IMPROVED ALUMINIUM BASED CASTING ALLOY

Abstract

An aluminium based alloy consisting essentially of (wt %): silicon: 4.0 to 8.5% magnesium: 0.07 to 0.3% titanium: 0.06 to 0.2% manganese: <0.05% iron: <0.15% chromium: <0.01% nickel: <0.01% copper: 2.5 to 4% zinc: 0 to 0.5% strontium: >0.001 to 0.03% beryllium: less than 0.0005% tin: less than 0.01% other elements (each): less than 0.05% each other elements total: less than 0.15% in total, and a balance of aluminium and other unavoidable impurities.

Claims

1. An aluminium based alloy consisting essentially of (wt %): TABLE-US-00012 silicon : 4.0 to 8.5% magnesium : 0.07 to 0.3% titanium : 0.06 to 0.2% manganese : <0.05% iron : <0.15% chromium : <0.01% nickel : <0.01% copper : 2.5 to 4% zinc : 0 to 0.5% strontium : >0.001 to 0.03% beryllium : less than 0.0005% tin : less than 0.01% other elements (each) : less than 0.05% each other elements total : less than 0.15% in total, and a balance of aluminium and other unavoidable impurities.

2. The aluminium based alloy of claim 1, wherein the composition is free of beryllium, rare earth elements, and free of chromium, and other transition metal elements not including Ti, Mn, Fe, Ni, Cu, Sr and Zn.

3. The aluminium based alloy of claim 1, wherein the silicon level is from 4.0 to 5.5 wt %, or about 5 wt %.

4. The aluminium based alloy of claim 1, wherein the silicon level is from 4.5 wt % to 7.5 wt %, or from 5.0 to 7.5 wt %.

5. The aluminium based alloy of claim 1, wherein copper is present at from 3 to 3.75 wt %.

6. The aluminium based alloy of claim 1, wherein the combined iron and manganese content is less than 0.1 wt %.

7. The aluminium based alloy of claim 1, wherein zinc is present at from 0.1 to 0.3 wt %.

8. The aluminium based alloy of claim 1, wherein magnesium is present at from 0.15 to 0.25 wt %.

9. The aluminium based alloy of claim 1, wherein strontium is present at from 0.01 to 0.025 wt %, or from 0.01 to 0.015 wt %.

10. The aluminium based alloy of claim 1, wherein strontium is present at from 0.001 to 0.008 wt %, or from 0.001 to 0.005 wt %.

11. The aluminium based alloy of claim 1, comprising a casting alloy for casting processes having a casting pressure of less than 10 bar.

12. The aluminium based alloy of claim 1, comprising a sand casting alloy or an investment casting alloy.

13. The aluminium based alloy of claim 1, comprising an ingot of alloy.

14. A method of fabricating an aluminium-based alloy product, the method comprising: providing an aluminium alloy melt from the aluminium-based alloy according to claim 1; and casting said aluminium alloy melt into a mould using a using a casting process having a casting pressure of less than 10 bar.

15. A method according to claim 14, wherein the casting process comprises one of sand casting or investment casting.

16. A cast product or component produced using the aluminium based alloy of claim 1.

17. A cast product or component according to claim 16, produced using a casting process having a casting pressure of less than 10 bar.

18. A cast product or component according to claim 16, comprising one of a sand cast product, or an investment cast product or a structural aerospace casting.

19. (canceled)

20. An aluminium alloy based casting comprising at least one aluminium based alloy according to claim 1.

21. The product, component or casting of claim 15, wherein the microstructure of the alloy includes dendrites with a dendrite arm spacing is less than 50 m, preferably between 10 m and 45 m.

22. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein:

[0041] FIG. 1 shows a thermal analysis scan of AlSiCu alloys in accordance with the present invention.

[0042] FIG. 2 shows the microstructure generated from an investment casting made from Alloy 1.

[0043] FIG. 3 shows the microstructure generated from an investment casting made from Alloy 2.

[0044] FIG. 4 shows the microstructure generated from an investment casting made from Alloy 3.

[0045] FIG. 5 shows the microstructure generated from an investment casting made from Alloy 4.

[0046] FIG. 6 shows the microstructure generated from an investment casting made from Alloy 5.

[0047] FIG. 7 shows a hardness time curve comparing age hardening after solution treatment at 490 C. and water quenching, comparing the hardening response of Alloy 4 and Alloy 5 when age hardened at 150 C.

DETAILED DESCRIPTION

[0048] The present invention provides an aluminium-silicon-copper based casting alloy which can be cast to produce a casting having high strength combined with good levels of tensile ductility.

[0049] Castings of the alloy may be produced by sand or investment castings, and other lower pressure casting processes and techniques. However, it should be appreciated that the alloys of the invention are not suitable for high pressure diecasting because it does not meet the requirement to avoid die soldering of (10Sr)+Fe+Mn>1, discussed above.

[0050] The castings may be produced by high integrity premium casting processes to achieve minimum levels of porosity and finer microstructures. The castings may be used together with chills or artificial cooling for critical locations to achieve fine microstructures. A wide variety of heat treatment procedures may be utilised, such as T4, T5, T6, T7, T8 or T9 tempers for example, depending on the desired result.

Examples

[0051] A series of experiments were undertaken to test the relative merit of five aluminium alloy compositions formulated in accordance with embodiments of the present invention, to establish the formability and properties of the alloys. Table 5 provides the composition of each of these experimental alloy compositions:

TABLE-US-00006 TABLE 5 Experimental Alloy Compositions (wt %) Experimental Alloys Alloy 1 Alloy 2 Alloy 3 Alloy 4 Alloy 5 Elements (wt %) (wt %) (wt %) (wt %) (wt %) Silicon 8.14 7.88 7.99 4.77 4.60 Iron 0.07 0.08 0.08 0.06 0.06 Copper 3.51 3.24 3.42 3.01 3.48 Manganese 0.002 0.002 0.002 0.001 0.001 Magnesium 0.25 0.24 0.25 0.11 0.29 Nickel 0.008 0.008 0.008 0.007 0.007 Zinc 0.30 0.29 0.25 0.19 0.19 Titanium 0.11 0.08 0.11 0.12 0.13 Strontium <0.001 0.02 0.03 0.02 0.02 Others, total <0.15 <0.15 <0.15 <0.15 <0.15 Aluminium Bal. Bal. Bal. Bal. Bal.

[0052] The alloy compositions were tested using thermal analysis scans to determine the cooling characteristics. Casting was conducted using investment casting and sand casting, prior to heat treatment and age hardening to T4 or T6 tempers. The microstructure and mechanic properties (tensile testing) were analysed.

[0053] FIG. 1 shows a thermal analysis scans showing cooling curves of AlSiCu alloys in accordance with the present invention. These were conducted using a standard thermal analysis method wherein a molded sand cast cup is used and the change in temperature is recorded as the metal cools. The sand molded cup for tracking the cooling curve is known as a Quik-Cup(without Te) commercially available from Heraeus Electronite, which is connected directly to a Picolog TC-08 datalogger. A small sample of molten aluminium is taken from the prepared melt and poured into the sand cup, which contains a thermocouple and enables an accurate logging of the change in temperature of the metal under standard conditions.

[0054] The resulting cooling curves are shown in FIG. 1. In this figure, the top curve corresponds to Alloy 1 from Table 5 whereas the bottom two curves correspond to Alloy 2 and 3 from Table 5, thereby highlighting differences in low iron alloys with or without Sr. The curves are characterized by three main features. (1) The onset of solidification occurs at around 600 C.; (2) there is a thermal arrest at around 555 C., and (3) solidification finishes at close to 500 C. The thermal arrest common in AlSiCu alloys containing iron, and forming -Al.sub.5FeSi at close to 575 C. is absent. The cooling curves are characterised by a relatively long duration where liquid phase is present in the alloy which is advantageous for interdendritic feeding. Where Sr is added, the onset of eutectic solidification occurs at a lower temperature and finishes at a higher temperature.

[0055] Experimental cast samples of Alloys 1 to 5 from Table 5 were produced using an investment casting process. Molds were production investment casting shells made by a typical silica system with two prime coats (Primecoat PLUS) and zircon stucco, followed by transition coats then backup and silica stucco coats. The total shell building time was four days including all drying cycles. After shell preparation the moulds were dewaxed by autoclaving before being fired, then preheated, if necessary, prior to pouring molten metal into them and allowing them to cool. For the determination of tensile properties, 4-bar trees with an external downsprue had a total of 16 cast to shape test bars added to each prior to shell building. The cast to shape test bars comply with the dimensional requirements of ASTM B557, for a 0.25 gage diameter. The molten alloys were prepared from various primary and secondary feedstocks, such as ingot, returns, master alloys and alloying elements added to the metal. The composition was verified with a Spectromaxx Spectrometer. The metal temperature prior to casting was typically 710 C.

[0056] FIG. 2 shows the as cast microstructure of Alloy 1, produced as an investment casting. After casting, the test pieces were removed from the cast tree and heat treated to a T6 temper. For this process, the alloy was solution treated for 22 h at 490 C. prior to water quenching and ageing 24 h at 150 C. FIG. 2 shows that the silicon structure is reasonably coarse despite the very long times of solution treatment. The dendrite arm spacing was measured to be 37 m. More typically, the silicon phase undergoes a process of fragmentation and Ostwald ripening but due to the lower temperature of solution treatment, this appears to have been mostly ineffective. This is also in contrast to the kind of behaviour observed for AlSiMg alloys, which undergo more thorough fragmentation when Sr is not present, but where solution treatment temperature is higher.

[0057] FIG. 3 shows the microstructure of Alloy 2 and FIG. 4 shows the microstructure of Alloy 3. For the purposes of comparison, these are shown together as they are a close representation of a repeated test. Alloy 2 displays a DAS of 16.3 m and Alloy 3 displays a DAS of 14.4 m and the microstructures are effectively identical. In difference to Alloy 1, the silicon phase is well distributed and fragmented in both alloys.

[0058] FIG. 5 shows the microstructure of Alloy 4 and FIG. 6 shows the microstructure of Alloy 5, both treated to a T6 temper in the same means as discussed above for Alloy 1. Here, the principle difference was in the Mg content of the two alloys, but they also were prepared using less than 5 wt % silicon. Each displayed a dendrite arm spacing of 23.9 m or 26.2 m respectively. Because of their lower silicon content, these alloys have a moderately different solidification behaviour to the alloys 1 to 3.

[0059] FIG. 7 shows a hardness-time curve describing the age hardening behaviour of alloys 4 and 5. The hardness-time curve was obtained using a Vickers hardness tester with a 10 kg load. Most importantly, the combination of high copper content and higher magnesium content work together in alloy 5 to produce improved strengthening.

[0060] Table 7 shows the tensile properties of the five alloys as investment castings in the as-cast condition for purposes of comparison. These tensile properties were obtained using the specimens cast to shape and tested in accordance with ASTM B557.

TABLE-US-00007 TABLE 7 Average as cast tensile properties of the five alloys tested. 0.2% Proof Alloy Stress Tensile Strength Elongation 1-As Cast 161 MPa 240 MPa 2% 2-As Cast 184 MPa 263 MPa 3% 3-As Cast 174 MPa 235 MPa 2% 4-As Cast 132 MPa 237 MPa 6% 5-As Cast 133 MPa 222 MPa 4%

[0061] Table 8 shows the average tensile properties for investment castings manufactured from each of the five alloys tested, heat treated to a T6 temper. The material was solution treated at 490 C. for 22 h, water quenched, then aged 24 h at 150 C. These tensile properties were obtained using the specimens cast to shape and tested in accordance with ASTM B557.

[0062] Table 8 also shows for Alloy 4, results for the investment cast alloy displaying a dendrite arm spacing of 23.9 m (T6 #1) or 38.4 m (T6 #2). The larger dendrite arm spacing shows a moderate reduction in tensile mechanical properties, however the level of strength achieved is still excellent. These tensile properties were obtained using the specimens cast to shape and tested in accordance with ASTM B557.

TABLE-US-00008 TABLE 8 Average T6 tensile properties of the five alloys tested 0.2% Proof Alloy Stress Tensile Strength Elongation 1-T6 405 MPa 476 MPa 2% 2-T6 414 MPa 483 MPa 3% 3-T6 390 MPa 476 MPa 5% 4-T6#1 310 MPa 413 MPa 11% 4-T6#2 306 MPa 397 MPa 8% 5-T6 395 MPa 452 MPa 5%

[0063] Table 9 shows the average tensile properties for Alloys 1 to 5, in the T4 temper. The material was solution treated at 490 C. for 22 h, water quenched, then aged a minimum of 14 days at 22 C. These tensile properties were obtained using the specimens cast to shape and tested in accordance with ASTM B557.

[0064] Similar to Table 8, results for the investment cast alloy displaying a dendrite arm spacing of 23.9 m (T4 #1) or 38.4 m (T4 #2) are presented. There is little difference in the result exhibiting the reduced dependence of the alloy on dendrite arm spacing. As may be readily observed from Table 8 and 9, the alloys of the invention (Alloy 2 to 5) develop their properties not through a singular addition or subtraction of only one element, rather they develop their mechanical properties by a combination of advantageous effects. In addition to the chemical composition of the alloys, they also require a moderate cooling rate and suitable grain refinement to develop the dendrite arm spacing, cell structure and microstructures shown in FIGS. 3 to 6.

TABLE-US-00009 TABLE 9 Average T4 tensile properties of alloys 1 to 5 0.2% Proof Alloy Stress Tensile Strength Elongation 1-T4 241 MPa 328 MPa 2% 2-T4 287 MPa 423 MPa 8% 3-T4 285 MPa 427 MPa 9% 4-T4#1 222 MPa 365 MPa 14% 4-T4#2 224 MPa 357 MPa 12% 5-T4 253 MPa 391 MPa 11%

[0065] Table 10 shows the composition of a test aerospace casting of a part where the test bars were integrally cast with the part.

TABLE-US-00010 TABLE 10 composition of alloy test bars and test casting. Elements Alloy 6 Silicon 7.7 Iron 0.09 Copper 3.07 Manganese 0.002 Magnesium 0.18 Nickel 0.008 Zinc 0.11 Titanium 0.08 Strontium 0.014 Others, total <0.15 Aluminium Bal.

[0066] Table 11 shows the mechanical properties of the test casting of Table 10 and the results of tensile testing a sand casting produced from the same alloy. These tensile properties were obtained using the specimens cast to shape and tested in accordance with ASTM B557 for the investment casting or AS1391 for the sand casting and were generated by third party testing at (Bureau Veritas Asset Integrity and Reliability Services Australia Pty. Ltd., Regency Park, South Australia).

[0067] The investment casting was produced with a shell temperature of 700 C. and a metal pour temperature of 720 C. The results of Table 11 were generated from alloy heat treated to a T6 temper, with a solution treatment of 24 h at 490 C., water quenched, and aged 24 h at 150 C. Each material meets the design requirements that would normally be considered suitable for alloy 201-T7, Class 2, but without the cost penalty associated with the use of silver in a cast alloy.

TABLE-US-00011 TABLE 11 Tensile properties from the Al-Si-Cu-Mg alloy shown in Table 10 for an investment casting produced with a shell temperature of 700 C. and a metal pour temperature of 720 C. 0.2% Proof Tensile Alloy Stress Strength Elongation 6-T6 (Investment Cast 352 MPa 416 MPa 7% integral testbars) 6-T6 Sand Casting 370 MPa 431 MPa 5%

[0068] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

[0069] Where the terms comprise, comprises, comprised or comprising are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.