IMPROVED ALUMINIUM BASED CASTING ALLOY

Description

PRIORITY CROSS-REFERENCE

The present patent application claims priority from Australian provisional patent application No. 2021902652 filed on 23 Aug. 2021, the contents of which should be understood to be incorporated into this specification by this reference.

TECHNICAL FIELD

The present invention relates to an aluminium based alloy for the manufacture of cast parts. The invention particularly relates to aluminium-silicon-copper based casting alloys suitable for sand or investment casting 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 low pressure casting processes.

BACKGROUND OF THE INVENTION

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.

Typical age hardenable 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, which are also age-hardenable alloys. The advantages of the Al—Si—X alloys include low cost and very good castability especially when using methods such as sand casting, low pressure casting, and investment casting. Conversely, alloys in the Al—Cu—X family of castings have relatively poor castability and are prone to hot tearing, but they may develop high levels of strength properties, especially when silver is added to their composition. These silver containing alloys have a significant cost penalty associated with their use, since even 0.5% silver can double the base price of a primary alloy. With respect to cast materials, typical minimum properties and conditions for delivery of common aerospace castings and their composition can be derived directly from international standards. For example, AMS-A-21180C (“Aluminum-Alloy Castings, High Strength”) is a widely used example that is also reflective of the contents of the MMPDS (Metallic Materials Properties Development and Standardization). The high strength aluminium alloy castings covered by AMS-A-21180C (2017) are intended for airframe, missile, and other applications where high strength, ductility, soundness and uniform composition within each casting are required. AMS-A-21180C covers the basic requirements for cast aerospace alloys A201-T7, 354-T6, C355-T6, A356-T6, A357-T6, D357-T6, and 359-T6. The composition of these is shown in Table 1. As may be appreciated there are variations on these alloys, such as D, E and F357 for example, which are slightly modified version of the base A357 alloy. A more complete overview of common alloys and their variants may be found in the Aluminum Association Pink Sheets (i.e. Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot).

TABLE 1
Chemical Composition of Cast Aerospace Aluminium Alloys
Alloy

Elements	A201.0	354.0	C355.0	A356.0	A357.0	359
Copper	4.0-	1.6-	1.0-	0.20	0.20	0.20
	5.0	2.0	1.5
Silicon	0.05	8.6-	4.5-	6.5-	6.5-	8.5-
		9.4	5.5	7.5	7.5	9.5
Iron	0.10	0.20	0.20	0.20	0.20	0.20
Manganese	0.20-	0.10	0.10	0.10	0.10	0.10
	0.40
Zinc	—	0.10	0.10	0.10	0.10	0.10
Magnesium	0.15-	0.40-	0.40-	0.25-	0.40-	0.50-
	0.35	0.6	0.6	0.45	0.7	0.7
Titanium	0.15-	0.20	0.20	0.20	0.20	0.20
	0.35
Beryllium	—	—	—	—	0.04-	—
					0.07
Silver	0.40-	—	—	—	—	—
	1.0
Others, each	0.03	0.05	0.05	0.05	0.05	0.05
Others, total	0.10	0.15	0.15	0.15	0.15	0.15
Aluminium	Bal.	Bal.	Bal.	Bal.	Bal.	Bal.

Within the scope of these six alloys, the minimum mechanical properties cut from designated areas of castings need to meet the most stringent criteria, in Strength Class 1, 2 or 3.

The tensile mechanical properties are then specified as minimums in accordance with Table 2. For the properties shown in Table 2, there is an allowance within the standard that permits the use of chills to achieve the mechanical properties. The use of chills in critical areas produces a finer microstructure or dendrite arm spacing, which can be required for the achievement of properties meeting the specification.

TABLE 2
Mechanical Properties of Specimens cut from designated areas of
castings (from Table 3 of AMS-A-21180C)

	Strength	Minimum	Minimum 0.2%	Minimum
	Class	Tensile	Proof Stress	Elongation
Alloy	Number	Strength (MPa)	(MPa)	(%)
A201.0-T7	1	414	345	3
	2	414	345	5
354.0-T6	1	324	248	3
	2	345	290	2
C355.0-T6	1	283	214	3
	2	303	228	3
	3	345	276	2
A356-T6	1	262	193	5
	2	276	207	3
	3	310	234	3
A357.0-T6	1	310	241	3
	2	345	276	5
359.0-T6	1	310	241	4
	2	324	262	3

In U.S. Pat. No. 8,409,374 by Lumley et. al, the means by which to heat treat high pressure diecast aluminium alloys without blistering or dimensional instability is taught. Al—Si—Cu—Mg alloys such as A360, A380 or C380 (see for example Table 3) were shown to be able to be heat treated by solution treating at far shorter times, and at lower temperatures, than what would be considered conventional for aluminium alloys such as A356-T6. For example, times as short as 15 minutes of solution treatment were effective in producing very good mechanical properties. Mechanical properties such as 0.2% proof stress displayed values above 350 MPa, and elongation values ≥3%, were recorded.

As may be appreciated, the requirements for high pressure diecasting alloys are that they must contain high levels of iron or combined iron+manganese, at around 1% total content, to avoid die soldering. Donohue and Lumley (R. J. Donahue & R. N. Lumley, Chapter 6, New Hypoeutectic/Hypereutectic die casting alloys and new permanent mold casting alloys that rely on strontium for their die soldering resistance, in Fundamentals of Aluminium Metallurgy, Recent Advances, R. N. Lumley ed., Woodhead Publishing, Duxford, UK, p. 173-216, 2018) reported a relationship governing die sticking or soldering tendency wherein the condition (10×Sr)+Mn+Fe>1 must be met in order to avoid die soldering. However, as may be appreciated, the heat treated properties achieved in high pressure die castings such as A380 or C380 are largely contingent on the extremely high solidification rate that is obtained in a high pressure diecasting (typically between 5° and 125° C./s depending on wall thickness) compared to sand castings (e.g. 0.1 to 5° C./s) or standard investment castings (e.g. 0.05 to 0.1° C./s). Premium investment castings typically involve process parameters wherein somewhat faster cooling rates may be attained, typically through heat transfer across the ceramic shell. In these instances, a cooling rate of around 0.5 to 5° C./s or faster may be attained. More generally however, the cooling rate can be estimated from the dendrite arm spacing of the material, using the equation SDAS=36.1×(CR)^−0.34. As a result, alloys such as the 380 variants shown in Table 3 are not considered suitable for casting methods other than high pressure diecasting. The slower cooling rate means that usable levels of tensile ductility are unable to be achieved. As may be appreciated, the dendrite arm spacing of a high pressure diecasting may be in the range of 1 to 5 μm, whereas a chilled sand casting may be in the range of 15 to 25 μm, and a standard investment casting may be in the range of 75 to 100 μm. By the use of premium investment casting processes, the dendrite arm spacing of an investment casting may be reduced to around 20 to 40 μm.

TABLE 3
Typical Al-Si-Cu diecasting alloys.
Alloy

	Elements	A360	A380	C380	F380
	Silicon	9.0-10.0	7.5-9.5	7.5-9.5	7.5-9.5
	Iron	1.3	1.3	1.3	0.4
	Copper	0.6	3.0-4.0	3.0-4.0	3.0-4.0
	Manganese	0.35	0.50	0.50	0.40
	Magnesium	0.4-0.6	0.1	0.1-0.3	0.50
	Nickel	0.50	0.50	0.50	0.50
	Zinc	0.5	3.0	3.0	1.0
	Tin	0.15	0.35	0.35	0.35
	Strontium	—	—	—	0.05
	Others, each	—	—	—	0.05
	Others, total	0.25	0.5	0.5	0.15
	Aluminium	Bal.	Bal.	Bal.	Bal.

Further Al—Si—Cu diecasting alloys include:

• • B380: composition is the same as A 380 but with a maximum 1.0% Zn;
  • D380: composition is the same as C380 but with a maximum 1.0% Zn;
  • E380: composition is the same as C380 but the lower limit of Mg is absent.

Table 4 summarizes similar alloys to Table 3 which are listed in the Aluminum Association pink sheets as registered alloys, but which are used as sand castings. Similar to Table 3, these all have high allowances of transition metal elements such as Fe, Mn, and Ni. Furthermore, ASTM B26-18e1 provides tensile mechanical properties for Alloy 319.0 in a heat treated T6 condition and has minimums of 140 MPa yield strength, 215 MPa tensile strength, and >1.5% elongation, for purposes of comparison.

TABLE 4
Al-Si-Cu Sand Casting Alloys registered with the Aluminum
Association (Values are maximums unless stated as a range).
Alloys

Elements	318.0	319.0	A319	B319	320
Silicon	5.5-6.5	5.5-6.5	5.5-6.5	5.5-6.5	5.0-8.0
Iron	1.0	1.0	1.0	1.0	1.2
Copper	3.0-4.0	3.0-4.0	3.0-4.0	3.0-4.0	2.0-4.0
Manganese	0.50	0.50	0.50	0.50	0.80
Magnesium	0.10-0.60	0.10	0.10	0.10-0.50	0.05-0.60
Nickel	0.35	0.35	0.35	0.35	0.35
Zinc	1.0	1.0	3.0	3.0	3.0
Titanium	0.25	0.25	0.25	0.25	0.25
Others, each	—	—	—	—	—
Others, total	0.50	0.50	0.50	0.50	0.50
Aluminium	Bal.	Bal.	Bal.	Bal.	Bal.

It would therefore be desirable to provide a new and/or improved aluminium-based alloy for manufacture of cast parts, particularly by sand casting or investment casting.

SUMMARY OF THE INVENTION

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.

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

	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.

The role of each of the elements of both 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. Compositions range within the limits of 4.0 to 8.5 wt %, but all maintain good fluidity to aid casting. The Si level is preferably from 4.0 to 7.5 wt %, more preferably 4.5 to 7.5 wt %. This corresponds to optimal casting conditions for the majority of instances and higher levels are not necessarily beneficial. In some embodiments, the Si level is 4.0 to 5.5 wt %, preferably about 5 wt %. In embodiments, it has been found that if the silicon can be maintained at around 5%, together with other elements chosen appropriately, the alloy of the invention is surprisingly less sensitive to cooling rate and can generate good strength and ductility levels at dendrite arm spacing such as between 35 and 40 μm.

Magnesium content of 0.07 to 0.3 wt % is a key part of the alloy of the invention. Greater additions of magnesium, such as >0.3 to 0.35 wt %, are not beneficial in the alloy of the current invention and may cause reductions in ductility or greater sensitivity to dendrite arm spacing. In embodiments, optimal concentration typically is found to be >0.1 wt % and <0.25 wt %, and more preferably around 0.15 to 0.25 wt %, preferably 0.15 to 0.2 wt %.

Titanium may be present in small but measurable quantities, of 0.06 up to 0.2 wt %, and its presence is critical for the efficacy of the present invention. Too much titanium in the alloys leads to negative effects that may reduce tensile elongation. Too little titanium and boron may cause the alloy to “run out” of grain refiner during the solidification process, impacting grain structure.

One of the other elements that can be present in the alloy is boron. Boron is present in the alloy in amounts less than 0.05 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.

In difference to the 380 type alloys shown in Table 3, in the current invention iron and manganese should ideally be kept as low as possible, and preferably, the combined iron and manganese content (sum of Fe+Mn) is preferably less than 0.1 wt %.

Copper is present to aid fluidity and to provide strengthening to the alloy by heat treatment, especially when in combination with magnesium and silicon. In general, Cu levels around 3 to 3.75 wt %, preferably 3 to 3.5 wt %, are optimal in the present invention, but levels as low as 2.5 wt % and as high as 4 wt % may also be considered in some applications. More generally however, the advantages of the alloy type are reduced compared to incumbent Al—Si—Mg alloys if the Cu content is too low or too high.

Zinc may be present at levels up to 0.5%, but its preferable to maintain its level below 0.3 wt %. Higher levels should be avoided. In embodiments, zinc is present from 0.1 to 0.3 wt %. Zinc in these levels is not detrimental to the efficacy of the invention. Higher levels create a weight or density penalty so should be avoided wherever possible. The copper+zinc content must be less than 4 wt % and preferably less than 3.6 wt %.

Strontium is known as a modifier to silicon in cast aluminium alloys, and has been found in this work to have a more significant effect in the Al—Si—Cu alloys of the invention compared to Al—Si—Mg alloys such as A356 and A357. Whilst not wanting to be limited to any one theory, the presence of strontium may be crucial to promote modification of the silicon present. Here it is also important to note, that F380 alloy as shown in Table 3, has had Sr added to prevent die soldering rather than to change the morphology of the silicon phase. The presence of strontium in the range of >0.001 to 0.03 wt % and preferably 0.01 to 0.025 wt % is necessary to achieve the best combinations of mechanical properties in the alloy. In some embodiments, strontium is present at from 0.01 to 0.015 wt %. Too much strontium increases porosity and may be detrimental to the outcome of the invention. However, there are instances where Sr is detrimental to the surface finish of investment castings through a metal-mould reaction initiated by strontium, and in these cases the strontium content must be maintained between 0.001 and 0.008 wt %, preferably 0.001 to 0.005 wt %. In some embodiments, the strontium content is present at from 0.001 to 0.007 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. Similarly, tin should be omitted entirely within the alloy of the invention or restricted to only trace levels as specified. Due to toxicity and environmental concerns regarding Cr, it is preferable to limit Cr content to a minimum, preferably <0.002%.

The content of nickel in the aluminium based alloy is controlled to be less than 0.01 wt %.

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, Fe, Ni, Cu, Sr and Zn.

The alloy of the present invention is most 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 low pressure casting. Generally, the alloy of the present invention comprises a casting alloy for casting processes having a casting pressure of less than 10 bar. In most embodiments, the casting pressure will be no more than 5 bar, and typically no more than 2 bar. In many embodiments, the casting pressure will be around atmospheric pressure up to around 2 bar. For example, sand casting or investment casting are conducted around atmospheric pressure. The alloy can therefore comprise a sand casting alloy or an investment casting alloy.

The alloy can be cast into any suitable shape or form. In some embodiments, the aluminium based alloy comprising and/or is cast as an ingot of alloy.

Castings of the alloy are also suitable for and 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.

However, it should be appreciated that the alloys of the invention are not suitable for high pressure diecasting. The alloy is unsuitable for pressure diecasting because it does not meet the requirement of (10×Sr)+Fe+Mn>1, mentioned earlier.

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 using a using a casting process having a casting pressure of less than 10 bar.

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. 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. A wide variety of heat treatment procedures may be utilised, such as T4, T5, T6, T7, T8 or T9 tempers, depending on the desired material properties and result.

A third aspect of the present invention provides a cast product produced using the aluminium based alloy of the first aspect of the present invention. That product is preferably produced using a casting process having a casting pressure of less than 10 bar and often less than one bar such as investment casting and sand casting. In most embodiments, the casting pressure will be no more than 5 bar, and typically no more than 2 bar. In many embodiments, the casting pressure will be around atmospheric pressure up to around 2 bar.

The present invention can be used to produce various cast products, such as a sand cast product, an investment 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 aerospace casting. The alloy can be cast into any suitable shape or form. In some embodiments, the cast product comprises an ingot of alloy.

The cast product can have various morphologies. In particular embodiments, the microstructure of the alloy includes dendrites (is dendritic), and has a dendrite arm spacing of less than 50 μm, preferably between 10 μm and 45 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

FIG. 1 shows a thermal analysis scan of Al—Si—Cu alloys in accordance with the present invention.

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

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

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

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

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

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

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.

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 (10×Sr)+Fe+Mn>1, discussed above.

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

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 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.

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.

FIG. 1 shows a thermal analysis scans showing cooling curves of Al—Si—Cu 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.

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 Al—Si—Cu alloys containing iron, and forming β-Al₅FeSi 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.

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.

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 μm. 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 Al—Si—Mg alloys, which undergo more thorough fragmentation when Sr is not present, but where solution treatment temperature is higher.

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 μm and Alloy 3 displays a DAS of 14.4 μm and the microstructures are effectively identical. In difference to Alloy 1, the silicon phase is well distributed and fragmented in both alloys.

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 μm or 26.2 μm respectively. Because of their lower silicon content, these alloys have a moderately different solidification behaviour to the alloys 1 to 3.

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.

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 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%

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.

Table 8 also shows for Alloy 4, results for the investment cast alloy displaying a dendrite arm spacing of 23.9 μm (T6 #1) or 38.4 μm (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 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%

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.

Similar to Table 8, results for the investment cast alloy displaying a dendrite arm spacing of 23.9 μm (T4 #1) or 38.4 μm (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 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%

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

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.

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).

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 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%

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.

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.