MAGNESIUM BASED ALLOY

FIELD OF THE INVENTION

The present invention relates to magnesium based alloys and, more particularly, to magnesium based alloys which can be cast by high pressure die casting (HPDC).

BACKGROUND TO THE INVENTION

With the increasing need to limit fuel consumption and reduce harmful emissions into the atmosphere, automobile manufacturers are seeking to develop more fuel efficient vehicles. Reducing the overall weight of vehicles is a key to achieving this goal. Major contributors to the weight of any vehicle are the engine and other components of the powertrain. The most significant component of the engine is the cylinder block, which makes up 20-25% of the total engine weight. In the past significant weight savings were made by introducing aluminium alloy cylinder blocks to replace traditional grey iron blocks, and further weight reductions of the order of 40% could be achieved if a magnesium alloy that could withstand the temperatures and stresses generated during engine operation was used. Development of such an alloy, which combines the desired elevated temperature mechanical properties with a cost effective production process, is necessary before viable magnesium engine block manufacturing can be considered.

HPDC is a highly productive process for mass production of light alloy components. While the casting integrity of sand casting and low pressure/gravity permanent mould castings is generally higher than HPDC, HPDC is a less expensive technology for higher volume mass production. HPDC is gaining popularity among automobile manufacturers in North America and is the predominant process used for casting aluminium alloy engine blocks in Europe and Asia. In recent years, the search for an elevated temperature magnesium alloy has focused primarily on the HPDC processing route and several alloys have been developed. HPDC is considered to be a good option for achieving high productivity rates and thus reducing the cost of manufacture.

WO2006/105594 relates to a magnesium based alloy consisting of, by weight:

1.5-4.0% rare earth element(s),

0.3-0.8% zinc,

0.02-0.1% aluminium,

4-25 ppm beryllium,

0-0.2% zirconium,

0-0.3% manganese,

0-0.5% yttrium,

0-0.1% calcium, and

the remainder being magnesium except for incidental impurities.

Alloys according to WO2006/105594 have demonstrated excellent high temperature creep properties but have proven somewhat difficult to die cast. The present inventors have ascertained that fluidity and hot tearing resistance during die casting and the oxidation resistance of the molten alloy is improved by increasing the proportion of lanthanum in alloys according to WO2006/105594.

Throughout this specification the expression “rare earth” is to be understood to mean any element or combination of elements with atomic numbers 57 to 71, ie. lanthanum (La) to lutetium (Lu).

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a magnesium based alloy consisting of, by weight:

2-5% rare earth elements, wherein the alloy contains lanthanum and cerium as rare earth elements and the lanthanum content is greater than the cerium content;

0.2-0.8% zinc;

0.02-0.15% aluminium;

0-0.5% yttrium or gadolinium;

0-0.2% zirconium;

0-0.3% manganese;

0-0.1% calcium;

0-25 ppm beryllium; and

the remainder being magnesium except for incidental impurities.

The total lanthanum and cerium content of the alloy is preferably 1.5-3.5% by weight, more preferably 1.8-3.0%, and most preferably 2.0-2.8%. Without wishing to be bound by theory, the lanthanum and cerium improve the castability and also the creep strength of the alloy. Again, without wishing to be bound by theory, a greater lanthanum content than cerium content further improves the castability of the alloy, particularly the hot tearing resistance of the alloy. Higher ratios of lanthanum to cerium typically give the alloy greater ductility and even greater resistance to hot tearing. Typically, a higher total lanthanum and cerium content is beneficial to the creep resistance of the alloy with a concomitant reduction in the ductility of the alloy.

The rare earth element content of the alloy may optionally contain neodymium, in which embodiment the rare earth element content is predominantly lanthanum, cerium and neodymium. Without wishing to be bound by theory, the inclusion of neodymium improves the creep resistance of the alloy. However, the neodymium content of the alloy may be reduced to improve the castability of the alloy, in particular its resistance to hot tearing. When present, the neodymium content is preferably 0.5-2.0% by weight of the alloy, more preferably 0.5-1.5% by weight, more preferably about 1% by weight.

Various of the rare earth elements are typically derived from a lanthanum misch metal containing lanthanum, cerium, optionally neodymium, a modest amount of praseodymium (Pr) and trace amounts of other rare earths. In another embodiment, the rare earth elements can be derived from a cerium misch metal, together with pure lanthanum to provide the greater lanthanum content relative to the cerium content. For alloys that require a low cerium content, the rare earth elements may be derived from a commercial purity source of lanthanum.

The neodymium may be derived from one or both of the above misch metals, a pure source of neodymium, didymium (a neodymium rich neodymium-praseodymium alloy) or any combination thereof.

Yttrium is an optional component which may be included. Without wishing to be bound by theory, the inclusion of yttrium is believed to be beneficial for both melt protection and creep resistance. However, the yttrium content of the alloy may be reduced to improve the castability of the alloy, in particular its resistance to hot tearing. When present, the yttrium content is preferably 0.005%-0.5% by weight, more preferably 0.01-0.4% by weight, more preferably 0.05-0.3% by weight, and most preferably 0.1-0.2% by weight.

The lanthanum or cerium misch metal from which the rare earth elements are derived may optionally also contain yttrium. The yttrium content may thus be derived from these misch metals. The yttrium content may also be derived from a pure source of yttrium, a magnesium-yttrium master alloy or any combination thereof with or without the misch metals.

Gadolinium is an optional element which may be included. Without wishing to be bound by theory, the inclusion of gadolinium is believed to be beneficial to both creep resistance and the oxidation resistance of the melt. The gadolinium addition may be made instead of an yttrium addition. The gadolinium addition may however be made in combination with an yttrium addition. When present, the gadolinium content is preferably 0.005%-0.5% by weight, more preferably 0.01-0.4% by weight, more preferably 0.05-0.3% by weight, and most preferably 0.1-0.2% by weight.

Preferably, alloys according to the present invention contain at least 94.0% magnesium, more preferably 95-96% magnesium, and most preferably about 95.3-95.7% magnesium.

The zinc content is 0.2-0.8% by weight, preferably 0.2-0.6%, more preferably about 0.4%.

The aluminium content is preferably 0.05-0.15% by weight, more preferably 0.08-0.12% by weight, more preferably about 0.1% by weight. Without wishing to be bound by theory, the inclusion of these small amounts of aluminium in the alloys of the present invention is believed to improve the creep properties of the alloys.

The beryllium content is 0-25 ppm. When present, the beryllium content is preferably 4-20 ppm, more preferably 4-15 ppm, more preferably 6-13 ppm, such as 8-12 ppm although beryllium is preferably absent when yttrium is present as yttrium has a similar effect to beryllium at low yttrium levels. When present, beryllium would typically be introduced by way of an aluminium-beryllium master alloy, such as an Al-5% Be alloy. Without wishing to be bound by theory, the inclusion of beryllium is believed to improve the die castability of the alloy. Again, without wishing to be bound by theory, the inclusion of beryllium is also believed to improve the oxidation resistance of the molten alloy and in particular improves the retention of the rare earth element(s) in the alloys against oxidation losses.

Reduction in iron content can be achieved by addition of zirconium which precipitates iron from the molten alloy. Accordingly, the zirconium contents specified herein are residual zirconium contents. However, it is to be noted that zirconium may be incorporated at two different stages. Firstly, on manufacture of the alloy and secondly, following remelting of the alloy prior to casting. Preferably, the zirconium content will be the minimum amount required to achieve satisfactory iron removal. Typically, the zirconium content will be less than 0.1%.

Manganese is an optional component of the alloy. When present, the manganese content will typically be about 0.1%.

Calcium (Ca) is an optional component which may be included, especially in circumstances where adequate melt protection through cover gas atmosphere control is not possible. This is particularly the case when the casting process does not involve a closed system.

Ideally, the incidental impurity content is zero but it is to be appreciated that this is essentially impossible. Accordingly, it is preferred that the incidental impurity content is less than 0.15%, more preferably less than 0.1%, more preferably less than 0.01%, and still more preferably less than 0.001%.

In a second aspect, the present invention provides an engine block for an internal combustion engine produced by high pressure die casting an alloy according to the first aspect of the present invention.

In a third aspect, the present invention provides a component of an automotive powertrain formed from an alloy according to the first aspect of the present invention.

The component of the powertrain may be the engine block or a portion of an engine such as a cover, sump or brackets.

The component of the powertrain may be the transmission housing or another transmission component.

Specific reference is made above to powertrains but it is to be noted that alloys of the present invention may find use in other elevated temperature applications as well as in low temperature applications. Specific reference is also made above to HPDC but it is to be noted that alloys of the present invention may be cast by techniques other than HPDC including thixomoulding, thixocasting, permanent mould casting and sand casting.

In a fourth aspect, the present invention provides an article formed from an alloy according to the first aspect of the present invention.

EXAMPLES
Example 1

A high-Nd variant die casting alloy has a composition:

1.8 wt. % Nd

0.7 wt. % Ce

0.4 wt. % La

0.6 wt. % Zn

balance Mg

This alloy was removed from a proprietary cover gas protection known as AM-cover by immersing a cylinder with a 10 mm diameter hole in the bottom. Dry air at 2 l/min was introduced to the top of the cylinder. The base of the cylinder was immersed into the molten alloy to a depth of 50 mm and the condition of the surface of the melt was observed.

For this high-Nd alloy, the new molten surface turned black almost instantly and blooms of flaming magnesium occurred shortly afterwards.

The addition of 53 ppm of yttrium via a 43% yttrium-57% magnesium master alloy to the melt dramatically changed the oxidation behaviour of the melt. When the cylinder was inserted into the melt, the melt surface stayed bright and shiny for 50 seconds before spot burning was initiated. For an addition of 250 ppm yttrium, the resistance to the onset of burning was also excellent.

A similar effect is also experienced with the addition of gadolinium to the melt instead of yttrium. A gadolinium addition of 310 ppm was sufficient to delay the onset of spot burning in the cylinder test for 60 seconds but is not as efficient as yttrium for this purpose.

Higher lanthanum variants of the alloy have been observed to behave in a different manner to the high-Nd variants. Test work was conducted on the oxidation behaviour of a high-La variant of the alloy containing:

1.6 wt. % La

0.9 wt. % Nd

1.1 wt. % Ce

0.6 wt. % Zn

balance Mg

The aforementioned cylinder test was again used. In removing the melt from the protective atmosphere and into dry air, the alloy remained bright and shiny with no sign of oxidation or burning after 40 seconds. This alloy had a similar melt protection behaviour to the high-Nd variant of the alloy with the addition of 50-100 ppm of yttrium. Yttrium addition to this high-La version of the alloy is not required for melt protection purposes.

Example 2

Ten alloys were prepared and chemical analyses of the alloys are set out in Table 1 below. The rare earths were added as a cerium-based misch metal (which contained cerium, lanthanum and some neodymium) and elemental lanthanum and neodymium. The yttrium and zinc were added in their elemental forms. The beryllium was added as an aluminium-beryllium master alloy. The aluminium was added as this master alloy supplemented with elemental aluminium or where beryllium was not added, as elemental aluminium alone. The zirconium was added through a proprietary Mg—Zr master alloy known as AM-cast. The balance of the alloys was magnesium except for incidental impurities. Standard melt handling procedures were used throughout preparation of the alloys.

TABLE 1

Alloys Prepared

wt. %
wt. %
wt. %
wt. %
wt. %
ppm
wt. %
ppm
wt. % Zr

Alloy
Nd
Ce
La
Y
Zn
Be
Al
Fe
(total)

A
1.47
0.49
1.71
<0.005
0.59
<1
0.008
7
0.097

B
1.50
0.50
1.73
0.052
0.61
<1
0.008
8
0.080

C
1.35
0.47
1.70
0.037
0.60
<1
0.030
6
0.052

D
1.34
0.46
1.73
0.033
0.61
<1
0.055
5
0.040

E
1.33
0.46
1.73
0.027
0.61
<1
0.10
3
0.018

F
1.38
0.47
1.73
0.016
0.61
<1
0.59
7
0.018

G
0.88
1.13
1.87
<0.01
0.41
4
0.07
13
NA

H
0.84
1.13
1.84
0.23
0.46
12
0.05
19
NA

I
1.62
0.66
0.37
<0.005
0.50
2
0.02
12
NA

J
1.69
0.28
0.68
<0.005
0.43
3
0.05
22
NA

(NA: not analysed)

FIG. 1 shows the creep results for 177° C. and 90 MPa for Alloys A, B, C, D, E and F. This set of creep curves illustrates the dramatic effect that compositional variations had on creep performance in alloys of the present invention. The control alloy (Alloy A) displayed a relatively poor creep resistance under the imposed test conditions, entering into tertiary creep quite early in the test (<50 hours) and ending with 1.3% creep strain when the test was terminated at 600 hours. This was consistent with previous results for other alloy variants that contained no Al/Be addition for melt protection.

With the addition of yttrium (˜0.05 wt. %) the creep response improved substantially (Alloy B). Although both Alloy A and Alloy B reached 0.1% creep strain at approximately the same time, 62 hours and 60 hours respectively, the onset of tertiary creep was delayed until much later in the test for Alloy B.

The addition of a small amount of aluminium (˜0.03 wt. %) produced a significant improvement in the creep response (Alloy C). This alloy did not reach 0.1% creep strain under the imposed test conditions until ˜500 hours and did not appear to have gone into tertiary creep up to the time of the termination of the test (600 h). With an additional amount of aluminium (−0.06 wt. %) a further improvement in the creep properties was observed (Alloy D), which did not reach 0.1% creep strain at all during the duration of the test (0.04% creep strain after 600 hours). With a further increase in the aluminium content (Alloy E, ˜0.1 wt. %) the creep resistance began to decline (0.16 creep strain in ˜190 hours), although this was still considered to be relatively good. Finally, with a significant increase in the aluminium content (Alloy F, ˜0.6 wt. %) the creep response of the alloy deteriorated totally. Alloy F was considered to have very poor creep resistance under the imposed test conditions. These results confirm that aluminium is an important micro-alloying addition in obtaining excellent creep properties.

FIG. 2 shows the creep results for 177° C. and 90 MPa for Alloys G and H. Both Alloys G and H had delayed tertiary creep to beyond the duration of the test. The creep resistance of Alloy H, as shown in FIG. 2, compared favourably to Alloy X prepared in accordance with WO2006/105594 and having a composition by weight of:

0.68%
zinc,

1.89%
neodymium,

0.56%
cerium,

0.33%
lanthanum,

<0.005%
yttrium,

0.05%
aluminium,

<5 ppm
iron

12 ppm
beryllium

with the balance magnesium except for incidental impurities.

Tensile properties were measured in accordance with ASTM E8 at 20 and 177° C. in air using an Instron Universal Testing Machine. Samples were held at temperature for 10 minutes prior to testing. The test specimens had a circular cross section (5.6 mm diameter), with a gauge length of 25 mm.

Tensile test results for various samples of the alloys are set out in Table 2.

TABLE 2

Tensile Test Data

20° C.
177° C.

0.2% Proof,

0.2% Proof,

Alloy
MPa
UTS, MPa
% Elong.
MPa
UTS, MPa
% Elong.

A
166.8 ± 1.6
175.6 ± 0.6
1.3 ± 0.5
129.1 ± 6.2
158.7 ± 11.8
6.6 ± 2.8

B
165.2 ± 3.1
171.7 ± 4.8
1.4 ± 0.5
125.5 ± 4.1
153.4 ± 8.1
5.3 ± 1.5

C
160.4 ± 5.8
171.7 ± 7.7
1.5 ± 0.5
124.2 ± 2.1
150.0 ± 0.6
5.4 ± 0.7

D
158.5 ± 4.5
175.4 ± 2.9
1.8 ± 0.6
123.4 ± 3.2
143.0 ± 5.2
3.9 ± 0.5

E
150.8 ± 1.9
170.0 ± 5.5
1.5 ± 0.6
121.3 ± 3.6
145.2 ± 5.8
4.8 ± 1.8

F
140.0 ± 1.4
173.4 ± 4.7
1.7 ± 0.6
106.1 ± 1.5
130.9 ± 3.4
3.7 ± 0.9

G
175.5 ± 2.6
183.7 ± 4.1
2.4 ± 0.9
118.7 ± 1.3
151.8 ± 2.6
6.7 ± 0.8

H
176.2 ± 1.6
179.3 ± 1.8
2.0 ± 0.3
132.4 ± 1.8
167.6 ± 3.8
7.5 ± 1.1

It is noted that Alloy G and Alloy H in particular both had very good castability. The processing window for which sound castings can be obtained is much wider for these two alloys than for Alloy X referred to above. For good casting quality an alloy requires a low susceptibility to hot tearing, good die filling characteristics and reduced susceptibility to the formation of defects at the intersection of flow fronts in the die.

A castability test die was developed to assess the castability of a wide range of alloys in high pressure die casting (HPDC). Castings from the die are shown in FIG. 3. The die was designed to have a complex shape such that it would be extremely difficult to produce good quality high pressure die castings using this die. FIG. 3(a) shows the channels of a three-part gating system on the right hand side of the casting (known in the art as “runners”) through which the molten alloy flows into the die. The “overflows” can be seen on the opposing side (the left hand side) of the casting to the runners. The overflows and runners are broken off after casting.

The castability test die was used to produce a casting of Alloy H. The as-cast surface quality of this casting of Alloy H is shown in FIG. 3(b).

Example 3

Alloys I, J and H (see Table 1, Example 2) were cast by high pressure die casting using the castability test die referred to above in Example 2 to study the effect of lanthanum and cerium on the castability of the alloy.

FIG. 4 shows the internal defect structure of the same section of the castings of (a) Alloy I, (b) Alloy J and (c) Alloy H. Alloy I (0.66% wt cerium, 0.37% wt lanthanum) was found to have a large amount of internal cracking after casting. By changing the lanthanum to cerium ratio to greater than 1:1 in Alloy J (0.68% wt lanthanum, 0.28% wt cerium) the amount of internal cracking can be seen in FIG. 4(b) to have been reduced and the overall quality of the casting improved. Further improvement in the castability was found for Alloy H which has a greater total lanthanum and cerium content (1.7% wt lanthanum, 1.1% wt cerium) as well as a ratio of lanthanum to cerium above 1:1 and a reduced neodymium content (0.7% wt neodymium compared to 1.62% wt neodymium in Alloy 1 and 1.69% wt in Alloy J). Almost no internal cracking was observed for the casting of Alloy H. It can also be seen in FIG. 4(c) that Alloy H has a good resistance to the formation of internal flow defects and hot tearing.

Without wishing to be bound by theory, the probable reason for this second observation can be explained with reference to FIG. 5 which shows the temperature versus fraction solid curves for Alloys I and H based on Gulliver-Scheil model calculations using the phase diagrams of magnesium with each of the individual rare earth elements assuming complete mixing within the alloy. Alloy H, which has a higher lanthanum content than Alloy I can be seen to have a shorter freezing range. This is known to reduce the susceptibility of the alloy to hot tearing. Alloy H also has an increased amount of eutectic over Alloy I. This is evidenced by the last part of solidification of the Alloys which is occurring at the same temperature. For Alloy H this occurs for a greater fraction of the alloy and thus for a longer period of time as compared to Alloy I. This further reduces the susceptibility of Alloy H to hot tearing. It is noted that lanthanum is more efficient than cerium in changing the solidification characteristics to reduce the alloy's susceptibility to hot tearing. This is because for alloys with the same total cerium plus lanthanum contents, the eutectic proportion is greater in solidifying lanthanum-rich alloys and the eutectic temperature is also higher.

Again, without wishing to be bound by theory, a reduction in flow lines when high pressure die casting using Alloy H as compared to Alloy I is also likely to be responsible for the reduction in internal cracking in Alloy H. Flow lines are formed during HPDC where flows of molten alloy from runners into the die meet the flow of other runners. Oxidation of the alloy occurs on the surfaces of these flows which meet to form the visible flow lines of oxidised alloy within the casting. Without wishing to be bound by theory, it is believed that the higher yttrium content in Alloy H is responsible for this effect as this improves the recovery rate of beryllium from the master alloy addition and also influences the beryllium's oxidation rate from the molten alloy.

FIG. 6 illustrates the improved surface appearance of HPDC castings from (a) Alloy I and (b) Alloy H, with the higher lanthanum and beryllium content alloy (Alloy H) having a much improved surface appearance.

Example 4

Five further alloys were prepared to study the effects of the neodymium addition. The alloys were prepared in accordance with the procedures describe above in Example 2. Table 3 below provide the chemical analysis of these further alloys (K—P).

TABLE 3

Alloys Prepared

wt. %
wt. %
wt. %
wt. %
wt. %
ppm
wt. %
ppm
wt. % Zr

Alloy
Nd
Ce
La
Y
Zn
Be
Al
Fe
(total)

K
0.01
0.52
1.49
0.05
0.41
<1
0.05
73
0.0

L
0.22
0.84
1.80
0.01
0.41
<1
0.023
108
0.0

M
0.45
0.53
1.52
0.03
0.41
<1
0.05
86
0.0

N
0.73
0.46
1.42
0.02
0.42
<1
0.04
107
0.0

P
0.93
0.39
1.42
0.04
0.42
<1
0.032
121
0.0

FIG. 7 shows the creep results for Alloy K to Alloy P at 177° C. and 90 MPa. It can be seen from FIG. 7 that the creep response improves with an increase in the neodymium content of the alloy (refer to Table 3). Alloy K, Alloy M, Alloy N and Alloy P also have very similar compositions in all the other alloying elements except for the neodymium content. The curves indicate that the neodymium content in the alloy should be greater than about 0.5 wt.% in order to obtain a creep response that is suitable for elevated temperature applications.

MAGNESIUM BASED ALLOY

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