METHOD OF FORMING A CAST ALUMINIUM ALLOY

Abstract

AlSiMg castings to provide enhanced mechanical properties for structural applications comprising (1) alloy optimisation with 8.5 to 12.5 wt. % Si, 0.46 to 1.0 wt. % Mg, 0.1 to 0.2 wt. % Ti, 0.05 to 0.25 wt. % Mn, 0.01 to 0.02 wt. % Sr, 0.004 to 0.1 wt. % B and other impurity elements of Cu, Fe, Zn each less than 0.15 wt. % and the balance of Al; (2) optimised melt treatment with appropriate melting, modification, degassing and grain refining; (3) appropriate type of grain refiner with optimised amount and method to add into the aluminium melt, and (4) optimised heat treatment process. When being utilized to make shape aluminium alloy castings with gravity casting process, the castings have been achieved the 0.2% offset yield strength of greater than 310 MPa, the ultimate tensile strength of greater than 365 MPa and the elongation of greater than 10%.

Claims

1.-11. (canceled)

12. An aluminium alloy comprising: From about 8.5 to about 12.5 wt. % Si; from about 0.46 to about 1.0 wt. % Mg; from about 0.1 to about 0.2 wt. % Ti; from about 0.05 to about 0.25 wt. % Mn; and less than about 0.05 wt. % Sn; the balance being Al and incidental impurities.

13. An aluminium alloy as claimed in claim 12, further comprising: grain refining additions of Ti, TiB2, AlB2, B, Be, Zr, Y, V, Nb, singly or in combination with one another in the range from about 0.001 to about 1.0 wt. %; Na and Sr, singly or in combination, in the range of from about 0.001 to about 0.10 wt. %; and P in the range from about 0.01 to about 0.30 wt. %.

14. A method of forming a cast aluminium alloy, comprising the steps of: (i) providing an aluminium alloy comprising from about 8.5 to about 12.5 wt. % Si, from about 0.46 to about 1.0 wt. % Mg, up to about 0.2 wt. % Ti, from about 0.05 to about 0.25 wt. % Mn, from about 0.002 to about 0.04 wt. % Sr, from about 0.001 to about 0.1 wt. % B and other impurity elements of Cu, Fe, Zn, each at less than about 0.15 wt. % with the balance being Al and incidental impurities; (ii) melting the alloy to yield an alloy melt; (iii) degassing the alloy melt by introducing into the alloy melt a gas comprising at least one of nitrogen, argon, or chlorine, or a mixture thereof to reduce dissolved hydrogen in the melt to a level of less than about 0.7 mL/100 g alloy melt; (iv) cleaning the alloy melt by adding about 25% Na2SiF6 and 75% C.sub.2Cl.sub.6 refining agents in an amount from about 0.01 to about 0.8 wt. %; (v) adding a grain refiner in the form of a TiB-containing master alloy, a B-containing master alloy, or an AlB master alloy with a boron content up to about 0.1 wt. % B, to yield an alloy having a eutectic silicon phase; (vi) refining and modifying the eutectic silicon phase by adding from about 0.002 to about 0.04 wt. % Sr in the form of an AlSr master alloy; (vii) carrying out a solution heat treatment at a temperature from about 520 C. to about 545 C. for a time from about 2 h to about 12 h; and (viii) carrying out an ageing heat treatment at a temperature from about 170 C. to about 200 C. for a time from about 2 h to about 8 h.

15. A method as claimed in claim 14, wherein the alloy of step (i) comprises: from about 8.5 to about 10.0 wt. % Si, from about 0.456 to about 0.65 wt. % Mg, from about 0.1 to about 0.15 wt. % Ti, less than about 0.15 wt. % Mn, from about 0.008 to about 0.02 wt. % Sr, from about 0.004 to about 0.04 wt. % B and other impurity elements of Cu, Fe, Zn, each at less than about 0.15 wt. % and the balance of Al and incidental impurities

16. A method as claimed in claim 14, wherein the dissolved hydrogen in step (iii) is reduced to a level of less than about 0.2 mL/100 g alloy melt.

17. A method as claimed in claim 14, wherein the grain refiner of step (v) includes up to about 3.5 wt. % Al.sub.3Ti.sub.3B or AlTi.sub.3B.

18. A method as claimed in claim 14, wherein the grain refiner of step (v) has a B content from about 0.004 to about 0.04 wt. %.

19. A method as claimed in claim 14, wherein in step (vi) the amount of Sr is from about 0.008 to about 0.02 wt. %.

20. A method as claimed in claim 14, wherein the solution heat treatment of step (vii) is carried out at a temperature from about 535 C. to about 540 C. for a time from about 8 h to about 10 h.

21. A method as claimed in claim 14, wherein the ageing heat treatment of step (viii) is carried out at a temperature of about 170 C. from about 7 h to about 8 h.

22. A method as claimed in claim 14, wherein the ageing heat treatment of step (viii) is carried out at a temperature from about 180 to about 190 C. from about 2 h to about 5 h.

Description

[0044] All the above and other characteristics and advantages of the invention will be further understood through the following illustrative and non-limitative description of embodiments thereof, with reference to the appended examples and drawings, in which:

[0045] FIG. 1 is a graph showing micro hardness of the alloy versus solution time at a solution temperature of 540 C.;

[0046] FIG. 2 is a graph showing micro hardness of the alloy versus ageing time at an ageing temperature of 170 C. after solution at 540 C. for 8 hours; and

[0047] FIG. 3 is a graph showing yield strength of the alloy versus ageing time at an ageing temperature of 170 C. after solution at 540 C. for 6-14 hours.

[0048] In one embodiment, an alloy system in accordance with the principles of the present invention is a modification of the Aluminum Association's alloy system 3XX. This modified alloy system generally comprises of Si in the range of 8.5 to 12.5 wt. % and Mg in the range of 0.3 to 0.7 wt. %, with one or more of Ti less than 0.2 wt. %, Mn less than 0.1 wt. %, Zn less than 0.1 wt. %, Sn less than 0.05 wt. %. In addition, the alloy could further include grain refining additions of Ti, TiB2, AlB2, B, Be, Zr, Y, V, Nb, singly or in combination with one another in the range of 0.001 to 1.0 wt. %, chemical modifiers such as Na and Sr, singly or in combination with one another in the range of 0.001 to about 0.20 wt. % and phase refiners such as P in the range of 0.01 to about 0.30 wt. %, and the balance of Al and incidental impurities.

[0049] In another embodiment, an alloy system in accordance with the principles of the present invention is a modification of the Aluminum Association's alloy system 3XX. This modified alloy system preferably comprises of 8.5 to 10.0 wt. % Si, 0.46 to 0.65 wt. % Mg, 0.1 to 0.15 wt. % Ti, less than 0.15 wt. % Mn, Sn less than 0.05 wt. %, and Zn less than 0.1 wt. %. In addition, the alloy further include grain refining additions of Ti, TiB2, AlB2, B, Be, Zr, Y, V, Nb, singly or in combination with one another in the range of 0.001 to 0.5 wt. %, most preferably grain refining additions of 0.1 to 0.5 wt. % Al3Ti3B master alloy comprising TiB2 and AlB2, chemical modifiers such as Na and Sr, singly or in combination with one another in the range of 0.001 to about 0.10 wt. % and phase refiners such as P in the range of 0.01 to about 0.20 wt. %, and the balance of Al and incidental impurities. In the present invention, the silicon can be used to improve the performance of the alloy casting, improve mobility and reduce hot cracking tendency, reduce shrinkage, improve air tightness. Magnesium's role is to improve its strength and toughness; cast, in addition to a small amount of magnesium dissolved in the -Al substrate body, mainly exists in the larger size of the Mg2Si phase, therefore, cast magnesium alloy on the mechanical properties of the obvious. The role of magnesium in the alloy is achieved by heat treatment; solution treatment, magnesium dissolved a matrix of precipitated Mg2Si during aging, the alloy strengthening.

[0050] Still in another embodiment, the castings will be cast using the conventional method of pouring the molten alloy mixture into a permanent, sand or investment type mold or alternatively cast using advanced techniques such as high pressure die casting or squeeze casting to produce a near net shape cast parts. Prior to casting, it is essential to have a proper degassing and grain refining. The casting AlSi alloys of the present invention may be resistance furnace smelting, alloying elements above the middle of its way and aluminium alloy added to the molten aluminium; with 25% Na.sub.2SiF.sub.6+75% C.sub.2Cl.sub.6 refining agents and with the rotary degassing unit with the use of the best refining effect in amount of 0.5-0.8% (mass percentage). TiB-containing refiner on AlSi alloy with good refinement effect, the best technology for the 720 C. adding 0.2-0.3 wt. % of refiner, insulation 8 min-15 min; Al-10Sr alloy has good metamorphism, added at 0.01-0.02 wt. % Sr, was added at the temperature 740 C.; heat treatment specification: 540 C. solid solution for 8-10 hours, 170 C. aging 7-8 hours; alloy having high strength and toughness, yield strength in exceed of 300 MPa and elongation in exceed of 9%.

[0051] Still in another embodiment, the casting is subjected to an appropriate heat treatment in accordance with the practice that involves the steps of solution heat treatment at temperatures approaching the solidus temperature of a given alloy; quenching into water or other appropriate media, and ageing at temperatures ranging from ambient to about 300 C.

[0052] Alternatively, a multiple stages solution process and multiple stages ageing process can be utilized. For example, in a two-step process, it includes primary ageing at a low temperature (e.g., less than about 190 C., preferably less than 160 C.) for an short period of time (e.g., longer than 1 hours but less than 10 hours, preferably about 2 hours) followed by secondary ageing at a high temperature (e.g., greater than about 100 C., preferably about 170 C.) for an extended period of time (e.g., longer than 2 hours but less than 48 hours, preferably about 8 hours).

[0053] Additional processing steps such as hot isostatic pressing, machining, surface modification and shot peening can be applied to further improve the casting alloys disclosed in the present invention. By utilizing the alloys of the present invention to form near net shape cast parts, significantly improved cast alloy properties can be achieved. For example, alloys, which embody the present invention, have been shown to have yield strengths (0.2% offset) in excess of 300 MPa and elongation in excess of 10%.

[0054] The present invention will be further described with reference to Examples:

Example 1: Gravity Casting

[0055] Four alloys of the compositions listed in Table 1 were cast into a permanent mold. The alloys also include an A356 and an A357 type cast aluminum alloys. The castings were made by weighting different elements with an appropriate ratio and melting them in a 12 kg clay-graphite crucible in an electric resistance furnace. When the melt was fully homogenised, it was subjected to degassing, during which Ar was blown into the melt by a commercial rotatory degasser adjusted at 350 rpm for 4 min. It should be mentioned that Al-10Sr alloy was added at 0.01-0.02 wt. % Sr before degassing. TiB-containing refiner was added at 720 C. with 0.005 wt. % of B and before pouring. Thereafter, the melt was poured into the boron nitride painted steel mould, designed based on ASTM B108 standard, to produce dog-bone shape tensile specimens. In a gravity casting using permanent mold, molten metal was poured into the steel mold which was already heated up to 400-460 C. Chemical composition analysis was carried out using the Foundry-Master Pro which is a high-performing optical emission spectrometer (OES).

[0056] Each of the four castings was solution heat treated at 540 C. for 8 hours, immediately quenched into ambient temperature water upon removal from the furnace and allowed to stabilize for several days. Ageing was optimized for each alloy by taking Vickers hardness measurements in accordance with the American Society for Testing and Materials (ASTM) standard E92-82 at selected time intervals for a wide range of temperatures. The optimized ageing process is ageing at 170 C. for 8 hours or ageing at 190 C. for 4 hours. The mechanical properties were further measured in accordance with ASTM B557 standard using an Instron 5500 Universal Electromechanical Testing Systems equipped with Bluehill software and a 50 kN load cell. All the tensile tests were performed at ambient temperature (25 C.). The gauge length of the extensometer was 50 mm and the ramp rate for extension was 1 mm/min. The mechanical properties of the four castings after solution and ageing treatment are listed in Table 2.

TABLE-US-00001 TABLE 1 Chemical composition (wt. %) of the alloys in EXAMPLE1 Alloy Si Mg Cu Fe Mn V Ti Sr B Al A356 6.99 0.35 0.00 0.11 0.06 0.016 0.14 0.015 0.002 Balance A357 7.01 0.50 0.00 0.11 0.06 0.017 0.14 0.015 0.002 Balance GC01 9.21 0.50 0.00 0.11 0.06 0.017 0.14 0.015 0.005 Balance GC02 9.72 0.50 0.00 0.11 0.07 0.018 0.14 0.015 0.005 Balance

[0057] As shown in Table 2, the developed alloys labelled as GC01 and GC02 display higher strengths and elongations over the commercially available A356 and A357 alloys. This is especially surprising given that the A357 alloy is by far the highest strength alloy in the AlSiMg cast alloy system. Moreover, the Mg content is higher in the A357 alloy, in which Mg content is 0.5-0.7 wt. %. Since published yield strength values (source: Metals Handbook Desk Edition. American Society for metals, H. E. Boyer and T. L. Gall, eds., 1985, pp. 6.48-6.62) for 357-T6 (0.55% Mg, yield strength 295 MPa) are about 18% greater than those obtained with alloy 356-T6 (same composition as the 357 with 0.35% Mg, yield strength 250 MPa). Clearly, the developed compositions are very potent in overcoming this large property disparity that is observed with slightly different Mg levels. If the Mg content were adjusted to the 0.60 weight percent level or above, it is likely that the strength of the developed alloys would be even greater. More importantly, the developed alloys show good ductility with an elongation higher than 10%. The good ductility of the developed alloys could be attributed to the increase of castability and the decrease of porosity level over the existing commercially available A356 and A357 alloys.

[0058] In a variation on the above-noted single step ageing treatment, a two-step ageing treatment consisting of an initial step of 150 C. for 2 hours followed by ageing at 180 C. for 6 hours was applied to the alloys. As ageing time progresses, the alloys attain yield levels that exceed that of aged at a single stage ageing. It is evident that a two-step ageing treatment could further widen the gap between the new alloys and the existing commercial alloy castings. The mechanical properties of the developed alloys labelled as GC01 and GC02 under solution and two-step ageing treatment are also listed in Table 2.

TABLE-US-00002 TABLE 2 Mechanical properties of the permanent mold casting alloys in EXAMPLE1 after heat treatment. Yield strength UTS Elongation Alloy Heat treatment (MPa) (MPa) (%) A356(Al7Si0.35Mg) 540 C./8 h + 170 C./8 h 250 310 8.5 A357(Al7Si0.5Mg) 540 C./8 h + 170 C./8 h 285 340 5.0 GC01(Al9.2Si0.5Mg) 540 C./8 h + 170 C./8 h 312 365 11.2 GC02(Al9.7Si0.5Mg) 540 C./8 h + 170 C./8 h 316 370 10.2 GC01(Al9.2Si0.5Mg) 540 C./8 h + 150 C./2 h + 80 C./6 h 317 370 10.8 GC02(Al9.7Si0.5Mg) 540 C./8 h + 150 C./2 h + 80 C./6 h 321 374 10.1

Example 2: Sand Casting

[0059] Four alloys of the compositions listed in Table 3 were cast into a sand mold. The alloys also include an A356 and an A357 type cast aluminum alloys. The castings were made by weighting different elements with an appropriate ratio and melting them in a 12 kg clay-graphite crucible in an electric resistance furnace. When the melt was fully homogenised, it was subjected to degassing, during which Ar was blown into the melt by a commercial rotatory degasser adjusted at 350 rpm for 4 min. It should be mentioned that Al-10Sr alloy was added at 0.01-0.02 wt. % Sr before degassing. TiB-containing refiner was added at 720 C. with 0.005 wt. % of B and before pouring. Thereafter, the melt was poured into the British standard sand mould, to produce dog-bone shape tensile specimens. In a gravity casting using sand mold, molten metal was poured into the sand mold which was at room temperature. Chemical composition analysis was carried out using the Foundry-Master Pro which is a high-performing optical emission spectrometer (OES).

[0060] Each of the four castings was solution heat treated at 540 C. for 8 hours, immediately quenched into ambient temperature water upon removal from the furnace and allowed to stabilize for several days. Ageing was optimized for each alloy by taking Vickers hardness measurements in accordance with the American Society for Testing and Materials (ASTM) standard E92-82 at selected time intervals for a wide range of temperatures. The optimized ageing process is at 170 C. for 8 hours or at 190 C. for 4 hours. The mechanical properties were further measured in accordance with ASTM B557 standard using an Instron 5500 Universal Electromechanical Testing Systems equipped with Bluehill software and a 50 kN load cell. All the tensile tests were performed at ambient temperature (25 C.). The gauge length of the extensometer was 50 mm and the ramp rate for extension was 1 mm/min. The mechanical properties of the four castings after solution and ageing treatment are listed in Table 4.

TABLE-US-00003 TABLE 3 Chemical composition of the alloys in EXAMPLE2. Alloy Si Mg Cu Fe Mn V Ti Sr B Al A356 7.08 0.35 0.00 0.11 0.06 0.016 0.14 0.015 0.002 Balance A357 7.05 0.50 0.00 0.11 0.06 0.016 0.14 0.015 0.002 Balance SC01 9.14 0.50 0.00 0.11 0.06 0.016 0.14 0.015 0.005 Balance SC02 9.76 0.50 0.00 0.11 0.07 0.016 0.14 0.015 0.005 Balance

[0061] As shown in Table 4, the commercially available A356 alloy displays a yield strength of 230 MPa and an UTS of 280 MPa with 0.35 wt. % Mg, and the commercially available A357 alloy shows a yield strength of 275 MPa and an UTS of 300 MPa with 0.5 wt. % Mg, while the developed alloys labelled SC01 and SC02 with 0.5 wt. % Mg display a remarkable increase of strength over the commercially available A356 and A357 alloys, with a yield strength above 295 MPa and an UTS above 325 MPa. This is especially surprising given that the A357 alloy is by far the highest strength alloy in the AlSiMg cast alloy system. More importantly, the Mg content is higher in the A357 alloy, in which Mg content is 0.5-0.7 wt. %, and the A357 alloy achieves higher strength over the A356 alloy at higher Mg content with significant decrease of elongation to be below 3%, while the developed alloys achieve higher strength over the A356 alloy without obvious decrease of elongation. Clearly, the developed compositions are very potent in overcoming this large property disparity that is observed with slightly different Mg levels. If the Mg content were adjusted to the 0.60 weight percent level or above, it is likely that the strength of the developed alloys would be even greater.

[0062] In a variation on the above-noted single step ageing treatment, a two-step ageing treatment consisting of an initial step of 150 C. for 2 hours followed by ageing at 180 C. for 6 hours was applied to the alloys. As ageing time progresses, the alloys attain yield levels that exceed that of aged at a single stage ageing. It is evident that a two-step ageing treatment could further widen the gap between the new alloys and the existing commercial alloy castings. The mechanical properties of the developed alloys labelled as SC01 and SC02 under solution and two-step ageing treatment are also listed in Table 4.

TABLE-US-00004 TABLE 4 Mechanical properties of the sand mold casting alloys in EXAMPLE2 after heat treatment. Yield strength UTS Elongation Alloy Heat treatment (MPa) (MPa) (%) A356(Al7Si0.35Mg) 540 C./8 h + 170 C./8 h 230 280 4.5 A357(Al7Si0.5Mg) 540 C./8 h + 170 C./8 h 275 300 3.0 SC01(Al9.1Si0.5Mg) 540 C./8 h + 170 C./8 h 300 325 4.5 SC02(Al9.8 Si0.5Mg) 540 C./8 h + 170 C./8 h 305 330 4.0 SC01(Al9.1Si0.5Mg) 540 C./8 h + 150 C./2 h + 80 C./6 h 305 331 4.5 SC01(Al9.8Si0.5Mg) 540 C./8 h + 150 C./2 h + 80 C./6 h 310 335 4.0

[0063] All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

[0064] The disclosures in UK patent application number 1713005.5, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.