HIGH STRENGTH, COMBUSTION-RESISTANT, TUBE-EXTRUDABLE AIRCRAFT-GRADE MAGNESIUM ALLOY

Abstract

Embodiments of the invention include magnesium-based alloys especially adapted for extrudable aerospace grade applications. Alloys of the invention provide excellent combinations of mechanical properties, good extrudability in hollow forms, and resistance to combustion.

Claims

1. A magnesium-based extrusion alloy composition comprising, by weight: 7.0%-11.0% Al, 0.1%-0.8% Zn, 0.15%-0.65% Mn, 0.6%-1.5% Ca, 0.05%-0.6% Y and a balance of Mg and unavoidable impurities.

2. The alloy according to claim 0 wherein: a content of said Mn is between about 0.15 wt % to 0.3 wt % of said alloy.

3. The alloy according to claim 0 wherein: a content of said Zn is between about 0.1 wt % to 0.35 wt % of said alloy.

4. The alloy according to claim 0 wherein: a content of said Zn is between about 0.4 wt % to 0.6 wt % of said alloy.

5. The alloy according to claim 0 wherein: a content of said Al is between about 8.3 wt % to 10 wt % of said alloy.

6. The alloy according to claim 0 wherein: a content of said Ca and Y is between about 0.75 wt % to 1.5 wt % of said alloy.

7. The alloy according to claim 0 wherein: a total combined content of said Al, Ca, and Y does not exceed 11 wt % of said alloy.

8. An alloy according to claim 1 wherein: said Ca and said Y are provided in intermetallic compounds.

9. The alloy, according to claim 8, wherein said intermetallic compounds of Ca and Y comprise: MgAlCa compounds and AlMnY compounds, respectively.

10. The alloy, according to claim 9, wherein: said MgAlCa intermetallic compound comprises: up of up to 57 wt % Al and up to 43 wt % Ca.

11. The alloy, according to claim 9, wherein: said AlMnY intermetallic compound comprises 40 wt % Al, 40 wt % Mn and 20 wt % Y.

12. The alloy according to claim 9 wherein: said Ca and Y intermetallic compounds contribute to flammability resistance of wrought products made from said alloy.

13. The alloy according to claim 0 wherein: said alloy comprises Ca in the form of intermetallic particles; said particles having an average diameter of about less than 1 m and finely distributed within said alloy.

14. The alloy, as claimed in claim 13, wherein: said intermetallic particles are formed in a wrought process, including extrusion, rolling, or forging.

15. The alloy, as claimed in claim 14, wherein: when said alloy is provided in a matrix phase, particles making up said alloy have an average diameter of about 10 m or less.

16. The alloy according to claim 1 wherein: said alloy comprises Ca intermetallic particles and said particles make up between about 1.0% to 5.0% of said alloy by volume.

17. The alloy, according to claim 1, wherein: said alloy has a tensile yield strength of at least 180 MPa and an ultimate tensile strength of at least 270 MPa.

18. The alloy, according to claim 1, wherein: said forged or drawn alloy has a tensile yield strength of at least 170 MPa, an ultimate tensile strength of at least 280 MPa and an elongation of at least 7% in tube forms

19. A method of making a product made from a magnesium-based alloy composition comprising the steps of: providing magnesium-based alloy composition comprising, by weight: 7.0%-11.0% Al, 0.1%-0.8% Zn, 0.15%-0.65% Mn, 0.6%-1.5% Ca, 0.05%-0.6% Y and a balance of Mg and unavoidable impurities; subjecting said alloy to extrusion wherein billets of said alloy 1 are hydraulically or mechanically forced through an orifice in a die to produce an extruded shape; or subjecting said alloy to rolling wherein billets of said alloy 1 are successively passed through rollers to produce a rolled sheet, plate, or simple shape; or subjecting said alloy to forging wherein billets of said alloy 1 are slowly compressed or quickly impacted with hammers or dies to produce a forged alloy.

20. The method, according to claim 19, wherein: said extrusion step comprises extruding said alloy into seamless tubes via extrusion of a hollow billet around a mandrel, or into structural tubes via extrusion of solid billets using porthole dies which split metal flow and subsequently merge the metal around a mandrel to form a hollow shape.

21. The method, according to claim 19, wherein: said extruded alloy has a tensile yield strength of at least 180 MPa and an ultimate tensile strength of at least 270 MPa.

22. The method according to claim 19, wherein: said extruded alloy has a tensile yield strength of at least 170 MPa, an ultimate tensile strength of at least 280 MPa and an elongation of at least 7% in tube forms.

23. A magnesium-based extrusion alloy composition consisting essentially of, by weight: 7.0%-11.0% Al, 0.1%-0.8% Zn, 0.15%-0.65% Mn, 0.6%-1.5% Ca, 0.05%-0.6% Y and a balance of Mg and unavoidable impurities.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 is an electron micrograph of an alloy of the invention showing an 1280 m by 960 m area of the alloy in a cast condition;

[0032] FIG. 2 is an electron micrograph of another alloy of the invention showing a 256 m by 192 m area of the alloy in an extruded condition; and

[0033] FIG. 3 shows photographs of extruded forms of the alloys.

DETAILED DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 shows an electron micrograph example of the alloy 10 of present invention in an as-cast state. FIG. 1 more specifically shows prominent features of the microstructure including a magnesium matrix phase 12 which makes up most of the microstructure and which contains dissolved aluminum and an aluminum and calcium rich phase 14 that is present in interdendritic spaces left by the magnesium matrix phase. The grain boundaries of the magnesium matrix are not well defined but have sizes well over 100 m. A 100 m scale is provided in the figure for size comparison. The aluminum and calcium rich phase is contiguous along some of these boundaries and exists in unbroken linear segments up to 100 m long. The microstructure also contains blocky AlMnY particles 16.

[0035] FIG. 2 shows another electron micrograph example of the alloy 10 of the present invention in an as-extruded state. The microstructure consists mainly of relatively fine Mg grains 18 having an average diameter of 8.9 m. A 500 m scale is provided in the figure for size comparison. The microstructure also contains stringers of Ca-rich particles 20, which have been broken up and distributed throughout the microstructure by the extrusion process. The Ca-rich particles 20 make up approximately 3.9% (by area) of the total microstructure and have an average diameter of 0.48 m. The thorough distribution of these Ca-rich particles helps pin grain boundaries of the matrix phase during and after extrusion, keeping the material's grain size small and contributing to the material's high strength. The small size of the Ca-rich particles makes them less likely to be a failure initiation site, meaning that there should be little detriment to the material's ductility. The microstructure also contains coarse or blocky AlMnY particles 22 that have not been broken up by the extrusion process and which do not serve to refine the microstructure.

[0036] FIG. 3 shows a photograph of examples of extrusions of the present invention. The present invention is extrudable in both tube form 24 and bar form 26. Other high strength magnesium alloys with aluminum as the principal alloying element are not typically extrudable in tube forms.

[0037] As mentioned, according to one preferred embodiment, the invention includes a magnesium alloy with alloying content, in weight percent, between 7.0% and 11.0% aluminum, between 0.1% and 0.8% zinc, between 0.15% and 0.65% manganese, between 0.6% and 1.5% calcium, and between 0.05% and 0.6% yttrium.

[0038] According to another feature of the invention, it includes an alloying addition of Mn between about 0.15% and 0.65% by weight. Furthermore, it is preferable that the alloy contain between 0.15% and 0.3% Mn by weight. Manganese is added to MgAl alloys to increase corrosion resistance by reducing the Fe impurity levels in the melt during primary processing; dissolved Fe content is very detrimental to Mg alloys' corrosion properties. In the present invention, this preferred range results in low Fe impurity levels and also low amounts of AlMn precipitates in the metal, contributing to the invention's superior workability.

[0039] According to yet another feature of the invention, it may include an alloying addition of Zn between about 0.1% and 0.8% by weight. In one embodiment, it is preferable that the alloy contain about 0.4% to 0.6% Zn. Zinc is added to the alloy for solid solution strengthening. In this preferred Zn content range, there is some effect of strengthening, but the Zn content is low enough to allow extrusion at moderate speeds and temperatures. In another embodiment of the present invention, it is instead preferable for the Zn content to be between 0.1% and 0.35% of the alloy by weight. This amount of Zn will allow some solid solution strengthening but will allow a slightly higher extrusion temperature, allowing the invention to be extruded at higher speeds.

[0040] According to yet another feature of the invention, it may include a Ca addition of between about 0.6 wt. % and 1.5 wt. % and a Y addition of between 0.05 wt. % and 0.6 wt. %. This combined Ca and Y addition imparts flammability resistance to the invention. The most obvious benefit is imparting flammability resistance in the final product, but equally important is the effect of flammability resistance during processing. For the present invention, billets of raw material can be safely preheated to higher temperatures than conventional alloys and can subsequently be extruded at higher temperatures. Higher working temperatures increase the workability of the present invention and allow it to be extruded into tubes and complicated shapes; similar tubes and shapes would be impossible to extrude in conventional high strength MgAl alloys.

[0041] According to yet another aspect of the invention, it may include a Ca addition resulting in the formation of MgAlCa intermetallic compounds which are broken up and finely dispersed by a wrought process. These particles contribute to grain refinement during extrusion and help mitigate grain growth immediately after extrusion or during subsequent heat treatment. It is preferred that the invention contain approximately 1.0% to 5.0% (by volume) of such particles and that such particles have an average diameter of less than 1m. Furthermore, it is preferred that the Mg matrix grains of the invention have an average diameter of less than about 10 m resulting from the finely distributed intermetallic compound content of the invention.

[0042] According to another aspect of the invention, it is preferred that components of the present invention are produced via extrusion. Any other wrought process which subjects a material to significant shear deformation, and which is capable of breaking up the aforementioned Ca-containing intermetallic particles of the present invention, may also be used, including forging or rolling. The extrusion process accomplishes this by using a hydraulic ram to force a billet through an orifice in the desired shape. The rolling process accomplishes this by using a set of flat rollers to impose successive thickness reduction of a plate or sheet or by using a set of shaped rollers to do the same for simple shapes. The forging process accomplishes this by slowly compressing or rapidly impacting a material one or more times with a hammer, die, or set of progressive dies.

[0043] According to yet another aspect of the invention, it may include a total amount of Ca and Y alloying addition be between about 0.75% and 1.5% of the alloy by weight. This amount of Ca and Y alloying addition has been found to give the present invention a favorable volume fraction of Ca-containing and Y-containing precipitates. This is sufficient for grain refinement purposes but will not contribute too much to brittleness. Further, it is preferred that the alloying content of Y be at the low end of the above prescribed 0.05 wt %-0.5 wt. % range, in order to minimize costs associated with rare earth alloying additions.

[0044] As set forth, a preferred embodiment of the invention provides an Al alloying addition of about 7.0 wt. % to 11.0 wt. %. This alloying addition may be more preferably an Al content of between about 8.3% and 10% of the alloy by weight. Such an amount of Al content has been found to give the present invention good mechanical properties from solid solution strengthening, while still remaining low enough in total alloying content so that the alloy can be easily extruded. Such an amount of Al content also gives the present invention a Mg.sub.17Al.sub.12 solvus very similar to conventional high strength MgAl alloys, meaning that the present invention has a potential for age hardening in a similar manner as conventional high strength MgAl alloys.

[0045] According to yet another aspect of the invention, it may include a total amount of Al, Ca, and Y alloying additions to not exceed about 11 wt % of the total alloy. This maximum ensures that the material will not become brittle and will maintain workability at safe working temperatures.

[0046] Several examples of the present invention demonstrate improved properties over existing commercial alloys. The material examples were produced by casting ingots and subsequent extrusion. Casting consisted of a fluxed process in a steel crucible, using high purity magnesium, aluminum, and zinc metals, manganese chloride, and magnesium-calcium and magnesium-yttrium master alloys as input materials. Molten alloys were gravity cast into permanent steel molds. The chemistry of each example alloy was verified via optical emission spectroscopy (OES). Alloy examples are listed in Table 1. Comparative Example 1 and Comparative Example 2 are commercial alloys AZ61 and AZ80 respectively. For these materials, the compositions listed in Table 1 are the nominal compositions for those alloys instead (not verified by OES), and they were produced using a larger scale direct chill casting process.

TABLE-US-00001 TABLE 1 Alloy Chemistries Composition, wt. % Alloy Al Ca Mn Y Zn Example 1 7.45 1.34 0.15 0.13 0.52 Example 2 8.34 1.41 0.17 0.14 0.62 Example 3 8.56 0.79 0.22 0.07 0.63 Example 4 9.42 0.71 0.23 0.09 0.65 Example 5 9.98 0.66 0.18 0.08 0.64 Example 6 8.69 0.82 0.15 0.41 0.6 Example 7 9.2 0.8 0.16 0.36 0.62 Example 8 9.97 0.83 0.24 0.56 0.6 Example 9 9.23 1.01 0.19 0.11 0.65 Example 10 9.54 0.63 0.15 0.42 0.65 Comparative 6.5 0 0.33 0 0.95 Example 1 Comparative 8.5 0 0.33 0 0.50 Example 2

[0047] Billets were scalped to two final diameters and sectioned to length for extrusion. Billets were preheated in a furnace for up to two hours prior to extrusion. Extrusion was carried out on a 500-ton extrusion press with three die geometries, which correlated to two extrusion profiles. Combinations of billet and die configurations are listed in Table 2. Often, a flat die for a round rod with an extrusion ratio of 25 is used to benchmark extrusion speeds. More complicated shapes and higher reduction ratios are more difficult to extrude, especially for porthole dies and hollow shapes, where higher pressure is required to overcome friction with greater surface area in the die. The choice of dies provided in this example is closer to an industrial setting, and it allows for demonstration of the present invention's superior extrudability in both bar and tube form.

TABLE-US-00002 TABLE 2 Extrusion geometries. Reduction ratio Billet Extrudate (extrusion diameter Die type geometry ratio) Alloys used 3.18 Porthole tube 30 mm outer 29.28 Examples 3 - die. diameter, 2 mm 10 wall thickness. 4.16 Porthole tube 30 mm outer 49.86 Examples 1 - die. diameter, 2 mm 2, Comp. wall thickness. Example 1 4.16 Flat die. 1.5 by 0.25 cross 36.25 All alloys section rectangular bar.

[0048] During extrusion, temperatures of the billet, die, and container varied. Temperatures were continually adjusted during experimentation in order to maximize extrusion speed for all example alloys. All temperatures were between 275 C. and 480 C. (527 F.-896 F.), but process temperatures were generally between 370 C. and 480 C. (698 F.-896 F.). Using these conditions and the die geometries in Table 2, the extrusion speeds and mechanical properties of extrudates are listed for bar form in Table 3 and are listed for tube form in Table 4.

TABLE-US-00003 TABLE 3 Extrusion and mechanical properties of example alloys in bar form. Maximum defect- Tensile Ultimate Elongation free extrusion Yield Tensile after speed (feet per Strength Strength fracture Alloy minute) (MPa) (MPa) (%) Example 1 11.1 188 298 12.72 Example 2 8.2 194 274 5.58 Example 3 8.2 190 301 12.05 Example 4 8.2 203 307 10.29 Example 5 5.7 217 290 4.16 Example 6 7.3 187 290 7.88 Example 7 6.9 197 276 5.81 Example 8 6.9 208 306 9.46 Example 9 5.4 201 300 8.66 Example 10 5.1 203 299 8.47 Comparative 6 165 275 9 Example 1 Comparative 7 195 295 8 Example 2

TABLE-US-00004 TABLE 4 Extrusion and mechanical properties of example alloys in tube form. Maximum defect- Tensile Ultimate Elongation free extrusion Yield Tensile after speed (feet per Strength Strength fracture Alloy minute) (MPa) (MPa) (%) Example 1 7 199 280 7.79 Example 2 5.6 194 319 9.94 Example 3 6.1 187 306 9.16 Example 4 5.8 188 308 7.44 Example 5 5.1 170 280 6.06 Example 6 6.1 171 295 9.21 Example 7 5.6 173 298 8.56 Example 8 5.3 192 295 6.15 Example 9 5.6 191 302 8.34 Example 10 5.6 183 296 7.36 Comparative 6 110 250 7 Example 1 Comparative N/A N/A N/A N/A Example 2

[0049] For all Examples of the present invention, strengths in Tables 3 and 4 were measured using the testing procedures described in ASTM B557-15. Tensile properties were measured in the extrusion direction of the material using samples that had 2 gage lengths, 0.50 widths, and nominal thicknesses of the material (listed in Table 2). Tensile Yield Strength was determined via the offset method detailed in ASTM B557-15 section 7.1.6, and it generally refers to the tensile stress that can be imposed on the material before it will permanently deform. Ultimate Tensile Strength is calculated as the maximum force the specimen will withstand, divided by its initial cross-sectional area. Elongation after fracture was determined per the method listed in ASTM B557-15 section 7.8.1, and it generally refers to the percent increase in the specimen's gage length after tension testing. For the commercial alloys AZ61 and AZ80 (Comparative Example 1 and Comparative Example 2, respectively), reference mechanical properties are taken from the relevant alloy specifications in ASTM B107.

[0050] Values for mechanical properties listed in Tables 3 and 4 are averages of test sets of up to 12 samples, with any sample discarded which had measurements at least 3 scaled median absolute deviations (MAD) lower than the median (about 2 standard deviations lower than the median), in order to prevent skewing the results due to a defect in the sample. Commercial alloy AZ80 (Comparative Example 2) is widely known to not be extrudable in tube or other hollow forms; as such, properties are not given for Comparative Example 2 in Table 4.

[0051] Tables 3 and 4 demonstrate that the present invention has excellent as-fabricated mechanical properties relative to commercial alloys. All Examples well exceed the yield strength of AZ61 (Comparative Example 1) in both tube and bar form. All Examples are also on par with AZ80 (Comparative Example 2) yield strength in bar form, with most Examples exceeding it. With the exception of Example 2, all Examples exceed the ultimate tensile strength (UTS) of AZ61 in bar form, with most Examples on par with or exceeding the UTS of AZ80. All Examples well exceed the UTS of AZ61 in tube form. In bar form, most Examples have elongation on par with or exceeding AZ80, with some examples exceeding the elongation of AZ61. In tube form, most Examples exceed the elongation of AZ61.

[0052] Tables 3 and 4 also demonstrate that the present alloys have excellent workability relative to commercial alloys. Most Examples have extrusion speeds on par with that of AZ61 (Comparative Example 1) or AZ80 (Comparative Example 2) in bar form. Some Examples significantly exceed these reference speeds, with Example 1 having an extrusion speed nearly double that of AZ61 in bar form. Unlike AZ80, all Examples are extrudable in tube form. In tube form, most Examples are on par with the extrusion speed of AZ61, with some Examples exceeding AZ61 speeds. In addition to simply being extrudable in tube form, it is especially significant that the present invention is able to maintain mechanical properties on par with AZ80, while being extrudable in tube form.

[0053] Selected alloys were tested for flammability by a third party, pursuant to the FAA Aircraft Materials Fire Test Handbook, Chapter 25 (Oil Burner Flammability Test for Magnesium Alloy Seat Structure). Test results are summarized in Table 5.

TABLE-US-00005 TABLE 5 Flammability Test Results Alloy Test Result Example 1 Pass - no burning Example 2 Not tested Example 3 Pass - no burning Example 4 Pass - no burning Example 5 Not tested Example 6 Not tested Example 7 Not tested Example 8 Pass - no burning Example 9 Pass - no burning Example 10 Pass - no burning

[0054] Comparative Example alloys 1 and 2 were not tested in this manner because they are known to be flammable and are known not to be self-extinguishing. The Example materials which were tested all pass the flammability test requirements and demonstrate clear superiority over conventional alloys, especially for applications which require flammability resistance. Referring to Table 1, the alloys which were tested reflect alloys with a combination of the highest and lowest Ca and Y content, which gives good confidence that all combinations of the present invention have significant flammability resistance.

[0055] The alloys described herein are robust and may be produced via many production methods. The preferred production method for Mg alloys according to the present work is direct chill (DC) casting followed by extrusion, but it is not intended that the present invention be limited to a specific process. Rather the process may be designed for several other casting and wrought processes not mentioned in detail. Such casting methods include but are not limited to sand casting, gravity die casting, tilt casting, low pressure die casting, high pressure die casting, strip casting, continuous casting, squeeze casting, centrifugal casting, thixomolding, and rheocasting. Such wrought processing methods include but are not limited to extrusion, rolling, and forging.