WO2009039581A1

WO2009039581A1 - Permanent mould cast magnesium alloy

Info

Publication number: WO2009039581A1
Application number: PCT/AU2008/001432
Authority: WO
Inventors: Mark Anthony Gibson
Original assignee: Cast Crc Limited
Priority date: 2007-09-28
Filing date: 2008-09-26
Publication date: 2009-04-02

Abstract

A magnesium based alloy consisting of, by weight: 1.2 - 1.4% neodymium; 0.5 - 0.8% rare earth element(s) with atomic number(s) 57 - 71, other than neodymium; 0.3 - 0.7% zinc; 0.3 - 1% zirconium; 0 - 0.5% yttrium; 0 - 0.3% manganese; 0 - 0.15% aluminium; 0 - 0.1% oxidation inhibiting element(s); and the balance being magnesium except for incidental impurities.

Description

PERMANENT MOULD CAST MAGNESIUM ALLOY

Field of the Invention

The present invention relates to magnesium alloys.

Background of the Invention

US patent no. 7,048,812 relates to magnesium alloys which are resistant to creep at high temperatures and which consist of, by weight: 1.4 - 1.9% neodymium; 0.8 - 1.2% rare earth element (s) with atomic number (s) 57 - 71, other than neodymium; 0.4 - 0.7% zinc; 0.3 - 1% zirconium; 0 - 0.3% manganese; 0 - 0.1% oxidation inhibiting element (s); with the remainder being magnesium except for incidental impurities.

It has now surprisingly been found that alloys containing less rare earths than the alloys of US 7,048,812 have a number of useful properties.

Summary of the Invention

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

1.2 - 1.4% neodymium;

0.5 - 0.8% rare earth element (s) with atomic number (s) 57 - 71, other than neodymium;

0.3 - 0.7% zinc,-

0.3 - 1% zirconium;

0 - 0.5% yttrium;

0 - 0.3% manganese;

0 - 0.15% aluminium;

0 - 0.1% oxidation inhibiting element (s); and the balance being magnesium except for incidental impurities .

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

1.2 - 1.4% neodymium;

0.3 - 0.7% zinc;

Substitute Sheet (Rule 26) RO/AU 0.3 - 1% zirconium;

0 - 0.5% yttrium;

0 - 0.3% manganese;

0 - 0.15% aluminium;

0 - 0.1% oxidation inhibiting element (s); no more than 0.15% titanium; no more than 0.15% hafnium; no more than 0.1% copper; no more than 0.1% nickel; no more than 0.1% silicon; no more than 0.1% silver; no more than 0.1% thorium,- no more than 0.01% iron,- no more than 0.005% strontium; and the balance being magnesium except for incidental impurities .

Preferably, alloys according to the present invention:

(a) contain less than 0.1% titanium, more preferably less than 0.05% titanium, more preferably less than 0.01% titanium, and most preferably substantially no titanium;

(b) contain less than 0.1% hafnium, more preferably less than 0.05% hafnium, more preferably less than 0.01% hafnium, and most preferably substantially no hafnium;

(c) contain less than 0.05% copper, more preferably less than 0.02% copper, more preferably less than 0.01% copper, and most preferably substantially no copper;

(d) contain less than 0.05% nickel, more preferably less than 0.02% nickel, more preferably less than 0.01% nickel, and most preferably substantially no nickel;

(e) contain less than 0.05% silicon, more preferably less than 0.02% silicon, more preferably less than 0.01% silicon, and most preferably substantially no silicon;

(f) contain less than 0.05% silver, more preferably less than 0.02% silver, more preferably less than 0.01% silver, and most preferably substantially no silver;

(g) contain less than 0.05% thorium, more preferably less than 0.02% thorium, more preferably less than 0.01% thorium, and most preferably substantially no thorium;

(h) contain less than 0.005% iron, most preferably substantially no iron; and

(i) contain less than 0.001% strontium, most preferably substantially no strontium.

Preferably, alloys according to the present invention contain at least 95% magnesium, more preferably 95.5-97% magnesium, and most preferably about 97% magnesium.

The neodymium content of 1.2 - 1.4% may be derived from pure neodymium, neodymium contained within a mixture of rare earths such as a misch metal, or a combination thereof .

Preferably, the rare earth (s) other than neodymium are cerium (Ce) , lanthanum (La) , or a mixture thereof. The rare earth (s) other than neodymium may be derived from pure rare earths, a mixture, of rare earths such as a misch metal or a combination thereof. Preferably, the rare earths other than neodymium are derived from pure lanthanum and a cerium misch metal containing cerium, lanthanum, optionally neodymium, a modest amount of praseodymium (Pr) and trace amounts of other rare earths .

The habit plane of the precipitating phase in Mg- Nd-Zn alloys is related to the zinc content, being prismatic at very low levels of Zn and basal at levels in excess of about lwt%. The best strength results are obtained at zinc levels which promote a combination of the two habit planes. Preferably, the zinc content is less than 0.6%, more preferably 0.3-0.5%, more preferably 0.35- 0.45%, most preferably about 0.4%.

Reduction in iron content can be achieved by addition of zirconium which precipitates iron from 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 melting of the alloy just prior to casting.

The elevated temperature properties of alloys of the present invention are reliant on adequate grain refinement and it is therefore necessary to maintain a level of zirconium in the melt beyond that required for iron removal. For desired tensile and compressive strength properties the grain size is preferably less than 200μm and more preferably less than 150μm. The relationship between creep resistance and grain size in alloys of the present invention is counter-intuitive . Conventional creep theory will predict that the creep resistance will decrease as the grain size decreases. However, alloys of the present invention have shown a minimum in creep . resistance at a grairi_si;ze_of_2.0_0-μm_mand._.improv_ements_in creep resistance at smaller grain sizes. For optimum creep resistance the grain size is preferably less than lOOμm and more preferably about 50μm. Preferably, the zirconium content will be the minimum amount required to achieve satisfactory iron removal and adequate grain refinement for the intended purpose. Typically, the zirconium content will be greater than 0.4%, preferably 0.4-0.6%, more preferably about 0.5%.

Manganese is an optional component of the alloy which may be included if there is a need for additional iron removal over and above that achieved by zirconium, especially if the zirconium levels are relatively low, for example below 0.4 wt%.

Elements which prevent or at least inhibit oxidation of molten alloy, such as beryllium (Be) and calcium (Ca) , are optional components 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%.

Alloys of the present invention may have a microstructure comprising equiaxed grains of magnesium based solid solution separated at the grain boundaries by a generally contiguous intergranular phase, the grains containing a uniform distribution of nano-scale precipitate platelets on more than one habit plane containing magnesium and neodymium, the intergranular phase consisting almost completely of rare earth elements, magnesium and a small amount of zinc, and the rare earth elements being substantially cerium and/or lanthanum.

The grains may contain clusters of small spherical and globular precipitates. The spherical clusters may comprise fine rod-like precipitates. The globular precipitates may be predominantly zirconium plus zinc with a Zr: Zn atomic ratio of approximately 2:1. The rod-like precipitates may be predominantly zirconium plus zinc with a Zr: Zn atomic ratio of approximately 2:1.

The expression "generally contiguous" as used in this specification is intended to mean that at least most of the intergranular phase is contiguous but that some gaps may exist between otherwise contiguous portions.

In a third aspect, the present invention provides a method of producing a magnesium alloy article, the method comprising subjecting to a heat treatment an article cast from an alloy according to the present invention. Preferably, the heat treatment comprises a solution treatment of 2-8 hours at about 525°C followed by an aging treatment of 2-6 hours at about 215°C, more preferably a solution treatment for about 4 hours at about 525⁰C followed by an aging treatment for about 4 hours at about 215°C.

Description of Preferred Embodiments of the Invention

Microstrueture

An alloy according to the present invention ("Inventive Alloy") of composition set out in Table 1 was low pressure, permanent-mould die cast into test specimens and heat-treated using an alternative thermal cycle to that specified for a standard T6 treatment. Instead of 8 hours, as specified for the standard T6 heat treatment, the solution treatment was performed only for 4 hours at 525°C, followed by the standard aging treatment of 4 hours at 215⁰C. The peak hardness generated by this alternative heat treatment was 72.5 ± 2.6 VHN. The composition of an alloy according to US patent no. 7,048,812 ("Comparative Alloy") is also set out in Table 1. The Comparative Alloy was subjected to a standard T6 heat treatment.

Table 1 Chemical analysis of Inventive Alloy and Comparative Alloy (all values in wt.%)

The microstructures of the Comparative Alloy and the Inventive Alloy are compared in Figure 1; both of which consist of equiaxed grains of a magnesium-rich phase and an intermetallic phase based on Mg₁₂RE that is located in the grain boundary region between the matrix grains . In addition, the microstructures display precipitate 'clouds', located generally in the central of each grain, that are a distinctive feature of heat treated alloys according to US patent no. 7,048,812. It can be seen that the Inventive Alloy material is well grain refined, with a smaller average grain size than the Comparative Alloy. In addition, although the volume fraction of the Mg₁₂RE intermetallic in the Inventive Alloy is rather low (see Figure Ib) , compared to that for Comparative Alloy (see Figure Ia) , the majority of the triple points between the grains are pinned by intermetallic particles and this is considered to be a significant element in the development of a creep resistant microstructure .

Tensile and Compression Properties

Tensile testing of the Inventive Alloy was conducted on standard, rectangular cross- section (6 mm wide, 3 mm thick and with a 25 mm gauge length), 'dogbone' specimens at various temperatures. Compression testing was conducted on cylindrical specimens (10 mm diameter and 15 mm height) at RT and 150⁰C. A minimum of three tests were conducted for each set of conditions . It can be seen from Tables 2 and 3 that the mechanical properties of the Inventive Alloy are, in general, as good as or better than those of the Comparative Alloy.

Table 2 Mechanical property data for Inventive Alloy at various test temperatures

# Data was determined at 15⁰C

Table 3 Mechanical property data for Comparative Alloy

Bolt Load Retention

BLR tests on the Inventive Alloy were conducted for 100 hours duration at temperatures of 150°C and 177°C, with applied loads of both 8 kN and 11 JcN at each temperature, and the results are summarised in Table 4 and Table 5 respectively (partly threaded M8 high tensile steel bolts, equipped with strain gauges, were used to measure BLR behaviour) . In general the BLR behaviour for the Inventive Alloy under the test conditions was excellent. The effect of the initial applied stress was more pronounced than the effect of the test temperature.

A comparison of overall BLR behaviour between the Comparative Alloy and the Inventive Alloy is contained in Figure 2. Figure 2 (a) shows that at an 8 kN load the two alloys display essentially the same behaviour at both temperatures. At the higher initial load of 11 kN the Inventive Alloy material is slightly better than the Comparative Alloy material. Both materials show anomalous behaviour at 150⁰C, with the % load retained at 11 kN load being greater than that at 8 kN load. This behaviour is repeated at 177°C for the Inventive Alloy material. The percentage of the initial load retained at the test temperature (Figure 2 (b) ) is effectively the same for both materials under all test conditions. This measure is an indication of the compressive creep behaviour of the materials, and it is clear that the amount of creep taking place in the 100 hour period is relatively insensitive to changes in initial load and test temperature. Therefore the BLR properties of the Inventive Alloy are equivalent to those of the Comparative Alloy.

Table 4 Characteristic BLR parameters of the Inventive Alloy tested at 150⁰C with either an 8 kN or 11 kN initial load.

Table 5 Characteristic BLR parameters of Inventive Alloy tested at 177°C with either an 8 kN or 11 kN initial load.

Tensile Creep Properties

Tensile creep tests were conducted on the Inventive Alloy using the same type of specimens as for the tensile testing. Under none of the imposed test conditions investigated did the material reach steady- state creep behaviour within the duration of the test and therefore a value for the secondary creep rate has not been determined. The creep performance of the Inventive Alloy is compared with that for the Comparative Alloy under the same test conditions (85 MPa and 177°C) in Figure 3. The results confirm that the tensile creep properties of the Inventive Alloy are exceptionally good up to at least 177°C.

High Cycle Fatigue

The high cycle fatigue properties of the Inventive Alloy material have been measured at room temperature, 120⁰C and 150⁰C on suitably prepared "dog- bone" specimens to determine the strength at 10⁷ cycles using the staircase method. A total of 57 tests were completed in the test program, 20 at 24⁰C (of which 16 formed part of the staircase) , 17 at 120⁰C (of which 13 formed part of the staircase) and 20 at 150⁰C (14 of which formed part of the staircase) . The results for the Inventive Alloy are compared with those for the Comparative Alloy in Table 6. It can be seen that the fatigue strength of the Inventive Alloy is superior to that of the Comparative Alloy at each temperature the two materials were tested.

Table 6 A comparison of the HCF data from the Inventive Alloy with that from the Comparative Alloy.

Further Comparative Alloys

Further Comparative Alloys 1, 2 and 3 were produced and tested to compare mechanical properties with the Inventive Alloy described above. Castings of Comparative Alloys 1-3 were produced by sand casting with a metal chill plate. The castings were then subjected to a standard T6 heat treatment as described above. Table 7 below sets out the compositions of the Further Comparative Alloys .

Table 7 - Chemical Analysis of Further Comparative Alloys

(all values in wt.%)

Material Nd Ce La Pr Zn Mn Zr (tot)

Alloy 1 1. 05 0. 31 0. 17 0. 03 0. 48 0. 06 0. 34

Alloy 2 0. 82 0. 21 0. 12 0. 02 0. 45 0. 04 0. 29

Alloy 3 0. 95 0. 56 0. 29 0. 06 0. 56 0. 04 0. 013

Samples of the Further Comparative 1-3 were subjected to tensile testing using "dog bone" specimens in accordance with the methodology described above. Table 8 below sets out the results from this tensile testing. Figure 4 provides a comparison of the creep curves for the Further Comparative Alloys as compared to that of the Inventive Alloy, tested under equivalent conditions (85MPa and 177°C) .

Table 8

Comparing the results in Table 8 to the results in Table 2 for the Inventive Alloy it is apparent from the tensile testing that the tensile creep properties of the three Further Comparative Alloys are inferior to that of the Inventive Alloy, in particular at 177°C. At this temperature, Figure 4 shows the steady state creep rates for the Further Comparative Alloys 1, 2 and 3 were in the order of IxIO^-9S^"1, IxIC⁹S^"1 and 2x 10^"9S^"1 respectively as compared to the steady state creep rate of the Inventive Alloy which is better than 8x 10^"11S^"1. It is believed that the inferior tensile creep properties of the Further Comparative Alloys are due to the low rare earth content of the alloys, in particular the low neodymium content. Whilst it is expected that there may be some influence of the casting process (sand casting as opposed to low pressure permanent mould die casting) on the difference in tensile properties between the Further Comparative Alloys and the Inventive Alloy, this is understood to not be significant.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

Claims

Claims1. A magnesium based alloy consisting of, by weight:

1.2 - 1.4% neodymium;

0.3 - 0.7% zinc;

0.3 - 1% zirconium;

0 - 0.5% yttrium;

0 - 0.3% manganese;

0 - 0.15% aluminium;

0 - 0.1% oxidation inhibiting element (s); and the balance being magnesium except for incidental impurities.

2. A magnesium alloy consisting of, by weight:

1.2 - 1.4% neodymium;

0.3 - 0.7% zinc;

0.3 - 1% zirconium;

0 - 0.5% yttrium;

0 - 0.3% manganese;

0 - 0.15% aluminium;

0 - 0.1% oxidation inhibiting element (s); no more than 0.15% titanium; no more than 0.15% hafnium; no more than 0.1% copper; no more than 0.1% nickel; no more than 0.1% silicon; no more than 0.1% silver; no more than 0.1% thorium; no more than 0.01% iron; no more than 0.005% strontium; and the balance being magnesium except for incidental impurities .

3. A magnesium based alloy as claimed in either claim 1 or 2, wherein the neodymium content is at least 1.3%.

4. A magnesium based alloy as claimed in any one of the preceding claims, wherein the neodymium content is about 1.4%.

5. A magnesium based alloy as claimed in any one of the preceding claims wherein the rare earth element content other than neodymium is at least 0.6%.

6. A magnesium based alloy as claimed in any one of the preceding claims, wherein the rare earth element content other than neodymium is less than 0.8%.

7. A magnesium based alloy as claimed in any one of the preceding claims, wherein the rare earth element content other than neodymium is 0.65-0.75%.

8. A magnesium based alloy as claimed in any one of the preceding claims, wherein the rare earth element content other than neodymium is substantially cerium and/or lanthanum.

9. A magnesium based alloy as claimed in any one of the preceding claims, wherein the alloy contains at least 95% magnesium.

10. A magnesium based alloy as claimed in any one of the preceding claims, wherein the alloy contains about 97% magnesium.

11. A magnesium based alloy as claimed in any one of the preceding claims, wherein the zinc content is less than 0.6%.

12. A magnesium based alloy as claimed in any one of the preceding claims, wherein the zinc content is 0.3-0.5%.

13. A magnesium based alloy as claimed in any one of the preceding claims, wherein the zirconium content is greater than 0.4%.

14. A magnesium based alloy as claimed in any one of the preceding claims, wherein the zirconium content is 0.4- 0.6%.

15. A magnesium based alloy as claimed in any one of the preceding claims, wherein the oxidation inhibiting elements comprise calcium and/or beryllium.

16. A method of producing a magnesium article, the method comprising subjecting to a heat treatment an article cast from a magnesium based alloy according to any one of the preceding claims .

17. A method of producing a magnesium alloy article as claimed in claim 16, wherein the heat treatment comprises a solution treatment of 2-8 hours at about 525°C followed by an ageing treatment of 2-6 hours at about 215°C.