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A Method to set process parameters of local squeeze in HPDC

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2012 IOP Conf. Ser.: Mater. Sci. Eng. 33 012001

(http://iopscience.iop.org/1757-899X/33/1/012001)

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A Method to set process parameters of local squeeze in HPDC

I Ohnaka1, JD Zhu

2, A Sugiyama

1 and F Kinoshita

3

1

  Osaka 560-0003, JapanieSol Co./Osaka University, Toyonaka-shi, Higashitoyonaka-cho 1-32-12,

2Multi-Flow Software CO.LTD, Canada

3

 QUALICA Inc. , Suita-shi, Japan

E-mail:ohnaka@ams.eng.osaka-u.ac.jp

Abstract. In HPDC processes, pressurization of the mushy regions via local squeeze pins and

 plunger during the intensification stage is very important to decrease porosity defects. In orderto better understand the process and to be able to set the process parameters more properly, we

developed a simulation code to solve the flow field of mushy regions and porosity formation

 based on the following main assumptions: 1) Local squeeze pins and plunger tip can be treated

as pressure boundaries, while the squeeze pins are treated as moving boundaries. 2)The

intensification and local squeeze pressure can propagate inside regions where the solid fraction

is less than critical values. 3) In the pressurized regions, the cast metal is treated as a

 Newtonian fluid and both the solid and liquid flow together when the solid fraction is low, andthe D'Arcy flow only exists when the solid fraction goes higher. 3) Only the liquid flows in the

unpressurized regions, following the D'Arcy's law. 4) Porosities grow in elements when the

 pressure decreases below critical values depending on the element condition. The simulation

was applied to HPDC castings and showed that the pressure-duration was similar to the

measured one though the simulated pressure drop in the pressurized region was much smallerthan the measured one. The pressure drop, pressure-duration and pin-penetration depth were

discussed and the simulation seems to be helpful to determine local squeeze parameters. Future

challenges are also discussed.

1. IntroductionIn high pressure die-casting(HPDC), pressurization of the mushy region is very important to decrease

the porosity defects, because many gas bubbles are entrained during mold filling and risers to feedsolidification contraction are not available. In particular, local squeeze, namely pressurization withlocal squeeze pins is often applied on thick-wall regions of the casting prone to porosity defects[1].

Because better propagation of applied pressure decreases porosity more significantly, several workshave been reported on pressure propagation and the effect of pressure on the quality of castings[2-8].However, very few can be found on porosity simulation in HPDC[9-11], especially the simulation of

effect of local squeeze pins [12,13].This paper presents a practical simulation method of the pressure propagation and porosity

formation to properly set process parameters of local squeeze.

2. Simulation methodBecause exact simulation of pressure propagation in the HPDC is very difficult, followingassumptions and equations have been used in the simulation:

1

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6) The temperature field is solved with the thermal energy balance equation described in Ref[14-16],neglecting the convection term. The latent heat of fusion is considered by the temperature recovery

method.7) The volume change of gas entrapped during mold filling follows the equation of state for ideal gas:

0PV m RT  =   (6)

where P : pressure of the gas[Pa], V  : volume of the gas[m3

 R], : gas constant[J/kgK],

T  : temperature[K],0

m : mass of the gas[kg].

8) The penetration depth of squeeze pins is calculated from the volume shrink during solidification oftheir territories bounded by the critical solid fraction. The compression of porosities is alsoconsidered. The pin penetration terminates when the solid fraction ahead of the pins, say 5mm fromthe pin tip, becomes greater than a critical value.

9) The thermal properties of squeeze pins are the same with those of the casting and there is nothermal resistance between the casting and the squeeze pins, while the region where the pins existis treated as a solid region. Solidification simulation for the case where a pin was inserted by 8mmshowed that the fraction solid ahead of the pin tip rapidly increased at the very early stage, but soonthe solidification rate decreased approaching rather similar value to the case where no pin exists.

Figure 2. Casting specimen used by Iwata et.al[4] Figure 3. Comparison of measured andsimulated pressure change

AC4C-1

Ref. 22

ADC12

Ref. 21

Measured

Simulated

Figure 4. Fraction solid and temperaturerelationship used in the simulation

Figure 5. Simulated pressure drops at themeasuring point

0  0.5 1.0 1.5 2.0 2.5 3.0

Time / s

-0

-4

-8

-12

   P  r  e  s  s  u  r  e

   /   M   P  a

0  0.5 1.0 1.5 2.0 2.5 3.0

Time / s

40

30

20

10

0

   P  r  e  s  s  u  r  e   d  r  o   /   k   P  a

AC4C

ADC12

AC4C-2

ADC12   S  o   l   i   d   F  r  a  c   t   i  o  n

Temperature /0C

AC4C-2AC4C-1

3

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This is because the pin temperature rapidly increased and kept due to its low thermal conductivity(SKD61) compared to that of the casting. Therefore, the assumption is reasonable for this work.

10) Porosity is calculated from the flow field assuming pores start to grow at elements when the

 pressure there becomes lower than a critical value. Once the critical pressure is reached, the pressure in the elements is set to zero and the net-loss of melt volume in these elements is

calculated and used as the increase of porosity volume (porosity growth).

3. Simulation of pressure propagation The measured data of Iwata et al.[4] were selected to evaluate the simulation accuracy of high pressure 

 propagation, because the casting dimensions are clearly described as in figure 2. They measured theFigure 3 compares the measured and simulated pressure changes with using solid fraction -

temperature relationship shown in figure 4. In the simulations the gate pressure was set as follows;0MPa at 0s, 40MPas at 0.3s, 40MPa between 0.3 and 3s, and 0MPa at 3.1s. Other data used in thesimulation can be found in table 1.

Figure 5 shows simulated pressure drops (pressure at the measuring point - pressure at the biscuit

[40MPa]) and figure 6 is the simulated flow field for the AC4C casting at 1.2s, showing a reasonableflow pattern. 

Although the simulated pressure drop is much smaller than the measured one, the pressure-duration

is rather similar between the simulation and measurement as shown in figure 3. Further, the pressuredrop strongly depends on the solid fraction - temperature relationship of casting alloys as shown infigure 3 and figure 5, though the ADC12 casting showed a larger drop unlike the experimental result.

The reasons for the difference in the pressure drop may be as follows:1) Boundary conditionActually the boundary at the plunger tip is not a uniform pressure boundary. This is because the tipmovement is controlled by the surrounding of the biscuit (P in figure 6) when the surroundingsolidifies. If its strength is high enough, the force of the tip is balanced with the reaction force of thesurrounding, resulting in pressure drop in the central part with lower strength. Solving such problemrequires a structural analysis with considering the thermo-mechanical behaviour of the solidified part

and the contact of the casting with the mold.

Physical parameters AC4C ADC12

Density [kg/m3  2400] 2480

Specific Heat[kJ/(kgK)] 1.19 1.19

Thermal cond.[W/mK] 100 100

Latent heat [kJ/kg] 425 495

Liquidus Temp.[℃] 615 580

Solidus Temp.[℃] 562 520

Ejection Temp.[[℃] 620 600

Shrinkage coef. [%] 6 4

gate f  , sqz f    0.9 0.99

S d   in Eq.4 and 5 [μm]  20 20

0µ  aS   in equation 5  200 200

SC  f   in equation 5 [-]  0.45 0.45

Table 1. Physical properties used in the simulation

Figure 6. Simulated flow pattern at 1.2s for

AC4C alloy (Cross-section AA' in figure 2)

P

4

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2) Basic equation and viscosityThe basic equation (equation 2) considers only the shear resistance which strongly depends on the

flow velocity. Because the velocity of the flow is usually low, resultant pressure drop is also low.Although a higher effective viscosity causes a higher pressure drop, even a viscosity 1000 times largerthan coal-tar did not cause such high pressure drops as measured.3) Measurement errorAlthough the pressure sensor was set at the boundary between the casting and mold with a heat-insulating-sheet insert, the solidification near the sensor may decrease the pressure on the sensor whenthe solidified region has a strength that varies with alloy composition. The displacement of the sensordue to the mold expansion may also contributes to the pressure drop.

Currently it is not clear yet which is the main reason caused the difference in the pressure drops.However, the estimation of the pressure-duration may be more important than that of the pressure drop, because the former affects the casting quality more significantly [4].

The reason why the pressure-duration affects the porosity defects may be explained as follows.

From assumption 6, equation 7 can be used to calculate the gas volume change if the temperaturechange is small;

0 0 /V PV P=   (7) 

where0

P   is the initial gas pressure, namely, the pressure at the end of mold filling, and is usually

much lower than the intensification pressure, P , for example0

P   ≈   1MPa and P ≈  40MPa.

Therefore the entrapped gas becomes very small in a very short period. When the pressure decreases,

however, the gas in the porosities expands. With a longer pressure-duration, the progress ofsolidification during this period will be larger and the gas will expand less. This may be supported bythe fact that a kind of extrusion of cast metal occurred when a squeeze pin was pulled out tooearly( figure 7), while the effect of pressure decrease induced by the pulling-out cannot be excluded.

Another hypothesis for the effect of pressure-duration might be gas absorption into the liquid phase

during the period. However this effect may be small, because the melt surface of the gas bubbles may be covered with an oxide film preventing the gas diffusion, and the pressure-duration was in the order

of a few seconds in this particular experiment.Based on the fact that complete blocking of the pressure propagation requires a certain thickness of

solidified shell, the pressure-duration in simulation was determined by the time when the solid fractionat 1.7mm from the mold, where the sensor is attached reached the critical solid fraction. Sensitivitywas also checked by changing the distance from 1.7mm to 0.8mm, and it was found that the pressure-duration becomes 20% shorter. As can be seen in figure 3, both simulated and measured results show asame trend, the pressure-duration of AC4C is shorter than ADC12. This is mainly due to the smallerlatent heat and higher solidus temperature of AC4C as compared with those of ADC12(table 1).Because usual castings produced by the HPDC have many thin and thick parts unlike the castingshown in figure 2, the pressure-duration may be determined mainly by the solidification of thin parts

and the starting time to push the pins. Therefore, the proposed method using the critical solid fraction,

gate f    sqz f and may be useful as shown below.

4. Local squeeze castingSimulation was carried out on a local squeeze casting of ADC12 where two squeeze pins were set onthe thick parts as shown in figures 8 and 10. Although the gas entrapment during mold filling wassimulated (the method is reported in [11]), it's effect didn't appear in the porosity simulation. This is because the entrapped gas was compressed to very small volume during the intensification stage. It is

reported that the local squeeze drastically decreased the porosity defects as shown in figure 8[1]. In theactual casting, the gate pressure of 52MPa was applied until 3s and squeeze pin pressure of 195MPafrom 1s to various pulling-out times. The pin-diameter was 25mm(total pin cross-sectional area was

980mm2

).

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  Figure 9 shows the simulation result for the case where the local squeeze is not applied, showingmany porosities formed in the product. When the local squeeze is applied from 3.5s to the end of the

solidification, most of the defects are eliminated(figure 10) just as the observation(figure 8). Thesqueeze-pin-penetration depth was about 14 mm as shown in figure 11( when the solidificationshrinkage coefficient is 5%, it increases to 17.8mm). It is greatly affected by the solidification at thehead of the pins, because the pin movement is terminated when the solid fraction becomes greater thanthe critical value.

Figure 7 Section through the center of squeeze

 pins in figure 10, showing kind of extrusion ofthe metal (Pull-out time: 12s after mold filling [1]).

Figure 10 Simulated porosity with local squeezeFigure 9 Simulated porosity without

local squeeze

Figure 8 Section through the center of squeeze pins in Fig.10, showing good quality(squeeze starting time: 3s , pulling-out time:20s)

Pin 2

Pin 1

Gate

Time / s

Figure 11 Simulated travel curve of the squeeze pins

0 5 10 15 20 25 30 35

16

12

8

4

0

   D   i  s   t  a  n  c  e   /  m  m

6

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The pin-penetration depth measured for the casting ranged from 20 to 27 mm depending on thesqueeze starting time[1], for example, 27mm for 2s, 22mm for 5s. If the penetration depth is

calculated from the volume of solidification shrinkage of the product (475x10

3

mm

3

), it is about19x103mm3

  However, the value of over 20mm for the starting time after 4 seconds cannot be explained only bythe solidification shrinkage. The reasons for the larger penetration depth may be as follows;

, hence the penetration depth is 19mm. However, it should be less than 19mm as thesimulation shows, because solidification proceeds before starting the squeeze in the product. The largevalue of 27mm that was obtained when the squeeze starting time was 2 seconds after the mold fillingmay be due to the pushing back of the plunger tip, because the intensification time was 3 seconds andthe squeeze pressure was higher than the plunger tip pressure.

1) Mold expansionUsually the mold temperature increases until 10 or 20 second after the mold filling depending on themass of casting and mold cooling conditions. Therefore, the mold should expand after the mold filling,resulting in increase of casting volume and hence the penetration depth. This also could affect the

 porosity defects.2) Compression of porositiesIf it took time to compress the porosities for some reasons, the porosities could remain even after theintensification stage and increase the penetration depth.

Although we cannot reject the latter, the former might be more possible.

5. Concluding remarks A simulation method for setting process parameters after mold filling in HPDC has been developedand the comparison of simulation with observations showed the followings: 1) The pressure-durationwas similar to the measured one and determined by solidification at the measuring point. 2) In the pressurized region, no significant pressure drop was obtained from the simulation. 3) The pressure propagation varies with the solid fraction-temperature relationship, though simulated pressure drops were much smaller than measured ones. 4) Why the longer pressure-duration causes smaller porositydefects may be explained by the suppression of the expansion of the compressed porosity gas due tothe progress of solidification during the pressure-duration.

Although the proposed simulation method should use proper critical solid fractions to determine the pressurized regions and termination of squeeze pins, and cannot consider the mold expansion, it canroughly estimate the interaction between the plunger and squeeze pin, time variation of the pin penetration depth and degree of porosities. Therefore, it seems the simulation is helpful to set the process parameters.

However, there are still many challenges to improve the simulation accuracy such as visco-plasticanalysis as a contact problem between the casting and mold, which enable the simulation withoutusing the critical solid fractions.  The consideration of the pin material properties and thermal

resistance between the pin and casting are also required. 

References[1]  Zhu J, Yokoyama H, Ohnaka I, Murakami T, and Cockcroft S,

High-pressure Die Casting Processes by Using JSCAST, 2009 Modeling of Casting, Welding and

Prediction of Casting Defects in

 Advanced Solidification Processes XI I , (Warrendale, TMS ), 361-368[2]   Nishi N, Sasaki H, Hirahara T and Takahashi Y , 1988 Imono 60 777-783[3]  Iwamoto N and I.Kuboki 1994 Trans. Japan Die Casting Conference, JD94-11 p 94[4]  Iwata Y,Sugiyama Y, Iwahori H and Awano Y, 2000 J.Japan Foundry Engineering 72 263-267[5]  Ikeda S, Matsumoto Y, Murakami M, Xiong S, and Hu B 2008 Relation btween Casting

Conditions and the Molten Metal Pressure in a Die Mold , Trans. 2008 Japan Die Casting

Congress,  JD08-18, 119-123

[6] 

Kato E, Nishiyama N, Nomura H, Asai K and Tanigawa S, 2002 J. JFES   74 370-376[7]  Tanigawa S, Asai K, Yang Y, Nomura H and Kato E, 2003 J.JFES   75 525-531

7

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http://slidepdf.com/reader/full/1757-899x331012001squeeze-pins 9/9

[8]  Ueki T,Tashiro M,Tajima A,Kato H,Yageta K and Kambe H, 2010 Measurement of melt and die

 behavior in die-casting process, Trans. 2010 Japan Die Casting Congress, JD10-25, 157-162

[9] 

Kubo K 1997 Method for Numerically Predicting Casting Defects, NADCA, T97-015[10]  Kubo K, Asao H, and Deki N, 2008 Progress in Computer Simulation of Die Casting,  Japan Die

Casting Congress, JD08-30 197-203[11]  Kimatsuka A, Ohnaka I, Zhu J, Sugiyama A and Kuroki Y,Mold Filling Simulation for Predicting

Gas Porosity 2006 Modeling of Casting, Welding and Advanced Solidification Processes XI I ,(Warrendale, TMS ), 603-610

[12]  Li S, Sannakanishi S, Tsumori M, Akase M and Anzai K, 2003 Method for Numerical Prediction

of Porosity Defects in Squeeze Casting, Trans. NADCA, T03-14[13]  Li S, Mine K,Sannakanishi S and Anzai K 2006 Qunatitative Simulation of Shrinkage Porosity by

Pressure in Squeeze Casting, Japan Die Casting Congress, JD06-30, 199-204[14]  Ohnaka I, 1979 Classification of Numerical Methods for Transient Heat Transfer Problems and

Improved Inner Nodal Point Method ,  ISIJ , 65 1737-1746

[15] 

Zhu J and Ohnaka I, 1996 Three Dimensional Computer Simulation of Mold Filling of Casting byDirect Finite Difference Method ”, J. of JFS , 68 668-673

[16]  J.D.Zhu and I.Ohnaka, 1995 Treatment of Free Surface Boundary Condition for the Simulation of

Mold Filling,  Modeling of Casting, Welding and   Advanced Solidification Processes VI???,(Warrendale, TMS ) 359-372

[17]  Ergan S, Fluid flow through packed columns, Chem.Eng.Prog., 1952 48  89-93[18]  Mori Y and Ototake N, On the Viscosity of Suspensions, J. Chemical Engineering of Japan, 1956

20 488-494[19]  Ohnaka I, Zhu J, Sugiyama A and Kinoshita F, 2011 Proc. 11th Asian Foundry Congress,

Foundry Inst. Chinese Mechanical Eng. Soc. Guanzhou China, 152-157[20]  Sugiyama Y, Iwahori H, Yonekura K and Oguchi Y, 1994 IMONO 66 412-417[21] Iimai H, Kameyama Y, Suzuki Y, Nagayama Y, Kato E and Nomura H, 2001 Denso Technical

Review, 6(2) 100-106[22] Sim J, Jang Y, Moon J, Kim J, Min K and H C, 2009 ISIJ International , 49 1700-1709

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