Note: Descriptions are shown in the official language in which they were submitted.
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"Combination of casting process and alloy composition"
The present invention relates to a process for casting a magnesium alloy
consisting of aluminium, zinc and manganese, and the balance being
magnesium and unavoidable impurities, the total impurity level being below
given % by weight.
Magnesium-based alloys are widely used as cast parts in automotive industries,
and with increasing importance in 3C components (3C: computers, cameras
and communications). Magnesium-based alloy cast parts can be produced by
conventional casting methods, which include die-casting, sand casting,
permanent and semi-permanent mould casting, plaster-mould 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 are such as Mg-Al-
alloys or Mg-AI-Zn-alloys with <0.5% Mn, mainly Mg-9%Al-l%Zn (designated
AZ91), Mg-6%Al (AM60) and Mg-5%Al (AM50).
WO 2006/000022 Al describes a magnesium-based alloy containing zinc,
aluminium, calcium and/or beryllium or optionally manganese by which is
provided an attempt to improve the surface finish of cast magnesium
components. The WO reference is, however, not particularly concerned with the
castability of the alloy.
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With the present invention is provided to provide relatively low cost
magnesium-
based alloy with improved surface finish and improved castability.
The invention is characterized by an alloy containing
10,00 - 13.00 % by weight of aluminium,
0.00 - 10.00 % by weight of zinc, and
5.00 -13.00 % by weight of aluminium,
10.00 - 22.00 % by weight of zinc,
also containing
0.10 - 0.5 % by weight of manganese,
and the balance being magnesium and unavoidable impurities, the total impurity
level being below 0,1 % by weight, whereby
^ the alloy is cast in a die the temperature of which is controlled in the
range of
150-340 C,
= the die is filled in a time which expressed in milliseconds is equal to the
product of a number between 2 and 300 multiplied by the average part
thickness expressed in millimeter,
= the static metal pressures being maintained during casting between 20-70
MPa and may be subsequently intensified up to 180 MPa, as defined in the
attached independent claim 1.
Dependent claims 2 - 11 define preferred embodiments of the invention.
By using the combination of a specified Mg-AI-Zn alloy with the special
casting
process as defined above, products may be made having excellent surface
finish, reasonable ductility and acceptable mechanical properties as well as
corrosion properties.
Preferably the aluminium content is between 5.00 and 13.00 % by weight. If
less than 10.00% Al is present the Zn content is restricted to 10.00 - 22.00 %
by weight. Lower Zn contents give poorer combination of castablity and surface
finish.
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If more than 10.00% Al is present, the range of Zn can be extended to 0.00
- 22.00% still giving satisfactory castability and surface finish.
For applications requiring a minimum of ductility the composition of the alloy
is selected in such a way that the aluminium content is between 10.00 and
12.00 % by weight and the Zn-content is between 0.00 and 4.00 % by
weight. Alloys with equivalent castability and surface finish can be prepared
if the composition of the alloy is such that the aluminium content is between
6.00 and 12.00 % by weight and the Zn-content is between 10.00 and
22.00% by weight. These alloys offer the advantages of lower casting
temperature.
Accordingly, the invention comprises a process for casting a magnesium
alloy consisting of 5.00 - 13.00 % by weight of aluminium, 0.00 - 22.00 %
by weight of zinc, also containing 0.10 - 0.5 % by weight of manganese,
and the balance being magnesium and unavoidable impurities, the total
impurity level being below 0.1 % by weight, wherein the alloy is cast in a
die in which the temperature is controlled in the range of 150 - 340 C, the
die is filled in a time which expressed in milliseconds, is equal to the
product
of a number between 2 and 300 multiplied by the average part thickness
expressed in millimetres, the static metal pressures being maintained during
casting is between 20-70 MPa and may subsequently be intensified up to
180 MPa.
The present invention will be further described in the following by means of
examples and with reference to the attached drawings where:
Figs. 1 A, B each shows schematically cold chamber and hot chamber die
casting machines, respectively,
Fig. 2 is a diagram showing the relationship between the solidification
rate and the microstructure (grain size and secondary dendrite
arm spacing) of cast Mg alloys,
Fig 3 is a diagram showing the grain size vs. ductility of Mg alloys,
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Fig. 4 is a diagram showing the grain size vs. tensile yield strength of
Mg alloys,
Fig. 5 shows a chart from a prior art reference, G. S Foerster; "New
developments in magnesium die casting", IMA proceedings
1976 p. 35-39, who split the composition range into a
castable -, a brittle - and a hot cracking region,
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Fig. 6 shows the Mg-rich corner of the Mg-AI-Zn phase diagram with
lines of constant liquidus temperature,
Fig. 7 shows a diagram with the fraction solid (expressed in % by
weight) on the horizontal axis versus the temperature ( C) on the
vertical axis for three different Mg alloys,
Figs. 8 - 10 show three different Mg alloy components being cast with three
different dies,
Fig. 11 is a diagram showing casting defects, average number of cracks
and defect ribs on the box die, Fig. 8, plotted as lines of equal
number of defects in a diagram, where the Zn content is plotted
along the x-axis and the Al content along the y-axis,
Fig. 12 is a diagram showing surface finish represented as a rating from 1
to 5 on the box die, Fig. 8, plotted as lines of equal rating in a
diagram, where the Zn content is plotted along the x-axis and the
Al content along the y-axis,
Fig. 13 is a diagram showing where the z-axis is representing the tensile
strength expressed in MPa, while the x and y-axes are
representing the Al and Zn contents, respectively, and where the
ductility is represented as lines of equal % elongation in the same
diagram,
Fig. 14 is a diagram showing corrosion rates in terms of weight loss being
represented as lines of equal corrosion rates (mg/cm2/day), where
the Zn content is plotted along the y-axis and the Al content along
the x-axis.
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In Figs. 1A and 1B there are schematically shown cold chamber and hot
chamber die castings machines respectively, each machine has a die 10, 20
provided with a hydraulic clamping system 11, 21, respectively.
5 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 as is
shown in Fig. 1 A, an auxiliary system for metering of the metal to the
horizontal
shot cylinder is required. The hot chamber machine, however, shown in Fig. 1
B, uses a vertical piston system 12, 23 directly in the molten alloy.
To obtain the excellent performance of the Mg-AI-Zn 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
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 180 MPa may be used, especially with thicker walled
castings. With this casting method the resulting cooling rate of the component
is
typically in the range of 10-1000 C/s depending on the thickness of the
component being cast.
In Fig. 2 there is shown the relationship between the solidification range and
the
microstructure of a cast alloy. 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 spacing expressed in pm is shown, whereas on the
right hand vertical scale the grain diameter expressed in pm is shown. Line 30
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indicates the grain size obtained, whereas line 31 is the obtained value for
the
secondary dendrite arm spacing.
With die casting grain refining is obtained by the cooling rate. As mentioned
above cooling rates in the range of 10-1000 C/s are normally achieved. This
typically results in grain sizes in the range of 5-100 gm.
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 average grain size has been represented expressed in pm, whereas
the vertical axis gives the relative elongation expressed in %. In the graph
there
are shown two different compositions, first pure Mg, line 35 and an Mg-alloy
designated AZ91 (Mg-9% Al, 1 % Zn), 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 d 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.
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.
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In the 3C industry extremely thin-walled components for e.g. lap-top and cell
phone housings, often less than 0.5 mm, are cast. This puts strong
requirements on the ability of the alloy to fill the mould and at the same
time
provide a smooth and shiny surface. AZ91 is the most common alloy for these
applications, mainly due to the better castability compared to AM50 and AM60.
However, the surfaces of thin walled components of AZ91 are often not
satisfactory. Usually, a conversion coating is applied to these components.
With
a less shiny surface sometimes including areas with segregation of elements,
multiple layers of coating has to be used. Generally, the better surface
quality,
the less coating is needed.
Mg-AI-Zn alloys with 0-10 wt% Al and 0-35 wt% Zn were examined in the
1970's (G. S Foerster; "New developments in magnesium die casting", IMA
proceedings 1976 pp. 35-39). The chart shown in Fig. 5, from Foerster's paper,
split the composition range into a castable -, a brittle - and a hot cracking
region. The alloys described in Australian patent WO 2006/000022 Al that
provide an attempt to improve the surface finish, are mainly inside the
castable
region of Fig. 5. The alloy composition ranges of the present invention are
mainly outside the composition ranges described in the prior art (Fig. 5) and
completely outside those described in patent WO 2006/000022 Al. During the
tests that will be explained later it became evident that the alloys of the
present
invention represent considerable improvements over the earlier described
alloys
in terms of die filling, die sticking and hot cracking. These are all crucial
features
in die casting of complex thin-walled components.
The Mg-AI-Zn alloys with the Al and Zn content as specified in the present
invention will start to solidify around 600 C, depending on the Al and Zn
content. This is indicated in Fig. 6 where lines of constant liquidus
temperature
in the Mg-corner of the Mg-AI-Zn phase diagram are shown. As a result, the
casting temperature, typically 70 C above the liquidus, can be significantly
lower than for the conventional AM50, AM60 and AZ91 alloys. Due to the fact
that the eutectic Mg17AI12 phase melts at around 420 C, the conventional Mg-Al
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alloys like AM50, AM60 and AZ91 will have a solidification range of nearly
200 C as shown in the annexed Fig. 7 which shows the fraction solid
(expressed in % by weight) on the horizontal axis versus the temperature ( C)
on the vertical axis for three different alloys. Specifically, AZ91 starts to
solidify
at 600 C and is completely solidified at 420 C. Increasing the Al content to
14%
as in alloy AZ141, the start of solidification occurs at around 570 C while
solidification is complete at 420 C. Due to the significant presence of Zn the
alloy AZ85 solidifies in the range 590-350 C. Since Zn in the Mg-AI-Zn alloy
modifies the eutectic Mg17AI12 phase, the alloy will solidify completely at
temperatures significantly lower than 420 C as is the case for the
conventional
alloys AM50, AM60 and AZ91.
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. 7. With the large amount of Zn in addition to Al in the present
alloys there is an even larger amount of (modified) eutectic present at the
end of
solidification, explaining the improved castability of the invented Mg-AI-Zn
alloys.
Magnesium alloys tend to ignite and oxidize (burn) in the molten state unless
protected by cover gases such as SF6 and dry air with or without C02, or SO2
and dry air. The oxidation aggravates with increasing temperature. Usually,
small amounts of beryllium (10-15 ppm by weight) are also added to reduce the
oxidation. Beryllium is known to form toxic substances and should be used with
care. Especially the treatment of dross and sludge from the cleaning of
crucibles requires considerable safety precautions due to an enrichment of Be-
compounds in dross/sludge. One advantage of the present invention is that the
alloy can be cast at temperatures significantly lower than for conventional
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alloys, thereby reducing the need for cover gases. For the same reason,
beryllium additions can be kept at a minimum.
The lower casting temperatures compared to conventional alloys also offer
significant advantages as the lifetime of the metering system, the shot
cylinder
and the die will all be improved. With hot chamber die casting in particular,
the
lifetime of the gooseneck will be significantly extended. The alloys with
lower
casting temperature also have a potential for reducing the cycle time, thereby
improving the productivity of the die casting operation.
Example 1
In order to evaluate the influence of the alloying elements and a number of Mg-
alloys have been prepared and cast in three different dies:
= The box die with ribs, Fig. 8
= The plate/bar die, Fig. 9
= The three plate die, Fig. 10
The alloy compositions and the casting temperatures are shown in Table 1
below.
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Table 1
Al wt% Zn wt%) Casting Al (wt%) Zn (wt%) Casting
( ) ( Temp C Temp C
AM20 2 0 710 AZ85 8 5 670
AZ21 2 1 710 AM90 9 0 670
AZ22 2 2 705 AZ91 9 1 670
AZ2-3.5 2 3,5 700 AZ96 9 6 650
AM40 4 0 700 AZ99 9 9 640
AZ41 4 1 695 AZ9-12 9 12 620
AZ42 4 2 695 AZ9-18 9 18 585
AZ4-3.5 4 3,5 690 AZ9-22 9 22 560
AZ45 4 5 680 AM100 10 0 660
AZ4-14 4 14 650 AZ10-1 10 1 660
AZ4-18 4 18 630 AZ10-2 10 2 660
AM60 6 0 680 AZ10-3.5 10 3,5 650
AZ61 6 1 680 AZ10-5 10 5 650
AZ62 6 2 680 AM120 12 0 650
AZ63 6 3 680 AZ12-1 12 1 650
AZ6-3.5 6 3,5 680 AZ12-2 12 2 640
AZ65 6 5 670 AZ12-3.5 12 3,5 640
AZ66 6 6 670 AZ12-5 12 5 630
AZ6-12 6 12 640 AZ12-6 12 6 630
AZ6-18 6 18 610 AZ12-12 12 12 590
AZ71 7 1 680 AZ12-18 12 18 550
AZ72 7 2 680 AM140 14 0 640
AM80 8 0 680 AZ14-1 14 1 630
AZ81 8 1 680 AZ14-2 14 2 630
AZ82 8 2 670 JAZ14-3.5 14 3,5 620
AZ8-3.5 8 3,5 670 AZ14-5 14 5 610
Details of the casting parameters are given in Table 2 below.
5
Table 2
Velocity 1 Velocity 2 Braking Calculated
(m/s) (m/s) (m/s) Fill Time
(ms)
Die 1 Tensile specimen 0,5 5 3 50
Die 2 Three Plate 0,5 5 2,5 53
Die 3 Box 0,5 5 3 40
No intensification pressure was used.
10 The performed tests are the following:
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Evaluation of casting defects
Visual inspection was undertaken on 10 arbitrary boxes from each alloy.
Defects were grouped as
= Defect ribs including incomplete filling and cold shuts
= Hot tears counted on nodes
= End cracks
Evaluation of surface finish
Surface finish was inspected visually by several persons independently,
and rated from 1 to 5 (5 best).
Tensile strength and ductility
Test-bars of 6 mm diameter in accordance to ASTM B557M have been
made, and the following test conditions have been used:
= 10 kN Instron test machine
= Room temperature
= At least 10 parallels
= Strain rate
- 1.5 mm/min up to 0.5% strain,
- 10 mm/min above 0.5% strain
= Testing in accordance with ISO 6892
Corrosion properties
The corrosion tests were done according to ASTM 13117.
Example 2
Casting defects average number of cracks and defect ribs are plotted in Fig.
11
as lines of equal number of defects in a diagram where the Zn content is
plotted
along the x-axis and the Al content along the y-axis. It is seen that the
lowest
numbers of cracks are found in the regions with low Zn (<3%) and high Zn
(>10%). It is seen that that particularly good alloys in .terms of casting
defects
are found with Al in the range of 8-10 % by weight and with Zn < 2 % by
weight;
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the lower Zn the better. Also, alloys with Al in the range of 7-12 % by weight
and
Zn in the range 12-18 % by weight exhibit very few casting defects.
Example 3
Surface finish represented as a rating from 1 to 5 is plotted in Fig 12 as
lines of
equal rating in a diagram where the Zn content is plotted along the x-axis and
the Al content along the y-axis. It is seen that the best regions in terms of
surface finish rating are found with Al >11 % by weight and Zn < 3 % by
weight;
the lower Zn the better. Also, a region roughly defined by 8-12 % Al by weight
and >10 % Zn by weight provides alloys with superior surface finish.
Example 4
For a number of compositions the strength and elongation have been measured
at room temperature. The results are shown in Fig. 13. Here, the z-axis is
representing the tensile strength expressed in MPa, whereas the x and y-axes
are representing the Al and Zn contents, respectively. The ductility is
represented as lines of equal elongation in the same diagram. Generally it is
seen that tensile strength expressed in MPa, increases with increasing content
of alloying elements. The effect of increasing Al (% by weight) is
significantly
greater than the effect of Zn. Fig. 13 also indicates that the ductility in
terms of
% elongation decreases with increasing content of alloying elements. As an
example, the line indicating 3 % elongation extends almost linearly from 12 %
Al
by weight and 0 % Zn to 0 % Al and 18 % Zn by weight.
Example 5
For a number of compositions the corrosion properties have been defined in
accordance to ASTM 13117. In this test a great amount of data has been
incorporated in order to define the influence of the Zn-content versus the AI-
content. The results are shown in Fig. 14.
In this figure corrosion rates in terms of weight loss is represented as lines
of
equal corrosion rates (mg/cm2/day), in a diagram where the Zn content is
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plotted along the y-axis and the Al content along the x-axis. It is seen that
for Zn
contents lower than approximately 8 % by weight, the corrosion rates decrease
with increasing Al content and are practically independent of the Zn content,
whereas for Zn contents above approximately 12 % by weight the corrosion
rates increases slightly with increasing Zn content and are practically
independent of the Al content. The region defined by 8-12 % by weight of Zn
represents a transition. Specifically, at 0% Zn the corrosion rate decreases
from
about 0.09 mg/cm2/day at 4 % Al by weight to approximately 0.03 mg/cm2/day
at 9% Al by weight. At constant 9% Al by weight the corrosion rate increases
to
0.05 mg/cm2/day at 8% Zn by weight and 0.11 mg/cm2/day at 14% Zn by
weight.
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.
