1. lln-Situ Temperature
!Measurements in
Low-Pressure p,ermanent-
Mold Casting
F. Paray
McGill University
Montreal, Quebec, CANADA
J. Clements
Grenville Castings, Ltd.
Merrickville, Ontario, CANADA
B. Kulunk
Timminco Metals
Haley, Ontario, CANADA
J.E. Gruz[eski
Mc Gm University
Montreal, Quebec, CANADA
ABSTRACT
The low-pressure casting process is widely used, as it allows a
rapidproduction ofcomponents close to thefinalnear-net shape
with a very good casting yield. Depending on the property
requirements ofthefinal product, it is often necessary to control
casting soundness. Internal porosity can cause a loss ofpressure
tightness, a critical factor in parts, such as engine blocks and
manifolds, which are required to keep separate various gases
and fluids.
Fundamental to an understanding and control ofporosity is
a knowledge of the thermal conditions that prevail during the
solidification ofthe casting. It is, therefore, necessary to acquire
thermal data during casting solidification. A low-pressure cast-
ing machine and die were instrumented to obtain in-situ thermal
analysis curves during the solidification offlat plates ofthick-
ness varying from 118 to 314 in., in strontium-modified and
unmodified 356 and 319 alloys.
This paper describes that effort and some of the obtained
results. The soundness of the castings was investigated; some
plates were x-rayed and porosity distributions were determined
along the length.
INTRODUCTION
The properties ofa cast productdepend critically on the quality ofthe
casting, itself. Porosity causes costly scrap loss and can limit the use
of castings in certain applications. The presence of porosity, inevi-
table to a certain extent in any casting, can have a detrimental effect
in terms ofsurface quality and a deleterious effect on the mechanical
properties and corrosion resistance. Internal porosity can cause a loss
ofpressure tightness, a critical factor in parts, such as engine blocks
and manifo]ds, which are i;equired to keep S·eparate various gases and
fluids. Poros~ty occurs in cast aluminum aHoys due to shrinkage,
resulting from the volume decrease accompanying solidification,
AFS Transactions 97-55
and due to the evolution of dissolved hydrogen, resulting from the
decrease in solubility in the solid compared to the liquid metal.
Shrinkage and gas porosity can occur separately or together to
produce undesirable features of castings. The formation of porosity
in Al-Si alloys is also controlled by other factors, l-4 such as grain
refinement, modification, inclusion content, cooling rate and alloy
chemistry. Strontium is used as a modifier for eutectic silicon in the
Al-Si alloy family. In the automotive industry, there is an interest in
the increased use ofSr to improve mechanical properties, to enhance
machinability of castings and, in some cases, to control shrinkage.
However, some foundries are still reluctant to adopt strontium
because they associate it with an increase in porosity.
Casting industries are v1ery interested in the prediction ofporosity
without carrying out costly trial-and-error processes. Over the past
decades, many efforts have been made in the development ofmodels
for the simulation of solidification phenomena in castings. A project
is currently underway at McGill University, in collaboration with
industrial partners, to study the feeding range of Sr-modified low-
pressure permanentmold cast alloys for the automotive industry, and
to develop criteria functions that should allow the prediction of the
thermal conditions necessary to maintain porosity below some
critical predetermined level. This will allow casting and die design-
ers to better deal with the problem of porosity.
In the present study, experimental data required for the calibra-
tion of the model used to simulate the solidification pattern were
produced. In order to determine criteria functions, it was necessary
to acquire thermal data during casting solidification. A low-pressure
casting machine and die were instrumented to obtain in-situ thermal
analysis curves during casting production. This paper describes that
effort and some of the obtained results. The thermal data obtained
will not only be used for modeling purposes, but will also provide
useful information on the operation oflow-pressure permanent mold
casting machines..
EXPERIMENTAL PROCEIDUIRE
Low-Pressure Permanent-Mold Machine
A low-pressure permanent-mold (LPPM) machine consists of two
main parts: 1) the hydraulic casting unit witll tlle die and the ejection
system, and 2) below it, a furnace that is a pressure-tight chamber
containing a crucible.
The hydraulic casting unit is a simple four-bar hydraulic press.
The bottom and top halves we mounted on the fixed lower platen and
the moving middle platen, respectively. The lower platen has an
opening through which the feed tube passes and makes a direct liquid
metal contact with the bottom die half. A feed tube ofcast iron, with
an inside diameter of 5 in. (127 mm) was used. Protection must be
provided or the molten aluminum will attack and dissolve the cast
iron, contaminating the casting alloy.. In this case, a refractory
material (Foseco Kornn) was brushed on the surface of the tube as a
coating.
The sealed chamber is resistance-heated and contains a crucible
of about 500 pound capacity. Initially, the crucible was charged with
molten metal by moving back the upper frame consisting of the
hydraulic mechanism, the die and the ejection system, and by
removing the furnace cover. Recharging is done without removing
the entire unit. The molten metal is poured through a spout into the
furnace of the LPPM machine, and this opening is sealed after
transfer to allow the pressurization ofthe electric resistance furnace.
791
2. The casting cycle starts when the mold is closed. At this point, a
3-psi pressure was applied, causing the molten metal to rise steadily
up through the feed tube into the die. This pressure is held for a time
to provide liquid feed metal to feed solidification shrinkage. The
furnace chamber is then released to the atmosphere and the casting
is allowed to cool down:inthe die, to ensure it has sufficient strength
for ejection from the die. At theend ofthe cooling period, the middle
platen moves up, thus extracting the casting from the bottom fixed
portion ofthe mold. As the moving middle platen is moved further
up, theejectorpins are activatedand push the casting outofthe mold.
The Die
An experimental die was designed for this study to produce castings
with different thicknesses, in order to cover the usual range of wall
sizes found in LPPM castings. The die produces four plates of
different thicknesses: 118 in. (3.18 mm), 1/4 in. (6.35 mm), 112 in.
(12.70 mm) and 3/4 in. (19.05 mm). This geometry was chosen
because the flat plate is a basic fonn found in many castings and
it is a simple design for thermal modeling. The choice of exact
dimensions was limited by the maximum size of die that could be
mounted on the LPPM casting machine. The length and the width of
plates are 11 in. (270.40 mm) and 4 in. (101.60 mm), respectively,
and! the thickness ratio between the gates andthe plates is 2/3 foreach
plate.
A photograph ofthe casting is shownin Fig. l. The mold consists
of two halves. Each plate cavity is inclined to avoid air entrapment,
and! the parting lines correspond to the tops of the plates. During the
advance of the molten metal into the die, the air escapes through the
vents and parting lines of the die. The surfaces of the die in contact
with the molten metal were coated by spraying a commercially
available die coat product (Foseco Dycote 34ESS).
In the LPPM process, the die must be preheated before each run,
anddietemperatureis an experimental variableto consider. A special
gas heater having the same shape as the die was made to evenly
preheat the die. A special tool was also designedto collectthe casting
after ejection from the die.
Rg. 1. Casting With its four plates of different thicknesses.
792
Thermal Analysis
It was necessary to be able to record the thermal history ofa casting
during solidification at different locations within the casting. The
low~pressure casting machine and die were instrumented to obtain
in-situ thermal analysis curves during the solidification of the flat
plates with different thicknesses. This necessitates the insertion of
thermocouples through thedie wall andinto the castinginsuch away
that the thermocouples were located in the area of interest, but they
could be removed from the casting when it was ejected from the
machine.
The thermocouples were inserted from the top half of the die,
which was fixed onthemoving platen..Foreachplate,fourholes were
drilled along the longitudinal axis from the feeding to the free end at
the mid-width. Theirdistancesfromthefeedingend were 1in. (25.40
mm) for the thermocouple location #4, 4.75 in. (120.65 mm) for the
location #3, 7.25 in. (184.15 mm) for the location #2 and 10 in.
(254.00 mm) for the location #1.
The thermocouples, held in place by compression fittings, were
aligned with the die ejection pins, so that they were pulled out of the
casting when it was ejected from the d:ite. The tips of the thermo-
couples were arranged at mid-height in each plate cavity, and a
graphite die coat brushed onto the thermocouple tips prevented
sticking in the casting. This application ofgraphite was found to be
critical to ensure long thermocouple life. Without it, the wires would
solder to the casting and pull out.
Pliable, type K, grounded thermocouples with a 1116 in. (1.59
mm) diameter were used. These were installed before mounting the
die on the machine,. and they remained in pface during an entire
testing campaign. Thus, thermal information couldbe recorded from
the first casting to the last one produced, but the thermocouples were
more exposed to damage and could not be replaced if they did fail.
Some :lfailmres did occur, and, over a typica] two-day testing period,
roughly four of the 16 thermocouples might be expected to have
failed by the end ofthe tests. During the preheatingofthe die,. the tips
of the thermocouples were protected by specially designed small
cups, to avoid the heat of a direct flame. In addition, ceramic
thermocoupleconnectors werepreferredto plastic ones, as they were
found to better support the heat.
The thermocouples were numbered and were connected via
chrome]-alumel wires extensions to two multichannel data acquisi-
tion units to record the temperature profiles with time dming the
solidification process. For each plate, the thermocouples were num-
bered from 1to4; the thermocouple with the lowestnumber was near
the free end, opposite to the mgate. The numbers correspond to the
thermocouple locations described previously.
The Alloys
Two commercial alloys were investigated in this study: 356 repre-
sentative of the Al-Si-Mg alloy system with 7.30 wt% Si and 0.34
wt% Mg; and 319 from the Al-Si-Cu family with 6.25 wt% Si and
3.62 wt% Cu. Castings containing 180 ppm of strontium were also
produced.
Gas Level
A quantitative version ofthe reduced pressure test (Straube-Pfeiffer
test) devdoped at McGill5,6 was used to determine the gas level.
Constant volume samples with a riser were produced and the hydro-
gen level of the melt was determined :from the density of these
AFS Transactions
3. samplesandcalibration curves. Twogas levels were used: the normal
gas level of the as-me[ted metal and a degassed level obtained by
using arotary impellerdegasser. For356 aHoy, ilies,e levds were 0.31
mlH2/100 g Alarnd0.14mlH2/100 g, respectively; for 319alloy, they
were 0.25 mlH2/100 g Al and 0.12 mm2/H>O g.
Pomsity Determination
The quantification of the porosity was done by density measure-
ments. Theplateswere sectioned alongthe lengthto use thecenterline
slice. This slice was cut into 30x15 mm (1. l8x0.59 in.) rectangular
b]ocks having the plate thickness.
RESULTS .AND DISCUSSION
Castiing Cycle
Casting variables, such as melt temperature, pressure time and
cooling time, were sdected., based on the experience of the machine
operator. A casting cycle consists of a combination of three steps: a
period when the pressure is applied, a time for cooling and a time
when the mold is opened for the casting ejection. The cast time, or
pressure time and the cool time, are selected by the operator and are
changed several times during the transient period of the production
run. They are then maintained at constant values when the operator
fee]s that he has reached the steady state.. In the steady-state period,
thepressure/cooUopen times used were90sec/80sec/30secand90sec/
90sec/31sec for 356 and 319 aHoys, respectively. On average, a
casting was produced every three minutes and 25 sec, about 17
castings per hour.
The thermal behavior of the casting and the mold determines
production parameters, such as the number ofcastings produced per
hour and the "equilibrium'' mold temperature. The rate of cooling
determines the time required for the aluminum alloy to completely
solidify and r:each a temperature that will allow ejection without
mechanical damage to the casting. Depending on the mold design, a
certain number ofcasting cycles will be required to reach the steady-
state period when the die temperature is more or [ess constant. and
a constant casting cycle can be maintained.
Thermal1 Analysi:s Study
The temperature at different locations in the plates was recorded, as
a function of time, during the production run, that is, during both
transient and steady-stateperiods ofthe casting process. A very large
number of thermal analysis curves was produced in this study and
only a selection of thos,e will be discussed here.
T,emperature versus time curves are presentedin Figs. 2 and 3 for
the Sr-modified 356 alloy and unmodified 319 alloy. The graphs
present the results obtained at location #4 (ilie closest to the feeding
end) and at location#1(the furthest from the gate) inthe 3/4-in. plate.
These temperature-time plots are typical of the cooling curves
obtained for 356 and 319 aUoys, and accurately r;eflect the time
variation of temperature during the production run.
Significantdifferences wereobservedbetweenthecoolingcurves
obtained for the very first castings and those recorded at the end of
the production run. This obs,ervation confirms the existence of a
transient period in the LPPM casting process, in terms of thermal
behavior, and it can be expected that castings produced during the
transient will be very different in properties from those produced
under steady-state conditions.
AIFS Tlransacti1ons
At the beginning, the casting cycle is not constant; the time
interval between cycles varies, as do the highest and lowest tempera-
tures measured for each casting cycle. Moreover, the solidification
time at a specific location, defined as the time elapsed between the
liquidus and solidus temperatures, increases during ilie beginning of
the production run. For example, for the fifth casting in the 3/4-in.
plate, the solidification times were 50 sec, 74 sec and 65 sec at
locations #1, #2 and #4. For the sixteenth casting, they were 69 sec,
119 sec and 107 sec, respectively.
Thermal characteristics, such as solidus and liquidus tempera-
tures, eutectic temperature and solidification time can be determined
from the curves, for all ilie plates at different locations. These values
represent valuable thermal data required for the calibration of any
eventual model to simulate the solidification pattern. Additional
information can also be obtained from the cooling curves, such as
times when the pressure was applied, when the cooling starts, and
when the mold is opened. These were recorded for each casting.
Tables 1 and 2 present some examples of temperatures all: different
steps in the process for 356 and 319 alloys, respectively. In the
steady-state period, the pressure time should be selected, in order to
hav,e a temperature lower than the eutectic temperature at the end of
the pressure time, in order to take advantage ofthe pressure in terms
of soundness.
Table 1.
Temperatures ("C) at Different Steps of Casting Cycle
for 356 Alloy With Sr
casting melt step* 314" plate 112" plate 114" plate
mnperablre
location 4 location 1 location 4 location 4
p 302 270 340
#1 747 c 562 567 S44
0 442 467 411
p 276 279 349
#10 757 c 584 S72 S47
0 S2S 493 464
p 321 306 383
#20 7Sl c 565 S62 S48
0 520 482 470
": wnen pressn11e ts appne 1, auratton 4U sec, :m sec, ~u sec tor castmg l, W,Z(
C : when cooling starts, duration 60 sec, 80 sec, 80 sec for casting 1, 10, 20
0 : when the mold opens
Table2.
Temperatures ("C) at Differ:ent Steps of Casting Cycle
for 319 Alloy With No Sr
-
401
403
377
S16
414
382
444
410
casting melt step* 314" plare 112" plate 114" plate
temperature
location 4 location 1 loc.ation4 location 3
p 22[ 195 260 247
#1 743 c 561 561 614 461
0 403 460 350 284
p 242 221 291 287
#10 744 c 561 561 507 436
0 461 449 403 314
p 264 I
247 325 )04
#20 745 c 560 I
556 491 385
0 471 459 ·420 325
p 305 267 3S7 322
#30 747 c 564 11
560 522 405
0 494 480 44S 348
: wnen p.resswe is appuea, ouration 'IQ sec, w sec,'~ sec, 9U sec tor casting 1, II, zo, ::Kl
C : when cooling starts, duration SO sec, 60 sec, 90 sec , 90 sec for casting L, rn, 20, 30
0 : when the mold opens
793
5. Meta~I Temperature
In the LPPM process. the melt temperature is controlled via the
temperature of the sealed chamber, which is monitored during a
production run. In these experiments, the metal temperature was
directly measured by plunging a temperature probe into the melt For
each production run, measurements were t~en just before the first
casting and after each ten castings produced. In all cases, the metal
temperature was approximately constant, fluctuating within a few
degrees. An exception was observed for the unmodified 356 alloy
(with normal gas level) where the temperature increased from 746C
to 763C and then to 789C. This variation in temperature was
significant and was reflected in the thermal analysis, as discussed
further.
Thefluid flow phenomenaandtemperature distribution/variation
of the molten metal during mold filling have great effects on the
quality of the casting obtained. Thermal information on the molten
metal during filling is important to determine operating conditions,
such as pouring temperature or cooling system design, which may be
required in the metallic die to avoid problems, such as misruns or
localized hot spots. The highest temperature on each cooling curv,e
obtained indicates, at each thermocouple location, the drop in metal
temperature while filling the mold. Examining ilie highest tempera-
tures recorded by an thermocouples, as a function of time, will give
some indication as to when the transientperiod is completed for each
plate or thermocouple location.
Because the molten metal does not contact all the thermocouples
at the same time, and for eas,eofcomparison, the results were plotted
as afunction ofcasting number. Figure4 presentssome results for the
356 alloy. For each plate, the highest temperature recorded at
locations #2 and #4 were plotted. In general, during a productionrun,
the highest temperature recorded at any location first increased
during a transient period and then reached a plateau. However, for the
unmodified 356 alloy, this plateau was more difficult to reach, due to
the fact that ilie melt ternperatme increased with time.
As can also be seen in Fig. 4, a significant difference exists
between the highest temperature measured at location #4 and the one
measured at location #2 in a plate. Along the distance of 6.25 in.
(158.75 mm), which separates these two Bocations, a metal tempera-
ture loss of several degrees occurs. It should be noted that, in both
cases, without and with Sr, the highest temperature recorded at
location #2 in the l/4-in. plate is very low, even lower than what was
measured by the thermocouple furthest from the in-gate, near the free
end (location #1). where the plateau was around 635C.
The thinner the pfate, the lower was the metal temperature at the
feeding end. Similar results were observed for the 319 alloy. as
shown in Fig..5a. where the highest temperature at the same location
(location #3) in the 3/4 in. and 1/4-in. plates are compared. Again, on
average, the loss in temperature for the metal during mold filling is
smallerduring the steady-statte period, compared to the beginning of
the production run: a difference of about 50°C. A significant differ-
ence exists between the highest temperature recorded at different
locations in the same plate (Fig. 5b).
From these measurements, it is possible to calculate the flow
velocity in a plate. Forthe3/4-in. plate, the average flow velocity was
128 mm/sec and 138 mm/sec for the 356 alloy,.without and with Sr.
The filling time was, therefore, 2.11 sec and 1.96 sec, respectively.
For the 319 alloy, the corresponding values were 102 mm/sec and
114 mm/sec, or filling times of2.65 sec and 2.37 sec.
AFS Transactions
Influence of Mold Design
The mold temperature is not uniform and continues to fluctuate, even
after the so-called steady-state regime is r~ached. At this point, the
casting cycle is held constant by ilie operator, in terms of pressure
time and cooling time, andthe mold opening time is kept as short and
constant as possible, in order to minimize temperature loss. Tem-
perature measurements were made with an infrared laser gun on the
outside surfaces of the mold for several casting cycles in the steady-
state period. At a given location, the temperature varies slightly with
thecasting cycle. The temperatures for the mold section correspond-
ing to the thin plate were significantly lower than what was recorded
from the other sides of the mold. A casting geometry, as used here,
with the thickest plate being six times thicker than ilie thinnest one,
requires a nonsymmetric mold, and this nonsymmetry affects the
thermal equilibrium. Consequently, the thin cavity was not always
completely filled, as observed from the 200 castings produced.
Casting Soundness and Porosity
For each production run, several castings were randomly selected
from among those produced during the steady-state r'egime. The
plates were separated from the gates and radiographed, in order to
obtain a general idea of the soundness and overall porosity distribu-
tion. In general, when defects were present,they were observed inthe
middle of the plate, along the length. The results obtained from the
porosity determination are in good agreement with those obtained by
radiography.
356 Alloy
Figure 6 presents some x-rays of 112-in. plates of 356 alloy, cast
without and with Sr, at two gas levels. Strontium is neutral in terms
of soundness, while degassing promotes larger gate defects in the
plates. No significant difference was observed from plate to plate
(i.e., casting to casting).This was confirmed by the porosity distribu-
tion curves determined from density measurements of small blocks
taken from the middle part of the plates along the length. For each
condition, six plates wereused.Individual curves werefrrst obtained,
and the average values were calculated.
Figure 7 compares the variation ofaverage porosity, according to
the distance from the feeding end. With the exception of the 3/4-in.
plates, where some plates exhibit a gate defect and some do not, a
good r;eproducibility was obtained from curve to curve (i.e. plate to
plate), and the average curves reflect well the general behavior. With
the exception ofthe first 75 mm, where degassing promotesa feeding
end defect, it can be observed that degassing lowers the porosity.
Strontium was neutral in terms of porosity, but it did have a
microstructural effect, producing good modification, except in the
thick plate where uneven structures with areas of fine modified and
areas of partially modified silicon were observed.
319 AUoy
Average porosity curves are presented in Fig. 8. Much higher
porosity levels than in the previous alloy were found, due to the
longer freezing range of the alloy. Degassing generates or accentu-
ates a gate region defect. As shown in Fig. 9, the addition of Sr has
significantly improved the soundness ofthe 1/2-in. plate, making the
porosity fine and dispersed. From the structure of the porosity, it is
evident that the spongy areas are due to an inability to feed this long-
freezing range alloy. In addition to its positive effect on porosity,
strontium partially modified the silicon and caused the CuAh to be
more massive.
795
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6 'JOO
(a)
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··········f--~~---~~--1---···········--------·--··--·-
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casting number
Rg. 5. Highest temperatures recorded for the unmodified 319 alloy:
(a) location 3; (b) 314-in. plate.
Hot tearing was observed in the 319 alloy. This began during
cooling, and crack growth continued as the casting cooled after
removal from the mold. In general, all the cracks fonned at about the
same location in the plates, near the feed end, perpendicular to the
length of the plate. The percentage of plates having a crack was
determined, and the crack size was measured as the linear distance
from one endofthe crackto the other. An average cracksize was then
calculated for each condition.
As can be seen in Table 3, Sr has a very strong positive effect on
elimination of hot tearing. In the presence of Sr, and without
degassing, no 112 in. or 314-in. plate contained a crack, while in the
absence of Sr, 13% and 27% of the plates had cracks, respectively.
The worst case occurred for the 114-in. plate, where 87% contained
cracks if no Sr was present, compared to only 7% for the Sr-
containing alloy. In addition, a significant difference in crack size is
seen.
When the melt was degassed, more plates did crack, but Sr
continued to have a positive effect on hot tearing. Cracks in p1ates
containing Sr were shorter, narrower, and more shallow that those in
the absence of Sr. Frequently, the cracks in Sr-free pfates extended
through the entire thickness of the plate, from the top to bottom
surface (Fig. 10). Such deep cracks were not observed ifthe melt was
strontium treated.
AFS Transactions
(6a) no Sr, normal gas
(6b) no Sr, degassed
(6c) w;th Sr, normal gas
(6d) with Sr, degassed
Fig. 6. X-rays for the 112-in. plates for the 356 alloy.
797
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,o 25 '° 75 100 125 150 175 200 m 250 21.s 300
distaDM from feeding end (mm)
(b)
13
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0 2S 50 75 100 125 1'0 175 200 225 2SO 275 300
distance from feeding ,eJJd (mm)
(c)
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0 25 so 1s 100 125 150 11s 200 225 250 ns 300
distance from feeding 1
end (mm)
(d)
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10 --·····,··-··········--····-·-----··-····-·-··-·····-·----------·-··-·······
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tit I !t., ..........,...........................................................
Ii;,..
·t' -,&:...., •••••.••••••.•..•••••.....•..•.••.••...••..••.••.••.•.•••.•.•.•
,5
I
5
4 .......,........ -··················-······························-·-·-----
2
1
1Q.._.....____--'---'---.__C:.a.;;.....i....__.;;....w..'--'M-11~._~
0 25 50 15 100 125 1'0 175 200 225 250 2'75 300
distance from feediog end (mm)
(e)
1s,..--------------------------------------
:-,:·::-:::J-·~:-.:~-----t:_-:::-::::::::::::
9 ·-·----~----·········,··········-·········-···--····,··········-·············
'Wt,I ·------····,····,······--------·············----------····················
.mt -- ---------------------------······---------········---------····---·-··
.... --·-t----------·-···················-·--···············--·--·-··-····'•
'...... ......................................···--··· --···-···· ..... ···-·
I l
4 ··-----· .-·-···································--·-·········-············
s ........,.. ,,·······························- ------ .......................,...
2 ···········~-et-s.....;,.;..,.e~
1 ..............~,---···········""····· .........,... ,.... ············--·---
~b-·'llt" '& ~
oL-....1........J~~~--...1........a.i::lltl:.:~....J=.:=~::.:!t:s;~_J
0 25 50 15 1!00 125 150 175 100 225 250 'Z15 300
distance &om feeding end (mm)
Ur-------------------------------------
:::-:::::•J:~:·:·~:J::::::_:::::.::::::::
' -----··-··-······ ····- .......... ,........................ ,.................. .
Wt 8 ................................................... ,....................... .
I
I 7 ----·-- ...,.- . .,.•,.••--•-••··-·~-•w•••·~-•-••••••-•••••·-·•••••••-••••••••••••·•-
0: ' ···ft~·-···-····-----·-············-··--------·······-·-··-----···········,
! 5 ...l---····················· ··-······································--··-'
I !
4 J·····••·····-·--·-··················--······················--········1
3 -A,.-···-···.,-·-------···············-··- -············· ····------- ---- ·······'
2
I
~1L...-·._-.J...-._--_-- J..A..:__.J.._---1--1---.l.Bl~.:.......L........J~::l!t.:illl:li:L-...I
,0 2S SO 75 100 125 lSO 175 200 225 250 'Z15 300
distance &om feeding end (mm)
Fig. 7., Porosity distribution for the plates of 356 alloy: (a) no Sr, 314-in. plate; (b) no Sr, 112-in. plate; (c) no Sr, 1/4-in. plate;
(d) with Sr, 314-in. plate; (e) with Sr, 112-in. plate; (f) with Sr, 114-in. plate.
798 AFS Transactions
10. (9a) no Sr, normal gas
(9b) no Sr, degassed
(9c) with Sr, normal gas
(9d) with Sr, degassed
Fig. 9. X-rays for the 112-in. plates for the 319 alloy.
800
Casting Yield
Higher metal yield, easier cleaning of the casting, fewer scrap
castings, better surface definition and consistent dimensional accu-
racy on production runs are advantages to all ofthe permanent mold
processes. The low-pressure casting process is a process, both
technically and economically, that bridges the gap between the
gravity (permanent mold) and high-pressure die casting processes.
Its advantages over permanent mold casting are twofold. First, it
allows a relatively nonturbulent filling of the mold or die cavity,
reducing defects such as oxide inclusions and air entrapment. Sec-
ond, since the excess of metal in the feed tube drains back into the
furnace, the casting yield is significantly greater and the need for
bulky risers is eliminated.
The casting yield for the production runs of 356 and 319 alloys
done in this study was found to be about 89% in all cases. An increase
of 2-3% in the casting yieldl was obtained after the transient period
was over.
Table 3.
Hot Tearing ObseNed in Plates of319 Alloy
Condition 'Thickness Average crack siz.e (mm) % of plates with crack
118" - 0
no Sr 1148 26.3 ± 9.4 87
nonnal gas lfl9 16.8 ± 8.2 13
3W 17.8 ± 12.4 27
118· 8 3
with Sr 1149 8.5 ± 3.5 7
normal gas 112· - 0
314• - 0
1/8" - 0
no Sr 114" 30.1 ± 9.5 90
degassed 112· 14.7 ± 7.4 67
3149 15.5 ± 6.7 37
1/8" - 0
with Sr 1/4" 18.7 ± 7.9 63
degassed 112· 20.4 ± 7.1 23
314• 5 3
Fig. 10. Portion of hot crack formed in 114 in. 319 alloy plate
without strontium. Total crack length: 35 mm.
I
I
AFS Transactions
11. SUMMARY
Although the LPPM casting process is an automated process in
whlch the casting is ejected from the die after solidification, it has
been possible to instrument the unit for therma] analysis. Tempera-
ture profiles with time dluring the solidification process for all
castings produced were obtained in-situ and! provided useful infor-
mation. Typical parameters, such as eutectic temperature, solidus
and liquidus tempe.!iatureand solidificationtime, were measured and
can be used foF the:rrnal modeling.
On the other hand, information on the operation of the LPPM
machine was obtained: a transient period was well identified., in
terms ofthermal behaviorofthe metal. Moreover, the temperature at
any step of the process could be determined at any location of the
thermocouples. Porosity profiles indicate that Sr has a neutral effect
on the soundness of 356 alloy, but dinrinishes porosity in the 319
alLoy and can significantly reduce hot tearing in tlris alloy.
AFS Transactions
ACKNOWLEDGMENTS
The authors wishto acknowledge the financial support ofthe Natural
Sciences and Engineering Research Council of Canada (NSERC),
Grenville Castings, Ltd. and Timminco Metals, a division of
Timminco, Ltd.
REFERENCES
1. J.E. Gruzlesk:i and B.M. Closset; The Treatment ofLiquid Aluminum-
Silicon Alloys, American Foundrymen's Society, Des Plaines, Illinois,
1990.
2. D. Emadiand J.E. Grusleski; "TheEffects ofCasting andMeltVariables
onPorosityinDirectionally SolidifiedAl-SiAlloys,"AFSTransactions,
vol W2, 1994.
3. G. Laslaz and P. Laty; "Gas Porosity and Metal Cleanliness in Alumi-
num Casting Alloys," AFS Transactions, vol 99, 1991, pp 83-90.
4. D. Emadi; "Porosity Formation in Sr Modified Al-Si Alloys," Ph.D
Thesis, McGill University, Montreal, Canada, Feb 1995.
5. W. La-Orchan, M.H. Mulazimoglu and J.E. Gruzleski; "Constant Vol-
ume Risered Mold for Reduced Pressure Test," AFS Transactions, vol
101, 1993, pp 253-259.
6. W. La-Orchan; "TheQuantificationoftheReducedPressure Test," Ph.D
Thesis, McGill University, Montreal, Canada, Sep 1994.
801
![lln-Situ Temperature
!Measurements in
Low-Pressure p,ermanent-
Mold Casting
F. Paray
McGill University
Montreal, Quebec, CANADA
J. Clements
Grenville Castings, Ltd.
Merrickville, Ontario, CANADA
B. Kulunk
Timminco Metals
Haley, Ontario, CANADA
J.E. Gruz[eski
Mc Gm University
Montreal, Quebec, CANADA
ABSTRACT
The low-pressure casting process is widely used, as it allows a
rapidproduction ofcomponents close to thefinalnear-net shape
with a very good casting yield. Depending on the property
requirements ofthefinal product, it is often necessary to control
casting soundness. Internal porosity can cause a loss ofpressure
tightness, a critical factor in parts, such as engine blocks and
manifolds, which are required to keep separate various gases
and fluids.
Fundamental to an understanding and control ofporosity is
a knowledge of the thermal conditions that prevail during the
solidification ofthe casting. It is, therefore, necessary to acquire
thermal data during casting solidification. A low-pressure cast-
ing machine and die were instrumented to obtain in-situ thermal
analysis curves during the solidification offlat plates ofthick-
ness varying from 118 to 314 in., in strontium-modified and
unmodified 356 and 319 alloys.
This paper describes that effort and some of the obtained
results. The soundness of the castings was investigated; some
plates were x-rayed and porosity distributions were determined
along the length.
INTRODUCTION
The properties ofa cast productdepend critically on the quality ofthe
casting, itself. Porosity causes costly scrap loss and can limit the use
of castings in certain applications. The presence of porosity, inevi-
table to a certain extent in any casting, can have a detrimental effect
in terms ofsurface quality and a deleterious effect on the mechanical
properties and corrosion resistance. Internal porosity can cause a loss
ofpressure tightness, a critical factor in parts, such as engine blocks
and manifo]ds, which are i;equired to keep S·eparate various gases and
fluids. Poros~ty occurs in cast aluminum aHoys due to shrinkage,
resulting from the volume decrease accompanying solidification,
AFS Transactions 97-55
and due to the evolution of dissolved hydrogen, resulting from the
decrease in solubility in the solid compared to the liquid metal.
Shrinkage and gas porosity can occur separately or together to
produce undesirable features of castings. The formation of porosity
in Al-Si alloys is also controlled by other factors, l-4 such as grain
refinement, modification, inclusion content, cooling rate and alloy
chemistry. Strontium is used as a modifier for eutectic silicon in the
Al-Si alloy family. In the automotive industry, there is an interest in
the increased use ofSr to improve mechanical properties, to enhance
machinability of castings and, in some cases, to control shrinkage.
However, some foundries are still reluctant to adopt strontium
because they associate it with an increase in porosity.
Casting industries are v1ery interested in the prediction ofporosity
without carrying out costly trial-and-error processes. Over the past
decades, many efforts have been made in the development ofmodels
for the simulation of solidification phenomena in castings. A project
is currently underway at McGill University, in collaboration with
industrial partners, to study the feeding range of Sr-modified low-
pressure permanentmold cast alloys for the automotive industry, and
to develop criteria functions that should allow the prediction of the
thermal conditions necessary to maintain porosity below some
critical predetermined level. This will allow casting and die design-
ers to better deal with the problem of porosity.
In the present study, experimental data required for the calibra-
tion of the model used to simulate the solidification pattern were
produced. In order to determine criteria functions, it was necessary
to acquire thermal data during casting solidification. A low-pressure
casting machine and die were instrumented to obtain in-situ thermal
analysis curves during casting production. This paper describes that
effort and some of the obtained results. The thermal data obtained
will not only be used for modeling purposes, but will also provide
useful information on the operation oflow-pressure permanent mold
casting machines..
EXPERIMENTAL PROCEIDUIRE
Low-Pressure Permanent-Mold Machine
A low-pressure permanent-mold (LPPM) machine consists of two
main parts: 1) the hydraulic casting unit witll tlle die and the ejection
system, and 2) below it, a furnace that is a pressure-tight chamber
containing a crucible.
The hydraulic casting unit is a simple four-bar hydraulic press.
The bottom and top halves we mounted on the fixed lower platen and
the moving middle platen, respectively. The lower platen has an
opening through which the feed tube passes and makes a direct liquid
metal contact with the bottom die half. A feed tube ofcast iron, with
an inside diameter of 5 in. (127 mm) was used. Protection must be
provided or the molten aluminum will attack and dissolve the cast
iron, contaminating the casting alloy.. In this case, a refractory
material (Foseco Kornn) was brushed on the surface of the tube as a
coating.
The sealed chamber is resistance-heated and contains a crucible
of about 500 pound capacity. Initially, the crucible was charged with
molten metal by moving back the upper frame consisting of the
hydraulic mechanism, the die and the ejection system, and by
removing the furnace cover. Recharging is done without removing
the entire unit. The molten metal is poured through a spout into the
furnace of the LPPM machine, and this opening is sealed after
transfer to allow the pressurization ofthe electric resistance furnace.
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![The casting cycle starts when the mold is closed. At this point, a
3-psi pressure was applied, causing the molten metal to rise steadily
up through the feed tube into the die. This pressure is held for a time
to provide liquid feed metal to feed solidification shrinkage. The
furnace chamber is then released to the atmosphere and the casting
is allowed to cool down:inthe die, to ensure it has sufficient strength
for ejection from the die. At theend ofthe cooling period, the middle
platen moves up, thus extracting the casting from the bottom fixed
portion ofthe mold. As the moving middle platen is moved further
up, theejectorpins are activatedand push the casting outofthe mold.
The Die
An experimental die was designed for this study to produce castings
with different thicknesses, in order to cover the usual range of wall
sizes found in LPPM castings. The die produces four plates of
different thicknesses: 118 in. (3.18 mm), 1/4 in. (6.35 mm), 112 in.
(12.70 mm) and 3/4 in. (19.05 mm). This geometry was chosen
because the flat plate is a basic fonn found in many castings and
it is a simple design for thermal modeling. The choice of exact
dimensions was limited by the maximum size of die that could be
mounted on the LPPM casting machine. The length and the width of
plates are 11 in. (270.40 mm) and 4 in. (101.60 mm), respectively,
and! the thickness ratio between the gates andthe plates is 2/3 foreach
plate.
A photograph ofthe casting is shownin Fig. l. The mold consists
of two halves. Each plate cavity is inclined to avoid air entrapment,
and! the parting lines correspond to the tops of the plates. During the
advance of the molten metal into the die, the air escapes through the
vents and parting lines of the die. The surfaces of the die in contact
with the molten metal were coated by spraying a commercially
available die coat product (Foseco Dycote 34ESS).
In the LPPM process, the die must be preheated before each run,
anddietemperatureis an experimental variableto consider. A special
gas heater having the same shape as the die was made to evenly
preheat the die. A special tool was also designedto collectthe casting
after ejection from the die.
Rg. 1. Casting With its four plates of different thicknesses.
792
Thermal Analysis
It was necessary to be able to record the thermal history ofa casting
during solidification at different locations within the casting. The
low~pressure casting machine and die were instrumented to obtain
in-situ thermal analysis curves during the solidification of the flat
plates with different thicknesses. This necessitates the insertion of
thermocouples through thedie wall andinto the castinginsuch away
that the thermocouples were located in the area of interest, but they
could be removed from the casting when it was ejected from the
machine.
The thermocouples were inserted from the top half of the die,
which was fixed onthemoving platen..Foreachplate,fourholes were
drilled along the longitudinal axis from the feeding to the free end at
the mid-width. Theirdistancesfromthefeedingend were 1in. (25.40
mm) for the thermocouple location #4, 4.75 in. (120.65 mm) for the
location #3, 7.25 in. (184.15 mm) for the location #2 and 10 in.
(254.00 mm) for the location #1.
The thermocouples, held in place by compression fittings, were
aligned with the die ejection pins, so that they were pulled out of the
casting when it was ejected from the d:ite. The tips of the thermo-
couples were arranged at mid-height in each plate cavity, and a
graphite die coat brushed onto the thermocouple tips prevented
sticking in the casting. This application ofgraphite was found to be
critical to ensure long thermocouple life. Without it, the wires would
solder to the casting and pull out.
Pliable, type K, grounded thermocouples with a 1116 in. (1.59
mm) diameter were used. These were installed before mounting the
die on the machine,. and they remained in pface during an entire
testing campaign. Thus, thermal information couldbe recorded from
the first casting to the last one produced, but the thermocouples were
more exposed to damage and could not be replaced if they did fail.
Some :lfailmres did occur, and, over a typica] two-day testing period,
roughly four of the 16 thermocouples might be expected to have
failed by the end ofthe tests. During the preheatingofthe die,. the tips
of the thermocouples were protected by specially designed small
cups, to avoid the heat of a direct flame. In addition, ceramic
thermocoupleconnectors werepreferredto plastic ones, as they were
found to better support the heat.
The thermocouples were numbered and were connected via
chrome]-alumel wires extensions to two multichannel data acquisi-
tion units to record the temperature profiles with time dming the
solidification process. For each plate, the thermocouples were num-
bered from 1to4; the thermocouple with the lowestnumber was near
the free end, opposite to the mgate. The numbers correspond to the
thermocouple locations described previously.
The Alloys
Two commercial alloys were investigated in this study: 356 repre-
sentative of the Al-Si-Mg alloy system with 7.30 wt% Si and 0.34
wt% Mg; and 319 from the Al-Si-Cu family with 6.25 wt% Si and
3.62 wt% Cu. Castings containing 180 ppm of strontium were also
produced.
Gas Level
A quantitative version ofthe reduced pressure test (Straube-Pfeiffer
test) devdoped at McGill5,6 was used to determine the gas level.
Constant volume samples with a riser were produced and the hydro-
gen level of the melt was determined :from the density of these
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