Improved castable magnesium alloys

Abstract

The present invention relates to a magnesium alloy comprising: (a) 1.32-1.8 wt% Gd, (b) 2-3.6 wt% Nd, (c) 0.55-0.7 wt% Zr, (d) 0.20-0.40 wt% Zn, and (d) at least 85 wt% Mg, with a ratio (wt%) of Gd:Nd between 0.40 and 0.63. The present invention also relates to an aircraft component comprising the magnesium alloy, and to a method for producing the magnesium alloy.

Description

[Technical field]

[0001] The present invention relates to high strength castable magnesium alloys, as well as methods for making such alloys and articles containing such alloys. [Background technology]

[0002] (Background technology)

[0003] Magnesium alloys containing rare earth metals and zirconium (Mg-RE-Zr alloys) are regularly used in aerospace and other specialty applications. In such applications, strength-to-weight ratio is a key factor in material selection. Generally, the higher the strength-to-weight ratio (i.e., the greater the strength of two materials of the same density), the more desirable it is. Examples of applications where strength-to-weight ratio is important include drive train components (gearboxes, housing covers, and similar items), engine components (cases, covers, moving parts components, etc.), and structural components (panels, body parts, etc.) in both conventional and electric aircraft. Similar material requirements apply to spacecraft and satellite applications where the total weight of the spacecraft / satellite determines the cost of launching into space.

[0004] Even within the family of Mg-RE-Zr alloys, properties can vary widely and multiple subfamilies exist. By selecting certain rare earth elements and adding small amounts of others, certain aspects of the alloy's performance can be enhanced. There are many examples of alloys that belong to broader groups. The general benefits of the various rare earth metals are covered in textbooks (e.g., "Light Alloys", 4th Edition, Ian Polmear, pg. 265, Figure 5.17). It is generally accepted that rare earth metals improve strength up to the limit of their solid solubility in the alloy, and that further increases in content result in a decrease in properties.

[0005] Examples of such alloys are disclosed in the applicant's earlier U.S. Patent No. 7,935,304, which describes an alloy containing 2-4.5% by weight neodymium, 0.2-7.0% by weight of at least one rare earth metal having atomic numbers between 62 and 71, up to 1.3% by weight zinc, 0.2-0.7% by weight zirconium, and optionally one or more other minor components.

[0006] Alloys falling within this definition are defined in AMS 4429B as having the following compositions: [Table 1]

[0007] There is a need for improved castable Mg-RE-Zr alloys, particularly alloys having improved strength properties. Summary of the Invention

[0008] (Summary of the invention)

[0009] The present invention relates to (a) 1.32-1.8 wt% Gd; (b) 2-3.6 wt% Nd; (c) 0.55-0.7 wt% Zr; (d) 0.20 to 0.40 wt% Zn; (d) at least 85 wt. % Mg; Including, The magnesium alloy has a Gd:Nd ratio (wt%) of 0.40 to 0.63.

[0010] Surprisingly, it has been found by the inventors that magnesium alloys having the above composition and a Gd:Nd (wt%) ratio of 0.40-0.63 have improved strength, especially improved ultimate tensile strength (UTS). In some embodiments, the alloy can be a cast magnesium alloy, more specifically a sand cast alloy. In particular, the cast magnesium alloy can have a thickness of 5-350 mm. In some embodiments, the cast magnesium alloy can have a thickness of 25-350 mm, more specifically 100-350 mm, even more specifically 200-350 mm. In other embodiments, the cast magnesium alloy can have a thickness of 5-100 mm, more specifically 10-100 mm. In the context of the present invention, the term "thickness" is used to mean the smallest dimension of the cast magnesium alloy. Surprisingly, it has been found by the inventors that the alloy of the present invention has improved retention of its properties in thicker castings. In some embodiments, the alloy can be a wrought magnesium alloy. The wrought alloy can be a wrought alloy or an extruded alloy.

[0011] In particular, the magnesium alloy may have an ultimate tensile strength (UTS) of at least 310 MPa measured according to ASTM B557M-15 when sand cast into a 200 mm x 200 mm x 25.4 mm plate and T6 aged. More particularly, the UTS may be at least 315 MPa, even more particularly at least 316 MPa, and even more particularly at least 320 MPa. In particular, the UTS may be 350 MPa or less.

[0012] In the context of the present invention, the term "ageing" is used to refer to a process in which a magnesium alloy is heated to a temperature above room temperature, held at that temperature for a period of time, and then returned to room temperature (i.e., about 25°C). In particular, the ageing may be a T6 ageing treatment. Such processes are known in the art and generally involve heating the magnesium alloy to a temperature of 515°C-524°C, holding at that temperature for a period of time (for solution treatment), optionally quenching (i.e. allowing it to cool to a temperature lower than the temperature to which it was heated, e.g., room temperature (about 25°C)), and reheating the magnesium alloy to a temperature of 200°C-204°C and holding at that temperature for a period of time (for precipitation heat treatment).

[0013] In the context of the present invention, the term "alloy" is used to mean a composition made by mixing and fusing two or more metallic elements by melting them together, mixing, and resolidifying. Thus, in the context of the alloys of the present invention, all the elements mentioned are in metallic form (e.g., not present as salts).

[0014] In particular, the ratio of Gd:Nd (wt%) may be 0.40 to 0.61. More specifically, the ratio of Gd:Nd (wt%) may be 0.40 to 0.60. Even more specifically, the ratio of Gd:Nd (wt%) may be 0.40 to 0.57.

[0015] In particular, the magnesium alloy may include 1.35-1.8 wt% Gd, more specifically 1.40-1.75 wt% Gd, and even more specifically 1.40-1.70 wt% Gd.

[0016] More specifically, the magnesium alloy may include 2.2-3.4 wt% Nd, and even more specifically 2.6-3.1 wt% Nd.

[0017] In particular, the magnesium alloy may include 0.6-0.7 wt% Zr, and more particularly 0.60-0.70 wt% Zr.

[0018] More specifically, the magnesium alloy may include at least 90 wt% Mg, and even more specifically at least 92 wt% Mg, in some embodiments, the remainder of the alloy may be magnesium and incidental impurities.

[0019] In particular, the magnesium alloy may optionally include (i) up to 0.4 wt% rare earth metals other than Gd and Nd, (ii) up to 0.05 wt% Ag, (iii) up to 0.01 wt% Cu, (iv) up to 0.010 wt% Fe, (v) up to 0.0020 wt% Ni, (vi) up to 0.01 wt% other elements, the balance being magnesium. The rare earth metals other than Gd and Nd may include Ce, La, and Pr.

[0020] The term "rare earth metals" is used in connection with the present invention to refer to the 15 lanthanide elements, as well as Sc and Y.

[0021] In particular, the magnesium alloy, when sand cast into a 200 mm x 200 mm x 25.4 mm plate and subjected to a T6 aging treatment, may have an average planar grain size of at least 10 μm, more specifically 10 to 100 μm, and even more specifically 20 to 80 μm. The average planar grain size was measured on a representative section of the magnesium alloy using the Hein linear intercept method as specified in ASTM E112-13. In this context, the term "representative section" is used to mean a section selected to represent the average condition within the alloy. Typically, such sections should not be taken from areas that have been subjected to shearing, sintering, or other processes that alter the grain structure.

[0022] It is known in the art that grain size affects the performance of a material. The Hall-Petch relationship indicates that UTS can be increased by reducing the grain size. There are other benefits to fine grain size. However, grain sizes below 10 μm (known as ultra-fine) are generally difficult to achieve in large industrially used castings. Such large castings typically have complex shapes and average wall thicknesses of more than 6 mm. To achieve ultra-fine grain size in such materials, processes such as forming / machining the material or using high solidification rate processes (atomization, high pressure die casting, etc.) must be employed. However, these processes are not suitable for many industrial / aerospace components (i.e., the primary application of the alloys of the present invention) where sand casting is the preferred manufacturing method due to practical considerations. Therefore, for the alloys of the present invention, it is desirable to achieve the required properties (e.g., improved UTS) in materials with grain sizes larger than 20 μm and typically less than 100 μm. The grain size of many commercial magnesium alloys, e.g. Mg-Al alloys, is strongly dependent on the solidification rate, with increasing cooling rate resulting in smaller grain size (see e.g. LA Dobrzan'ski, M. Kro'l, T. Tan'ski, Effect of cooling rate and aluminum contents on the Mg-Al-Zn alloys' structure and mechanical properties, Journal of Achievements in Materials and Manufacturing Engineering 43 / 2(2010)613-633). It has been found by the inventors (e.g. UTS) that the alloys of the present invention can achieve improved (i.e. smaller) average grain size and mechanical properties when sand cast (i.e. slow cooling rate process) with e.g. 25.4 mm wall thickness.

[0023] More specifically, the magnesium alloy may include two or more crystalline phases, i.e., the magnesium alloy may be heterogeneous. In some embodiments, the alloy may include a first crystalline phase and a second crystalline phase. The first crystalline phase may differ from the second crystalline phase with respect to its crystalline structure and / or its composition. In particular, the magnesium alloy may include at least 97%, more specifically at least 98%, in particular at least 98.5%, and more particularly at least 99% of the first crystalline phase when sand cast into a 200 mm x 200 mm x 25.4 mm plate and subjected to a T6 aging treatment. In this context, the percentages represent the area of ​​a two-dimensional scanning electron microscope (SEM) microstructural image of the magnesium alloy.

[0024] Surprisingly, the inventors have discovered that the alloys of the present invention have a relatively low Zn content as specified herein, which increases the first phase content (and decreases the second phase content). This improves the UTS of the alloy. This is contrary to the general knowledge in the art that the higher the Zn content, the higher the UTS. An example of this is found in International Standard ASTM B60-15, which describes ZK41A, ZK51A, and ZK61A sand casting alloys. These are Mg-Zn-Zr alloys that are nominally identical except for the Zn levels of about 4.3, about 4.6, and about 6%, respectively (the midpoint of the specification range). The minimum UTS specifications for these alloys are 200, 234, and 276 MPa, respectively, which shows how increasing the Zn content increases the UTS values.

[0025] The present invention also relates to an aircraft or spacecraft component (e.g., a conventional aircraft or electric aircraft component) comprising the above-mentioned magnesium alloy. More specifically, the aircraft part may be an electric aircraft part. The term "aircraft" includes airplanes and helicopters. The term "spacecraft" also includes satellites. The inclusion of Gd in the alloy may provide a neutron absorption effect, which may be beneficial in environments with increased levels of neutrons (e.g., space). In particular, the aircraft component may be a drive train component, an engine component, or a structural component. More specifically, the drive train component may be a gear box, a housing, or a cover. In particular, the engine component may be a case, a cover, or a moving component. More specifically, the structural component may be a panel or a body part.

[0026] The present invention also provides (a) heating Mg, Gd, Nd, Zr and Zn to form a molten magnesium alloy containing 1.32-1.8 wt% Gd, 2-3.6 wt% Nd, 0.55-0.7 wt% Zr, 0.20-0.40 wt% Zn and at least 85 wt% Mg, with a Gd:Nd ratio (wt%) of 0.40-0.63; (b) mixing the resulting molten magnesium alloy; (c) casting the magnesium alloy; The present invention also relates to a method for producing a magnesium alloy comprising the steps of:

[0027] In particular, the method may be for producing a magnesium alloy as defined above. Any other necessary components in the resulting alloy (e.g. those listed in the previous paragraph describing the alloy) may be added in the heating step (a). More particularly, the heating step may be carried out at a temperature equal to or higher than 650°C (i.e. the melting point of pure magnesium) and even more particularly below 1090°C (the boiling point of pure magnesium). In particular, the temperature range may be between 650°C and 850°C, more particularly between 700°C and 800°C, even more particularly between 750°C and 780°C. More particularly, in step (b) the resulting alloy may be completely melted and / or dissolved.

[0028] More specifically, in step (a), the resulting alloy may be fully melted. In particular, prior to melting in step (a), the alloy components may be present in elemental form or as one or more alloys.

[0029] In particular, in step (c), casting may include pouring the molten magnesium alloy into a mold, which may be a die mold, a permanent mold, a sand mold, an investment mold, a direct chill casting (DC) mold, or other mold.

[0030] After step (c), the method may include one or more additional steps of (d) extruding, (e) forging, (f) rolling, (g) machining.

[0031] The invention is further described with reference to the following drawings, which are not intended to limit the scope of the invention as claimed. [Brief description of the drawings]

[0032] [Figure 1] 1 is a graph of ultimate tensile strength (UTS) versus Gd:Nd ratio (wt %) for the alloys of Examples 1-9. [Diagram 2] 1 is a graph of ultimate tensile strength (UTS) versus Zn content (wt%) for the alloys of Examples 10 to 16. [Diagram 3] FIG. 1 shows the microstructure of the alloy of Example 5 measured by scanning electron microscope (SEM). [Figure 4] FIG. 4 is the image of FIG. 3 after processing to identify the first and second phases of the alloy. [Diagram 5] 1 is a graph of ultimate tensile strength (UTS) versus area percent of second phase for alloys of Examples 1-16. [Figure 6] 1 is a graph of second phase area % versus Gd:Nd ratio (wt %) for the alloys of Examples 1-16. EXAMPLES

[0033] (Example)

[0034] Magnesium alloy compositions were prepared by combining the ingredients in the amounts listed in Table 1 below. These compositions were then melted by heating to 750°C-780°C. The melt was then poured into a 200mmx200mmx25.4mm mold and subjected to a T6 aging treatment. The ultimate tensile strength (UTS) of the alloys was then tested according to ASTM B557M-15.

[0035] [Table 2]

[0036] Figure 1 is a graph of UTS versus Gd:Nd ratio for the samples in Table 1. It clearly shows that the samples of the present invention, i.e., samples having the claimed Gd:Nd ratios (wt%), have improved UTS.

[0037] FIG. 3 is an SEM image of the optical microstructure of the magnesium alloy of Example 5 (i.e., cast into a 200 mm×200 mm×25.4 mm mold and subjected to a T6 aging treatment). It shows that the alloy has a nearly equiaxed grain structure. The alloy is almost entirely single phase with only a few small regions of secondary phases present. An SEM image of the same alloy is shown in FIG. 4. Small amounts of the secondary phases appear as white regions in FIG. 4.

[0038] Additional magnesium alloy compositions were prepared by combining the ingredients in the amounts listed in Table 2 below. These compositions were prepared to demonstrate the effect of Zn content on the magnesium alloys of the present invention. The compositions were melted by heating to 750°C to 780°C. The melt was then poured into a 200mm x 200mm x 25.4mm mold and subjected to a T6 aging treatment. The ultimate tensile strength (UTS) of the alloys was then tested according to ASTM B557M-15.

[0039] [Table 3]

[0040] Figure 2 is a graph of UTS versus Zn content for Examples 10-16 in Table 2. The data shows that magnesium alloys having Zn contents outside the claimed ranges exhibit reduced strength properties.

[0041] The percentage of the second phase was measured by generating optical two-dimensional SEM images of each alloy (i.e., cast into 200 mm x 200 mm x 25.4 mm molds and subjected to a T6 heat treatment). The two phases have different atomic compositions and appear as contrast in the backscattered electron SEM images. By using image processing software, the initial SEM images can be converted to black and white images. The first phase appears as black and the areas of the second phase appear as white. The software is used to measure the area of ​​the second phase (white) as a percentage of the total area. This is the type of image shown in Figure 4. At least two images were taken for each alloy and the average percentage was calculated. These results are shown in Table 3 below.

[0042] [Table 4]

[0043] The percentage second phase values ​​in Table 3 are shown versus the UTS value for each alloy in Figure 5. The graph in Figure 5 shows the trend of decreasing UTS as the amount of second phase increases.

[0044] A further graph is shown in Figure 6. This graph plots the Gd:Nd ratio against the area percentage of the second phase for each example. For ease of reference, the data points are plotted as circles (Zn values ​​less than 0.5, i.e., within the range of the alloys of the present invention) and squares (Zn values ​​greater than 0.5, i.e., beyond the range of the alloys of the present invention). For the circles (alloys with Zn content within the range of the alloys of the present invention), the percentage of second phase is higher and thus the UTS is lower for alloys with Gd:Nd ratios above the values ​​required for the alloys of the present invention. The square data points (alloys with Zn content outside the range of the alloys of the present invention) show a higher percentage of second phase and therefore a lower UTS for alloys with Gd:Nd ratios within the range required for the alloys of the present invention but with Zn content above the range required for the alloys of the present invention.

Claims

1. (a) 1.32 to 1.8 wt% of Gd, and (b) 2 to 3.6 wt% of Nd, and (c) 0.55 to 0.7 wt% of Zr, and (d) 0.20 to 0.40 wt% of Zn, and (e) at least 85 wt% of Mg, comprising, wherein the ratio of Gd:Nd (wt%) is 0.40 to 0.63, a magnesium alloy.

2. The magnesium alloy according to claim 1, which is sand-cast into a plate of 200 mm × 200 mm × 25.4 mm and subjected to T6 aging treatment, and has a tensile strength of at least 310 MPa measured according to ASTM B557M-15.

3. The magnesium alloy according to claim 2, which is sand-cast into a plate of 200 mm × 200 mm × 25.4 mm and subjected to T6 aging treatment, and has a tensile strength of at least 315 MPa measured according to ASTM B557M-15.

4. The magnesium alloy according to claim 1, wherein the ratio of Gd:Nd (wt%) is 0.40 to 0.

61.

5. The magnesium alloy according to claim 4, wherein the ratio of Gd:Nd (wt%) is 0.40 to 0.

57.

6. The magnesium alloy according to claim 1, wherein the alloy contains 1.35 to 1.8 wt% of Gd.

7. The magnesium alloy according to claim 5, wherein the alloy contains 1.40 to 1.70 wt% of Gd.

8. The magnesium alloy according to claim 1, wherein the alloy contains 2.2 to 3.4 wt% of Nd.

9. The magnesium alloy according to claim 8, wherein the alloy contains 2.6 to 3.1 wt% of Nd.

10. The magnesium alloy according to claim 1, wherein the alloy contains 0.6 to 0.7 wt% of Zr.

11. The magnesium alloy according to claim 1, wherein the alloy contains at least 90 wt% of Mg.

12. The magnesium alloy according to claim 11, wherein the alloy contains at least 92 wt% of Mg.

13. The magnesium alloy according to claim 1, wherein the alloy optionally contains (i) rare earth metals other than Gd and Nd up to 0.4 wt%, (ii) Ag up to 0.05 wt%, (iii) Cu up to 0.01 wt%, (iv) Fe up to 0.010 wt%, (v) Ni up to 0.0020 wt%, (vi) other elements up to 0.01 wt%, and the balance is magnesium.

14. The magnesium alloy according to claim 1, wherein the alloy is sand-cast into a plate of 200 mm × 200 mm × 25.4 mm and has an average planar particle size of 10 to 100 μm measured in accordance with ASTM E112-13 when subjected to T6 aging treatment.

15. The magnesium alloy according to claim 14, wherein the alloy is sand-cast into a plate of 200 mm × 200 mm × 25.4 mm and has an average planar particle size of 20 to 80 μm measured in accordance with ASTM E112-13 when subjected to T6 aging treatment.

16. The magnesium alloy according to claim 1, wherein the alloy includes a first crystal phase and a second crystal phase.

17. The magnesium alloy according to claim 16, wherein the alloy is sand-cast into a plate of 200 mm × 200 mm × 25.4 mm and has at least 98% of the first crystal phase when subjected to T6 aging treatment.

18. An aircraft component comprising the magnesium alloy according to any one of claims 1 to 17.

19. The aircraft component according to claim 18, wherein the aircraft component is a drive train component, an engine component, or a structural component.

20. The aircraft component according to claim 18, wherein the aircraft component is an electric aircraft component.

21. The aircraft component according to claim 19, wherein the aircraft component is an electric aircraft component.

22. (a) heating Mg, Gd, Nd, Zr, and Zn to form a molten magnesium alloy containing 1.32 to 1.8 wt% of Gd, 2 to 3.6 wt% of Nd, 0.55 to 0.7 wt% of Zr, 0.20 to 0.40 wt% of Zn, and at least 85 wt% of Mg, and having a Gd:Nd ratio (wt%) of 0.40 to 0.63; (b) mixing the obtained molten magnesium alloy; (c) casting the magnesium alloy; A method for manufacturing the magnesium alloy according to any one of claims 1 to 17, comprising the steps of.

Images