Lppm Filling Chiesa

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Measuring Mold Cavity Filling Time in Low Pressure Permanent MoldCasting of Aluminum A356 Parts

F. Chiesa , and N. GigureCentre de Mtallurgie du Qubec, Trois-Rivires, Qubec

B. DuchesneCollge de Trois-Rivires, Trois-Rivires, Qubec

J. Baril

Technologie de lAluminium et du Magnsium, Trois-Rivires, Qubec

Copyright 2011 American Foundry SocietyABSTRACT

In the present work, a great number of castings werepoured by the Low Pressure Permanent Mold processfor a wide range of filling times and pouringtemperatures. One casting was a 2mm-wall (0.080 in.)cover weighing 0.13kg (0.27lb) and the other was a7kg (15.4lbs) bell housing with walls varying inthickness from 6 to 25mm (0.25 to 1 in.).

Type K thermocouple wires (0.12mm diameter[0.0045 in.], with a response time of 0.1s) wereconnected to a data logger (20 readings per second) todetect the passage of the liquid metal front, allowingan accurate measurement of the filling time (0.05s).

As the filling time is obtained by subtracting thepassage times recorded by two identicalthermocouples, the time lags cancels out with nodetrimental effect on the accuracy.

This allowed to determine a slowing factor (SF)defined as the ratio of the actual measured filling timeto the filling time calculated based on the staticequilibrium level. Slowing factor was found to varyfrom 1, for very slow rates of filling, to values greaterthan 2 for extremely steep pressure ramps. For typicalindustrial production conditions, SF is in the range 1.3to 1.6 and appears to be closer to 1.0 for thin castings.

It was found that the pouring temperature had littleeffect on SF except for extremely low pouringtemperatures (below 700C or 1292F).

INTRODUCTION

The Low Pressure Permanent Mold casting (LPPM)process is used for producing aluminum andmagnesium parts. It is a novel process for pouring

magnesium alloys,1,2 but a mature one as far asaluminum alloys are concerned.3The principle of theLPPM process is shown in Figure 1. The liquid metal,located under the mold, is pushed up a transfer tube byapplying a gas pressure on the surface of the melt.This process presents a host of advantages over gravitycasting, including:

a) Tranquil and perfectly controlled bottom filling ofthe mold cavity as illustrated in Figure 1;

b) Superior feeding without risers. In LPPM, thetypical feeding pressure is 900mB versus 100-200mB for riser fed gravity castings;

c) Thinner walls may be obtained as compared togravity filling (as thin as 2mm);

d) The yield is typically 85%, versus 60% in gravitycasting, leading to less returns in the melt, hence acleaner metal with consequent energy savings andreduced melting furnace capacity (tons/h);

e) The liquid metal is cleaner because it is extractedfrom underneath the melt surface; and

f) No metal handling by the operator results in betterergonomics and a perfectly repeated filling at eachcycle.

Among these advantages, the control of the filling isthe most spectacular: by varying the rate of increase ofthe pressure applied on the melt, the filling may besped up or slowed down at will with great ease.

Filling in the LPPM process has been previously

studied. 4,5However, the actual time necessary to fillthe mold cavity is not known because the pressureapplied on the melt cannot be strictly related to thelevel of the melt in the mold as discussed in thefollowing section.

Paper 11-003.pdf, Page 1 of 8AFS Proceedings 2011 American Foundry Society, Schaumburg, IL USA


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Fig. 1. Principle of LPPM casting.

RELATING PRESSURE AND MELT LEVEL

The LPPM casting process allows the production ofhigh integrity light metal parts with a high

productivity. LPPM permits very close control of thefilling process. However, the mold cavity beingclosed, it is difficult to accurately know the filling timeof the mold cavity, as easily as it is in the case of an

open cavity typical of the gravity poured permanentmolds. The known pressure rise applied on the surfaceof the melt allows a calculation of theoretical fillingtime, which is always substantially less than the actualfilling time, particularly at high rates. The knowledgeof this actual filling time is necessary to the modelingof the process, and more importantly, for thecomprehension of phenomena such as superheatlosses, air entrapment and turbulence, leading tocasting defects such as misruns, cold-shuts andsagging of the cope surface.

The equilibrium level of the metal (i.e. when the liquid

metal is at rest) in the transfer tube and the moldcavity depends only on the pressure applied on thesurface of the melt inside the pressure tight crucible:This level will be called the theoretical level. Eachincrease in pressure of 1 mB will result in the liquidaluminum level rising by 4mm.

Consequently, the equilibrium altitude, or theoreticallevel can be mathematically derived using the simplelaw stating that the difference in pressure p betweentwo points with a difference in altitude of h (in m) is

equal to p (in Pa) =.g.h, where is the density ofthe fluid (2600 kg.m-3for liquid aluminum) and g theacceleration of gravity (9.81 m.s-2)

However, this will not be the case in real life becauseof the following reasons:

a) Part of the pressure force is used to accelerate the

liquid metal at the entrance of the transfer tube andto counter viscous forces, so that the actual metallevel will be lower than the theoretical level.

b) Depending on the filling rate with respect to theventing of the mold, the difficulty in expelling theair from the mold cavity will result in a pressure

build up which will prevent the liquid metal levelto rise as fast as it should.

For instance, let us assume that the pressure rampshown in Figure 2 is applied on the melt surface (0-500mB in 10s, or 50mB/s), while the top of the moldis 1500mm above the melt surface. This means that the

theoretical level will rise at a rate of 200 mm/s asindicated by the blue line in Figure 3. The top of themold (at altitude 1500mm) will thus be reached after7.5s, when the pressure is 375mB as shown by thewhite arrow in Figure 2.

Fig. 2. Typical pressure curve in LPPM casting.

Fig. 3. Melt level progression corresponding to thepressure curve of Fig.2 as a function of venting.

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16 18

time, s

metalheightabove

potlevel,cm

theoretical (4mm/mB) well vented mold poorly vented mold



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However, in a normally vented mold, this relativelysteep pressure rise will result in a lag mainly due to anair pressure build up in the mold cavity. The actualmelt level progression in the mold will follow the redcurve in Figure 3 while a poorly vented mold willresult in the very slow filling depicted by the yellowcurve in the same figure.

Figure 4 shows typical curves of the pressure build upin the mold cavity recorded by a manometer duringfour consecutive fillings of the LPPM cast bellhousing referred to in the next chapter; in this

particular case, the rate in pressure rise over thesurface of the liquid metal was 15mB/s. The curves inFigure 4 recorded during these four consecutive cyclesindicate that the backpressure varied between 25 and40mB under identical pouring conditions; this scatteris probably due to variations in mold tightness at eachmold closing.

0

10

20

30

40

50

0 1 2 3 4 5 6 7 8 9 10 11

time, s

pressure,mB

Fig. 4. Pressure build-up in cavity during filling.

FILLING A 7kg (15.4lbs)A356 BELL HOUSINGThe filling times of a 7kg finished casting weremeasured for different pressure ramps and the SFobtained by dividing the measured filling time by thetheoretical filling time corresponding tometallostatic equilibrium.

The filling times were measured by detecting thearrival of the flow via two quick responsethermocouples encapsulated in thin copper sheaths;they were located as indicated in Figure 5 and therecording rate was 20 readings per second.

The LPPM press used in our filling experiments isshown in Figure 6 and schematized in Figure 7 wherethe correspondence between the pressure applied onthe melt and the altitude is indicated.

Fig. 5. Location of the two thermocouple tipsallowed measuring of the filling times (red dots).

Fig. 6. LPPM press with close-up on mold (at right).

Figure 7 states that when the crucible is full, assuminga very slow rise in pressure over the melt, the moldcavity will start filling when the pressure reaches88mB, and will finish filling when the pressurereaches 143mB.



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Fig. 7. Relationship between pressure and level.

However, as explained above, the actual pressures willbe greater than 88 and 143mB, all the more since thefilling is fast and the venting of the mold is poor.

The measurement of the filling times was done in thecourse of a campaign where the 40 bell housingsshown in Figure 8 were produced. Nine initial pourswere necessary to run in the mold and reach a dynamicthermal equilibrium.

Figure 9 shows the recorded temperature cycling in themold during the whole campaign (49 pours); the

positions of the thermocouples inserted in the mold arerepresented by a red dot on the models shown at twodifferent view angles in Figure 9.

Fig. 8. Production of the casting campaign whenfilling times were measured (40 castings).

Fig. 9. Recorded mold temperature cycling at locations indicated by the two red dots on the casting models.



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300

320

340

360

380

400

420

440

10 12 14 16 18 20 22 24

time (s)

te

mperature(C)

EXPERIMENTAL RESULTS (BELL HOUSING)

A typical response from the two thermocouples usedto measure the filling time is shown in Figure 10. It

pertains to pour #34 with a melt temperature of 745C(1373F), a pressure ramp of 0-300mB in 25s (12mB/s)

corresponding to a theoretical filling time of 4.7s. Thedetailed analysis of the curve shows that the fillingtime is 21.3-14.9= 6.4s. The slowing factor is thusequal to SF=6.4s/4.7s=1.36 in this particular instance.

Slowing factor was calculated in a similar fashion for19 normal pours, i.e. pours at a temperature between745C and 755C(1373F and 1391F) and pressureramps of 15mB/s and 12mB/s, or 0-300mB pressurerise in 20s and 25s respectively. The results are shownin Figure 11 for 12 pours at 15mB/s and 7 pours at12mB/s.

It shows that under normal conditions, the SF isabout 1.5 with a higher dispersion when the filling isfaster. This would entail that a more reproduciblefilling is achieved when the filling is slower.

Fig. 10. Typical response from the thermocouplesmeasuring the filling time. (pour#34)(See Fig.5)

Fig.11.Slowing factor for normal filling times (Bellhousings).

The SF was also calculated for extreme values offilling time, namely for pressure ramps of 0-300mB in5, 10, 15, 35, 60, 75 and 90s and pouring temperaturesof 720C, 695C, 685C, 681C and 664C(1328F, 1283F,

1265F, 1258F, and 1227F respectively). This is shownin Figures 12 and 13.

Fig. 12. Slowing factor over a wide range of fillingtimes (1:18 ratio.)(Bell housings)

The leftmost bar in the graph of Figure 12 indicatesthat for 0-300mB pressure ramp duration of 5s, thetheoretical filling time is 0.9s and the slowing factor isequal to 2.27; hence the measured filling time has been2.1s.

Likewise, the rightmost bar in the same graphcorresponds to a 0-300mB pressure ramp duration of90s, i.e. a very slow rate of filling; in this case, the

theoretical filling time is 16.5s, the SF is 1.10,calculated from the measured filling time of 18.1s. Thegraph shows, quite expectedly, that SF is close to 1 forvery slow filling times; it increases up to 2.3 forextremely steep pressure ramps; this results from thegrowing difficulty in expelling the air entrapped in themold cavity. For normal ramp durations of 5 to 10s,the value of SF lies around 1.5. The graph on Figure13 lumps the results obtained for normal 0-300mB

pressure ramp durations of 15s, 20s, and 25s and for arange of pouring temperatures comprised between

300

350

400

450

500

550

0 10 20 30 40 50

time (s)

temperature(C)



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664C and 750C (1227F and 1382F). It shows that theSF increases slightly as the pouring temperature drops,

probably due to the increased viscosity of the melt.

0

0.5

1

1.5

2

660 680 700 720 740 760

pouring temperature (C)

slowingfactor,SF

Fig. 13. Slowing factor over a wide range ofpouring temperatures for a normal range offilling times.(Bell housings)

EXPERIMENTAL RESULTS (THIN CASTING)

Similar filling times were measured when pouring thinwall castings, one of which is shown in place in themold and after ejection in Figure 14. The clustercomprises two mirror-copy covers used to encase anairplane seat adjustment device. The massive feedingand gating system represents 70% of the total weightof the cluster.

Fig. 14. Cover castings (1kg two-part cluster).

Figure 15 shows the 72 clusters which were poured inone campaign for a range of 0-300mB pressure rampdurations of 2s (150mB/s), 4s, 6s, 8s, 12s, 16s, and 20s(15mB/s).

Fig. 15. Clusters poured in the present study.Three thermocouples represented in red in Figure 16were inserted in the mold to record the thermal

history; their responses are shown in Figure 18. Thefilling time was measured by fast responsethermocouples in green on the same figure separated

by a vertical distance of 211mm or 8.3 in. (Figure 17).In the same manner as was done for the bell housings,dividing the actual measured filling times by thetheoretical filling times allowed to determine the SFfor pouring temperatures of 720C, 740C and

760C(1328F, 1364F, and 1400F) . The results areplotted in Figure 19 for these 3 pouring temperatures.Looking at the three leftmost bars indicates that, for avery steep pressure ramp of 0-300mB lasting 2s, thetheoretical filling time is 0.45s and the slowing factorsare 4.7, 2.8 and 2.3 for pouring temperatures of 720,740 and 760C. This translates to actual filling time of2.1, 1.2 and 1.0s respectively. However, for slower(and more reasonable) filling rates, SF is always lessthan 1.5 and did not depend much on the pouringtemperature between 720C and 760C(1328F and1400F).Similarly to what was observed in the bell housing

experiments, and for the same reason, SF decreases asthe filling is slower. The actual filling time isgenerally closer to the theoretical filling time than forthe bell housing experiments; this may be explained

by the fact that a much lesser amount of air must beexpelled from the cavity, especially in the final stageof the filling process.

Fig. 16. Thermocouples inserted in mold.

Fig. 17. Vertical distance separating the twothermocouples measuring the fill time.



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Temperature at 3 locations in the mold during the casting campaign

300

350

400

450

500

550

600

10:20 10:50 11:20 11:50 12:20 12:51 13:21 13:51 14:21 14:52

time (hrs:min)

temperature(C)

Fig. 18. Response of the thermocouples represented in red on Figure 16.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

slo

wingfactor,SF

2/0.45 4/0.9 6/1.3 8/1.75 12/2.6 16/3.5 20/4.5

time to reach 300mB (s) / theoretical filling time ( s)

760C

740C

720C

Fig. 19. Slowing factor over a wide range of filling times (1:10.)(Thin casting)

CONCLUSIONS

By measuring the filling time of a thin and a bulkycasting under a range of process conditions, it was

possible to determine the SF, ratio of the actual fillingtime to the theoretical filling time based on the staticlevel equilibrium of the melt. SF allows a calculationof the filling time of the mold cavity after the pressureramp of the LPPM process is selected. The knowledgeof SF is very useful to input a realistic mold fillingtime when modeling the filling of a LPPM mold.

A SF value of 1 indicates that the actual filling time isequal to the theoretical time; this corresponds to anextremely slow mold filling, when no back pressure

builds up inside the mold cavity. A value of SF of 2means that the actual filling time is twice the timecalculated based on the pressure ramp applied on themelt and the height of the casting.



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