electromagnetic force of the EMS

was measured with a gaussmeter at three positions. Fig. 3 shows

thevariations in themagnetic inductiondensity measured at three

positions insidetheEMSasa functionof thecurrent. Themeasured

magnetic inductiondensity wasproportional to the increase in the

current at each position. At a current of 60A, the magnetic induc-

tion densities at the upper, middle, and lower positions were 640,

680and 1120G, respectively.Thestirringforceinducedshearstress

in themolten aluminium, which controlled thegrowthof dendrite

arms which form during solidification. Thus, it controlled thegrain

size of the solid phase and made the grains globular.

Fig. 4 shows the process to make a semi-solid slurry. First, the

cup is inserted intothe EMS, and a ladle isused toscoop themoltenmetal from the furnace. Then, stirring starts as the electric current

is applied while themoltenmetal is poured into the EMS cup. (The

molten metal is at a temperature of 680 ◦C in the furnace, 635 ◦C

in the ladle, and 620 ◦C in the cup.) Stirring is performed until the

temperature of the moltenmetal in the cup cools to596 ◦C, i.e., the

solid fraction ( fs) is 40%. It takes about 78s of stirring to reach this

level. The variables for the stirring experiment were the molten

metal temperature at the start of stirring (T S ), the stirring current

( A), and the stirring time (t ). Bae et al. (2007) suggested that fine

and globular solid particles can formwhen the moltenmetal tem-

perature at the start of stirring is below 655 ◦C, the stirring current

is 60A, and the stirring time is 60s based on an electromagnetic

stirring experiment using A356. Therefore, in the present experi-

ment, the molten metal temperature was 620◦

C, and the stirringcurrent was 60A, as given in Table 3.

Stainless steel 304 was used for the EMS cup. Stainless steel is

nonmagnetic because it is austenitic, so it is not affected by elec-

tromagnetic forces. In addition, stainless steel 304doesnotdeform

even at temperatures over 700 ◦C. Seo et al. (2002) performed a

compression experiment ona semi-solid slurryand examinedhow

changes intheheight anddiameter of thebilletaffecttheliquidseg-

regation. They concluded that a larger billetdiametermakes it less

likelyforporesand liquidsegregationtooccur. Inaddition, a greater

billet lengthincreasesthevoid content gets. Thus, after theamount

ofmaterial required forthinplate formingwasconsidered,the EMS

cup was designed to have a similar diameter to the inner diameter

of the die sleeve (60mm) and a much lower height. Fig. 5(a) and

(b) illustrates the shape dimensions of the EMS cup and actual cup,

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C.K. Jin et al./ Journal of Materials Processing Technology 224(2015) 156–168 159

Fig. 4. EMS process for fabricating semi-solid slurrywith fine andglobular solid particles.

 Table 3

Experimental conditions for semi-solid slurry fabrication.

Parameters Values

Stirring method Electromagnetic stirring

Stirring current (A) 60A

Temperature at thestart of 

stirring (T S )

620 ◦C

Temperature at thefinish

of stirring (T F )/solid

fraction ( fs)

596 ◦C/40%

Stirring time (t ) 60 s

Fig. 5. Geometries of stirring cup and slurry: (a) section of cup, (b) photo of cup,and (c)photoof semi-solid slurry.

respectively. Fig. 5(c) shows the semi-solid slurry. The cup is 2mm

thick and 100mm long. Because the molten metal rotates during

stirring because of the stirring force, the molten metal will flow

over the cup if the cup is filled to the inlet. Therefore, the cup was

filled to a height of 90mm with the moltenmetal. The volume of 

the fabricated semi-solid slurrywas165,597mm3, and the volume

of thethinplatemodel calculated using thespacefinder functionof 

the software UGNX6 was 161,304mm3. The amount of semi-solid

slurrywas adequate to fill the thin cavity.

 2.4. Indirect rheo-casting process

Thesemi-solidslurry( fs=40%)producedbytheEMSexperiment

was injected into the die in a 200t hydraulic press for the thin

plate forming experiment. Fig. 6 shows the 200t hydraulic press

with the installed die for thin plates. Table 4 and Fig. 7 present the

experimental conditions for the thin plate forming process and a

general diagramof theprocess, respectively. Thedie’s temperature

was maintained at 280–290 ◦C with a cartridge heater. The pres-

sure of the punch was set to 200MPa, and two speed were used:

30 and 300mm/s. The semi-solid slurry was injected into the die

and compressed with the punch; the pressurewas maintained for

5s. To ensure that the formedplate would not be deformed by the

ejector pin, the die was left open for 10s for some solidification.

Then, the plate was taken out of the die and immediately cooled in

water.

Fig. 6. 200-t hydraulic presswith die installed forthin plate fabrication.

 Table 4

Experimental conditions for indirect rheo-castingwith semi-solid slurry.

Parameters Value

Temperature and solid fraction

of semi-solid slurry

596 ◦C and 40%

Temperature of die 280–290 ◦C

Pressure of press punch (P P ) 200MPa

Velocity of press punch (V P ) 30 and 300mm/s

Lubricant for die cavity Graphite

Condition1 V P  = 300mm/s, without lubricant(m f  = 0.9)

Condition2 V P  = 300mm/s, graphite lubricant (m f  = 0.4)

Condition3 V P  = 30mm/s, graphite lubricant (m f  = 0.4)

Pinsky et al. (1984) discovered that segregation of the solid

and liquid phases increases with friction between the die and

material in compression and ring compression experiments on

semi-solid Sn–Pb alloy. Kang et al. (1999) performed compres-sion experiments on semi-solid materials at different strain rates

and concluded that the solid phase is evenly distributed at high

strain rates,which leads to less liquidsegregation. Therefore, three

forming experiments were performed to analyse the formability,

microstructure, and mechanical properties for two friction con-

ditions between the cavity and semi-solid slurry and two punch

speeds. A graphite lubricantwas sprayed in thecavityfor theform-

ing experiment with two punch speeds, and the experiment was

performedwith a non-lubricated cavity to analyse the segregation

of the solid and liquid phases aswell as defects for different cavity

friction states. Experiment condition1wasa non-lubricated cavity

(m f =0.9)witha punch speed of300mm/s.Conditions2 and 3were

a graphite-lubricatedcavity(m f =0.4)withpunchspeedsof 300and

30mm/s, respectively. These conditions are listed in Table 4. Five

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Fig. 7. Schematicdiagram of indirect rheo-castingprocess: (a) input semi-solid slurry, (b) forming, and(c) ejecting.

testswere performedundereach experiment conditiontoproduce

a total of 15 thin plate samples.

 2.5. Measurement of microstructure and mechanical properties

The size andglobularity of the solid particles in themicrostruc-

ture were measured using an image analyser. The equivalent

diameter (average size) of the solid particles is defined by Eq. (3)

below,andtheshape factor (globularity) represents theroundnessof the particles and is defined by Eq. (4). When R=1, a particle has

a perfectly circular shape; for a less globular particle, R>1 because

it hasa longer circumference than a circular particlewith thesame

area.

D =

 4 A

  (3)

R =P 2

4A  (4)

here D, R,  A, and  p are the diameter, radius, area, and perimeter,

respectively, of a globule.

Fig. 8. Mould-fillingbehaviour of semi-solid slurryaccording to gate shape: (a) straight gate and (b)fan gate.

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Fig. 9. (a)Detail designof fan gate system(unit: mm)and (b)material age.

In order to analyse the mechanical properties of the formed

thin plate, a tensile test was performed on specimens fabricated

withdifferentcavity locations.The tensile specimenswereflatwith

a 25mm gage length and 6mm width; there were fabricated in

accordance with the ASTM E 8M specifications (sub-size speci-men). The specimens were set to be as thick as the sample thin

plate. For the tensile test, a 25-t MTS was used, and the strain rate

wasset to1mm/min. Foraccuratemeasurement of theelongation,

an extensometer was used; this is a method to contact speci-

mens. The tensile test specimenswere used tomeasure theVickers

hardness. Each specimen was tested three times and five times

for tensile and hardness, respectively. The results were averaged,and the maximum and minimum values are presented with an

error bar.

Fig. 10. Mould-fillingbehaviour of semi-solid slurry formodelwith twooverflows at side: (a)velocity and (b)temperature.

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3. Result and discussion

 3.1. Simulation for die design

To design the gate shape for a thin plate, the biscuit was set

to 60mm, which was equivalent to the inner diameter of sleeve.

Fig. 8(a) shows the filling behaviour of the semi-solid slurry in a

die cavitywith a straight gate. As shown in the simulation results,

the slurry filled the cavity in a straight line along the gate’s shape

because of its viscosity. The sides of the cavity were filled by the

backflow of the fluid, which reached the end of the cavity and

returned. This may cause incompletefilling on the sides of the cav-

ity. Thus, the gatewas expanded to a fan type so that the material

could fill up the entire cavity after passing through the gate. The

length of the gate was shortened to avoid the semi-solid slurry

solidifying within the cavity duringfilling. Fig. 8(b) shows that the

entirewidth of the cavitywasfilled as thematerial passed through

the gate.

After a fanshapewasselected,specificdetailswere designed for

the gate. As shown by the gate cross-section in Fig. 9, the gatewas

18mm thick at thepartconnectedto thebiscuit andthennarrowed

closerto thecavity.Foreasyremovalofthe formedthinplate,5◦ and

6◦ gradientswere setfor thebiscuit’stop andbottomparts, respec-

tively, along the parting line of the die. Each corner was rounded.

When themoltenmetalcooledtoa semi-solid slurry, theupperpart

of the slurryhad a poor surface state because of the air inflow from

outside; the surface of the slurry was 3 ◦C lower than the centre

because of heat transfer. Because the upper part of the semi-solid

slurry was injected into the die first, the poor upper part of slurry

mayflowintothe cavityandcauseincompletefilling. Toaccount for

this possibility, the gate was located 4mmhigher than the bottom

part of thebiscuit (i.e., around the semi-solid slurryupper part), as

shown in Fig. 9(a). The poor upper part (further solidified) of the

slurry does not fill the cavity and is placed at the area of the semi-

solid slurryupperpart. Fig. 9(b) shows thematerial ageresults; the

initial material did not flow into the cavity. The material age refers

to the elapsed time for material inflow; a longer time indicates an

earlier material inflow.

Thefan gate causedthematerial tofillthe entirecavity.Theflow

rate at thesidesof thecavity increasedaswell, asshowninFig.8(b).

Because the flow at the sides of the cavity reaches the end of the

cavity faster than the flow at the centre of the cavity, the gas or air

in thecentre cannotbe released through thedie’spartinglineandis

lockedinside thecavityinstead.The formedsample surfacemay be

rough at the end of the cavity because of defects such as pores, air

pockets, and flow marks. In addition, an uneven filling speed may

cause the solid and liquidphases to segregate. Therefore, to obtain

a uniform flow rate within the cavity, two overflows at the sides

of the cavity were designed. Fig. 10 shows the simulation results

for a model with two overflows placed at the sides of the cavity.

As the faster flow on the cavity sides was led to the overflows,

the cavity was evenly filled overall. However, when the material

passed through the cavity’s centre, the velocity and temperature

dramatically dropped. After the cavity was totally filled, the tem-

perature at the end of the cavitywas below585 ◦C. These may lead

to incomplete filling at the end of the cavity in an actual forming

experiment,which cancause casting defects such as surface cracks

and pores. Because the material portion that solidifies at temper-

atures below 585 ◦C at the end of the cavity can also be removed

with an overflow, five overflows were placed at the cavity end, as

Fig. 11. Mould-fillingbehaviour (temperature) of semi-solid slurryfor modelwith twooverflowsat side andfiveoverflows at end.

Fig. 12. Pressure distributionin cavity formodels: (a) twooverflows at side andfive overflows at end, (b) four overflowsat side andfive overflows at end.

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C.K. Jin et al./ Journal of Materials Processing Technology 224(2015) 156–168 163

Fig. 13. Indirect rheo-castingdie for forming thin plateswith semi-solid slurry: (a) photo of real die cavity and (b) die structure.

shown in Fig. 11. The material part that solidified at below 585

◦

Cfilled the five overflows at the cavity end, and the temperature in

the cavitywas maintained at 590 ◦C.

Fig. 12(a) shows the pressure distribution in the cavity. The

pressure distributionwassignificantlyuneven. Thepressure at the

sides was about atmospheric pressure (1013mbar). Applied pres-

sure was lost, and some areas were subjected to pressures over

10,000mbar.The fiveoverflowsat theendof cavitywere subjected

to atmospheric pressure. The applied pressurewasnot transferred

to the cavity endbecause of the uneven pressure distribution. Thisresult shows that incomplete filling may occur at the entrance

of overflows in an actual forming experiment. In order to solve

this problem, two overflows were added at the sides, as shown

in Fig. 12(b). A pressure of over 4000mbarwas evenly distributed

over the cavitywith the two addedoverflows.

Based on the simulation results, the cavity shape and die struc-

ture were designed as shown in Fig. 13(a) and (b), respectively.

Because thedie’stemperatureshouldstay at280–290◦Cduringthe

Fig. 14. Microstructuresof semi-solid slurry(a) with and (b)without EMS.

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Fig. 15. Volume fraction, equivalent diameter, and roundness of solid particles in

semi-solid slurrywith andwithout EMS.

forming experiment, three andfourФ20mm holesweremachined

in the upper and bottom dies, respectively, as cartridge heater

holes. Then, Ф1.8mm holes were machined at the centres of the

upper and bottom die, and a K-type thermocouple was inserted

for measurement and control of the die temperature. For easy

removal of thethinplatesampleafterforming,Ф14mmholeswere

machined for ejector pins: one at the sleeve, six at the cavity, and

one for each overflow. The diameter of the punch for compress-

ing the semi-solid slurry was 60mm, which was the same as the

sleeve’s inner diameter. The upper part of the punchhad a diame-

ter of 100mmtoprevent the punch frombuckling during repeated

experiments. To prevent the aluminium alloy from sticking to the

die surface and enhance the surface hardness, the punch, sleeve,

and cavitywere heat-treated through nitriding.

Fig. 16. Thin plate samples fabricated with different punch speeds (V P ) and friction states of cavity: (a) 30mm/s,without lubricant; (b) 300mm/s, graphite lubricant; and

(c) 30mm/s, graphite lubricant.

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Fig. 17. Thickness of formedthin plates.

 3.2. Microstructures of semi-solid slurry

Before the thin plate forming experiment, the semi-solid slurryneeded to be cooled down immediately in water for analysis of 

themicrostructure. Fig. 14 shows themicrostructures of the semi-

solid slurry stirred with an EMS and not stirred but cooled down

to 596 ◦C. The microstructure showed primary -Al for the solid

phase and a eutectic phase for what used to be the liquid phase

in the solid–liquid state. In the microstructure with stirring, fine

and globular solid particles were distributed evenly at the centre

of the semi-solid slurry because the uneven growth of particles

wascontrolled (Fig. 14(a)).At the sides of the semi-solid slurry, the

cup’s surfacewasabout3 ◦C lower at the centre because of the heat

transfer during stirring. Thus, the solid particleswere coarser than

at thecentre. For themicrostructurewithout stirring, a large num-

ber of rosette particles, dendrites, and some fine globular particles

were distributed (Fig. 14(b)). The moltenmetal grew as a dendrite

structure until it cooleddown to596 ◦C. The grain sizewas coarser

around the sides of the material.

Fig. 15 shows the volume fraction, equivalent diameter, and

roundness of the solid particles in the semi-solid slurry with and

without an EMS. At thecentre, theequivalent diameter andround-

ness of thesolidparticlesof thesemi-solid slurrywithanEMSwere

75mand1.5,respectively.Thoseof thesolidparticlesof thesemi-

solid slurrywithout an EMSwere 110mand2.3, respectively. On

the sides, the equivalent diameter and roundness of the solid par-

ticles of the semi-solid slurry with an EMS were 82m and 1.65,

respectively. These of the solid particles of the semi-solid slurry

without an EMSwere 119mand3.0, respectively. Thesemi-solid

slurry with an EMS had volume fractions of 45% at the centre and

41% at the sides. The semi-solid slurrywithout anEMS had volume

fractions of 43% at the centre and 40% at the sides.

The results confirmed that a semi-solid slurry with fine and

globular solid particles can be obtained with an EMS. If a semi-

solid slurry with uneven and coarse particles is injected into the

die and then compressed by a punch, incomplete filling ormisrun

(short shot) can be caused by the poor fluidity, and the mechan-

ical properties of the formed sample will be low because of the

poor microstructure. Therefore, a semi-solid slurry produced with

an EMS was used in the experiment on thin plate forming.

 3.3. Formability of thin plate

Fig. 16 illustrates a selection of the best thin plate samples fab-

ricated under each condition. The thin plate samples were formed

at two punch speeds (30 and 300mm/s) and cavity friction condi-

tions (with and without the graphite lubricant). The black part on

the surface of the plate sample is the graphite lubricant. Fig. 16(a)

shows a sample formed at a punch speed of 300mm/swithout the

graphite lubricantin thecavity (m f =0.9). Because semi-solidslurry

exhibits severe stickiness from its viscosity, the punch may getstuck in the sleeve if the slurry is compressed without a lubricant.

Therefore, in this experiment, a small amount of graphite lubri-

cant was sprayed within the die sleeve. The sample formed under

this condition had short shots at the sides of the end of the cav-

ity. As shown by the simulation result for the pressure in Fig. 5(b),

this short shot can be attributed to the decreased pressure at the

entrance of the overflow at the end of the cavity. Fig. 16(b) and(c)

shows the thin plate samples formed with a graphite lubricant in

the cavity (m f =0.4) at punch speeds of 300 and 30mm/s, respec-

tively. The thin plate sample formed at a punch speed of 300mm/s

with the graphite lubricant filled the cavity completely without

any short shots. Although only two overflows were connected at

the end of the cavity, all five overflows were formed in the actual

experiment, and the entrance of the overflow was broken and fell

off because of the ejection force from the sample being pulled out.

Formingata punchspeedof30mm/s ledtoashortshotattheendof 

the cavity, and six overflows resulted in short shots (Fig. 16(c)). As

shownby thesimulationresults forthefillingtemperaturein Fig.4,

this is the point where the material’s temperature dropped below

585 ◦C. Because the punch speed was 10 times less than the sim-

ulation condition, the temperature decreased even farther, which

may have caused incomplete forming.

Fig. 17 shows the thickness of the formed thin plate samples.

Groups A (A1, A2, and A3) and B (B1, B2, B3) indicate the centre

and side positions, respectively, of the sample. A1 and B1 (near the

Fig. 18. Simulationresults of solidification behaviour.

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166 C.K. Jin et al. / Journal of Materials Processing Technology 224 (2015) 156–168

gate) were thicker thanA3and B3 (near the overflow), and group B

was thicker than group A. Thiswas presumed to be due to shrink-

age upon solidification. Fig. 18 shows the simulation results of the

solidificationbehaviourafterfilling.Similar tothefillingpatternsof 

thesemi-solid slurry, the solidification patterns in a contour shape

formed toward the gate at the overflow. Because positions B1 and

A1underwentsolidificationslowlycomparedtotheotherpositions

in the cavity, theycan beexpected tobe thicker. Incontrast, B3and

A3 solidified faster than the other positions, which suggests that

they should be thinner than the other positions. The thickness of 

the thin plate fabricated with graphite lubricant in the cavity at

a punch speed of 300mm/s was the nearest to the thickness of 

the die cavity; thus, it was the most precisely formedproduct. The

results for thin plate forming using a semi-solid slurry suggest that

the optimal conditions are a high punch speed of 300mm/s and

graphite lubricant in the cavity.

Fig.19. Microstructureof thinplatesamples fabricatedwithdifferentpunch speeds (V P ) andfrictionstates of cavity: (a)30mm/s,without lubricant; (b)300mm/s, graphite

lubricant; and (c) 30mm/s, graphite lubricant.

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 3.4. Microstructures of thin plate

Fig. 19(a) illustrates the microstructure of the sample formed

with a punchspeed of 300mm/s and with no graphite lubricant in

the cavity (condition 1). The microstructure shows that the solid

and liquid phases were severely segregated at positions F and E.

At position F (near gate), there were a few coarse dendrites from

coalescence;at position E (far from gate), therewere fewsolid par-

ticles. When there was high friction, the flow of solid particles was

difficult.Thus, a small part of thesolidphasemoved into thecavity,

and the liquid phase, which had better fluidity, filled the cavity. In

otherwords,high friction between the semi-solid material anddie

cavity cause a high degree of segregation between the solid and

liquid phases.

Fig. 19(b) and (c) shows the microstructures of the samples

formed at punch speeds of 300 and 30mm/s (conditions 1 and

2), respectively, with graphite lubricant in the cavity. The punch

speed of 300mm/s produced a finer grain size and denser distri-

bution of the solid phase than the punch speed of 30mm/s. At

30mm/s, the gap between solid particles was large because there

wasa lownumberofsolidparticleswithadiameterofover100m.

The porositywas high at position E. Fig. 20 shows thevolumefrac-

tion, equivalent diameter, and roundness of solid particles in the

samples formedat punch speedsof 300and 30mm/swithgraphite

lubricant. Forthe sampleformedat300mm/s,theequivalent diam-

eter, roundness, and volume fraction of solid particles at position

F were 65m, 1.25, and 45.5%, respectively. At position E, they

were 62m, 1.27, and 43.5%, respectively. For the sample formed

at 30mm/s, the equivalent diameter, roundness, and volume frac-

tion of the solid particles at position F were 72m, 1.45 and 36%,

respectively. At position E, they were 70m, 1.4, and 35%, respec-

tively. At position E, the solid phase was slightly smaller and more

globular, but the volume fractionwas lower than at position F. The

results show that themicrostructure of thin plate samples formed

at 300mm/s and 30mm/s were finer andmore globular compared

with themicrostructure of the semi-solid slurrywith EMS.

When a semi-solid slurrywas compressed with a punch, coarse

solid particles from coalescence broke off, and single solid parti-cles were deformed. Theseparticles and the liquid phase filled the

die cavity. Plastic deformation of the solid particles occurred by

forced convection during filling. A small and globular grain struc-

ture then formed. The moving velocity and distance of the solid

phase increase for smaller andmore globular grains.

 3.5. Mechanical properties of thin plate

Fig. 21 shows thetensile strength,elongation, andhardnessval-

ues of the samples formed under the three conditions. The sample

Fig. 20. Volume fraction, equivalent diameter, and roundness of solid particles in

samples formedat 300and30mm/s punchspeeds with graphite lubricant.

Fig. 21. Mechanical propertiesof fabricated samples.

formed under condition 2 had a tensile strength and elongation

of 216MPa and 10%, respectively, at position F and 209MPa and

8.5%, respectively, at position E. This sample hadVickers hardness

values of 72HV and 69HV at positions F and E, respectively. The

sample formedundercondition3hada tensile strength,elongation,and Vickers hardness of 160MPa, 4.5%, and 69.5HV, respectively,

atpositionF and148MPa, 2.5%, and68HV,respectively, atposition

E. These values were 56MPa, 5.5%, and 2.5HV lower at position F

and 61MPa, 6%, and 1HV lower at positionE compared to the sam-

ple under condition 2. The sample formed under condition 1 (i.e.,

formingwithoutgraphitelubrication of cavity)hadverylowtensile

strength and elongation of 132MPa and 2%, respectively, at posi-

tion F and 126MPa and 2%, respectively, at position E compared

to condition 2. However, the Vickers hardness was much higher

under condition 3 than condition 2 because of the large eutectic

phase which used to be the liquid phase. The Vickers hardness of 

the solid phase was about 56HV, and that of the liquid phase was

about 98HV.

The tensile strengthandelongation of thesample formedundercondition 2 at 300mm/s were higher than those of the sample

formed under condition 3 at 30mm/s. This was presumed to be

because the solid particleswere slightly smaller andmore densely

distributed. In particular, the elongation of the sample under con-

ditions 1 and 3 were significantly lower than that of the sample

under condition 2. This indicates that fewer solid particles in the

microstructure decrease the elongation. Under condition 3, even

though the solid particles in themicrostructure were mostly glob-

ular, there was a large number of solid particles with a diameter

of over 100m. This indicates that the number of solid particles

within the 1.2mm thick thin plate was much smaller than that

under condition 2. For themicrostructure under condition 1, solid

particleswererarelydistributedandwereconcentratedasdendrite

structures in one place. With regard to the difference in mechan-ical properties at different positions, position F (close to the gate)

showedslightly improvedpropertiescomparedtopositionE under

all threeconditions,butthemicrostructureswere similarat thetwo

locations.

Based on theanalysis of the formability, mechanical properties,

and microstructure of the thin plates formed at different punch

speeds andwithandwithout lubrication,condition2 (withgraphite

lubricant and a punch speedof300mm/s)was found tobe themost

appropriate.

4. Conclusions

An indirect rheo-casting die for 1.2mm thick thin plates was

designed through a filling simulation. Thin plates were formed

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168 C.K. Jin et al. / Journal of Materials Processing Technology 224 (2015) 156–168

under two friction conditions for thecavityand twopunchspeeds.

The effects of these parameters on the formability andmechanical

properties were analysed. The analytical and experimental results

are as follows:

(1) A fan-shapedgate allows the semi-solid slurry toevenlyfill the

entire thin cavity.

(2) Theincreasedflowratealongthecavity’s sidescanbecontrolled

by using overflows located at the sides of the cavity.

(3) The placement of overflows at the end of the cavity allows for

an even temperature and pressure (greater than atmospheric

pressure) distributionwithin the cavity.

(4) A semi-solid slurry having a solid fraction of 40% with fine

and globular solid particles can be obtained with an EMS. The

semi-solidwithout EMShadrosette particles andcoarser glob-

ular -Al particles. The equivalent diameter and roundness

of the solid particles of the semi-solid slurry with EMS were

about 75m and 1.5, respectively. Those of the solid parti-

cles of the semi-solid slurry with EMS were 110m and 2.3,

respectively.

(5) The cavity’s surface state experiences greater segregation

between the solid and liquid phases with increased friction.

When the frictionof the cavityis high, solid particles find it dif-

ficult tomove into thecavity, so theliquid phasewith relatively

good fluidity flows into the cavity.

(6) A higher punch speed for compression of the semi-solid

slurry produces better formability for the thin plate and

a microstructure with densely and evenly distributed solid

particles.

(7) The microstructure of formed thin plate was finer and more

globular compared with the microstructure of the semi-solid

slurry with EMS. Plastic deformation of the solid particles

occurred by forced convection during filling. A small and glob-

ular grain structure then formed. The moving velocity and

distance of the solid phase increase for smaller and more glob-

ular grains.

(8) When the semi-solid slurry was formed at a punch speed of 

300mm/s with a graphite lubricant, a thin platewithout segre-

gationbetween thesolidandliquid phasesor other defectswas

obtained with a tensile strength of 216MPa and elongation of 

10%.

 Acknowledgements

This work was supported by the National Research Foun-

dation of Korea (NRF) grant funded by the Korea government

(No. 2013R1A1A2062759). Ministry of knowledge Economy (No.

20104010100540), This study was also supported by the Korea

Research Foundation (KRF) grant fundedby theKorea government

(MEST) (grant No. 2012-0001204).

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