Comparing Low Pressure Permanent Mold Casting of

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Paper 09-028.pdf, Page 1 of 13
AFS Transactions 2009 © American Foundry Society, Schaumburg, IL USA
Comparing Low Pressure Permanent Mold Casting of Magnesium AZ91E
and Aluminum A356
F. Chiesa, G. Morin
Centre Intégré de Fonderie et de Métallurgie, Trois-Rivières, Québec
B.Duchesne
Collège de Trois-Rivières, Trois-Rivières, Québec
J. Baril
Technologie de l Aluminium et du Magnésium, Trois-Rivières, Québec
Copyright 2009 American Foundry Society
ABSTRACT
While the low pressure permanent mold (LPPM) casting of aluminum parts is commonplace, this process is virtually not used
with magnesium alloys for reasons very difficult to apprehend. In the present study, a step casting, 152mm wide, with plate
thicknesses of 6, 13 and 25mm and an automotive bell housing, 460x380x170mm in overall dimensions, were poured in a
low pressure permanent mold machine (LPPM), first in aluminum A356, then in magnesium AZ91E. The same molds and
gating systems were used for both metals.
The differences in behavior of the two alloys were highlighted and the metallurgical quality of the castings was assessed by
radiographic analysis and by measuring metallographic properties such as grain size, secondary dendrite arm spacing (DAS)
and level of micro voids at different locations in the castings. Samples were also excised from the castings and tested in
tension in the T6 metallurgical temper for a wide range of solidification conditions.
This study highlighted the challenges that would face a LPPM aluminum foundry which would wish to produce magnesium
parts. This work also compared the in-situ tensile properties of the two most widely used aluminum and magnesium foundry
alloys in a casting of variable wall thickness, rather than comparing textbook values obtained from separately cast specimens.
INTRODUCTION
Permanent mold casting of aluminum alloys is a mature process while this process is in its infancy for magnesium alloys. In
North America, shipped magnesium castings (mainly poured in alloy AZ91) represent only 2% of their aluminum counterpart
(mainly A356). Also, more than 90% of these magnesium castings are pressure die cast, a process normally not suitable to
produce structural castings. These figures highlight two facts: the exiguity of the magnesium market when compared to
aluminum, and the fact that the permanent mold process is very little used with magnesium. There is no rational explanation
for this, but for the fact that magnesium alloys are rarely chosen for structural components, as was the case for aluminum, 30
years ago. The mechanical properties of magnesium and aluminum alloys are similar, and the increasing demand for
magnesium structural castings should be expected to bring about the development of the permanent mold process. A review
conducted for the Structural Cast Magnesium Development Program of the United States Automotive Material Partnership
(USAMP) concluded that permanent mold could be a viable processing route to produce quality components of varying size
1
and thickness . Some of the problems facing the foundry when pouring magnesium in permanent molds have been
2
addressed . Over the past 5 years, breakthroughs have been made on bigger magnesium castings such as the BMW engine3
containing 12 kg of AJ62 alloy and the GM Corvette engine cradle4 with 11kg of AE44 alloy.
Magnesium presents the obvious advantage of lightness; some shortcomings of magnesium have been solved (corrosion
resistance), or are presently addressed (hot shortness via grain refining); however the present main obstacle is the absence of
a critical mass of foundries with magnesium casting capabilities; this results in a lack of options for the potential user of
magnesium permanent mold castings. Since most aluminum «high integrity » castings are poured in permanent mold, it is fair
to assume that it should become the process of choice for the production of magnesium structural castings.
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In our previous studies, the comparison of gravity permanent mold casting of magnesium and aluminum has been made on
laboratory tensile test samples, and later, on an automotive torque ball housing5. The present study consists in comparing
the production of step castings and automotive bell housings poured by low pressure permanent mold process (LPPM), first
in aluminum A356, then in magnesium AZ91E.
GEOMETRY OF THE TWO CASTINGS
The step casting and the automotive bell housing are shown in Figure 1 (left and right respectively). The step casting consists
of three 152mm wide plates, with thicknesses of 25, 13 and 6mm, and a 2mm thick overflow. The aluminum trimmed
castings weigh 1.35kg (step casting) and 6.5kg (bell housing), with casting weight to poured metal ratios of 76% and 87%
respectively. Of course, the magnesium castings are one third lighter than their aluminum counterpart.
152mm wide, 25, 13 and 6mm thick step casting
460x380x170mm automotive bell housing
Figure 1: Geometry of the two castings poured successively in aluminum A356 and magnesium AZ91E
THE MOLDS AND THEIR PREPARATION
The two steel mold halves (40kg+60kg) for the step casting are shown in Figure 2 as installed on the LLPM machine. The
mold included four air cooling lines, 10mm in diameter, and two 3mm diameter type K thermocouples, the locations of which
are indicated in the model of Figure 3. When artificial cooling was applied, the air flow rate was 18m3/s (SPTC),
corresponding to a velocity of about 10m/s in the channels, and a surface heat transfer coefficient of 450 W.m-2.°K-1 ; the
outlet air temperature was 175°C.
2 air cooling lines
Figure 2: Step casting mold with two air cooling lines installed on the LLPM machine platen
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Figure 3: Location of air cooling lines (red) and thermocouples (green) in the step casting mold
The equivalent pictures for the bell housing mold are shown in Figure 4 and Figure 5.
Figure 4: Bell housing mold halves installed on the LLPM machine platen
Figure 5: Location of the thermocouple (left)) and air cooling lines (right) in the bell housing mold
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The mold coating used with AZ91E was a commercial one, recently developed specifically for magnesium; it is applied in
two steps: a red primer coating and a sodium silicate free white top coating. The nature, spraying condition and purpose
of this coating have been explained in details in the literature6.
The mold was preheated at 200-230°C before the coating was applied as illustrated in Figure 6.
Figure 6: Preheating the step casting mold at 200-230°C (left) before spraying the coating (right)
In the case of magnesium, the top coat had to be rebuilt by spraying a thin white coat every 4 cycles. The coating typical
thickness was in the range 60-100µm, for aluminum and magnesium as well.
POURING AND EJECTING THE CASTINGS
Figure 7 shows typical pressure cycles applied to the melt surface for magnesium and aluminum (step casting). The
difference in the pressure level accounts for the difference in density between magnesium and aluminum. The first ramp
corresponds to the filling of the transfer tube and mold. An arrest in the pressure rise at 300mB for magnesium and 450mB
for aluminum will minimize flashes at the parting line. The final pressure applied corresponds to a pressure head of 3 meters
of liquid metal, which explains the exceptional feeding capability of the process, as long as directional solidification is
ensured.
Mg AZ91E
Al A356
Figure 7: Typical LPPM pressure cycle applied for magnesium and aluminum (Step casting)
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The pouring temperature was always maintained in the vicinity of 750°C (±5°C) for both alloys. No melt treatment was
applied to the aluminum A356 melt from primary ingots, while a hexachlorethane grain refining treatment was performed on
the magnesium AZ91E melt. The ejection, shown in Figure 8 for the two castings, occurred after a time varying between 3
and 5 minutes depending on the run.
Figure 8: Ejection of a magnesium step casting (left) and of a magnesium bell housing (right)
COMPARING THE ALUMINUM A356 AND AZ91E MAGNESIUM STEP CASTINGS
THERMAL REGIME IN THE MOLD
Two temperatures were recorded at the top and bottom of the step casting mold (see position in Figure 3). The
thermocouples recordings are shown in Figure 9. The aluminum run reported started at a rate of one casting every 4 minutes;
the cooling channels were activated after 20min, producing a drop of temperature of 40°C in the mold; after 65 min, the rate
of casting was increased to one every 3 min, provoking an increase in mold temperature and an increased temperature
gradient between the bottom and top of the mold.
The air cooling channels were activated throughout the magnesium campaign, and the rate was one casting ejection every 3
minutes. The graphs in Figure 9 show that under similar casting conditions (one casting per 3 min with air cooling), the mold
runs at a temperature 340-400°C with aluminum and 270-310°C with magnesium.
Aluminum A356, step casting
cooling starts at 20min - speed up at 65min
Mag AZ91E, step casting
(with air cooling)
450
temperature, °C
temperature, °C
450
400
350
300
400
350
300
250
250
0
0
10 20 30 40 50 60 70 80 90 100 110
10
20
30
40
time, min
time. min
Figure 9: Temperature cycling at the top and bottom of the mold for the step casting
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50
60
MICROSTRUCTURAL PROPERTIES IN THE A356 AND AZ91E LPPM STEP CASTINGS
ALUMINIUM A356
MAGNESIUM AZ91E
25mm thick plate
DAS: 40 m
25mm thick plate
microporosity 0.38%
13mm thick plate
DAS : 27 m
microshrinkage:0.73%
13mm thick plate
microporosity: 0.02%
6mm thick plate
DAS= 25.0 m
grain size: 88 m
grain size: 80 m
microshrinkage: 0.15%
6mm thick plate
grain size: 77 m
Figure 10: Micrographs in the three plates of the aluminum A356 and magnesium AZ91E step castings
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Typical microstructures in the 3 steps of the aluminum A356 and magnesium AZ91E castings are shown in Figure 10. The
secondary dendrite arm spacing (DAS) varied from 25 to 40µm in the A356 casting while the grain size did not vary much
(from 77 to 88µm) in the AZ91E plates. In the aluminum alloy too, the grain size was not found to depend much on the
solidification time; it varied between 800 and 900 m.
It is not convenient to measure the DAS in grain refined AZ91E as only a few dendrite arms are contained in one grain. In
magnesium alloys, grain size should be the indicator of metallurgical quality while DAS is a better choice to assess
metallurgical quality in aluminum-silicon alloys.
The aluminum step castings exhibited less % micro-voids (from 0.01 to 0.38%) than the magnesium ones (0.15 to 0.73%).
Tests were performed on non grain refined AZ91E, so that dendrite arm spacing could be measured. These tests also allowed
to assess the effectiveness of the grain refining treatment. It was found that the hexachlorethane refining of the magnesium
melt had divided the grain size by 4 in the 25mm plate, and by 2 in the 6mm plate. Similarly to what takes place in Al-Si
alloys, the secondary dendrite arm spacing (DAS) in AZ91E decreases with plate thickness (hence with solidification time).
For equivalent solidification times, the DAS is smaller in magnesium: 17, 21 and 27 m in the 6, 13 and 25mm plates
respectively, versus 25, 27 and 40 m for aluminum.
MECHANICAL PROPERTIES IN THE 3 PLATES OF THE STEP CASTINGS (A356 and AZ91E)
Flat tensile test bars were excised from two aluminum step castings as indicated on the sketch of Figure 11. Thus, 5 bars,
6mmx13mm in section, were cut from the 25mm thick plate, 4 bars, 13mmx13mm in section from the 13mm plate and 4 bars
6mmx13mm in section, from the 6mm plate. Similar tensile bars were cut from two magnesium step castings.
Figure 11: Flat tensile test bars excised from the 3 plates of the step casting (red samples from 25mm plate)
The tensile properties were measured after a standard T6 heat treatment was applied to the aluminum (ASTMB597) and
magnesium (ASTM B661) castings; the results are listed in Table 1; the values are the average of at least 6 tensile results.
Because the gage length was the same for all samples (25mm), the measured elongation on the bigger 13mx13mm test bars
might be higher than they would have been on a 6mmx13mm bar.
Table 1
Tensile Properties in the 25mm, 13mm and 6mm Plates of the A356 and AZ91E Step Castings
Average tensile properties (T6 temper)
25mm
13mm
6mm
A356-T6 versus AZ91E-T6
Plate thickness
A356
AZ91E
A356
AZ91E
A356
AZ91E
Yield strength
204 MPa
107 MPa
216 MPa
105 MPa
213 MPa
107 MPa
Ultimate tensile stength, UTS
253 MPa
198 MPa
282 MPa
208 MPa
302 MPa
213 MPa
3.2%
4.1%
5.5%
4.9%
10.5%
5.8%
329 MPa
290 MPa
393 MPa
312 MPa
455 MPa
328 MPa
Alloy
Elongation in 25mm, E
Quality Index UTS+150 Log E
The results in Table 1 show that the yield strength does not vary much with the thickness of the plate for both aluminum
A356 and magnesium AZ91E, while the elongation and ultimate tensile strength are higher when the solidification times are
short (i.e. for the thinner plates). In order to lump the tensile properties into one convenient number, the notion of Quality
index defined for aluminum7 was used.
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COMPARING THE ALUMINUM A356 AND AZ91E MAGNESIUM LPPM POURED BELL HOUSINGS
THERMAL REGIME IN THE MOLD
Artificial air cooling was activated in some of the bell housing runs. However, the results reported in this paper were
obtained when no artificial cooling was applied. For both alloys, the time elapse between pours was maintained as close to 5
minutes as possible. With magnesium, ejection occurred 2 minutes after filling instead of 3 minutes with aluminum; however,
a 0.5% SF 6 protective gas must be flushed prior to pouring magnesium. Two minutes were allowed for mold opening,
ejection, mold cleaning and coating retouch when necessary; overall, the production rate was higher for magnesium.
The temperature was recorded inside the mold, at a location indicated by a red dot on Figure 5 (left). The recordings for
aluminum A356 and magnesium AZ91E are shown in Figure 12.
Magnesium AZ91E, bell housing
550
550
500
500
temperature, °C
temperature, °C
Alum inum A356, bell housing
450
400
350
300
250
450
400
350
300
250
200
200
0
10
20
30
40
50
60
0
tim e, m in
10
20
30
40
50
60
time, min
Figure 12: Temperature cycling in the mold for the bell housing
Similarly to what was observed in the step casting runs, the equilibrium temperature is some 50 °C higher when pouring
aluminum. However, due to the bigger size of the bell housing, it takes about 6 pours to bring the mold to its equilibrium
temperature, versus 4 for the step casting mold.
STRUCTURAL AND MECHANICAL PROPERTIES IN THE A356 AND AZ91E BELL HOUSINGS
Metallographic samples were excised from an aluminum A356 and a magnesium AZ91E bell housings at locations T, F and
W shown in Figure 13. The predicted solidification times for the aluminum A356 bell housing are also indicated by the color
code in the same figure.
Figure 13: Predicted solidification times in aluminum A356 - Location of excised samples (F, T and W)
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In the aluminum parts, the secondary dendrite arm spacing (DAS) and level of microporosity were measured by image
analysis; similarly, the grain size and level of microshrinkage were measured at the same locations in the magnesium parts.
The micrographs and the results are shown in Figure 14 for both alloys. Contrary to what was observed in the step castings,
the microvoid level is less in magnesium. Quite expectedly, the DAS in aluminum A356 is increased at higher solidification
times; however, in keeping with what was observed in the step casting, the grain size does not depend much on wall
thickness in magnesium AZ91E.
ALUMINIUM A356
MAGNESIUM AZ91E
25mm bracket (T)
DAS: 37 m
25mm bracket (T)
microporosity 0.52%
13mm flange (F)
DAS : 29 m
microshrinkage:0.06%
13mm flange (F)
6mm thick wall (W)
DAS= 27 m
grain size: 62 m
grain size: 77 m
6mm thick wall (W)
grain size: 66 m
Figure 14: Micrographs at locations T, F and W of the A356 and AZ91E bell housings
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As sketched in Figure 15, 6mm diameter tensile test bars were machined at locations F and T while 13mmx6mm flat
specimens were cut at location W.
Figure 15: Location of excised tensile bars in the aluminum and magnesium bell housings
The results of the tensile tests in the T6 condition are listed in Table 2. In this table, the term fineness refers to the DAS for
aluminum A356 and to the grain size for magnesium AZ91E.
Table 2
T6 condition
Tensile Properties at Locations T, F and W of theA356 and AZ91E Bell Housings
YS
UTS
El
fineness/microvoid
Q
T A356
213 MPa
260 MPa
3.9%
37um/0.52%
349 MPa
T AZ91E
182 MPa
240 MPa
6.5%
62µm/0.06%
362 MPa
F A356
203 MPa
270 MPa
5.1%
29um/0.26%
376 MPa
F AZ91E
150 MPa
222 MPa
3.0%
77µm/0.19%
294 MPa
W A356
207 MPa
279 MPa
5.7%
27um/0.36%
392 MPa
W AZ91E
145 MPa
242 MPa
2.3%
66µm/0.17%
296 MPa
Similarly to what had been observed in the step castings, the tensile properties are higher in aluminum A356. Also, the
strength is higher in faster solidifying sections for aluminum A356; however, in the magnesium AZ91E castings, the highest
tensile properties were observed in the thicker 25mm bracket (Sample T). In view of these unexpected results, tensile tests
were repeated on 4 additional housings and it was confirmed that surprisingly high tensile strengths and elongations were
obtained in this part of the AZ91E bell housing. It is noteworthy that this part of the casting is very well fed, resulting in an
extremely low micro-shrinkage (0.06% versus 0.19% and 0.17% at locations F and W); however, it cannot totally explain
the extremely high tensile properties observed in the 25mm thick section T.
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DIFFERENCES IN CASTING CHARACTERISTICS OF ALUMINUM A356 AND MAGNESIUM AZ91E
In spite of the fact that the mold runs at a lower temperature with magnesium, the capability of magnesium AZ91E in filling
thin sections was always superior to that of aluminum A356; this is illustrated by the picture in Figure 16, where the 1mm
thick vent of the step casting filled very easily when pouring magnesium, which was not the case when pouring aluminum.
Figure 16: Step casting showing the good filling characteristics of magnesium AZ91E
The original bell housing design (up to now cast in sand mold) exhibited hot spots when poured by the low pressure process,
as indicated by the simulation at le left of Figure 17. These hot spots appeared as slumped surfaces in aluminum A356 and
under the form of concentrated shrinkage cavity (piping) in the AZ91E castings.
predicted hot spots
aluminum A356
magnesium AZ91E
Figure 17: Difference in the appearance of hot spot defects in the aluminum and magnesium housings
This may be explained by the higher ability for AZ91E to provide interdendritic flow until the last stages of solidification,
while in aluminum A356, the start of eutectic solidification hampers this process after 60% of melt has solidified. Also the
higher thermal conductivity of aluminum widens the mushy zone with a similar consequence.
In order to eliminate the hot spots identified in the modeling and observed on the castings, four modifications were made to
the initial design. Excess metal masses were trimmed and padding was added at selected locations in order to ensure
directional solidification. Contrary to the gravity or tilt pour process, risering goes against the nature of the LLPM process;
consequently, for a given casting geometry, only local artificial chilling and mold coating can be used to ensure proper
feeding of the casting.
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Some hot cracking was observed in the early magnesium castings poured into a cold mold, which was not the case in the
aluminum castings. Typical hot tears in a AZ91E bell housing are shown in Figure 18.
Figure 18: Appearance of hot tears in the early run in pours of the magnesium AZ91Ecampaign
CONCLUSIONS
By pouring series of step castings and automotive housings in aluminum A356 and magnesium AZ91E by the low pressure
permanent mold process, the following conclusions could be drawn:
1) For all the castings investigated, the level of micro void was always much lower than 1% in both aluminum A356 and
magnesium AZ91E, insuring an excellent radiographic quality for this type of defect (Frame 3 or less per ASTM E155)
2) The tensile properties were higher in the aluminum A356 castings by about 20%. Tensile properties were surprisingly high
in the thick (25mm) bracket of the magnesium AZ91E bell housings
3) When properly grain refined (grain size < 100µm), hot tearing was not a problem in AZ91E castings.
4) The measurement of the secondary dendrite arm spacing DAS is only practical in aluminum A356; DAS is mathematically
tied to the local solidification time. On the contrary, in magnesium AZ91E, the grain size is a more convenient marker of the
metallurgical quality than the DAS;
5) In spite of the lower mold temperature, magnesium AZ91E exhibited a better mold filling capability ( fluidity ) than
aluminum A356
6) Shrinkage cavities are more concentrated in magnesium AZ91E parts; they are more spread out in aluminum A356
castings and often appear as depressed surfaces
ACKNOWLEDGMENTS
The authors wish to acknowledge the contribution of Ministère du Développement Économique, de l Innovation et de
l Exportation from Québec and of the Canadian Foundation for Innovation to the CIFM infrastructure which made this
project possible. Part of the operating expenses for this study were covered by the Québec government Programme d Aide à
la Recherche Technologique . The authors are also indebted to TMA (Technologie de l Aluminium et du Magnésium) for
making their facilities and personnel available for the casting runs.
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4.
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7.
Emadi D., Davies K.G., Saddayapan M., Sahoo M.; A Review of Mechanical Properties and Casting Process
Capabilities for Magnesium Alloys ; MTL Report 2003-12 (TR-R), Natural Resource Canada (2003)
Bergeron R., Chiesa F., Morin G., Experience in Casting AZ91E in Permanent Molds , Proceedings of the 13th
Magnesium Automotive and end User Seminar, 11 p., September 22-23, 2005, Aalen, Germany, (2005)
Kluting M., Landerl C.; The new BMW Inline Six-cylinder Spark-ignition Engine , Concept and construction, MTZ
Worldwide, 5p., (Nov. 2004)
Li N., Osborne R.J, Cox B.M., Penrod D.E.; Structural Cast Magnesium Engine Cradle , AFS Transactions, paper 06127, 13p, (2006)
Chiesa F, Duchesne B., Morin G., Baril J.; Permanent Mold Casting of Magnesium versus Aluminum , FonderieFondeur d aujourd hui , Vol. 266, 13p., (June-July 2007)
LaFay V.C., Robison S.; Selection and Application of Permanent Mold Coatings for Magnesium , AFS Transactions,
Vol. 114, paper 07-077, 13p, (2007)
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paper, Vol. 107, pp 811-818 (1999)
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Comparing Low Pressure Permanent Mold Casting of

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