US20090196787A1

US20090196787A1 - Magnesium alloy

Info

Publication number: US20090196787A1
Application number: US12/023,105
Authority: US
Inventors: Randy S. Beals
Original assignee: Individual
Current assignee: FCA US LLC
Priority date: 2008-01-31
Filing date: 2008-01-31
Publication date: 2009-08-06

Abstract

An improved, castable, magnesium-based alloy having good creep resistance, good corrosion resistance, with improved tensile strength and elongation; methods for preparing the alloys; methods for preparing articles from the alloys; and articles, machines, and other devices comprising the same.

Description

FIELD

The present invention relates to creep-resistant magnesium-based alloys and more particularly, to castable magnesium-based alloys with improved tensile strength and elongation.

BACKGROUND

Magnesium alloys are increasingly used as lightweight materials in automotive and aerospace vehicles. Magnesium alloys can offer a high strength to weight ratio for a given component, providing structural rigidity with reduced weight. Typically die-cast under high pressure, magnesium alloy structural components are gaining market share, partly in response to the need to minimize fuel consumption in light of continued high fuel costs. As a result, such alloys are used to produce, e.g., transmission casings, cylinder heads, engine blocks, gearboxes, valve covers, cam covers, intake manifolds, and steering components.
One drawback associated with some magnesium based alloys is known as creep. Creep occurs when a material continues to deform under constant stress and temperature. Creep resistance is a desirable characteristic for use of magnesium based alloys in power train components. Creep resistance under compressive load and temperature is important in order to maintain bolt torque and dimensional stability of cast bodies during vehicle operation. Another important property for alloy used in rigid, structure components is a high tensile strength. However, known magnesium alloys do not exhibit an optimal combination of castability, creep resistance, tensile strength, and corrosion resistance.
Because there is an increasing need for improved, high-strength magnesium alloys, it would be desirable to provide a relatively low-cost magnesium alloy that exhibits high castability, and that has good creep resistance and a high level of ultimate tensile strength, while exhibiting a high percent elongation ands good corrosion resistance.

SUMMARY

The present disclosure describes a relatively low-cost magnesium alloy that exhibits high castability, and that has good creep resistance and a high level of ultimate tensile strength, while exhibiting a high percent elongation ands good corrosion resistance. The present disclosure further provides:
Creep-resistant, castable magnesium alloys containing, by weight: from about 6 to about 7.5% aluminum from about 1 to about 3.5% rare earth metals; from about 0.2 to about 0.45% zinc; from about 0.15 to about 0.25% manganese from about 0.01 to about 0.9% calcium; from zero to about 0.001% beryllium; from about 0.01 to about 0.12% tin; from about 0.01 to about 0.11% strontium; and the balance being magnesium and, optionally, impurities commonly found in magnesium alloys.
Such alloys in which the aluminum is present at from about 6.5 to about 7.5% by weight, or from about 7 to about 7.5% by weight; such alloys in which the zinc is present at from about 0.35 to about 0.45% by weight, or from about 0.35 to about 0.4% by weight. Such alloys that exhibit an ultimate tensile strength of about or at least 39 ksi, as measured according to ASTM B557; and/or a percent elongation at break of more than 13% as measured according to ASTM D-1708.
Creep-resistant, castable magnesium alloys containing, by weight: from about 6.5 to about 7.5% aluminum; from about 1.5 to about 2.5% rare earth metals; from about 0.35 to about 0.45% zinc; from about 0.15 to about 0.25% manganese; from about 0.01 to about 0.1% calcium; about 0.001% beryllium; from zero to about 0.01% tin; from zero to about 0.01% strontium; and the balance being magnesium and, optionally, impurities commonly found in magnesium alloys.
Creep-resistant, castable magnesium allows containing, by weight: from about 7 to about 7.2% aluminum; from about 1.5 to about 1.75% rare earth metals; from about 0.35 to about 0.4% zinc; from about 015 to about 0.2% manganese; from about 0.01 to about 0.02% calcium; from about 0.0005 to about 0.001% beryllium; about 0.01% tin; from zero to about 0.01% strontium; and the balance being magnesium and, optionally, impurities commonly found in magnesium alloys.
Such alloys in which the alloy contains, by weight: about 7.18% aluminum; about 1.62% rare earth metals; about 0.355% zinc; about 0.17% manganese; about 0.01% calcium; about 0.0007% beryllium; about 001% tin; from zero to about 0.01% strontium; and the remainder magnesium and, optionally, impurities commonly found in magnesium alloys.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is a graphic of mechanical properties, combining a bar chart showing yield strength (YS) and ultimate tensile strength (UTS) of, and a graph showing percent elongation (% Elong) of, alloys according to various embodiments hereof (e.g., Series 2), compared to those for, e.g., aluminum and comparative examples (e.g., Series 6).

FIG. 2 is a graph showing creep resistance of alloys according to various embodiments hereof (e.g., Series 2), compared to that for commercially available, creep-resistant magnesium alloys (e.g., AS31) ands that for comparative examples (e.g., Series 6). Creep strain was monitored over time for comparable samples maintained at 150° C. under 8.5 ksi of stress.

FIG. 3 is a graph showing creep resistance differences among a number of the most creep-resistant samples presented in FIG. 2, at bottom.

FIG. 4 is a graph showing creep resistance of alloys according to various embodiments hereof (e.g., Series 2), compared to that for commercially available, creep-resistant magnesium alloys (e.g., AS31) and that for comparative examples (e.g., Series 6). Creep strain was monitored over time for comparable samples maintained at 180° C. under 8.5 ksi of stress.

FIG. 5 is a graph showing creep resistance differences among a number of the most creep-resistant samples presented in FIG. 4, at bottom.

DETAILED DESCRIPTION

The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, is application, or uses.
Values stated herein can be recited as approximate values, but can alternatively be recited with any desired degree of precision by addition of further zeros, after the decimal point, following the last-stated significant digit. Unless otherwise stated, percent values are reported herein as percent by weight (wt. %).
Although described herein using open-ended language, such as “comprising,” the subject matter disclosed herein can alternatively be described using more closed language, such as “consisting of” or “consisting essentially of” Where “consisting essentially of” is utilized herein to describe a component, it refers to the recited component free of other components in amounts that would interfere with the function of that component or that would have a significantly deleterious effect on the composition or article in which the component is used, i.e. which would make the composition or article significantly less fit for its intended purpose either in its physical or chemical properties.
As used herein, a “rare earth” component of an alloy refers to a combination of rare earth elements that includes cerium, lanthanum, and neodymium, and can contain praseodymium and/or small amounts (i.e. a minority by weight) of other rare earth elements, e.g., yttrium. In various embodiments, a rare earth component of an alloy hereof can comprise about 50 wt. % cerium, and about 50 wt. % of a combination of lanthanum and neodymium. In some embodiments, a rare earth component can comprise about 50 wt. % cerium, and about 50 wt % of a combination of lanthanum, neodymium, and praseodymium; the combination of La, Nd, and Pr can, in some embodiments, comprise about 20 wt. % or less, about 15 wt. % or less, or about 10 wt. % or less Pr. A rare earth component can be prepared by combining rare earth elements, or can more typically be obtained directly by recovering a rare earth mixture from an ore or ore mixture. The term “rare earth element” refers to any one of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). In some embodiments, a single rare earth element or a different rare earth element combination can be used in place of the Ce, La, and Nd combination, or the Ce, La, Nd, and Pr combination.
In various embodiments, a magnesium alloy hereof contains, in addition to magnesium, between 7 and 15 wt. % other elements including, aluminum, rare earth, zinc, manganese, calcium, and beryllium, and in some embodiments, tin.
The aluminum can be present in an alloy hereof in an amount of from about 6 to about 7.5 wt. %, or more typically in an approximate range of 6.5 to 7.5 wt. %, or 7 to 7.5 wt. %. In various embodiments, the aluminum content can be, e.g., about or less than 7.5, 7.4, 7.3, or 7.2 wt. %, and about or greater than 7 wt. %. In some embodiments, the aluminum content can be in an approximate range of 7 to 7.2 wt. %, or 7.1 to 7.2 wt. %.
The rare earth component, i.e. rare earth metals, can be present in an alloy hereof in an amount of from about 1 to about 3.5 wt. %, or more typically in an approximate range of 1.3 to 1.8 wt. %, 1 to 2.5 wt. %, 1.5 to 2.5 wt. %, or 1.5 to 2 wt. %. In various embodiments, the rare earth component content can be, e.g., about or less than 1.8, 1.75, 1.7, or 1.65 wt. %, and about or greater than 1.2, 1.3, 1.35, 1.4, or 1.45 wt. %. In some embodiments, the aluminum content can be in an approximate range of 1.3 to 1.8 wt. %, 1.35 to 1.75 wt. %, or 1.5 to 165 wt. %.
The zinc can be present in an alloy hereof in an amount at from about 0.2 to 0.45 wt. %, or more typically in an approximate range of 0.25 to 0.45 wt. %, 0.3 to 0.45 wt. %, 0.35 to 0.45 wt. %, or 0.35 to 0.4 wt. %.
The manganese can be present in an alloy hereof in an amount of from about 0.2 to 0.45 wt. %, or more typically in an approximate range of 0.15 wt. % up to 0.4, 0.35, 0.3, or 0.25 wt. %. In some embodiments, the manganese content can be in an approximate range of 0.15 to 0.25 wt. % or 0.15 to 0.2 wt %.
The calcium can be present in an alloy hereof in an amount of from about 0.001 to 0.9 wt. %, or more typically in an approximate range of 0.002 to 0.5, 0.005 to 0.2, 0.005 to 0.1, 0.01 to 0.1, 0.01 to 0.05, or 0.01 to 0.02 wt %. In some embodiments, the manganese content can be about 0.01 wt. %.
The beryllium can be present in an alloy hereof in an amount of from about zero to 0.001 wt. %, or more typically in an approximate range of 0.00001 to 0.001 wt. %, 0.0001 to 0.001 wt. %, 0.0005 to 0.001 wt. %, or 0.0006 to 0.0008 wt. %. In some embodiments, the beryllium content can be about 0.0007 wt. %.
The tin, where used, can be present in an alloy hereof in an amount of from about 0.001 to 0.12 wt. %, or more typically in an approximate range of 0.001 to 0.1 wt. %, 0.002 to 0.08 wt. %, 0.005 to 0.05 wt. %, or 0.008 to 0.02 wt. %. Thus, tin can be present in an amount of from zero to 0.12 wt. % or from zero to 0.1 wt. %. In some embodiments, the tin content can be in an approximate range of 0.01 to 0.12 wt. %, or can be about 0.01 wt. %.
The strontium, where used, can be present in an alloy hereof in an amount of from about 0.001 to 0.11 wt. %, or more typically in an approximate range of 0.001 to 0.1 wt. %, 0.002 to 0.08 wt. %, 0.005 to 0.05 wt. %, or 0.008 to 0.02 wt. %. Thus, strontium can be present in an amount of from zero to 0.11 wt. % or from zero to 0.1 wt. %. In some embodiments, the strontium content can be in an approximate range of 0.01 to 0.11 wt. % or can be about 0.01 wt. %.
In various embodiments, a magnesium alloy hereof can have the following ranges of constituents:


Aluminum	about 6 to about 7.5%, or about 6.0 to about 7.5%
Rare Earth	about 1 to about 3.5%, or about 1.0 to about 3.5%
Zinc	about 0.2 to about 0.45%, or about 0.20 to about 0.45%
Manganese	about 0.15 to about 0.25%
Calcium	about 0.01 to about 0.9%, or about 0.01 to about 0.90%
Beryllium	zero to about 0.001%
Tin	about 0.01 to about 0.12%
Strontium	about 0.01 to about 0.11%

and the remainder magnesium and, optionally, impurities commonly found in magnesium alloys.

In some embodiments, a magnesium alloy hereof can have the following ranges of constituents:


Aluminum	about 6.5 to about 7.5%
Rare Earth	about 1.5 to about 2.5%
Zinc	about 0.35 to about 0.45%
Manganese	about 0.15 to about 0.25%
Calcium	about 0.01 to about 0.1%, or about 0.01 to about 0.10%
Beryllium	about 0.001%
Tin	zero to about 0.01%
Strontium	zero to about 0.01%

and the remainder magnesium and, optionally, impurities commonly found in magnesium alloys.


	Aluminum	about 7 to about 7.2%
	Rare Earth	about 1.5 to about 1.75%
	Zinc	about 0.35 to about 0.4%
	Manganese	about 0.15 to about 0.2%
	Calcium	about 0.01 to about 0.02%
	Beryllium	about 0.0005 to about 0.001%
	Tin	about 0.01%
	Strontium	about 0.01%

	and the remainder magnesium and, optionally, impurities commonly found in magnesium alloys.


	Aluminum	about 7.18%
	Rare Earth	about 1.62%
	Zinc	about 0.355%
	Manganese	about 0.17%
	Calcium	about 0.01%
	Beryllium	about 0.0007%
	Tin	about 0.01%
	Strontium	about 0.01%

	and the remainder magnesium and, optionally, impurities commonly found in magnesium alloys.

Various commercially available magnesium alloys contain a significant content of silica, strontium and/or tin. Although one or more of these can be present in a magnesium alloy hereof, in some embodiments, a magnesium alloy hereof can be silica-free, or strontium-free, or tin-free, or a combination thereof.
A magnesium alloy hereof can, and typically does, contain impurities commonly found in magnesium alloys, in typical, impurity-level amounts; in various embodiments, the concentration(s) thereof can be described, e.g., as an incidental, nominal, or trace amount. Common impurities can include one or more of Cu, Fe, and Ni. In various embodiments, the total content of such impurities in an alloy hereof can be less than 0.5 wt. %, less than or about 0.2 wt. %, or less than or about 0.1 wt. %. Tolerance levels for such impurities can be found, e.g., in B. A. Shaw (rev.), “Corrosion Resistance of Magnesium Alloys,” in ASM Handbook, Vol. 13A, pp. 692-696 (2003) (ASM International) (see, e.g., Table 6 thereof), incorporated by reference. Various embodiments of an alloy hereof can contain impurities at or below such tolerance levels.

Properties

In various embodiments, a magnesium alloy hereof can exhibit an ultimate tensile strength off about or at least 39 ksi, as measured according to ASTM B557, and a percent elongation at break of more than 13% as measured according to ASTM D-1708. In addition, the estimated cost of production of such an alloy hereof is estimated to be about 10% less than that for a comparable, commercially available, castable, magnesium-based alloy (Hydra Magnesium AE44) (data not shown).

Uses

In various embodiments, a magnesium alloy hereof can be die-cast, sand-cast, wrought, and/or machined, in producing an article for an automotive vehicle or other device, or in producing any other object. Die casting is contemplated as a technique that can be employed for many common uses. Any of the conditions and methods known in the art as useful for casting, working, machining, or otherwise treating magnesium alloys can be employed herein to produce articles e.g., components for automotive or aeronautic vehicles, such as drive train components or engine components. Thus, articles, machines and other devices comprising or formed from an alloy according to various embodiments hereof are contemplated. Methods are also contemplated that involve alloying the elements described for alloys hereof to prepare arm alloy therefrom; and methods that involve preparing an article, machine, or other device from an alloy hereof, e.g., by casting, working machining, or otherwise treating an alloy according to various embodiments hereof to have the shape of a desired article, machines or other device.

EXAMPLES

Magnesium-based alloys according to various embodiments of the present invention and comparable comparative alloys (see Table 1), as well as commercially available magnesium-based alloys (see Table 2) and aluminum samples (Aluminum A380) for comparison, are cast from the same mold and subjected to several identical tests.

TABLE 1

Magnesium-Based Alloys Prepared for Testing
Magnesium Alloy Compositions: Elements Alloyed with Magnesium

Component

Series

0	Series 1	Series 2	Series 3	Series 4	Series 5	Series 6

Mn	0.17	0.15	0.17	0.2	0.18	0.16	0.23
Zn	0.405	0.385	0.355	0.345	0.305	0.2	0.2
Al	6.92	7.075	7.18	6.82	6.145	6.3	6.205
Be	0.0008	0.0007	0.0007	0.0008	0.0006	0.0008	0.0006
Ca	0.01	0.01	0.01	0.15	0.12	0.71	0.89
Rare Earth	1.315	1.385	1.62	1.745	1.995	1.27	1.38
Sn	0.01	0.01	0.01	0.01	0.01	0.125	0.12
Sr	0.01	0.01	0.01	0.01	0.01	0.11	0.10

TABLE 2

Commercially Available Magnesium-Based Alloys for Testing
Commercial Alloy Compositions: Elements Alloyed with Magnesium

Component	MRI 153	AE44	AJ62	MRI 230	AXJ530	AS31

Mn	0.26	0.2	0.3	0.3	0.5	0.27
Zn	0.69	0.03	0.03	0.08	0.08	0.09
Al	8	3.6	6.5	6.5	6	3.62
Ca	0.83	NA	NA	2.3	2.5	NA
Rare Earth	0.16	3.69	NA	NA	NA	NA
Sr	0.14	0.01	0.85	0.4	0.4	NA
Si	0.02	0.1	0.05	0.05	0.05	0.94
Sn	0.004	NA	NA		1	NA	NA

Commercially available magnesium alloys MRI 153, AE44, AJ62, MRI 230, AXJ530, and AS31 are well known, commonly available, and often found in automotive drive train components. Information on MRI 153 and MRI 230 can be obtained from the Magnesium Research Institute (Beer-Sheva, Israel). Information on other commercially available alloys can be obtained, e.g., from Norsk Hydra Magnesium GmbH (Livonia, Mich., USA).
Ultimate tensile strength (UTS) tests and yield strength (YS) tests are performed according to ASTM B-557 (results thereof are reported in ksi, wherein 1 ksi=1000 pounds per square inch). Percent elongation at break is also measured (% Elong), according to ASTM D-1708. Results are shown in Table 3 and in FIG. 1.

TABLE 3

Results of Strength Testing

	Alloy	YS	UTS	% Elong

Series

0	21.6	34.4	7.8
	Series 1	21.2	40.85	15.2
	Series 2	21.6	40.3	14.6
	Series 3	22	38.15	11.3
	Series 4	17.2	27.1	5.74
	Series 5	22	34.7	7.3
	Series 6	22.9	35.8	8.7
	Hydro AE44	21	36	10
	Aluminum A380	23	46	3.5

As shown, an embodiment of the present subject matter (Series 2) can exhibit an ultimate tensile strength of about or at least 40 ksi, as measured according to ASTM B-557, and a percent elongation at break of more than 14% as measured according to ASTM D-1708.

Creep resistance tests are performed according to ASTM E-139-83. Results are shown in FIGS. 2-5. Corrosion resistance tests are performed according to ASTM B-3117, using a 240-hour salt spray technique. Good corrosion resistance is seen for samples of Series 0-6 magnesium-based alloys (data not shown).
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the sprit and scope of the invention.

Claims

1. A creep-resistant, castable magnesium alloy comprising, by weight:

from about 6 to about 7.5% aluminum;

from about 1 to about 3.5% rare earth metals;

from about 0.2 to about 0.45% zinc;

from about 0.15 to about 0.25% manganese;

from about 0.01 to about 0.9% calcium;

from zero to about 0.001% beryllium;

from about 0.01 to about 0.12% tin;

from about 0.01 to about 0.11% strontium;

and the balance being magnesium and, optionally, impurities commonly found in magnesium alloys.

2. A creep-resistant, castable magnesium alloy comprising, by weight:

from about 6.5, to about 7.5% aluminum;

from about 1.5 to about 2.5% rare earth metals;

from about 0.35 to about 0.45% zinc;

from about 0.15 to about 0.25% manganese;

from about 0.01 to about 0.1% calcium;

about 0.001% beryllium;

from zero to about 0.01 tin;

from zero to about 0.01% strontium;

3. A creep-resistant, castable magnesium alloy comprising, by weight:

from about 7 to about 7.2% aluminum;

from about 1.5 to about 1.75% rare earth metals;

from about 0.35 to about 0.4% zinc;

from about 0.15 to about 0.2% manganese;

from about 0.01 to about 0.02% calcium;

form about 0.0005 to about 0.001% beryllium;

about 0.01% tin;

4. The allay according to claim 3, wherein the alloy comprises, by weight:

about 7.18% aluminum;

about 1.62% rare earth metals;

about 0.355% zinc;

about 0.17% manganese;

about 0.01% calcium;

about 0.0007% beryllium;

about 0.01% tin;

and the remainder magnesium and, optionally, impurities commonly found in magnesium alloys.

5. The alloy according to claim 1, wherein the alloy comprises from about 6.5 to about 7.5% by weight aluminum.

6. The alloy according to claim 5, wherein the alloy comprises from about 7 to about 7.5% by weight aluminum.

7. The alloy according to claim 1, wherein the alloy comprises from about 0.35 to about 0.45%: by weight zinc.

8. The alloy according to claim 7, wherein the alloy comprises from about 0.35 to about 0.4% by weight zinc.

9. The alloy according to claim 1, wherein the alloy exhibits an ultimate tensile strength of about or at least 39 ksi, as measured according to ASTM B-557.

10. The alloy according to claim 1, wherein the alloy exhibits a percent elongation at break of more than 13% as measured according to ASTM D-1708.