Aluminium casting alloy displaying improved thermal conductivity

Abstract

The present invention provides an aluminium based alloy consisting essentially of a weight percentage composition of: silicon: 1.5 to 2.5 % magnesium: 0.1 to 0.6 % titanium: 0.06 to 0.4 % manganese: <0.6 % iron: <0.6 % chromium: <0.01 % nickel: <0.01 % vanadium: <0.02 % copper: <0.05 % zinc: <0.05 % strontium: < 0.03 % beryllium : < 0.0005 % tin: < 0.01 % boron: < 0.10 % other elements (each) : less than 0.10 % each other elements: less than 0.20 % in total, and a balance of aluminium and other unavoidable impurities.

Description

ALUMINIUM CASTING ALLOY DISPLAYING IMPROVED THERMAL CONDUCTIVITYPRIORITY CROSS-REFERENCE

[0001] The present patent application claims priority from Australian provisional patent application No. 2021902651 filed on 23 August 2021 , the contents of which should be understood to be incorporated into this specification by this reference.TECHNICAL FIELD

[0002] The present invention relates to an aluminium based alloy for the manufacture of cast parts. The invention is particularly applicable to sand or investment castings and it will be convenient to hereinafter disclose the invention in relation to that exemplary application. However, it is to be appreciated that the invention is not limited to that application and could be used in a number of casting processes including die casting processes such as high pressure die casting.BACKGROUND OF THE INVENTION

[0003] The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

[0004] Many aluminium castings are based around the Al-Si-X alloying system, with a range of alloying elements present. Other, less common alloy systems include those based around the Al-Cu-X system. Both are known as age hardenable alloy castings. In particular, the Al-Si-X alloys are low cost to produce and have good castability especially when using methods such as high pressure die casting, sand casting, low pressure casting, and investment casting.Thermal Conductivity of Metals and Alloys

[0005] The thermal conductivity of a material is equivalent to the quantity of heat, AQ transmitted during time At through a thickness x, in a direction normal to asurface with area A, per unit area of A, due to a temperature difference A T, under steady state conditions and when the heat transfer is dependent only on the temperature gradient. The thermal conductivity (W / m.K) is thus reliant on the thermal diffusivity, and is related to it directly via the relationship:K=aCp / ? where K is thermal conductivity, a is thermal diffusivity (m2 / s), Cp is specific heat, (J / kg.K), and p is density in g / cm3. These parameters may be determined readily by using methods as outlined in ASTM E1461 .

[0006] In metals, the total thermal conductivity is the sum of electronic thermal conductivity (Ke) and phonon (lattice) thermal conductivity (KP), such that:K = Ke + Kp

[0007] In pure metals phonon thermal conductivity is relatively minor, but is significant in alloys and compounds. Metals contain charge carriers, specifically electrons, which contribute most significantly to the electronic thermal conductivity, Ke. The inverse of conductivity, resistivity, in metals results directly from impediments to the mobility of electrons, and occurs as a result of electron scattering. Three principal electron scattering processes affect the electrical and thermal conductivities in metals. These are: (1 ) lattice defects such as solute atoms present in the metallic lattice; (2) electrons deflected via phonons (lattice vibrations); and (3) electrons interacting with each other. If several distinct scattering mechanisms are present, then the overall resistivity is the sum of each individual scattering mechanism.

[0008] In general, with a rise in temperature, both the number of carrier electrons and contribution of lattice vibrations increase. Thus, thermal conductivity of a metal is expected to increase. However, because of greater lattice vibrations, electron mobility decreases, and the combined effect of these factors leads to varying effects in different metals. Thermal conductivity of pure Al for example, changes only slightly over the technologically important ranges from below ambient up to 200 °C.

[0009] In addition to the direct influence of temperature on thermal conductivity, the role of constituent alloying elements in the aluminium alloy is important. Thermal conductivity becomes reduced with alloying additions over wide temperature ranges, since scattering is increased. More generally, improved thermal conductivity of aluminium arises when the alloying content is low, together with the residual alloying elements present in solid solution being reduced. For this reason, it is also important that the heat treatment procedures employed are specific to the alloys developed so as to remove as much solute atoms from the aluminium solid solution as is possible. The other consequence of this is that the alloy itself may become harder and stronger so develops properties that are desirable for engineering applications.

[0010] The thermal conductivity of current aluminium casting alloys at room temperature may be readily found from literature sources. A summary of available data is presented in Table 1 (page 4). Depending on the alloy, the thermal conductivity at ambient temperature or 22°C may range from less than 90 W / m.K up to above 160 W / m.K.Emissivity of Aluminium Surfaces.

[0011] The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Emissivity is defined as the ratio of the energy radiated from a material's surface to that radiated from a perfect emitter, known as a blackbody, at the same temperature and wavelength and under the same viewing conditions. It is a dimensionless number between 0 (for a perfect reflector) and 1 (for a perfect emitter). It is well understood that the emissivity coefficient of an aluminium surface varies with its condition. The emissivity coefficient of polished aluminium for example is reported to be around 0.05, and that of rough aluminium around 0.07. To improve the emissivity of an aluminium surface, it may be painted or anodized. The emissivity coefficient of a blue or black anodized aluminium component for example is then reported to be greater than 0.85 (and typically around 0.9). Importantly, it may be appreciated by a person normally skilled in the art, that a conventional aluminium-silicon based casting alloy is also not able to be successfully anodized because of aphenomena known as silicon smutting, where the surface of the cast material becomes grey or greyish brown.

[0012] It is clearly preferable that any aluminium alloy which is used in thermal management applications, should preferably have adequate mechanical properties, high thermal conductivity, and be capable of demonstrating high emissivity.

[0013] Furthermore, although electrical conductivity and thermal conductivity have similarities, it is important to appreciate that they are not the same measure and are not proportionate over temperature scales. It is common for example that electrical conductivity will decrease with increasing temperature, but thermal conductivity may display different trends. Identifying alloys that display increasing thermal conductivity when temperature increases is beneficial because it means that in applications such as electronics enclosures, that heat dissipation becomes more efficient as the substrate alloy gets hotter.

[0014] Table 1 - Literature Values for Thermal Conductivity of Al Casting AlloysSC= sand cast, PM = permanent mold cast, IC = investment cast

[0015] The patent literature provides examples of aluminium-silicon based alloy compositions that have been developed that have problematic compositional or mechanical properties. For example, US patent application No. 2019 / 0127824 teaches an alloy range in which the alloy has greater than 45% IACS (e.g. 45 to 55%) and the alloy reportedly has yield strengths typically in the range of 120- 175MPa. The alloy disclosed has Si (1 to 4.5%), Mg (0.3 to 0.5%), TiB2 (0.02 to 0.07%), Fe<0.1 %, Zn<0.01 %, Cu<0.01 % and Mn<0.01 %. As may be appreciated, the maximum solid solubility of silicon in Aluminium is 1 .65 % at 577 °C, and the maximum solubility of Mg2Si in aluminium is at about 1 .85 % at 595 °C. Titanium diboride (TiB2) as a compound is about 30 to 31 wt% Boron and 69 to 70 %Ti so that the relative weight percentage of titanium and boron individually in an aluminium alloy can be established. US2019 / 0127824 teaches that preferred composition ranges for silicon are from 1 to 1.3% Si, or from 3.8 to 4.3%. However, alloys with less than 1 .5% Si can be expected to form almost complete solid solutions on solution treatment at around 540 °C, with limited or no residual silicon. Higher silicon contents such as 4 wt% lead to substantial residual silicon remaining in the matrix material, and may provide improved castability. Moreover, the structure of the alloy and mechanical properties are then dependent on the speed of solidification due to the importance of the dendrite arm spacing. Such alloys are notably amenable to rapid solidification such as by high pressure diecasting. In International Patent publication No. W02020 / 028730 similar alloys compositions were cast and tested such as Al- 1 Si-0.4Mg-0.03Ti or AI-3.6Si-0.4Mg-0.03Ti. In these cases particular note was made that the alloys may be subject to hot tearing (i.e. Al- 1 Si-0.4Mg-0.03Ti) or to poor fluidity (i.e. AI-3.6Si-0.4Mg-0.03Ti) and were considered as having problematic properties for many cast product applications including high performance automobile parts.

[0016] To this end, it would therefore be desirable to provide a new and / or improved aluminium alloy which can be used in thermal management applications.SUMMARY OF THE INVENTION

[0017] The present invention provides an aluminium-silicon based casting alloy which provides medium to high tensile properties, good thermal conductivity combined with an ability to be conventionally anodized. Castings of the alloy may be produced by any suitable casting method available.

[0018] A first aspect of the present invention provides an aluminium based alloy consisting essentially of a weight percentage composition of: silicon 1 .5 to 2.5 % magnesium 0.1 to 0.6 % titanium 0.06 to 0.4 % manganese <0.6 % iron <0.6 % chromium <0.01 % nickel <0.01 % vanadium <0.02 % copper <0.05 % zinc <0.05 % strontium < 0.03 % beryllium < 0.0005 % tin < 0.01 % boron < 0.10 % other elements (each) less than 0.10 % each other elements less than 0.20 % in total and a balance of aluminium and other unavoidable impurities.

[0019] The role of each of the elements of the alloy and the manufacture of the casting of the invention now will be discussed in turn.

[0020] Silicon is required in the alloy to depress the melting temperature, aid fluidity and increase strength via heat treatment. Compositions of the invention range within the limits of 1 .5 to 2.5 wt%, which is sufficient to provide castabilityof the alloy in combination with other elements. In some embodiments, the Si level is from 1 .5 to 2.2 wt%, preferably 1 .5 to 2.0 wt%. In other embodiments, the Si level is from 1 .8 to 2.5 wt%. The Si level preferably is from 1 .7 to 2.2 wt%, for example about 1 .7 wt%. Importantly, at this level, the silicon is not present in high enough quantity to adversely impact the ability to be anodized, but is beneficial for casting and for age hardening by normal heat treatment processes.

[0021] Magnesium content of 0.1 to 0.6 wt% is an important part of the alloy of the invention. Greater additions of magnesium are not beneficial. Optimal concentration is found to be between 0.2 wt% and 0.4 wt%, and preferably around 0.2 to 0.35 wt%. In embodiments, the Mg composition is from 0.2 to 0.3 wt%. In other embodiments, the Mg composition is from 0.3 to 0.5 wt%. Because both Si and Mg content influences the final result of thermal conductivity, as a general principle, the thermal conductivity at room temperature will be optimal when these two soluble elements are low. However, it is also a requirement that there needs to be sufficient age hardening elements to provide a functional cast product that responds favourably to heat treatment.

[0022] The titanium content of the alloy should be between 0.06 to 0.4 %. Titanium may be present as a grain refiner (such as from commercially available products (e.g. Tibor)) in small but measurable quantities, of 0.06 up to 0.4 wt%. Typically, boron is present together with the Ti, normally in a ratio of 5:1 or 3:1 for example, depending on the composition of the master alloy added to the alloy. As may be appreciated, any commercially sourced grain refiner whose Ti:B ratio is greater than 2.2:1 , has significant amounts of free titanium present in the molten metal which, when used either alone or in conjunction with other elements. Whilst not wishing to be limited to any one theory, the inventor has has found that this titanium content to be important to castability, fluidity and solidification behaviour in the current alloy ranges. On the other hand, it must also be considered that titanium may also negatively influence thermal conductivity if it remains in solid solution, meaning that strict control of free titanium is necessary, since it does not easily precipitate as compounds during conventional heat treatments. In embodiments, the amount of free titanium in the alloy is greater than 0.04 wt%, and preferably the amount of free titanium is greater than 0.15%.

[0023] The amount of boron present in the alloy is less than 0.10%, preferably less than 0.03 wt%. In embodiments, wherein the amount of boron present in the alloy is greater than 0.015 wt%.

[0024] Iron and manganese are not required for the alloy of the invention unless the alloy is to be high pressure die cast. For sand castings or investment castings, iron and manganese must be as low as possible, preferably less than 0.15 wt% iron and less than 0.05 wt% Mn. For sand casting alloy or investment casting alloy embodiments, the manganese content is preferably <0.02 wt%, and / or the iron content is preferably <0.15 wt%. For die castings, enough transition metal elements such as Mn and Fe are required to prevent die sticking. If the alloy as a die casting requires high ductility, Mn may be present; if this is not relevant, iron can be introduced instead which will provide a better result for thermal conductivity. Some iron is likely to be present in the starting aluminium used to make the alloys meaning the cost can be maintained at a reduced level by having an iron tolerance. In high pressure die casting alloy embodiments, manganese is preferably present at 0.4 to 0.6 wt% and / or iron is preferably present at 0.05 to 0.3 wt%.

[0025] Due to toxicity and environmental concerns regarding Cr, it is preferable to limit Cr content to a minimum, near to trace and preferably <0.002 wt%.

[0026] Nickel preferably is kept at a low level specifically less than 0.01 %. Vanadium must be maintained at less than 0.02% and preferably less than 0.015%. Both elements may influence the thermal conductivity. Vanadium in particular must be kept as low as possible, at a level preferably less than 0.02%.

[0027] Copper and zinc should not be present above the 0.05 wt% level to ensure thermal conductivity stays optimized. Preferably, zinc is present at <0.01 wt%. More generally however, the advantages of the alloy type are reduced if copper or zinc are present as they are both highly soluble in the aluminium solid solution and increase scattering.

[0028] Strontium is known as a modifier to silicon in cast aluminium alloys and has been found to have a desirable effect in the alloys of the invention. The presence of strontium can promote a fine distribution of residual silicon in the alloy and can also be also important to fluidity and castability of the alloy. The strontium content of the alloy is < 0.03 %. In embodiments, strontium is present at from 0.001 to 0.015 wt%.

[0029] Beryllium is known to provide various advantages to aluminium alloys, particularly in changing the morphology of iron bearing phases. It is however highly toxic and should not be permitted or included in the alloy. Tin should be omitted entirely within the alloy of the invention or restricted to only trace levels as specified. Thus, in embodiments, the composition is free of beryllium, rare earth elements, and free of chromium and other transition metal elements not including (i.e. with the exception of) Ti, Mn, Ni, V, Fe, Cu, Sr and Zn at the levels specified above.

[0030] The alloy of the present invention is most highly suited to the processes of investment casting and sand casting, but variations on the invention have been found to have utility with other casting techniques such as die casting (e.g. high pressure die casting) when it meets the requirement of containing sufficient transition metal elements such as Mn or Fe or Sr to avoid die sticking, mentioned earlier.

[0031] A second aspect of the present invention provides a method of fabricating an aluminium-based alloy product, the method comprising: providing an aluminium alloy melt from the aluminium-based alloy according to the first aspect of the present invention; and casting said aluminium alloy melt into a mould to produce a casting.

[0032] Again, the method of this second aspect is highly suited to the processes of investment casting and sand casting but may also find utility with other casting techniques such as gravity casting or die casting. It should be appreciated, that the cast alloy can be subjected to any number of secondary treatment processes including but not limited to heat treatment including tempering, annealing or thelike, age hardening, solution heat treatment or the like. As with any casting, the casting can be machined and finished appropriately.

[0033] Heat treatment can be used to improve the properties of the casting. In some embodiments, the method can further include the step of: heat treating the casting to a T4, T5, T6, T7, T8 or T9 temper.

[0034] Embodiments of the aluminium-silicon based alloy according to the present invention can be anodized. This is a surprising advantage of the alloy of the present invention, which appears overcome the previously discussed silicon smutting disadvantages of a conventional aluminium-silicon based alloy. Thus, in some embodiments, the method can further include the step of anodizing the casting, which may be polished or machined. The casting is preferably anodized green, blue or black but may also be anodized other colours, for example clear.

[0035] A third aspect of the present invention provides a cast product comprising the aluminium based alloy of the first aspect of the present invention. That product is preferably cast using a casting process such as investment casting, sand casting or die casting - for example a high pressure die casting. However, variations on the invention have been found to have utility with other casting techniques.

[0036] The present invention can be used to produce various cast products, such as a sand cast product, an investment cast product, a die cast product, a high pressure die cast product, or an aluminium alloy based casting. In exemplary forms, the alloy is used to form a product or component cast comprising a structural casting. Again, in some embodiments, the casting / cast product or component can be polished and in some embodiments the casting can be polished and anodized. In some embodiments, the casting / cast product or component can be heat treated to a T4, T5, T6, T7, T8 or T9 temper.

[0037] The alloy can be cast into any suitable shape or form. In some embodiments, the cast product comprises an ingot of alloy.

[0038] A fourth aspect of the present invention provides an ingot produced using the aluminium based alloy of the first aspect of the present invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0039] 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:

[0040] Figure 1 shows hardness-time curves to demonstrate the T6 age hardening response of the alloys of the invention.

[0041] Figure 2 shows hardness-time curves to demonstrate the T7 age hardening response of the alloys of the invention.

[0042] Figure 3 shows the T6 microstructure generated from investment castings made from Alloys 1 to 4 (Table 2).

[0043] Figure 4 shows the T6 microstructure from the sand castings manufactured either a) with or b) without chills.

[0044] Figure 5 shows the outcomes of thermal conductivity testing for Alloys 1 to 4 over the temperature range of 23 °C up to 250 °C, generated following the methods of standard ASTM E1461 -13 for thermal diffusivity and ASTM 1269-18 for specific heat.

[0045] Figure 6 shows examples of investment casting of Alloys 1 to 4 that have been anodized blue.

[0046] Figure 7 shows examples of investment castings of Alloys 1 to 4 that have been anodized black.DETAILED DESCRIPTION

[0047] The present invention provides an aluminium-silicon based casting alloy which provides medium to high tensile properties, good thermal conductivity combined with an ability to be conventionally anodized.

[0048] Castings of the alloy may be produced by any casting method available. As previously indicated, the alloy of the present invention is most highly suited to the processes of investment casting and sand casting, but variations on the invention have been found to have utility with other casting techniques such as high pressure die casting when it meets the requirement of containing sufficient transition metal elements such as Mn or Fe together with Sr to avoid die sticking, mentioned earlier.

[0049] 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. The alloy may be produced in any conventional heat treated condition, such as generic T4, T5, T6, T7, T8 or T9 tempers.EXAMPLES

[0050] A series of experiments were undertaken to test the relative merit of seven aluminium alloy compositions formulated in accordance with embodiments of the present invention, to establish the formability and properties of the alloys. Table 2 provides the composition of each of these experimental alloy compositions.

[0051] The composition of these seven experimental alloys were formulated in view of the thermal conductivity of known Al casting alloys as set out in Table 1 in the background to the invention section of this specification. Table 1 shows literature values at room temperature for thermal conductivity of a range of aluminium casting alloys. It may be seen from Table 1 that across the range of casting alloys that most fall within the range of 90 W / m.K to around 160 W / m.K.

[0052] Table 2 shows the compositions of seven alloys which are examples investigated leading to the present invention. From the results shown in Table 2 the approximate amounts of TiB2 may also be calculated as being greater than 0.06 wt.%

[0053] Table 2 - Experimental Alloy Compositions (wt%)

[0054] The principal difference between Alloys 1 to 4 lies in the magnesium content. Variations within the ranges tested are present for titanium and strontium. Alloy 5 was a repeat of Alloy 2, with a reduced strontium content, and used for examining chill effects in sand castings. Alloy 6 was used for testing the feasibility of high pressure die casting versions of the alloy of the invention, where manganese was purposefully added to the alloy to prevent die sticking or soldering.

[0055] Experimental cast samples of each of Alloys 1 to 5 from Table 2 were cast using an investment casting process. Moulds 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, where necessary, prior to pouring molten metal into them and allowing solidification to occur. In addition to trial investment castings, Alloy 5 was also prepared as sand castings using normal sand moulding methods suitable for aluminium, but examples also were produced either with or without chills. The investment casting moulds for determination of tensile mechanical properties were 4-bar trees with a separate downsprue and bottom filter, thatcomprised 16 test bars per mould where each tensile test bar was cast to shape in accordance with the dimensions outlined for a 0.25” diameter gage from ASTM B557. Subsequent testing of these samples after removal from the trees was done in accordance with the same standard. For sand cast test pieces, a simple sand mould was cast with two test pieces per mould that after heat treatment, were machined and tested in accordance with Australian Standard AS1391 -2007 for a 10 mm gage diameter and 50 mm gage length.

[0056] Molten metal was prepared from additions of elements and master alloys (AI-3Ti-1 B and AI-10Sr) to pure aluminium. Composition was verified with a Spectromaxx Spectrometer. For Alloys 1 to 5, the metal temperature prior to casting was 750 °C, for investment castings the shell temperature was 680 to 700 °C and for sand castings the casting temperature was 750 °C but with ambient temperature sand moulds.

[0057] Figure 1 shows the T6 hardness-time curve for the alloys 1 to 4. For these alloys, heat treatment was conducted following standard procedures in an air circulating furnace. A solution treatment temperature of 540 °C was used for 16 h, followed by water quenching, and then age hardening at 177 °C. The hardness-time curve was obtained using a Vickers hardness tester with a 10kg load. Figure 1 indicates all four alloys display a good age hardening response, with the outcome being very highly dependent on the magnesium concentration present. Most notably, there is no advantage of using 0.9 wt% Mg compared to 0.6 wt% Mg.

[0058] Figure 2 shows the T7 hardness time curve for the Alloys 1 to 4. For these alloys, heat treatment was conducted following standard procedures and a solution treatment temperature of 540 °C for 16h, followed by water quenching, and then age hardening at 220 °C. A Vickers hardness tester with a 10 kg load was used.

[0059] T able 3 shows average tensile mechanical properties of Alloys 1 to 4 made as investment castings, then heat treated to a T6 temper (4h at 177°C) or a T7temper (2 h at 220 °C) following solution treatment and water quenching. These tensile properties were obtained following the procedures of ASTM B557.

[0060] Table 3 - average tensile mechanical properties of alloys 1 to 4

[0061] Table 4 shows average tensile mechanical properties of Alloys 1 to 4 made as investment castings and given alternate T6 tempers. These tensile properties were obtained using the procedures of ASTM B557. The results shown in Table 4 are for the same alloys as in Table 3, but with times of ageing being 1 h, 2 h or 3 h as noted.

[0062] Table 4 - average tensile mechanical properties of Alloys 1 to 4

[0063] Table 5 shows tensile mechanical properties of Alloy 1 and 2 made as investment castings given alternate T6 tempers. These tensile properties were obtained following the procedures of ASTM B557. Test data was generated by third party testing at (Bureau Veritas Asset Integrity and Reliability Services Australia Pty. Ltd., Regency Park, South Australia).

[0064] For these results (in Table 5), the solution treatment temperature was 560 °C for 16 h, followed by water quenching and then ageing 1 h at 177°C. Here it will be seen that the tensile properties, in particular the tensile elongation, is further increased above the results shown in Table 4.

[0065] Table 5 - tensile mechanical properties of Alloy 1 and 2

[0066] Figure 3 shows the T6 microstructure generated from investment castings made from Alloys 1 to 4. The composition corresponding to Alloys 1 to 4 is noted within the image. Alloys with 0.2 or 0.4 wt% Mg show limited residual as-cast microstructure in the images, but they are characteristically cast microstructures. The Alloy 3 with 0.6 wt% Mg shows additional residual features from casting and Alloy 4 with 0.9 wt% Mg exhibits a greater amount of residual cast microstructure, reflecting the lower tensile properties recorded.

[0067] Table 6 shows tensile mechanical properties generated from Alloys 1 to 4 made as sand castings and heat treated to a T6 temper. Solution treatment was 540 °C for 16h, followed by water quenching and ageing at 177 °C for 4h. These tensile properties were obtained following procedures outlined in AS1391 . Test data was generated by third party testing at (Bureau Veritas Asset Integrity and Reliability Services Australia Pty. Ltd., Regency Park, South Australia).

[0068] Table 6 -Tensile mechanical properties generated from Alloys 1 to 4

[0069] T able 7 shows tensile mechanical properties generated from Alloy 5, made as sand castings and heat treated to different T6 tempers. These tensile properties were obtained following the procedures outlined in AS1391 . Test data was generated by third party testing at (Bureau Veritas Asset Integrity and Reliability Services Australia Pty. Ltd., Regency Park, South Australia).

[0070] For each of the examples shown in T able 7, there is a comparison between sand castings containing chills (1 h-C, 2h-C, 4h-C) and those without chills (1 h- NC, 2h-NC, 4h-NC). The digit (1 ,2,4) represents the time of ageing at 177°C. Importantly, for the ageing duration of 2 or 4h, there appears to be no advantage of using chills over not using chills and the elongation is above 10% in each case.

[0072] Figure 4 shows the T6 microstructure from the sand castings manufactured either with (Figure 4a) or without chills (Figure 4b). There is little obvious difference between the two alloys despite the apparent difference in cooling rates.

[0073] Figure 5 shows the outcomes of thermal conductivity testing for Alloys 1 to 4 (heat treated to a T6 temper as the same method in Table 3) over the temperature range of 23 °C up to 250 °C, generated by independent testing by (Thermophysical Properties Research Laboratory, Inc., West Lafeyette, Indiana, USA and using the method of standard ASTM E1461 -13 (Thermal Diffusivity) and ASTM 1269-18 (Specific Heat). In Figure 5 it can be seen that for each of the four alloys tested (Alloys 1 to 4) that the thermal conductivity increases with increasing temperature. Alloy 2 displayed a thermal conductivity value at 23 °C of 174 W / m.K, which rose to 202 W / m.K at 250 °C. Alloy 1 displayed a thermal conductivity value at 23 °C of 187 W / m.K, which rose to 215 W / m.K at 250 °C. In difference to the thermal conductivity of pure aluminium, the thermal conductivity of the alloys increased with increasing temperature.

[0074] Table 8 shows the outcomes of tensile testing as-cast and T5 high pressure die castings, made from Alloy 6. These tensile properties were obtained following standard AS1391 -2007 on samples cast to shape. Test data was generated by third party testing at (Bureau Veritas Asset Integrity and Reliability Services Australia Pty. Ltd., Regency Park, South Australia).

[0075] For the T5 temper presented in Table 8, the samples were aged 4h at 177 °C following casting.

[0076] Table 8- Alloy 6 tensile testing results

[0077] For the manufacture of die castings corresponding to the composition of Alloy 6, a Toshiba horizontal cold chamber die casting machine was used with a metal velocity at the gate of 86 m / s and pressure on the shot biscuit during intensification of 60 MPa. In this process three test bars were produced on each shot with two cylindrical test bars and one flat bar produced. Only the cylindrical samples were tensile tested. The alloy did not show a tendency for die sticking and 100 shots were successfully cast in the run. The alloy was degassed withargon prior to casting and cast at 730 °C. Die heaters were maintained at around 140 °C.

[0078] Table 9 shows results from tensile testing aluminium plates of Alloy 7. The alloy was solution treated 16 h at 560 °C, followed by ageing 1 ,2 and 4h. In this case, the castings were of a configuration where two plates ~28mm thick were cast together from a common central downsprue. Each plate weighed 7 kg and had approximate dimensions of 380x240x28 mm. No chills or rapid solidification techniques were employed. These tensile properties were obtained following standard AS1391 -2007 on samples machined from the castings. Test data was generated by third party testing at (Bureau Veritas Asset Integrity and Reliability Services Australia Pty. Ltd., Regency Park, South Australia).

[0079] Table 9. Alloy 7 Tensile Testing Results; 28 mm Wall Sand Castings.

[0080] The proportion of the aluminium alloy in the molten state that contains free or elemental titanium was tested using Alloy 4. In this experiment, the alloy was manufactured using a commercial AI-3Ti-B grain refiner to alloy titanium into the melt. The composition of the alloy was measured directly prior to casting, and then again following a period of around 5.5 hours settling time. As may be appreciated, the density of TiB2 particles (4.52 g / cm3) and AhTi (3.40 g / cm3) is higher than of the molten aluminium (approx. 2.35 g / cm3at 750°C) meaning that a proportion of these particles are expected to sink to the bottom of the crucible during settling time after degassing. As may be appreciated, the solubility of Ti in molten aluminium at 750 °C is approximately 0.34 % so very few AhTi particles should exist at an equilibrium condition for a composition containing a total of 0.25% Ti. What is then left in the molten aluminium is free titanium. Table 10 shows the results of this test, and it can be concluded that approximately 0.13% Ti is free in the melt. Free titanium in the molten metal is critical to the fluidity and performance of the alloy range disclosed, and works to promote excellent mouldfilling and minimise hot tearing of the alloys of the invention. As may be appreciated, the efficacy of the alloy itself is not due to titanium in isolation, rather it is due entirely to the synergistic effect of all contained elements together.

[0081] Table 10. Composition of Alloy 4 after 5 h settling time compared to test directly after degassing (wt%).

[0082] Figure 6 shows an example of Alloys 1 to 4 investment cast and then heat treated to a T6 temper in accordance with Table 3, machined by skimming the test piece and anodized blue. No special procedures were employed, and the alloy was anodized using the same industrial process that would be normal for a wrought alloy such as 6061. Anodizing was conducted at a commercial facility with no special instructions (Collins Anodic Treatment (Vic) Pty Ltd), and according to a standard anodizing process, MIL-A-8625F Type II Class 2 (Blue Anodize). The results show that an advantageous anodized coating can be surprisingly achieved on an investment cast product produced from an aluminium-silicon based alloy according to the present invention.

[0083] Figure 7 shows examples of Alloys 1 to 4, investment cast and heat treated to a T7 temper in accordance with Table 3, machined by skimming the test pieceand anodized black. No special procedures were employed and the alloy was anodized using the same industrial process that would be normal for a wrought alloy such as 6061. Anodizing was conducted at a commercial facility with no special instructions (Collins Anodic Treatment (Vic) Pty Ltd), and according to a standard anodizing process, MIL-A-8625F Type II Class 2 (Black Anodize). These results again show that an advantageous anodized coating can be surprisingly achieved on an investment cast product produced from an aluminium-silicon based alloy according to the present invention.

[0084] 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.

[0085] 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.

Claims

23CLAIMS:1 . An aluminium based alloy consisting essentially of (wt%): silicon 1 .5 to 2.5 % magnesium 0.1 to 0.6 % titanium 0.06 to 0.4% manganese <0.6 % iron <0.6 % chromium <0.01 % nickel <0.01 % vanadium <0.02 % copper <0.05 % zinc <0.05 % strontium < 0.03 % beryllium < 0.0005 % tin < 0.01 % boron < 0.10 % other elements (each) less than 0.10 % each other elements less than 0.20 % 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, Cu, Ni, V, Sr and Zn at the levels specified in claim 1 .

3. The aluminium based alloy of claim 1 or 2, wherein the amount of free titanium is greater than 0.04 wt%.

4. The aluminium based alloy of any one of claims 1 to 3 wherein the amount of free titanium is greater than 0.15%.

5. The aluminium based alloy of any one of claims 1 to 4, wherein the amount of boron present is greater than 0.015%.

6. The aluminium based alloy of any one of claims 1 to 5, wherein the amount of boron present is less than 0.03%.

7. The aluminium based alloy of any one of claimsl to 6, wherein the silicon level is from 1 .5 to 2.2 wt%, preferably 1 .5 to 2.0 wt%, more preferably about 1 .7 wt%.

8. The aluminium based alloy of any one of claims 1 to 7, wherein the silicon is from 1 .8 to 2.5 wt%9. The aluminium based alloy of any one of claims 1 to 8, wherein magnesium is present at from 0.2 to 0.4 wt%, preferably from 0.2 to 0.3 wt%.

10. The aluminium based alloy of any one of claims 1 to 8, wherein magnesium is present at from 0.3 to 0.5 wt%.1 1. The aluminium based alloy of any one of claims 1 to 10, wherein zinc is present at <0.01 wt%.

12. The aluminium based alloy of any one of claims 1 to 1 1 , wherein strontium is present at from 0.01 to 0.015 wt%.

13. The aluminium based alloy of any one of claims 1 to 12, wherein manganese is <0.02 wt%.

14. The aluminium based alloy of any one of claims 1 to 13, wherein iron is <0.15 wt%.

15. The aluminium based alloy of any one of claims 1 to 14, comprising a sand casting alloy or an investment casting alloy.

16. The aluminium based alloy of any one of claims 1 to 14, wherein manganese is present at 0.4 to 0.6 wt%.

17. The aluminium based alloy of any one of claims 1 to 14, wherein iron is present at 0.05 to 0.3 wt%.

18. The aluminium based alloy of claim 16 or 17, comprising a high pressure die casting alloy.

19. A method of fabricating an aluminium-based alloy product, the method comprising: providing an aluminium alloy melt from the aluminium-based alloy according to any one of claims 1 to 18; and casting said aluminium alloy melt into a mould to produce a casting.

20. A method according to claim 19, wherein the casting process comprises one of sand casting, investment casting, gravity casting or die casting.21 . A method according to claim 19 or 20, further including the step of: anodizing the casting.

22. A method according to any one of claims 19 to 21 , further including the step of: heat treating the casting to a T4, T5, T6, T7, T8 or T9 temper.

23. A cast product or component comprising the aluminium based alloy of any one of claims 1 to 18.

24. A cast product or component according to claim 23, comprising a sand cast product, an investment cast product, or a high pressure die cast product.

25. A cast product or component according to claim 23 or 24, comprising a structural casting.

26. A cast product or component according to any one of claims 23 to 25, wherein the cast product or component is anodized, preferably anodized green, blue or black.2627. A cast product or component according to any one of claims 23 to 26, wherein the cast product or component is heat treated to a T4, T5, T6, T7, T8 or T9 temper.

28. An ingot produced using the aluminium based alloy of any of claims 1 to 18.

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