Note: Descriptions are shown in the official language in which they were submitted.
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A combination of casting process and alloy compositions resulting in cast
parts with superior combination of elevated temperature creep properties,
ductility and corrosion performance.
The invention relates to a process for casting a magnesium alloy consisting of
2,0 - 6,00 % by weight of aluminium,
3,00 - 8,00 % by weight of rare earth metals (RE-metals),
the ratio of the amount of RE-metals to the amount of aluminium
expressed as % by weight being larger than 0,8,
at least 40 % by weight of the RE-metals being cerium,
less than 0,5 % by weight of manganese,
less than 1,00 % by weight of zinc,
less than 0,01 % by weight of calcium
less than 0,01 % by weight of strontium
and the balance being magnesium and unavoidable impurities, the total
impurity level being below 0,1 % by weight.
Magnesium-based alloys are widely used as cast parts in the aerospace and
automotive industries. Magnesium-based alloy cast parts can be produced by
conventional casting methods, which include die-casting, sand casting,
permanent and semi-permanent mold casting, plaster-mold casting and
investment casting.
Mg-based alloys demonstrate a number of particularly advantageous properties
that have prompted an increased demand for magnesium-based alloy cast parts
in the automotive industry. These properties include low density, high
strength-
to-weight ratio, good castability, easy machinability and good damping
characteristics.
Most common magnesium die-casting alloys such as Mg-Al-alloys or Mg-AI-Zn-
alloys are known to lose their creep resistance at temperatures above 120 C.
Mg-AI-Si alloys have been developed for higher temperature applications and
offer only a limited improvement in creep resistance. Alloys of the Mg-Al-Ca
and
Mg-Al-Sr system offer a further improvement in creep resistance, but a great
disadvantage with these alloys is problems with castability. This is
particularly a
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problem with high metal velocities impinging directly onto the die surface,
the so-
called water hammer effect.
It is known that the alloy AE48 (4% AP, 2-3 % RE) offers a significant
improvement in elevated temperatures properties and corrosion.
Mg-Al alloys containing elements like Sr and Ca offer a further improvement in
creep properties, however at the cost of reduced castability. Alloys of the Mg-
Al-
Ca and Mg-Al-Sr system offer a further improvement in creep resistance, but a
io great disadvantage with these alloys are problems with castability. This is
particularly a problem with high metal velocities impinging directly onto the
die
surface, the so-called water hammer effect.
In the annex Figures 1A and 1B there are schematically shown cold chambers
and hot chambers die castings machines respectively each machine has a die
10, 20 provided with a hydraulic damping system 11, 21 respectively.
Molten metal is introduced into the die by means of a shot cylinder 12, 22
provided with a piston 13, 23 respectively. In the cold chamber system an
auxiliary system for metering of the metal to the horizontal shot cylinder is
2o required. The hot chamber machine (Fig. 1 B) uses a vertical piston system
(12,
23) directly in the molten alloy.
To obtain the excellent performance of the Mg-Al-Re alloys , it is mandatory
that
the alloys are cast under extremely rapid cooling conditions. This is the case
for
the high pressure die casting process. The steel die 10, 20 is equipped with
an
oil (or water) cooling system controlling the die temperature in the range of
200-
300 C. A prerequisite for good quality is a short die filling time to avoid
solidification of metal during filling. A die filling time in the order of 10-
2 s x
average part thickness (mm) is recommended. This is obtained by forcing the
3o alloy through a gate with high speeds typically in the range 30-300 m/s.
Plunger
velocities up to 10 m/s with sufficiently large diameters are being used to
obtain
the desired volume flows in the shot cylinder for the short filling times
needed. It
is common to use static metal pressures 20-70 MPa and subsequent pressure
intensification up to 150 MPa. With this casting method the resulting cooling
rate
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of the'component is typically in the range of 10-1000 C/s depending on the
thickness of the component being cast. For AE alloys this is a key factor in
determining the properties, both because of general high cooling rate of the
part,
and in particular the extremely high cooling rate of the surface layer.
In the annexed Fig. 2 there is shown the relationship between the
solidification
range and the microstructure. On the horizontal axis there is shown the
solidification rate expressed as C/S and on the left hand vertical scale the
secondary dendrite arm spacings expressed in pm is shown, whereas the right
hand vertical scale the grain diameter expressed in pm is shown. Line 30
io indicates the grain size obtained, whereas line 31 is the.obtained value
for the
secondary dendrite arm spacings.
With die casting grain refining is obtained by the cooling rate. As mentioned
above cooling rates in the range of 10-1000 C/s is normally achieved. This
typically results in grain sizes in the range of 5-100 m.
It is well known that fine grain size is beneficial for the ductility of an
alloy. This
relationship is illustrated in the annexed Fig. 3, in which the relationship
between
grain size and relative elongation has been shown. On the horizontal axis the
arrange grain size has been represented expressed in pm, whereas the vertical
2o axis gives the relative elongation expressed in %. In the graph there are
shown
two different composition, first pure Mg, line 35 and a Mg-alloy designated
AZ91,
line 36.
It is also well known that fine grain size is beneficial for the tensile yield
strength
of an alloy. This relationship (Hall-Petch) is shown in the annexed Fig. 4. In
the
horizontal axis there is represented the grain diameter, expressed as d (-
0,5), in
which has been expressed in pm , and in the vertical axis there is shown the
tensile yield strength expressed in MPa.
It is therefore evident that the fine grain size provided by the very high
cooling
rates facilitated by the die casting process is a necessity for obtaining
tensile
strength and ductility.
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The castability term describes the ability of an alloy to be cast into a final
product
with required functionalities and properties. It generally contains 3
categories;
(1) the ability to form a part with all desired geometry features and
dimensions,
(2) the ability to produce a dense part with desired properties, and (3) the
effects
on die cast tooling, foundry equipment and die casting process efficiency.
The German Patent Application 2122148 describes alloys of the Mg-Al-RE type
mainly Mg-Al-RE alloys with RE content < 3wt%, although alloys with higher RE
content are discussed as well. It is known that the alloy AE42 (4% Al, 2-3%
RE)
io offers a significant improvement in elevated temperature properties and
corrosion properties. It is experienced that small RE additions to Mg-Al
alloys
lead to a significant improvement in corrosion properties, but a deterioration
in
the castability as problems with die sticking occur more frequently. In the
annexed Fig. 5 there is shown the regions of excellent, poor and very poor
castability in the Mg-Al-Re system. In the horizontal axis the amount of Al
expressed as % by weight is shown, whereas in the vertical axis the amount of
RE expressed in % by weight is shown. The line 40 is the line indicating the
solubility of RE at 680 C, whereas the line 41 indicates the solubility of RE
at
640 C. The region (dark) 42 represents the composition with very poor
castability. The region (intermediate) 43 represents the composition with poor
castability and the region 44 (light) represents the compositions with
excellent
castability. As illustrated in Fig. 5, the castability becomes worse as the RE
content of the alloy increases. However, as Fig. 5 indicates, there is a
region
with RE >3.5 wt% (the upper limit restricted by the solubility of RE), Al in
the
range 2.5% to 5.0% and furthermore described with a%RE/%AI ratio greater
than 0.8 where the high pressure die castability is excellent.
It is therefore an object of the present invention to provide relatively low
cost
magnesium-based alloys with improved elevated-temperature performance and
improved castability.
Due to the formation of AIxREy dispersoid phases, the compositions of the
present invention minimise the volume fraction of the brittle Mg AI12 phase
(The
RE/Al ratio in the dispersoid phases increases with increasing %RE/%Al content
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in the alloy). Due to the fact that the eutectic Mg AI12 phase melts at around
420 C, the conventional Mg-Al alloys like AM50, AM60 and AZ91 will have a
solidification range of nearly 200 C as shown in the annexed Fig. 6. Fig 6
shows the fraction solid (expressed in % by weight) on the horizontal axis
versus
5 the temperature ( C) on the vertical axis for a number of alloys. The Mg-Al-
RE
alloys with the %RE/%AI ratios as specified in the present invention will
solidify
completely at around 570 C, hence the solidification range is only
approximately
50 C.
io In general, increasing aluminium content in Mg-Al die casting alloys
improves
the die castability. This is due to the fact that Mg-Al alloys have a wide
solidification range, which makes them inherently difficult to cast unless a
sufficiently large amount of eutectic is present at the end of solidification.
This
can explain the good castability of AZ91 D consistent with the cooling curves
shown in Fig. 6. As the Al-content is reduced to 6, 5 and 2% in AM60, AM50 and
AM20, respectively, the remaining eutectic is decreasing to a level where
feeding becomes difficult during the final stages of solidification which
means,
for thick walled parts, microporosity and even larger voids can be present.
For
thin walled parts, the ability to feed during the final stages is less
important
(while alloy fluidity becomes the significant factor) since the volume
shrinkage is
partly taken up by thickness reduction due to shrinkage from the die walls.
The
AE44 and AE35 alloys show very different cooling characteristics from Mg-Al
alloys. The solidification interval is significantly smaller, indicating
concentrated
shrinkage porosity can be decreased during solidification. These alloys have
good fluidity during mold filling, and can thus easily be cast into final
products
with less casting defects. The castability of AE44 and AE35 is relatively
equal to
that of AZ91 D.
A further issue related to the narrow solidification interval is the fact that
the
commonly observed inverse segregation occurring in AZ91 D as well as AM
alloys will not occur. This is illustrated by the fact that AE alloys with
high RE
contents have a shiny surface without segregations of Mg-Al eutectic phase.
The surface layer solidifies during and immediately after die filling, and the
temperature will rapidly decrease below the solidus temperature, thereby
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preventing molten metal to be forced towards the die surface when shrinkage
starts. This will be beneficial to prevent reactions between the die wall and
molten metal, which could lead to die sticking.
An example with a wall thickness of about 3 mm showing three layers with
different microstructure in AE44 is given in the annexed Fig. 7. The surface
layer, having a thickness of approx. 50 m, consists of equiaxed grains with
size
about 10 m. This is a fairly small grain size, which can be explained by the
rapid cooling conditions on the die wall. The intermediate layer is about 100
m
io thick and is extremely fine grained. The morphology is different from the
former
and DAS in the range of 2-4 m is observed. The change in equilibrium melting
point due to pressure may explain this observation. When the metal becomes
pressurized the equilibrium melting point increases, i.e., the metal suddenly
becomes undercooled. In theory, this is the same for all Mg alloys, but there
remains a significant difference in the solidification characteristics among
the
alloys. The core consists of equiaxed grains of -20 m. The solidification of
the
core is restricted by the heat flow out of the core to the die. Both the heat
transport through the already solidified layer and the heat transfer over the
casting/die interface will give a slower cooling rate than the skin and thus a
coarser microstructure is formed.
When the RE content is low, or the %RE/%Al ratio is low like in AE42 or AE63,
there will be a possibility that eutectic Mg-Al is present that could
segregate to
the surface, and lead to sticking. This may explain why AE42 shows up with a
poorer castability.
In Fig. 8 there is shown a box die (upper) part of the drawing. Micrographs of
examples from node 3 (close to the gate) for alloys AM60, AM40, AE63, AE44
and AE35 as shown below. Hot cracks are observed in AM40 and AE63.
Fig. 8, have demonstrated that AE44 and AE35 are less susceptible to hot
tearing than AM alloys. This is explained from the fairly rapid solidification
of the
surface layer resulting in the relatively fine grained structure as described
above.
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Partly due to the fine grain structure and partly due to the absence of the
brittle
MgõAI12 phase this layer becomes very ductile, and is therefore able to deform
when thermal strains are developing during solidifaction. A surface layer with
coarser grains, as would typically appear in alloys with larger solidification
interval, and/or a Mg17Al12 rich layer, will have a much lower ductility and
would
tend to crack and form hot tears rather than deform.
Testing of large (-1.5m) thin walled parts (-3mm thick) has shown that the die
filling characteristics of AE44 and AE35 are excellent, and since long range
io feeding is not necessary for thin walled parts as discussed above, this
alloy is
expected to be a viable alternative for these types of components, where die
filling is of prime importance.
The properties of various AE alloys are explained from the observations that
Al
alone.provides the solid solution strengthening while RE combines with Al
forming dispersoid phases in the grain boundary regions. In the alloys AE44
and
AE35, the dispersoid phase (mainly AI2RE) constitutes a continuous 3D network,
effectively preventing creep arising from thermal activation and grain
boundary
sliding. This shown in Fig. 9 which are SEM-BEC (Backscatter Electronic
Composition) images showing the die cast microstructure of (from left to
right)
AE44, AE35 and AE63. While Al alone provides the solid solution
strengthening, RE combines with AL forming dispersoid phases in the grain
boundary regions.
A further enlargement of the SEM-BEC-images for AE 44 is shown in Fig. 10,
which. also shows the lamellar structure of AI,REy phases in AE44. As seen
from Fig. 10 the dispersoid AIxREy phases in the AE alloys consist of an
extremely fine lamellar structure. This structure of submicron lamellas are
stiffening the grain boundaries thereby preventing creep. On the other hand,
these lamellas are not brittle (or not as brittle as the eutectic Mg-Al) as
the die
cast AE44 alloy experience a ductility that is similar to AE42. In AE63, the
network (mainly AI>>RE3) becomes fragmented and the grain boundary regions
are probably influenced by a substantial amount of eutectic Mg-Al, reducing
the
ductility and the creep properties. In AE42 there is probably also a
significant
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amount of eutectic Mg-Al that limits the creep properties. The alloy AE35 has
slightly lower ductility than AE44, but still higher than AE63.
Numerous examples of mechanical properties including ductility, tensile
strength, creep resistance and corrosion properties of the AE alloys are shown
later. The unique combination of creep resistance and ductility compared to
existing alloys is illustrated in Fig. 11. In fig. 11 the ductility
(horizontal axis) is
shown as versus the creep resistance for a number of known Mg-alloys. The
zone 50 comprises AM-alloys, zones 51 AE-alloys, zone 52 AZ91-alloy and
io zone 53 other high temperature alloys. The AE alloys of the present
invention
are the only die casting alloys that combine ductility and elevated
temperature
properties in this way, and hence offer numerous new and unexplored
opportunities for constructors and designers particularly in the automotive
industry.
It is a more particular object to provide relatively low cost die casting
magnesium-aluminum-rare earth alloys with excellent castability, good creep
resistance and tensile yield strength and bolt-load retention, particularly at
elevated temperatures of at least 1500 C.
SUMMARY OF THE INVENTION
The present invention therefore provides :
~ the alloy is cast in a die the temperature of which is controlled in the
range of
180-340 C,
= the die is filled in a time which expressed in milliseconds is equal to the
product of a number between 5 and 500 multiplied by the average part
thickness expressed in millimeter,
= the static metal pressures being maintained during casting between 20-70
MPa and is subsequently intensified up to 180 MPa.
By using the combination of a specified Mg-Al-RE alloy with a special casting
process, products could be obtained having excellent creep resistance, at
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elevated temperature, high ductility and generally good mechanical properties
as well as corrosion properties.
In general a number of RE-metals can be used as alloying element, such as e.g.
Ce, La, Nd and or Pr and mixtures thereof. It is however preferred to use
cerium
in substantial amounts as this metal gives the best mechanical properties. Mn
is
added to improve the corrosion resistance but its addition is restricted due
to
limited solubility.
io Preferably the aluminium content is between 2,0 and 600% by weight, more
preferably between 2,60 and 4,50 % by weight.
If higher amounts of aluminium are present, this can easily lead to the
formation
of a Mg AI1Z-phases which is detrimental for the creep properties. Too low Al
is
negative for the castability.
With respect to the RE-metals it is preferred that the RE-content is between
3,50
and 7,00 % by weight, the upper limit being restricted by the solubility of RE
in
the Mg-Al-RE system as indicated in Fig.1.
If more than 3.50 % RE by weight is present, this gives a significant
improvement of the creep properties. More than 7.00 % by weight is not
practical because of the restricted solubility of RE-metals in liquid
magnesium-
aluminium alloys.
Furthermore, it is preferred that the RE/Al ratio is larger than 0.9.
For specific applications the composition of the alloy is selected in such a
way
that the aluminium content is between 3,6 and 4,5 % by weight and the RE-
content is between 3,6 and 4,5 % by weight, with the additional constraint
that
the RE/Al ratio is larger than 0,9.
This type of alloys can be used for applications up to 175 C while still
showing
excellent creep properties and tensile strength. Moreover this alloy does not
show any degradation of its properties due to ageing and has a good
castability.
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For applications above 175 C the composition of the alloy is such that the
aluminium content is between 2,6 and 3,5 % by weight and the RE-content is
greater than 4,6 % by weight.
5 Apart from the excellent creep properties and tensile strength this alloy
does not
show any degradation of properties due to ageing.
Preferably the RE-metals are selected from the group cerium, lanthanum,
neodymium and praseodymium.
The RE-metals are contributing to the ease of alloying, but also increase the
corrosion resistance, the creep resistance and improve the mechanical
properties.
Preferably the amount of lanthanum is at least 15 % by weight and more
preferably at least 20 % by weight of the total content of RE-metals,
Preferably
the amount of lanthanum is less than 35 % by weight of the total content of RE-
metals.
Preferably the amount of neodymium is at least 7 % by weight and more
preferably at least 10 % by weight of the total content of RE-metals.
Preferably
the amount of neodymium is less than 20 % by weight of the total content of RE-
metals.
Preferably the amount of praseodymium is at least 2 % by weight and more
preferably at least 4 % by weight of the total content of RE-metals.
Preferably
the amount of praseodymium is less than 10 % by weight. Of the total content
of
RE-metals.
Preferably the amount of cerium is greater than 50 % by weight of the total
content of RE-metals, preferably between 50 and 55 % by weight.
It is known that calcium and strontium give an increase in creep resistance,
and
the addition of at least 0,5 % weight of calcium will improve the tensile
strength.
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However, Ca and Sr should be avoided because even at very small
concentrations these elements lead to considerable sticking problems thereby
influencing the castability of the alloy.
The present invention is described in more detail with reference to the
following
example which are for purposes of illustration only and are not to be
understood
as indicating or implying any limitation on the brood invention described
herein.
io Example 1
In order to compose the influence of the alloying elements and a number of Mg-
alloys have been prepared with the compositions as given in table 1.
Of each alloy purposes a number of test bars has been made to do the testing
described in the following examples. The performed tests are the following
Tensile strength and ductility
Test-bars of 6 mm in accordance to ASTM have been made, and the
following
Test conditions has been used :
= 10 kN Instron machine
= Room temperature to 210 C
= At least 5 parallels at each temperature
= Strain rate
- 1.5 mm/min up to 0.5 % strain,
- 10 mm/min above 0.5 % strain
= Testing in accordance with ISO 6892
Tensile creep testing
For this text the following test material is used
= Diameter : 6 mm
= Gauge length : 32.8 mm
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= Radius of curvature : 9 mm
= Grip head diameter : 12 mm
= Total length : 125 mm
The testing is done in accordance with ASTM E 139
Stress relaxation testing
= Test material
- 12 mm diameter, 6 mm length
- Cut from arbitrary end of creep bars
= Testing in accordance with ASTM E328-86
Corrosion properties
The corrosion is tested according to ASTM 117.
Example 2
For a number of compositions the strength has been measured as a function
of the temperature.
The results are shown in Figures 12, 13 and 14. In these figures the y-axis
is representing the tensile strength expressed in MPa, whereas the x-axis is
representing the temperature expressed in degrees Celsius.
Example 3
For a number of compositions the Creep strain has been measured as a
function of the time.
The results are shown in Figures 15 and 16. In fig. 15 the measurement is
done at 175 C whit a 40 MPa-force, and in fig. 16 the measurement is done
at 150 C with a 90 MPa-forces.
In these figures the y-axis is representing the creep strain expressed in
percentage, whereas the x-axis is representing the time expressed in hours.
Example 4
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For a number of compositions according to table 1 the stress relaxation has
been defined, expressed as the remaining load versus the time. The results
are shown in fig. 17, 18 and 19.
In these figures the y-axis is representing the remaining load expressed in
percentage of initial load, whereas the x-axis is representing the time
expressed in hours.
Example 5
to For a number of compositions the corrosion properties have been defined in
accordance to ASTM B117. In this test a great amount of data has been
incorporated in order to define the influence of the RE-contest versus the
Al-contest. The results are shown in Fig. 20.
In this figure the y-axis is representing the RE-content expressed in % by
weight whereas the x-axis is representing the Al-content also expressed in %
by weight.
The border lines between the zones with different shades are representing
lines of equal corrosion resistances.
2o From these test results it is clear that a process for casting a magnesium
alloy
has been provided whereby products are obtained with a superior combination
of elevated temperature creep properties, ductility and corrosion performance.
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