WO2006125278A1

WO2006125278A1 - Hpdc magnesium alloy

Info

Publication number: WO2006125278A1
Application number: PCT/AU2006/000712
Authority: WO
Inventors: Mark Antony Gibson; Colleen Joyce Bettles
Original assignee: Cast Centre Pty Ltd
Priority date: 2005-05-26
Filing date: 2006-05-26
Publication date: 2006-11-30
Also published as: CN101228286A; DE112006001375T5; US20080193322A1; TW200643188A

Abstract

A magnesium - rare earth - yttrium - zinc alloy consists of 0.2-1.5% by weight zinc and rare earth(s) (RE) and yttrium in amounts which fall within a quadrangle defined by lines AB, BC, CD and DA wherein: A is 1.8% RE - 0.05% Y, B is 1.0% RE - 0.05% Y, C is 0.2% RE - 0.8% Y, and D is 1.8% RE - 0.8% Y.

Description

HPDC MAGNESIUM ALLOY

FIELD OF THE INVENTION

The present invention relates to magnesium alloys and, more particularly, to magnesium 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 the vehicles is a key to achieving this goal. Major contributors to the weight of any vehicle are the engine and other components of the power train. 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. The most common magnesium based HPDC alloys are AM50 (95% Mg, 5% Al) , AM60 (94% Mg, 6% Al) and AZ91 (90% Mg, 9% Al and 1% Zn) . Unfortunately, none of these alloys are suitable for use at elevated temperatures .

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.

US 3718460 which claims a priority date of 4 December 1967 relates to a magnesium-aluminium-silicon alloy which is "particularly adaptable to die casting" . The alloy consists "essentially of magnesium containing by weight from about 0.4 to 1.5 per cent silicon, from about 3.5 to about 7 per cent aluminium, up to about 1 per cent manganese and up to about 2 per cent zinc" . US 3718460 makes no mention of yttrium.

PCT/GB96/00261 (WO 96/24701) which claims a priority date of 6 February 1995 relates to a magnesium- zinc-rare earth (Mg-Zn-RE) alloy for HPDC in which the expression rare earth is specifically defined in terms of a range of elements but "is not intended to include elements such as yttrium." Yttrium is thus specifically excluded as an alloy component of this HPDC alloy.

US 6322644 which claims a priority date of 15 December 1999 relates to a magnesium-based diecast alloy having improved elevated temperature performance which consists of 2-9% aluminium, 0.5-7% strontium, 0-0.6% manganese, 0-0.35% zinc and the balance magnesium. No mention of yttrium is made in US 6322644. Various magnesium based alloys which contain yttrium have been proposed over the years .

GB 1067915 which claims a priority date of 26 October 1963 notes that "it has now been discovered that an addition of yttrium brings about a further refinement of the grain of a zirconium-containing magnesium alloy." The patent is broadly directed to magnesium alloys containing 0.1-1% zirconium (Zr), 0.1-10% yttrium (Y), and up to 10% of at least one additional alloying element selected from beryllium (Be) , lead (Pb) , cadmium (Cd) , calcium (Ca) , cerium (Ce) , copper (Cu) , silver (Ag) , thallium (Tl) , thorium (Th) , bismuth (Bi) and zinc (Zn) .

The magnesium alloy MLlO, developed in the former USSR, has been used for many years for cast parts intended for use in aircraft at temperatures up to 250⁰C. MLlO is a high strength Mg-Nd-Zn-Zr alloy. ML19 alloy is similarly based on the Mg-Nd-Zn-Zr system but additionally contains Y.

A paper by Mukhina et al entitled

"Investigation of the Microstructure and Properties of Castable Neodymium and Yttrium-Bearing Magnesium Alloys at Elevated Temperatures" published in Science and Heat Treatment" VoI 39, 1997, indicates typical compositions (% by weight) of MLlO and ML19 alloys are:

MLlO MLl9

Neodymium (Nd) 2.2 - - 2. 8 1.6 - 2.3

Yttrium (Y) Nil 1.4 - 2.2

Zirconium (Zr) 0.4 - - 1. 0 0.4 - 1.0

Zinc (Zn) 0.1 - - 0. 7 0.1.- 0.6

Magnesium (Mg) Balance Balance with impurity levels of:

Iron (Fe) < 0.01

Silicon (Si) < 0.03

Copper (Cu) < 0.03

Nickel (Ni) < 0.005

Aluminium (Al) < 0.02

Beryllium (Be) < 0.01

MLlO and ML19 are both sand casting alloys and neither has found commercial acceptance as a HPDC alloy.

GB 1378281 which claims a priority date of 14 March 1973 "relates to magnesium-based light structural alloys, particularly those for the production of parts subject to heating in service." The alloy contains 0.8- 6.0% Y, 0.5-4.0% Nd, 0.1-2.2% Zn, 0.31-1.1% Zr, up to 0.05% Cu, up to 0.2% manganese (Mn) and the balance Mg. A related US patent, US 4116731, claims an alloy of identical composition which is a "heat-treated and aged" alloy in which "no less than 50% of the total amount of neodymium and yttrium additions enters the solid solution after heat treatment" and the alloy, having been heat treated at approximately 535°C for 4-8 hours, is cooled in air and then aged at approximately 200⁰C for 12 hours.

US 4401621 which claims a priority date of 25 May 1981 relates to magnesium alloys consisting of:

"(a) from 1.5 to 10% by weight of an yttrium component consisting of at least 60% by weight of yttrium and the balance, if any, of heavy rare earth metals, and

(b) from 1 to 6% by weight of a neodymium component consisting of at least 60% by weight of neodymium, not more than 25% by weight of lanthanum and substantially all the balance, if any, of praseodymium, the remainder of the alloy consisting of magnesium." US 6767506 which claims a priority date of 10

January 2002 "relates to magnesium-based alloys suitable for applications at temperatures as high as 250-300⁰C" . Alloys according to US 6767506 contain 2.7-3.3% Nd, Y in amounts up to 2.6%, 0.2-0.8% Zr, 0.2-0.8% Zn, 0.03-0.25% Ca, 0-0.001% Be and at least 92% Mg. The alloys are said to be well adapted for sand casting, permanent mould casting and direct chill casting with subsequent extrusion and/or forging. There is no suggestion in US 6767506 that the alloys are suitable for HPDC. Mg-RE-Y alloys tend to be used as gravity and sand casting alloys which can be heat treated to achieve desired properties . They tend to have rather high additions of both RE and Y with the aim of having a phase at grain boundaries which is of the Mg-RE type and two precipitating phases, namely Mg_3-2Nd and Mg₂₄Ys- Y has a high solubility in Mg even at room temperature and so high levels of Y are necessary to achieve any significant level of precipitation. As far as the present inventors are aware, no Y containing Mg-based alloy has found commercial acceptance as a HPDC alloy.

SUMMARY OF THE INVENTION In a first aspect, the present invention provides a magnesium - rare earth - yttrium - zinc alloy consisting of: rare earth (s) (RE) and yttrium (Y) in amounts which fall within a quadrangle defined by lines AB, BC, CD and DA wherein:

A IS 1.8% RE - 0.05% Y,

B is 1.0% RE - 0.05% Y,

C is 0.2% RE - 0.8% Y, and

D is 1.8% RE - 0.8% Y; 0.2 - 1.5% zinc (Zn);

0 - 0.25% aluminium (Al); 0 - 0.2% zirconium (Zr); 0 - 0.3% manganese (Mn);

0 - 0.1% oxidation inhibiting element (s), and the balance being magnesium (Mg) except for incidental impurities. Unless otherwise stated, all percentages in this document are % by weight.

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) .

The quadrangle defined by lines AB, BC, CD and DA is illustrated in Figure 1 which is a plot of total rare earth content versus yttrium content . Preferably, alloys of the present invention contain: 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 0.1% strontium, and no more than 0.01% iron.

More 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; and

(h) contain less than 0.05% strontium, more preferably less than 0.02% strontium, more preferably less than 0.01% strontium, and most preferably substantially no strontium.

Preferably, alloys of the present invention contain rare earth (s) and yttrium in amounts which fall within a quadrangle defined by lines EF, FG, GH and HE wherein:

E is 1.5% RE - 0.3% Y,

F is 1.0% RE - 0.3% Y, G is 1.0% RE - 0.8% Y, and

H is 1.5% RE - 0.8% Y.

The quadrangle defined by lines EF, FG, GH and HE is illustrated in Figure 1 which is a plot of total rare earth content versus yttrium content.

Preferably, alloys according to the present invention contain at least 96.7% magnesium, more preferably 97-98.5% magnesium, and most preferably about 98% magnesium. Preferably, the rare earth component of alloys according to the first or second aspects of the present invention are selected from neodymium (Nd) , cerium (Ce) , lanthanum (La), or any mixture thereof.

Preferably, the neodymium content is greater than 0.2%, more preferably greater than 0.4%, more preferably 0.4-1.8% and most preferably 0.4-1.0%, although alloys of the present invention may contain no neodymium. The neodymium content 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 content of rare earth (s) other than neodymium is 0-1.6%, more preferably 0.5-1.0%, although alloys of the present invention may contain no rare earths other than neodymium. Preferably, any rare earth (s) other than neodymium are cerium, lanthanum, or a mixture thereof . 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, rare earths other than neodymium are derived from a cerium misch metal containing cerium, lanthanum, optionally neodymium, a modest amount of praseodymium (Pr) and trace amounts of other rare earths.

Without wishing to be bound by theory, the inclusion of yttrium is believed to be beneficial to melt protection, ductility and creep resistance.

Preferably, the zinc content is 0.2-0.7%, more preferably 0.3-0.5%, more preferably 0.4-0.6%. Zirconium is an optional component of alloys of the present invention. Reduction in iron content can be achieved by addition of zirconium which precipitates iron from molten alloy. Desirably, the alloys contain a minimum of iron. Preferably, alloys of the present invention contain less than 0.005% iron and, most preferably, substantially no iron. 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. Preferably, the zirconium content will be the minimum amount required to achieve satisfactory iron removal. Typically, the zirconium content will be about 0.1% or less.

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

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.

When present, the beryllium content is preferably less than 50ppm, more preferably 4-25 ppm, more preferably 4-20 ppm, more preferably 4-15 ppm, more preferably 6-13 ppm, such as 8-12 ppm. Beryllium would typically be introduced by way of an aluminium-beryllium master alloy, such as Al-5% Be, and thus aluminium may be present in small amounts up to 0.25%. Preferably, the aluminium content is less than 0.2%, more preferably less than 0.1%. Without wishing to be bound by theory, the inclusion of beryllium and/or calcium is believed to improve the die castability of the alloy.

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%. Surprisingly for HPDC alloys, at least some alloys of the present invention can benefit from heat treatments , such as a T6 heat treatment which would typically involve solution treatment at 450-550⁰C for up to 6 hours, followed by a quench, and then an artificial aging at 150-300⁰C for up to 24 hours.

In a second aspect, the present invention provides a component of an internal combustion engine formed from an alloy according to the first aspect of the present invention. The component of the internal combustion engine may be the engine block or a portion thereof such as a shroud.

Alloys according to a first aspect of the present invention may be cast by processes other than HPDC, such as sand casting or low pressure/gravity permanent mould casting.

In a third 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. Specific reference is made above to engine blocks but it is to be noted that alloys of the present invention may find use in other elevated temperature applications such as may be found in automotive power trains 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 moulding and sand casting .

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Example 1

Two alloys according to the present invention were prepared and chemical analyses of the alloys are set out in Table 1 below. The rare earths other than neodymium were added as a Ce-based misch metal which contained cerium, lanthanum and some neodymium. The extra neodymium and the zinc were added in their elemental forms. Standard melt handling procedures were used throughout preparation of the alloys .

Table 1 - Alloys Prepared

The tensile properties of Alloys A and B at room temperature and at 177°C are set out below in Table 2.

Table 2 - Tensile Properties

Creep tests were carried out on Alloys -A and B at a constant load of 90MPa and at a temperature of 177⁰C in the as-cast condition and in a T6 heat treated condition and at 75MPa and 200⁰C for Alloy A in the as- cast condition. The steady state creep rates are listed in Table 3.

Table 3 - Steady State Creep Rates

Figure 2 shows the creep results for 177⁰C and 90MPa for Alloys A and B in the as-cast condition. From Figure 2 it can be seen that, although the two alloys have similar secondary creep rates, Alloy A is considerably more resistant than Alloy B to instantaneous strain upon loading under these conditions.

Figure 3 shows the creep results for 177⁰C and 90MPa for Alloys A and B in a T6 heat treated condition. Alloys A and B were solution treated for 8 hours at 525⁰C, followed by a cold water quench, and then were aged at 215°C for 4 hours. Under these conditions Alloy A is also considerably more creep resistant than Alloy B.

The influence of a T6 heat treatment on the creep behaviour of HPDC test specimens for Alloy A and Alloy B of the present invention is illustrated by a comparison of Figure 2 with Figure 3. It can be seen that a T6 heat treatment provides little advantage to low Y content alloys (<0.4 wt.% Y). However, for compositions that contain 0.5 wt.% Y or greater, a T6 heat treatment can have a significant beneficial influence on the creep performance of the alloy. Example 2

A series of alloys according to the present invention were produced and their, compositions are listed in Table 4 which includes Alloys A and B referred to in Example 1.

Table 4 Chemical composition of Alloys A - V

For the purposes of mechanical property evaluation, test specimens were produced by the high pressure die casting (HPDC) of these alloys on a 250 tonne Toshiba cold chamber machine. The alloy properties that were evaluated include casting quality, as-cast microstructure, tensile strength at room temperature and . 177⁰C and creep behaviour at 177⁰C and 200⁰C. A typical example of the microstructure of an alloy according to the present invention (Alloy N) , in the as-cast condition, is shown in Figure 4. Due to the nature of HPDC there is a transition from a fine grain . structure, close to the surface of the cast specimen (the ^xskin'), to a coarser grain structure in the central region (the 'core' ) . However, both regions consist of primary magnesium-rich grains or dendrites with a Mg-RE intermetallic phase in the inter-granular and interdendritic regions.

Alloys of the present invention are non-burning and highly resistant to oxidation as shown in Figure 5.

Figures 5 (d) , 5 (e) and 5(f) relate to an alloy of composition very similar to that of Alloy H and Figures 5 (a) , 5 (b) and 5 (c) relate to an alloy of equivalent composition save for it containing no yttrium. Figures 5 (b) and 5(e) are macro images of polished sections through the centre of the castings shown in Figures 5 (a) and 5 (d) respectively which give an indication of the depth of penetration of the oxides that are formed on the surface into the interior of the castings. Figures 5(c) and 5(f) are equivalent higher magnification images of Figures 5 (b) and 5 (e) respectively. It can be seen in Figure 5(c) that the yttrium free alloy displays extensive penetration of oxide stringers; whereas, there is minimal penetration evident in Figure 5(f) of the alloy of the present invention.

The degree of surface oxidation and the depth of penetration of oxide stringers into the bulk of the casting are both greatly reduced for alloys with compositions typical of the present invention. This non- burning behaviour is very advantageous in all practical casting operations . A summary of the tensile test data for the alloys of the present invention are given in Table 5 and it can be seen that the tensile behaviour is reasonable at both test temperatures considered. Examples of the stress-strain curves for Alloy N, Alloy 0 and Alloy P are shown in Figure 6 and Figure 7 for tests conducted at room temperature and 177⁰C respectively. The influence of the Y content of the composition on the tensile stress-strain behaviour is shown in Figure 8, where it can be seen that the tensile strength is improved with increasing Y content.

Table 5 Typical tensile properties for the range of example alloys of the present invention at both room temperature and 177⁰C.

The steady-state creep rates for an alloy of the present invention (Alloy A, in the as-cast condition) under a number of different test conditions are contained in Table 6 and examples of the associated creep curves are also shown in Figure 9. Table 6 Steady-state creep rates for Alloy A of the present invention under various test conditions and under the same test condition for different heat treatment.

A summary of the steady-state creep rate under the same conditions of 177°C and 90 MPa for all the composition variations measured in the as-cast condition is contained in Table 7.

Table 7 Steady-state creep rate for a number of alloy variants of the present invention.

Selected creep curves, for alloys with increasing Y content, are shown in Figure 10, Figure 11 and Figure 12 for compositions that contain a low total rare earth (TRE) content (in the range 1.1-1.2 wt.%), a medium TRE content (in the range 1.3-1.4 wt.%) and a high TRE content (in the range 1.5-1.6 wt.%). It is a general observation that increasing the Y content of the alloys of the present invention results in a significant improvement to the creep behaviour observed under these test conditions (177⁰C and 90 MPa) . Alloy compositions with both a low TRE content and a low Y content display poorer creep performance under these stringent test conditions, as indicated by the curve for Alloy J in Figure 10. Such compositions are therefore better suited to less demanding applications in the powertrain than the engine block. For compositions that contain in excess of 0.45 wt. % Y the creep performance is very good as shown by Alloy A, Alloy C and Alloy T (see Figure 9, Figure 11 and Figure 12 respectively) .

It is to be clearly understood that although prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art in Australia or in any other country.

Claims

1. A magnesium - rare earth - yttrium - zinc alloy consisting of: rare earth (s) (RE) and yttrium in amounts which fall within a quadrangle defined by lines AB, BC, CD and DA wherein:

A is 1.8% RE - 0.05% Y,

B is 1.0% RE - 0.05% Y,

C is 0.2% RE - 0.8% Y, and D D iiss 1.8% RE - 0.8% Y;

0.2 - 1.5% zinc; 0 - 0.25% aluminium; 0 - 0.2% zirconium; 0 - 0.3% manganese; 0 - 0.1% oxidation inhibiting element (s), and the balance being magnesium (Mg) except for incidental impurities .

2. An alloy as claimed in claim 1 which contains rare earth (s) and yttrium in amounts which fall within a quadrangle defined by lines EF, FG, GH and HE wherein:

E is 1.5% RE - 0.3% Y, F is 1.0% RE - 0.3% Y, G is 1.0% RE - 0.8% Y, and

H is 1.5% RE - 0.8% Y.

3. An alloy as claimed in claim 1 or claim 2 containing at least 96.7% magnesium.

4. An alloy as claimed in any one of the preceding claims wherein the rare earth element (s) are selected from neodymium, cerium, lanthanum, praseodymium, or any combination thereof.

5. An alloy as claimed in any one of the preceding claims having a neodymium content of 0.4-1.0%.

6. An alloy as claimed in any one of the preceding claims wherein the content of rare earth (s) other than neodymium is 0.5-1.0%.

7. An alloy as claimed in any one of the preceding claims having a yttrium content of 0.1-1.6%.

8. An alloy as claimed in claim 7 having a yttrium content of 0.25-1.25%.

9. An alloy as claimed in claim 8 having a yttrium content of 0.5-1.0%.

10. An alloy as claimed in any one of the preceding claims having a zinc content of 0.2-0.7%.

11. An alloy as claimed in claim 10 having a zinc content of 0.4-0.6%.

12. An alloy as claimed in any one of the preceding claims containing aluminium in an amount less than 0.25%.

13. An alloy as claimed in any one of the preceding claims containing zirconium in an amount less than 0.2%.

14. An alloy as claimed in any one of the preceding claims containing manganese in an amount less than 0.3%.

15. An alloy as claimed in any one of the preceding claims containing beryllium in an amount less than 50ppm.

16. An alloy as claimed in any one of the preceding claims containing calcium in an amount less than 0.1%.

17. A component of an internal combustion engine or automotive power train formed from an alloy as claimed in any one of the preceding claims.

18. An engine block or portion thereof produced by high pressure die casting an alloy as claimed in any one of claims 1-16.