Journal of Materials Processing Technology 147 (2004) 45–50

Die design by filling analysis of semi-solid injection forgingprocess and their experimental investigation

D.H. Leea, P.K. Seoa, C.G. Kangb,∗a Department of Mechanical and Precision Engineering, Graduate School, Pusan National University, Pusan 609-735, South Korea

b Engineering Research Center for Net Shape and Die Manufacturing, Pusan National University, Pusan 609-735, South Korea

Received 21 March 2001; received in revised form 24 September 2003; accepted 24 October 2003

Abstract

Die design by computer simulation has some advantages compared to the conventional method that was performed by designer’sexperience and trial and error. The die filling and solidification phenomenon in semi-solid injection forging process were simulated byMAGMAsoft/thixo module. Furthermore, the die design for semi-solid injection forging was performed with the two kinds of geometricshapes. The effect of designed gate dimension on filling phenomenon was estimated by filling simulation. The calculated results were com-pared with experimental data. The free surface phenomenon obtained by experiment has good agreement with computer simulation results.The solidification affect much as porosity and shrinkage for designed semi-solid forging die had been predicted by computer simulation.The designed die for semi-solid injection forging had been applied to the production of the frame part used in air conditions system.© 2003 Elsevier B.V. All rights reserved.

Keywords: Semi-solid injection forging; Gating system; Free surface; Solidification; Rheology flow model; Three step reheating

1. Introduction

Semi-solid injection forging process is performed at atemperature between liquid and solid state has many ad-vantages such as good mechanical properties, better surfacefinish and less filling and solidification defects because oflaminar flow into the die cavity and less solidification time.In the productivity aspect, semi-solid injection forging pro-cess is a beneficial process which can get the same or moreproduction rate than conventional high-pressure die casting(HPDC) process[1].

The reported results on the die design associated withsemi-solid injection forging are limited. However, recently,modified algorithm which is used to die design of die castingand squeeze casting has been studied for process design ofsemi-solid injection forging. Backer[2] introduced the CAEalgorithm for semi-solid forming process modifying theconventional die casting things. Tims et al.[3] embody met-allurgical filling behavior of semi-solid materials by com-puter simulation. Lipinski and Flender[4] performed threedimensional filling analysis by new developed algorithm.

Recently, semi-solid injection forging processes such asthixocasting and thixoforging are quickly developing alter-

∗ Corresponding author.E-mail address: cgkang@pusan.ac.kr (C.G. Kang).

natives to traditional casting and forging processes. Becausethe development of semi-solid injection forging processes isdriven by the automotive industry’s demand for more com-plex light-weight parts as aluminum and magnesium withhigh mechanical properties. Computer simulation tools areplaying a pivotal role in the development of these processes,because unlike the traditional processes, these new processesare not supported by extensive bases[5–7]. Studies of diefilling and solidification analysis for semi-solid injectionforging process have not been investigated until now.

The main purpose of this study is to show the die designprocess to reduce the lead-time of the part development bysemi-solid injection forging process. The die filling and so-lidification analysis results by commercial package MAG-MAsoft with add-on module ‘thixo’ were applied to the diedesign to development Al frame which is one of the electrichome appliance parts. Furthermore, the die design processfor semi-solid injection forging will be applied to the devel-opment of lightweight automatic parts.

2. The die design for thixoforming process

Fig. 1 shows the semi-solid injection forging process thatconsists of fabricated raw material, reheating process of bil-let, billet handling, filling into the die cavity and solidifica-tion.

0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2003.10.023

46 D.H. Lee et al. / Journal of Materials Processing Technology 147 (2004) 45–50

The viscosity of semi-solid material has drasticallychanged in the mushy state(fs = 0.3–0.7) that the liquidphase and the globular solid phase co-exist in the semi-solidforming region. The material in static can be treated assolid state owing to the high viscosity but in shear force, theviscosity abruptly falls with rising shear rate. The stressesin semi-solid material are not only dependent on the currentstate of deformation but also on the current deformationhistory. This characteristic of semi-solid material is called‘thixotropic’ [8]. This research was proposed for the diedesign method for semi-solid injection forging process withflow characteristics like thixotropic.

2.1. Flow model

The dependency of viscosity on shear rate must be recog-nized to establish the rheology model of semi-solid material.The viscosity decreases as shear rate increases in semi-solidregion of aluminum. The flow behavior of non-Newtonianmaterials can be described as the basic shear diagrams. Theshear diagram for a pseudoplastic fluid as shown in Fig. 2describes a decrease in viscosity with increasing shear rate.But, shear diagram as shown in Fig. 2 is not helpful to ex-actly understand the thixotropic behavior. Fig. 3 shows therelationship between shear rate and viscosity which are ob-tained in three different regions; the lower Newtonian re-gion, the upper Newtonian region and the variable viscosityregion (power-law approximations apply here).

The most popular numerical analysis methods used fordie filling analysis are solution algorithm–volume of fluid,(SOLA–VOF), marker and cell (MAC) and simplifiedmarker and cell (SMAC) method. For all these algorithms,the calculation domain is divided into small rectangular ele-ments, hexahedrons and they are used in three dimensionalcases. Therefore, the calculation domain is often describedas zig-zag shape or stair-like contour. This may have nosignificant influence on the predicted flow pattern into thedie cavities if the calculation domain is bulky casting cav-ity. But, it may have a great significant influence on theflow passing through a narrow gating system, particularlywhen tapers and round corners are included in the gatingsystem design. Generally using a finer mesh could solvethese problems. However, the finer mesh has enormouslymore solving time and much cost.

The characteristics of MAGMAsoft used in this study areas follows:.

• Ease of physical interpretation of various steps of algo-rithms.

• Conservation of physical properties.• Better convergence than purely FEM or FDM.• Reduction of solving time.

Basic governing equations of MAGMAsoft which isadopted for control volume method are continuity equation,Navier–Stoke’s equation, energy equation and volume offluid (VOF) method as liquid forming process.

The rheology flow models for semi-solid injection forgingprocess are Ostwald–de Waele and Carreau–Yasuda rheol-ogy model. The rheology equations of Carreau-Yasuda andOstwald–de Waele are as follows[9]:

η = ρ{να + ν0 − να[1 + (λγ̇)a](n−1)/a} (1)

whereν is low shear rate viscosity (zero shear rate viscos-ity, m2/s),ν� the high shear rate viscosity (m2/s),λ the timeconstant of transient region in Fig. 3 (s),a the Yasuda coef-ficient andn is the sensitivity exponent of material

τxy = −m

∣∣∣∣

dvx

dy

∣∣∣∣

n−1 dvx

dy(2)

η = ρmγ̇n−1, τ = −ρmγ̇n (3)

where ρ is the density (kg/m3), η the apparent dynamicviscosity (Pa s),m the Ostwald–de Waele coefficient (m2/s),γ̇ the shear rate (s−1), n the Ostwald–de Waele exponentandτ is the shear stress (Pa).

Generally,Eq. (2) is known as the power law equation.Here, this equation becomes Newtonian flow model (m = µ)if Ostwald–de Waele exponent is 1. Kim and Kang[10] sim-ulated the filling into the die cavity and solidification withNewtonian and Ostwald–de Waele flow model in MAGMA-soft. Compared to the results of simulation and experimental,the Ostwald–de Waele flow model is more appropriate forsemi-solid forming process. Therefore, Ostwald-de Waelemodel was adopted as a rheology model in this study.

2.2. Modeling of product and simulation parameters

The application of semi-solid injection forging processthat is relatively a new forming process is quite limited asan air conditioning, braking, fuel delivery, suspension and akind of frame. In this study, the die filling and solidificationanalyses of semi-solid injection forging process by commer-cial package MAGMAsoft with add-on module ‘thixo’ wereapplied to the die design of frame which is one of the realelectric home appliance parts. The photographs of productwhich was formed by sand casting and high-pressure diecasting are shown in Fig. 4. The schematic drawing of thedie design for semi-solid injection forging process is shownin Fig. 5. This schematic drawing is the same as die cast-ing parts and the diameter of gating system was selected asthe design variables. Three dimensional solid models withCATIA and STL (streolithography) file used for geometrytransfer were shown in Fig. 5 as well.

The simulation was performed with A356 aluminum alloy,and thermophysical properties of A356 thixo material, whichis used in simulation, are shown in Fig. 6. Fig. 6(a) showsthe solid fraction that means the ratio of solidified portionto the total amount of alloy. At liquidus temperature,fs is 0,and at solidus temperature,fs is 1. Temperature-dependentexponent ofEq. (2), n, is shown in Fig. 6(b). Fig. 6(c) showsthe coefficient of apparent kinematic viscosity of the A356thixo alloy that can be temperature dependent.

D.H. Lee et al. / Journal of Materials Processing Technology 147 (2004) 45–50 47

Table 1Volume and mass of each modeled parts

Material Properties

Volume (l) Mass (kg)

Cast alloy 0.08917 0.238Permanent mold (1) 3.39782 26.589Permanent mold (2) 0.01785 0.140Inlet 0.01637 0.044Gating (runner) 0.04963 0.132Ingate 0.01067 0.028

The volume and mass of each modeled parts in simulationare listed inTable 1.

In this study, the initial conditions for numerical simula-tion are as followings. The initial die temperature is 250◦C,initial billet temperature is 577◦C (fs = 55%) and the punchspeed is 200 mm/s. The heat transfer coefficient for simula-tion is 500 W/m2 K between dies, and 1000 W/m2 K in cast-ing and die. The pressurized condition has lasted with 38 tfor 19 s. The number of total control volumes and metal cellswhich are generated by auto mesh generation in MAGMA-soft is 740,784 and 58,260 EA, respectively.

Figs. 7 and 8 show the die filling simulation results whichhave two kinds of gating system−25 and 18 mm diame-ter. The temperature distribution of the die cavity is about590◦C, and the pressure is distributed widely from 1000 to3000 bar. In the small diameter of gating system, the pressureis distributed uniformly within narrow range of pressure.From Figs. 7 and 8, the die filling behavior and temper-ature distribution of 70 and 90% filled state are similarlyexpressed in the large and small diameter of gating system.However, small diameter of gating system has shorter fillingtime and stable filling behavior than large diameter of gatingsystem because it has high velocity. Fig. 8 shows pressuredistribution in the die cavity during filling, small diameterof gating system has uniform and large pressure value afterfilling was completed. The small diameter of gating systemhas less possibility of unfilled zone and stable filling be-havior because of the high-pressure value in the die cavity.

To apply the numerical simulation results, the geometryof product was modified as same as cast iron which is usedfor semi-solid injection forging experiments in Fig. 4. Theschematic diagram of modified geometry, 3D model used forCATIA and STL file used for geometry transfer are shownin Fig. 9.

To minimize the deformation of die and product, the de-signed gating system was proposed. Because a part of sleeveis taken apart when products were pulled out after semi-solidinjection forging due to the structure of products, the sleeveshape was modified, as shown in Fig. 9. The volume ofsleeve increased in modified gating system rather than ini-tial gating system. Geometry of the cavity was modified assame as the sand casting geometry.

Table 2shows the volume and mass of each part (mod-ified model). Simulation parameters applied as same as the

Table 2Volume and mass of each modeled part (modified model)

Material Properties

Volume (l) Mass (kg)

Cast alloy 0.10771 0.287Permanent mold (1) 3.36836 26.359Permanent mold (2) 0.01005 0.079Inlet 0.04441 0.118Gating (runner) 0.05879 0.157Ingate 0.00166 0.004

semi-solid injection forging experiments. The pouring ratewas set with 1678.5 cm3/s by calculating pouring rate persecond in accordance with 370 mm/s of injection velocityin the semi-solid injection forging. The die temperaturewas 250◦C in accordance with preheating, and initial bil-let temperature was 577◦C (solid fraction,fs = 55%).The semi-solid injection forging experiments generate thetemperature loss of reheated billet handling, but the billettemperature was supposed to be constant because of theproblems of inputting the fixed quantity in the computer-aided engineering.

Midson et al.[11] introduced a proper semi-solid injectionforging condition, over the 5 m/s of the gate velocity, overthe 573◦C of initial billet temperature by experiments. In thebase of this, each gate velocity was calculated with initialgating shape and modified gating shape. In the mass betweeninitial gating shape and modified gating shape, as shownin Tables 1 and 2, the volume except permanent mold was0.149 l (149 cm3) and 0.168 l (168 cm3), respectively.

Vf = Wf

γ(4)

Ag = Vf

tf · Vg(5)

whereVf is the total volume of cavity (cm3), Wf the weightof cavity (g), γ the specific gravity of aluminum (g/cm3,Al = 2.6), Ag thegate area (cm3), tf the filling time (s) andVg is the gate velocity (cm/s)

From the result of calculating with initial gating shape,when diameter of gate is 25 and 18 mm, filling time is 0.193and 0.182 s, respectively. Filling time of modified gatingshape was supposed to take about 0.14 s and injection ve-locity increased to 370 mm/s. Therefore, each gate velocitywas calculated.Table 3shows the result of calculation.

Gate velocity 7.25 m/s of the shape like Fig. 9, as knownin Table 3, was fitted in the semi-solid injection forging.

Table 3Gate velocities in various gating systems

Gate diameter (mm) Vg (m/s)

Initial gating shape 1 25 1.57Initial gating shape 2 18 3.21Modified gating shape 21 7.25

48 D.H. Lee et al. / Journal of Materials Processing Technology 147 (2004) 45–50

2.3. Analysis results and discussions

Filling and solidification in the cavity were investigatedby using the designed gating system. Figs. 10 and 11show the generation of material group by using MAGMApre-processor and shapes after dividing element automati-cally. Material group in the analysis, such as cavity, gatingand inlet, etc, was indicated from the STL file in order torecognize the shape in MAGMAsoft pre-processor. Usingthe model function of package generated simple shape suchas a die.

The number of rectangular elements formed by dividingthe perpendicular coordinates is 591,668, and the numberof metal cells that lie within the melt is 63,751. Specifi-cation of workstation used for computer-aided engineeringwas R10000 CPU, 512MB memory of Indigo 2. The com-putation time by using the MAGMAthixo module was takenfor 10 days.Table 4shows properties used for semi-solidinjection forging simulation by using A356 alloy.

Ohnaka[12] introduced that the convection heat transfercoefficient was 1260 W/m2 K when liquid aluminum flowedon the die surface, but this analysis was carried out with1000 W/m2 K of the heat transfer coefficient between thesemi-solid injection forging billet and the die. The analysiswas performed with 500 W/m2 K of heat transfer coefficientin the upper die, lower die and core. Considering the rheo-logical behavior of semi-solid material, Ostwald–de Waeleflow model was used because it had a good agreement withexperimental results and simplicity. After completely fillingto die cavity inside, the pressurization condition for process-ing (P) and holding time (t) are 82 MPa and 19 s, respec-tively.

Figs. 12–14 show the filling patterns from 0.081 s at 60%filling to 0.136 s at 100% filling. Fig. 12 shows the tempera-ture, pressure and velocity distribution at 60 and 70% fillingstate with the modified gating system. The temperature offree surface increased about 2–3◦C after filling starts to thecross direction. The solid fraction of initial billet was 55%,

Table 4Properties used to semi-solid injection forging simulation by using A356alloy

Parameters Symbol Unit Values

Solidus temperature Ts◦C 547

Liquidus temperature Tl◦C 617

Latent heat Q kJ/kg 430.518Punch speed Vp mm/s 370Initial billet temperature Tb

◦C 577Initial die temperature Td

◦C 250hd: Heat transfer coefficient

between mold and moldK W/m2 K 500

hm: Heat transfer coefficientbetween material and mold

K W/m2 K 1000

Mesh number of controlvolumes

EA 591668

Mesh number of metal cells EA 63751Holding time t s 19

but because the temperature of thixoformed billet increasedafter passing the gating system, but the filling could showgood results by increasing the temperature. Pressure distri-bution has higher value at the sleeve than that of free surfacedue to resistance of the sleeve. As the filling was done, thepressure of sleeve was increased by degrees. The velocityindicated comparatively low value.

Figs. 13 and 14 show the temperature, pressure and ve-locity distribution at 75, 80, 85, and 100% filling state withthe designed gating system. As known in the results of fill-ing analysis, the flow characteristic in semi-solid injectionforging process such as filling defects like air entrapment byturbulence flow of the cavity inside was not found, therefore,the filling was done in sequence. Increased temperature afterpassing the gating system did not increase at 80% filled statefor 0.109 s, but as the filling started to the lower direction,the temperature of free surface increased by binding the ge-ometry shape. Increased temperature, which reached 590◦C(solid fraction: 45%), is the cause of the shape in the cavityand affected by pressurizing after filling was completed. Thecause of temperature increasing about 10–13◦C than initialbillet temperature was the change from mechanical energyand thermal energy by velocity gradient.

In 80% of filling, the considerable pressure was shownat the side where filling was finished. When the filling wasdone, the pressure indicated 5000 mbar, and it was almosttwice the value compared to the pressure in the middle offilling by the pressurizing effect. The pressure of free sur-face indicated comparatively uniform distribution and it wasabout 1500 mbar in the middle of filling. The die life wassupposed to show good results by uniform pressure distri-bution. Velocity distribution, like the pressure distribution,showed uniform value totally in the middle of filling, butthe cause of unstable velocity gradient after the filling wasdone as same as the cause of pressure distribution.

Fig. 15 shows sequences of mold filling traced bytracer particles, produced by MAGMAsoft itself, and thethixotropic flow pattern. Fig. 16 shows the temperaturedistribution at 70 and 90% filled state (section view). Thedifference of temperature gradient appeared at the rightand left base of section. The cause of generating differenceof temperature gradient is that the geometry modeling byfinite difference method is difficult compared with finite el-ement method, during remeshing process to calculation. Inorder to prevent these phenomena, the mesh size for finitedifference analysis should be small. If mesh size is small,the computational time is much needed. Therefore, the de-termination of mesh size and prediction of completion timefor computer-aided engineer of semi-solid injection forgingis very important. Therefore, in accordance with operationtime and the accuracy of shape recognition, the elementdivision had to be carried out.

Fig. 17 shows the temperature distribution at 60 and 80%of solidification state, and solidification starts from the at-tached die and progresses into the inner of products. Thesolidification is already completed in invisible parts from

D.H. Lee et al. / Journal of Materials Processing Technology 147 (2004) 45–50 49

Table 5Chemical composition of ATLHIX A357

Si Fe Cu Mn Mg Ni Zn Ti Pb

Min (%) 6.5 – – – 0.50 – – – –Max (%) 7.5 0.15 0.03 0.03 0.60 0.03 0.05 0.20 0.03

the results of solidification analysis. Shrinkages are seldomfound because solidification of the cavity was finished within10 s.

Fig. 18 shows the contour of solidification time. On thewhole, it appeared from the part attached to the die and innerpart in sequence. Shrinking defects by solidification seldomappeared when the supply rate and the porosity distributionwere considered totally. It is reasonable that the pressurizingtime was maintained for 19 s, because solidification time ofthe gate was found about 17 s after filling was completed.The final solidification position that has a high possibilityof generating the shrinkage defects took place at the sleeveand thin point of forged part. Therefore, the internal defectsdue to solidification delay were not forced for semi-solidinjection forging process.

3. Filling test

Filling test was performed with semi-solid injection forg-ing in order to verify the propriety of CAE method. Thebillet was heated to semi-solid state for the processes ofsemi-solid forging. Reheating process is very important toobtain the desired temperature because the microstructureof billet was controlled during inductive heating process. Incase of billet heating with inductive heating, heating timewas lower than that of electric furnace. During inductionheating of billet, heat transfer occurs by conduction from thesurface of billet. Therefore, in case of non-ferrous metals,like Al-alloy, thermal conductivity is comparatively high.Thus, application of inductive heating is not the problem.In case of metals with low thermal conductivity like steel(76 W/m K), problem such as superheating of surface can befound [12]. A remarkable characteristic of inductive heat-ing is that desired temperature distribution is obtained withmulti heating process. Repeatability and reappearance makeautomation in the aspect of manufacturing.

Initial solid fraction, which is designed by initial billettemperature, has an influence on semi-solid injection forgingprocesses. Three times of temperature increment and hold-ing during reheating process are given to obtain the homoge-nous temperature distribution in cross section area of billet,respectively[13,14]. Therefore, inductive heating methodsfor the control of temperature are essential to semi-solidinjection forging processes. The three-step reheating con-ditions were performed in this experiment[15]. The billetused for the filling test is ALTHIX A357 billet, which isfabricated by electro-magnetic stirring, from PECHINEY inFrance.Table 5 shows the chemical composition of AL-

THIX A357 billet. Fig. 19 shows the schematic diagram ofreheating conditions for the filling test[15]. Final temper-ature was established at 577◦C in regard of the differencebetween the temperature of input and output, as shown inTable 4.

After the stroke of main cylinder of hydraulic press wascontrolled by limit switch that is attached with the press,filling test was performed. Fig. 20 shows the die set andhigh-speed hydraulic press used for filling test. The dietemperature is controlled with cartridge heater. The mea-sured data during filling experiment are stroke, velocity andpressure. Fig. 21 shows the relationship between time andstroke during filling test. The difference in strokes betweencompletely filled and unfilled state is about 20 mm. Fig. 21shows the relationship between punch stroke and time forfilled and unfilled parts during filling test. The final pressureis measured with 86 MPa. Fig. 22 shows the comparison ofcomputational analysis and experimental results. The ini-tial temperature of semi-solid billet and aluminum alloy is577◦C and A357, respectively. Then, the punch velocity toexperimental of semi-solid forging is 300 mm/s. The anal-ysis results at 70, 80 and 92% have good agreement withexperimental results except for the shape of free surfaceat cavity inside. The causes of difference between analysisresults and experimental one at the shape of free surfaceduring filling are that theoretical filling ratio was not co-incided with experimental filling ratio due to measurementerror.

In the results of this study, applying the results of fill-ing and solidification analysis in semi-solid injection forgingprocesses to die design can remarkably reduce the time forthe product development. The sensitivity exponent of ma-terial (n value ofEq. (1)) which was measured as functionof temperature is necessary. That is because the reheatingof semi-solid billet is very sensitive to the injection forgingtemperature.

4. Conclusions

From this study the following can be summarized basedon the die design for semi-solid injection forging processesby using computer-aided engineering:

1. It was established that flow state in semi-solid injectionforging process could be easily investigated by using thecomputer-aided engineering.

2. The flow pattern in semi-solid material was much influ-enced by the gating shape rather than punch velocity anddie preheating temperature, and the optimal die designmethod was introduced in regard of gating velocity, dieand products deformation.

3. The results by filling test coincided with the results bycomputer-aided engineering, and Ostwald–de Waele flowmodel was suitable for the die design in semi-solid in-jection forging processes.

50 D.H. Lee et al. / Journal of Materials Processing Technology 147 (2004) 45–50

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