Aluminium Casting Alloy Displaying Improved Thermal Conductivity

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

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.

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.

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.

The thermal conductivity of a material is equivalent to the quantity of heat, ΔQ, transmitted during time Δt through a thickness x, in a direction normal to a surface with area A, per unit area of A, due to a temperature difference ΔT, 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:

where κ is thermal conductivity, α is thermal diffusivity (m/s), Cis specific heat, (J/kg·K), and ρ is density in g/cm. These parameters may be determined readily by using methods as outlined in ASTM E1461.

In metals, the total thermal conductivity is the sum of electronic thermal conductivity (κ) and phonon (lattice) thermal conductivity (κ), such that:

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.

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.

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.

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.

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(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 a phenomena known as silicon smutting, where the surface of the cast material becomes grey or greyish brown.

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.

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.

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 120-175 MPa. The alloy disclosed has Si (1 to 4.5%), Mg (0.3 to 0.5%), TiB(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 MgSi in aluminium is at about 1.85% at 595° C. Titanium diboride (TiB) 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. WO2020/028730 similar alloys compositions were cast and tested such as Al-1Si-0.4 Mg-0.03Ti or Al-3.6Si-0.4 Mg-0.03Ti. In these cases particular note was made that the alloys may be subject to hot tearing (i.e. Al-1 Si-0.4 Mg-0.03Ti) or to poor fluidity (i.e. Al-3.6Si-0.4 Mg-0.03Ti) and were considered as having problematic properties for many cast product applications including high performance automobile parts.

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.

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.

A first aspect of the present invention provides an aluminium based alloy consisting essentially of a weight percentage composition of:

and a balance of aluminium and other unavoidable impurities.

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.

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 castability of 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.

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.

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%.

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 %.

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 %.

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 %.

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%.

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.

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 %.

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.

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.

A second aspect of the present invention provides a method of fabricating an aluminium-based alloy product, the method comprising:

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 the like, age hardening, solution heat treatment or the like. As with any casting, the casting can be machined and finished appropriately.

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.

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.

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.

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.

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

A fourth aspect of the present invention provides an ingot produced using the aluminium based alloy of the first aspect of the present invention.

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 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.

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.

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.

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.

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 TiBmay also be calculated as being greater than 0.06 wt. %

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.