WO2015118312A2

WO2015118312A2 - Method of manufacture

Info

Publication number: WO2015118312A2
Application number: PCT/GB2015/050278
Authority: WO
Inventors: Ashley Edward BROUGH
Original assignee: Jbm International Limited
Priority date: 2014-02-04
Filing date: 2015-02-02
Publication date: 2015-08-13
Also published as: WO2015118312A3; GB201401911D0; GB2522719B; GB2522719A

Abstract

The present invention provides a method of manufacture of a structural aluminium alloy having a desired composition, the method comprising blending secondary aluminium obtained from one or more sources with primary aluminium and, optionally, one or more additional refining and/or alloying elements, in order to obtain the desired composition. To be accompanied when published by figure 1 of the drawings.

Description

Method of Manufacture

The present invention relates to the manufacture of aluminium alloys, in particular structural aluminium alloys, and components or articles made therefrom.

So-called structural aluminium alloys are used to make structural components, e.g. in the automotive industry. Examples of vehicle structural components include chassis parts, shock towers and parts for suspension systems. In the automotive industry, it is known to manufacture vehicle structural components by die casting structural aluminium alloys. By using aluminium, as opposed to steel, lighter components can be manufactured. As a consequence, the weight of a vehicle may be reduced with consequential benefits, e.g. in terms of reduced fuel consumption. The quality of a die cast structural component depends on the chemical composition and microstructure of the aluminium alloy used as well as the machine setting and the process selected. Ductility may be particularly important, particularly in components of complex design, e.g. a component with a complex geometry and thin walls. Thus, a structural casting alloy should typically have a high elongation to fracture in the cast state. In addition, the alloy should typically be easily weldable and flangeable, able to be riveted and have good corrosion resistance.

US 6,824,737 discloses an aluminium alloy suitable for die casting of components with high elongation in the cast state. As well as aluminium and unavoidable impurities, the alloy comprises 9.0 to 11.0 wt% silicon, 0.5 to 0.9 wt% manganese, max 0.06 wt% magnesium, 0.15 wt% iron, max 0.03 wt% copper, max 0.10 wt% zinc, max 0.15 wt% titanium, 0.05 to 0.5 wt% molybdenum and 30 to 300 ppm strontium or 5 to 30 ppm sodium and/or 1 to 30 ppm calcium for permanent refinement. An AlSi9Mn pressure die casting alloy within the scope of US 6,824,737 is produced and sold commercially by Rheinfelden as Castasil®-37. It is known for vehicle structural components to be die cast from Castasil®-37. Castasil®-37 is manufactured from "pure", primary aluminium, to which further alloying and refining elements are added to provide the required composition. Hence, aluminium casting alloys, such as the alloy disclosed in US 6,824,737 (e.g. Castasil®-37), typically may be relatively expensive and consequently may not be cost- competitive with other materials, in particular steel. This is because the chemistry and composition of the casting alloy may be tightly controlled and/or may require the addition of rare and expensive alloying elements such as molybdenum or strontium and/or may typically be manufactured only from primary sources of aluminium.

Accordingly, there is a need for a more economical and energy efficient method of manufacturing structural aluminium alloys, especially for use in automotive structural applications.

A first aspect of the invention provides a method of manufacture of a structural aluminium alloy having a desired composition, the method comprising:

• blending secondary aluminium obtained from one or more sources with primary aluminium and, optionally, one or more additional refining and/or alloying elements, in order to obtain the desired composition.

In an embodiment, the alloy may comprise 10 wt% or more of recycled aluminium. The alloy may comprise 25 wt% or more of recycled aluminium. The alloy may comprise less than or at least 50 wt% of recycled aluminium.

In an embodiment, the desired composition may comprise a hypo-eutectic aluminium- silicon alloy comprising more than 0.15 wt% iron.

Conveniently, the alloy may be a casting alloy, which may be suitable for die casting of structural components with high elongation state. For example, the alloy may be suitable for high pressure die casting (HPDC) of structural components. The structural aluminium alloy may contain a relatively high proportion of iron. Typically, in prior art structural aluminium alloys the iron content has been limited to a lower level, since iron may generally have been seen as a largely unwanted impurity. Surprisingly, however, we have found that the strength of the alloy may be improved by having a higher iron content.

In an embodiment, the structural aluminium alloy may comprise no more than 0.6 wt% iron, no more than 0.55 wt% iron, no more than 0.45 wt% iron, no more than 0.35 wt% iron or no more than 0.3 wt% iron. Alternatively or additionally, the structural aluminium alloy may comprise at least 0.16 wt% iron, at least 0.2 wt% iron, at least 0.25 wt% iron or at least 0.3 wt% iron. In an embodiment, the structural aluminium alloy may comprise from 0.16 wt% to 0.3 wt% iron.

The structural aluminium alloy may comprise no more than 0.01 wt% molybdenum or no more than 0.001 wt% molybdenum. In an embodiment, the structural aluminium alloy may be substantially free of molybdenum. Conveniently, no additional molybdenum may need to be added to the alloy during manufacture. This may be economically advantageous, since molybdenum is a relatively expensive alloying addition.

The structural aluminium alloy may comprise no more than 0.05 wt% strontium, no more than 0.01 wt% strontium or no more than 0.001 wt% strontium. The structural aluminium alloy may comprise up to 200 ppm strontium. Conveniently, no strontium may need to be added to the alloy during manufacture. This may be economically advantageous, since strontium is relatively rare and expensive.

Surprisingly, we have found that the addition of molybdenum and/or strontium during manufacture of the alloy may not be required in order to produce a hypo-eutectic aluminium-silicon alloy having a micro structure and mechanical properties suitable for die casting of components. The structural aluminium alloy may comprise at least 8.5 wt% silicon, e.g. 9 wt% silicon or more. The structural aluminium alloy may comprise up to 11 wt% silicon, e.g. 10.5 wt% silicon or less. The structural aluminium alloy may comprise at least 0.6 wt% manganese or at least 0.65 wt% manganese. Additionally or alternatively, the structural aluminium alloy may comprise up to 0.85 wt% manganese or up to 0.8 wt% manganese.

The structural aluminium alloy may comprise up to 0.1 wt% magnesium.

The structural aluminium alloy may comprise no more than 0.025 wt% of zinc. Typically, the structural aluminium alloy may comprise only trace amounts of zinc.

The structural aluminium alloy may comprise up to 0.25 wt% of other alloying and/or refining elements, which other alloying and/or refining elements may include one or more of chromium, nickel, copper, lead, tin and titanium. In an embodiment, chromium, nickel, lead, tin and titanium together may make up no more than 0.05 wt%, preferably no more than 0.04 wt%, of the structural aluminium alloy. Typically, the remaining balance of the structural aluminium alloy may be made up of aluminium and unavoidable impurities.

In an embodiment, the structural aluminium alloy may be a hypo-eutectic aluminium- silicon alloy comprising or consisting essentially of:

from 8.5 wt% to 11 wt% silicon;

more than 0.15 wt% and up to 0.6 wt% iron;

more than 0.6 wt% and up to 0.8 wt% manganese;

up to 0.25 wt% zinc;

up to 0.1 wt% magnesium;

up to 0.05 wt% strontium;

up to 0.01 wt% molybdenum; up to 0.25 wt% of other alloying and/or refining elements including one or more of chromium, nickel, copper, lead, tin and titanium; and

aluminium and unavoidable impurities as the remaining balance. In an embodiment, the ratio by weight of iron to manganese may be no less than 1:5 and/or no more than 4:5. The ratio by weight of iron to manganese may be no more than or no less than 1:2, 1:3 or 2:5. The ratio by weight of iron to manganese may be around 2:5. Surprisingly, we have found that by controlling the iron to manganese ratio such that it is relatively high, the performance and properties of the alloy may be acceptable for use in die casting of structural components, even if the alloy is substantially free of a grain refining element such as zinc, molybdenum, strontium, sodium or calcium. This may be economically advantageous, since grain refining elements typically may be relatively rare and/or expensive.

Advantageously, since the structural aluminium alloy may contain a relatively high proportion of iron and other impurities, increased use of aluminium from secondary sources, e.g. scrap, during manufacture is enabled. Significant cost and environmental benefits may be realised by making use of recycled aluminium during the manufacture of the structural aluminium alloy.

In an embodiment, as cast the structural aluminium alloy may have: a yield strength of 120 MPa or more or 130 MPa or more; and/or a tensile strength of 250 MPa or more; and/or an elongation of 7% or more.

In an embodiment, after a heat treatment, the structural aluminium alloy may have: a yield strength of 90 MPa or more; and/or a tensile strength of 175 MPa or more; and/or an elongation of 10% or more or 15% or more. In an embodiment, after a heat treatment, the structural aluminium alloy may have: a yield strength of 110 MPa; and/or a tensile strength of 185 MPa; and/or an elongation of 10% or more. The aluminium industry is split between the primary aluminium industry and the secondary aluminium industry. The primary aluminium industry makes "pure" or "raw" aluminium from bauxite by electrolysis, which is a highly energy-intensive process. Currently, for instance, bauxite is mined in Australia before being transported to Iceland, where aluminium is produced by electrolysis. This arrangement can make economic sense, due to the abundant, stable supply and low price of electricity in Iceland. However, it is clearly not ideal given the amount of shipping of material that is required. Typically, "pure" or "raw" aluminium contains no more than 0.1 wt% iron. A standard "pure" aluminium grade is PI 020, which contains 0.1 wt% iron and 0.2 wt% silicon contaminants in the aluminium.

The secondary aluminium industry is made up of producers who use waste or scrap from various sources to produce aluminium. Typical sources include "semis", e.g. old extrusions, old plates or old foil, and waste from several industries such as the automotive, aerospace, electronics and construction industries. The waste may comprise old, no-longer-wanted components, or waste material such as swarf, powder or cuttings produced when manufacturing aluminium articles. Another source of aluminium waste is new production scrap from the primary aluminium producers.

In an embodiment of the invention, the recycled aluminium may come from any available source. The recycled aluminium may come from more than one source, e.g. more than one site. The recycled aluminium may comprise more than one type and/or composition of waste or scrap aluminium. By mixing recycled aluminium from more than one source, it may be possible to control and/or vary the composition of the recycled aluminium, in order to achieve higher efficiencies, depending, for instance, on the desired composition of the structural aluminium alloy that is being produced and the relative cost, availability or abundance of different types of recycled, waste aluminium at the time. New production scrap from primary aluminium producers typically may have a reasonably carefully controlled composition. End of life scrap, or new production scrap, from secondary producers may have a much wider range of alloy compositions. For instance, typically an aluminium extrusion may contain 0.5 wt% iron, plate may contain 0.3 wt% iron and foil might contain 1 wt% iron.

The recycled aluminium may be selected so as to have a predetermined composition.

The method may comprise the preliminary step of measuring or determining the composition of the recycled aluminium.

If the composition of the recycled aluminium is known or has been determined, then the correct ratio of recycled aluminium to primary aluminium and any additional refining and/or alloying elements required to be blended together to provide the desired composition can be readily determined.

In an embodiment, the recycled aluminium may comprise up to 2 wt% iron or up to 1 wt% iron. In an embodiment, the recycled aluminium may be obtained from one or more sites remote from the location where the structural aluminium alloy is being manufactured.

In the manufacture of structural aluminium alloys traditionally there has been a prejudice against using recycled aluminium, due to concerns over perceived quality and/or potential difficulty in controlling the composition of the structural aluminium alloy. The present invention addresses or at least alleviates this prejudice.

A second aspect of the invention provides a method of manufacture of a structural component or article comprising:

· providing a structural aluminium alloy made according to the first aspect of the invention; and • casting, e.g. die casting, the structural component or article from the structural aluminium alloy.

The method may further comprise treating the cast component or article. For instance, the cast component or article may be annealed in accordance with a predetermined heat treatment schedule.

Annealing may improve the ductility of the structural component or article. Accordingly, the structural component or article may fail in a more ductile manner than it would if it had not been annealed.

Typically, annealing may improve the life cycle fatigue of the structural component or article, thereby improving in-service performance and lifetime, e.g. in relatively demanding automotive applications.

The predetermined heat treatment schedule may comprise annealing the cast article or component at a predetermined annealing temperature for a predetermined period of time. The predetermined heat treatment schedule may comprise annealing the cast article or component at a substantially constant annealing temperature.

A third aspect of the invention provides a structural component or article manufactured in accordance with the method of the second aspect of the invention.

The structural component may comprise a structural vehicle component. For instance, the structural vehicle component may comprise a shock tower, e.g. a front shock tower, an engine mount bracket, a chassis component or a suspension system component.

A fourth aspect of the invention provides a die casting tool or machine comprising a chamber containing a structural aluminium alloy, the chamber being in fluid communication with a mould cavity, wherein, in use, the structural aluminium alloy is urgable under pressure form the chamber to the mould cavity, wherein the structural aluminium alloy is manufactured in accordance with the method of the first aspect of the invention.

In order that the invention may be well understood, it will now be described by way of example only with reference to the accompanying drawings, in which:

Figures 1, 2 and 3 are optical micrographs of a first alloy manufactured according to the invention; Figures 4, 5 and 6 are optical micrographs of a second alloy manufactured according to the invention;

Figures 7, 8 and 9 are optical micrographs of a third alloy manufactured according to the invention;

Figures 10, 11 and 12 are optical micrographs of a sample of Castasil®-37;

Figure 13 is an analysis of variance (ANOVA) boxplot of measured mean yield strengths for alloys A, B and C manufactured according to the invention and Castasil®- 37;

Figure 14 is an analysis of variance (ANOVA) boxplot of measured mean tensile strengths for alloys A, B and C manufactured according to the invention and Castasil®- 37;

Figure 15 is an analysis of variance (ANOVA) boxplot of measured mean elongation of alloys A, B and C manufactured according to the invention and Castasil®-37.

Table 1 below details the composition of three example alloys (A, B and C) of an aluminium-silicon alloy manufactured according to the invention. The iron content in alloy A (0.16 wt%) is lower than in alloy B (0.31 wt%), which in turn is lower than in alloy C (0.55 wt%). The iron to manganese ratio by weight in alloy A is 0.22, in alloy B the ratio is 0.43 and in alloy C the ratio is 0.76. All three alloys A, B and C contain only trace amounts of molybdenum. The alloys A, B and C also have a very low strontium content of 70-190 ppm strontium. 25 wt% of the alloys A, B and C came from recycled aluminium. The recycled aluminium contained no more than 1 wt% iron. The alloy could contain a higher or lower proportion of recycled aluminium. The recycled aluminium could have a different iron content.

Table 1

As-cast samples of the alloys A, B and C were heat treated at 385°C for 25 minutes in accordance with a suitable heat treatment process for promoting ductile fracture behaviour.

The micro structure of the heat treated samples was then examined under an optical microscope. In general, the micro structure observed in the samples of alloys A, B and C comprised a-Al, Al-Si eutectic and (FeMn)₃Si₂Ali₅ particles. The (FeMn)₃Si₂Ali₅ particles were found to fall into two distinct size classes. Figures 1, 2 and 3 are optical micrographs of a sample of alloy A. The general microstructure shown comprises a-Al, Al-Si eutectic and (FeMn)₃Si₂Ali₅ particles. In Figure 2, a modified eutectic microstructure can be seen clearly. In Figure 3, "script" form (FeMn)₃Si₂Ali₅ particles can be seen clearly.

3005 (FeMn)₃Si₂Ali₅ particles were counted in the sample of alloy A. There were 8 large particles per mm of the sample. The average particle size of large particles was 18.13 μπι and the average particle size of small particles was 2.84 μπι. Figures 4, 5 and 6 are optical micrographs of a sample of alloy B. The general microstructure shown comprises a-Al, Al-Si eutectic and (FeMn)₃Si₂Ali₅ particles. In Figure 5, a modified eutectic microstructure can be seen clearly. In Figure 6, "script" form (FeMn)₃Si₂Ali₅ particles can be seen clearly. 4441 (FeMn)₃Si₂Ali₅ particles were counted in the sample of alloy B. There were 20 large particles per mm of the sample. The average particle size of large particles was 15.05 μπι and the average particle size of small particles was 2.64 μπι.

Figures 7, 8 and 9 are optical micrographs of a sample of alloy C. The general microstructure shown comprises a-Al, Al-Si eutectic and (FeMn)₃Si₂Ali₅ particles. In Figure 8, a modified eutectic microstructure can be seen clearly. In Figure 9, "script" form (FeMn)₃Si₂Ali₅ particles can be seen clearly.

3795 (FeMn)₃Si₂Ali₅ particles were counted in the sample of alloy C. There were 22 large particles per mm of the sample. The average particle size of large particles was 24.09 μπι and the average particle size of small particles was 2.80 μιη.

Figures 10, 11 and 12 are optical micrographs of a sample of Castasil®-37. Castasil®- 37 is an AlSi9Mn alloy produced by Rheinfelden. Castasil®-37 provides a useful comparison, as it is a speciality alloy used in die casting of structural components, e.g. front shock towers, in the automotive industry. The general microstructure shown in Figures 10, 11 and 12 comprises a-Al, Al-Si eutectic and a low level of (FeMn)₃Si₂Ali₅ particles. In Figure 11, a modified eutectic microstructure can be seen clearly. The average particle size of large (FeMn)₃Si₂Ali₅ particles was 11.20 μιη. The average particle size of small (FeMn)₃Si₂Ali₅ particles was 2.11 μιη.

A series of comparative studies were carried out, in which the properties and performance of heat-treated samples of the alloys A, B and C were compared with Castasil®-37. Castasil®-37 is an AlSi9Mn alloy produced by Rheinfelden. Castasil®- 37 was chosen as a suitable benchmark, because it is a speciality alloy used in die casting of structural components, e.g. front shock towers, in the automotive industry.

The heat treatment schedule was 25 minutes at 385°C.

Table 2 below summarises the results of tensile tests carried out on heat-treated samples of alloys A, B and C and on samples of Castasil®-37.

Table 2

The measured mean yield strengths (measured as 0.2% proof stress, R_po.₂) of heat- treated alloys A, B and C were 93 MPa, 96 MPa and 94 MPa respectively. By comparison, the measured mean yield strength of Castasil®-37 was 102 MPa. The measured mean yield strength of alloy A was 91% of the mean yield strength of Castasil®-37. The measured mean yield strength of alloy B was 94% of the mean yield strength of Castasil®-37. The measured mean yield strength of alloy C was 92% of the mean yield strength of Castasil®-37.

Figure 13 is an analysis of variance (ANOVA) boxplot, which shows that the measured mean yield strengths for alloys A, B and C are statistically different from the measured mean yield strength of Castasil®-37.

Nevertheless, while the measured mean yield strengths of alloys A, B and C are lower than that of Castasil®-37, the difference is not so great that alloys A, B and C would not be acceptable for use as a viable substitute for Castasil®-37 in some instances.

The measured mean tensile strengths (R_m) of heat-treated alloys A, B and C were 178 MPa, 183 MPa and 182 MPa respectively. By comparison, the measured mean tensile strength of Castasil®-37 was 201 MPa.

The measured mean tensile strength of alloy A was 89% of the mean tensile strength of Castasil®-37. The measured mean tensile strength of alloy B was 91% of the mean tensile strength of Castasil®-37. The measured mean tensile strength of alloy C was 91% of the mean yield strength of Castasil®-37.

Figure 14 is an analysis of variance (ANOVA) boxplot, which shows that the measured mean tensile strengths for alloys A, B and C are statistically different from the measured mean tensile strength of Castasil®-37. Nevertheless, while the measured mean tensile strengths of alloys A, B and C are lower than that of Castasil®-37, the difference is not so great that alloys A, B and C would not be acceptable for use as a viable substitute for Castasil®-37 in some instances.

The measured mean elongation of heat-treated alloys A, B and C were 21%, 15% and 17% respectively. By comparison, the measured tensile strength of Castasil®-37 was 15%. Figure 15 is an analysis of variance (ANOVA) boxplot, which shows that the measured mean elongation of alloys A, B and C are statistically identical to the measured mean elongation of Castasil®-37. Accordingly, alloys A, B and C could be acceptable for use as a viable substitute for Castasil®-37 in some instances. Table 3 summarises the results of a series of Vickers hardness (HVio) tests carried out on heat-treated samples of alloys A, B and C and on samples of Castasil®-37.

Table 3

The measured mean hardness (HVio) of heat-treated alloys A, B and C were 59.3, 62.8 and 61.7 respectively. The standard deviations in the results were 2.2, 6.0 and 2.1 for alloys A, B and C respectively. By comparison, the measured mean hardness of Castasil®-37 was 65.8 with a standard deviation of 2.1.

The measured mean hardness of alloy A was 90% of the mean hardness of Castasil®- 37. The measured mean hardness of alloy B was 95% of the mean hardness of Castasil®-37. The measured mean hardness of alloy C was 94% of the mean yield strength of Castasil®-37. While the measured mean hardnesses of alloys A, B and C are lower than that of Castasil®-37, the difference is not so great that alloys A, B and C would not be acceptable for use as a viable substitute for Castasil®-37 in some instances. Rivetability tests showed that the rivetability of alloys A, B and C was similar to that of Castasil®-37. Accordingly, the rivetability tests also indicated that alloys A, B and C could be acceptable for use as a viable substitute for Castasil®-37 in some instances. The mechanical performance and rivetability of the alloys A, B and C in these comparative studies were such as to indicate that the alloys A, B and C could be used as an acceptable substitute for Castasil®-37 in at least some instances. This could provide significant cost savings, since the aluminium- silicon alloy of the invention may be considerably cheaper to manufacture than Castasil®-37.

Table 4 below summarises the results of tensile tests carried out on as cast (pre-heat treatment) samples of alloys A, B and C.

Table 4

As cast alloy A had a measured mean yield strength (Rpo_.2) of 131.6 MPa, a measured mean tensile strength (R_m) of 260.6 MPa and a measured mean elongation of 9.8%. As cast alloy B had a measured mean yield strength (Rpo_.2) of 130.3 MPa, a measured mean tensile strength (R_m) of 260.8 MPa and a measured mean elongation of 9.1%.

As cast alloy C had a measured mean yield strength (Rpo_.2) of 132.8 MPa, a measured mean tensile strength (R_m) of 258.6 MPa and a measured mean elongation of 7.4%.

These measured mechanical properties for alloys A, B and C compare favourably with those of LM24, an aluminium pressure die casting alloy used in automotive chassis applications and non- structural powertrain components. As cast LM24 has a mean yield strength (Rpo_.2) of 100 MPa, a tensile strength (R_m) of 170 MPa and an elongation of 1%.

Accordingly, these results suggest that as cast alloys A, B and C could be a more than acceptable substitute for LM24 in chassis and/or powertrain applications, in particular for applications where improved mechanical properties may be desired. For instance, load-bearing capabilities and/or impact resistance of components may be improved by using as cast alloy A, B or C in place of LM24. In addition to being applicable to the manufacture of a structural aluminium alloy suitable for use in the manufacture, typically by high pressure die casting, of structural automotive components (e.g. any part of a vehicle chassis, shock towers, engine mount brackets and/or suspension system components), it is envisaged that the structural aluminium alloy made in accordance with a method of manufacture according to the present invention may be suitable for use in the manufacture of other, non-structural components such as powertrain components and body parts, e.g. panels. Accordingly, manufacture and assembly of a vehicle may be simplified by using components manufactured in accordance with the invention for a wider variety of applications. Moreover, while components or articles manufactured according to the invention may be especially useful in the automotive industry, they may also be useful in other manufacturing industries, e.g. train manufacture or the manufacture of components for the construction industry. Conveniently, the composition and chemistry of the alloy allows for increased use of secondary sources of aluminium, e.g. waste or scrap aluminium.

Advantageously, components or articles may be relatively cheap to manufacture, since fewer expensive alloying elements such as molybdenum or strontium are required and/or the aluminium- silicon alloy may be manufactured using aluminium from secondary sources. For example, secondary aluminium may be blended with primary aluminium during production of the alloy.

This may be more energy efficient, since it may take only around 5% of the energy to make one tonne of recycled aluminium (i.e. from a secondary source) that it does to make one tonne of primary aluminium. Consequently, it is envisaged that a structural aluminium alloy manufactured in accordance with the invention may be more cost-competitive with steel than known speciality aluminium casting alloys.

Claims

1. A method of manufacture of a structural aluminium alloy having a desired composition, the method comprising:

· blending secondary aluminium obtained from one or more sources with primary aluminium and, optionally, one or more additional refining and/or alloying elements, in order to obtain the desired composition.

2. A method according to claim 1, wherein the alloy comprises 10 wt% or more of recycled aluminium, 25 wt% or more of recycled aluminium and/or less than or at least

50 wt% of recycled aluminium.

3. A method according to claim 1 or claim 2, wherein the desired composition comprises a hypo-eutectic aluminium-silicon alloy comprising more than 0.15 wt% iron.

4. A method according to claim 1, claim 2 or claim 3, wherein the structural aluminium alloy comprises no more than 0.6 wt% iron, no more than 0.55 wt% iron, no more than 0.45 wt% iron, no more than 0.35 wt% iron or no more than 0.3 wt% iron.

5. A method according to any one of the preceding claims, wherein the structural aluminium alloy comprises at least 0.16 wt% iron, at least 0.2 wt% iron, at least 0.25 wt% iron or at least 0.3 wt% iron.

6. A method according to any one of the preceding claims, wherein the structural aluminium alloy comprises from 0.16 wt% to 0.3 wt% iron.

7. A method according to any one of the preceding claims, wherein the structural aluminium alloy comprises: no more than 0.01 wt% molybdenum or no more than 0.001 wt% molybdenum; and/or no more than 0.05 wt% strontium, no more than 0.01 wt% strontium or no more than 0.001 wt% strontium.

8. A method according to any one of the preceding claims, wherein the structural aluminium alloy comprises: at least 0.6 wt% manganese or at least 0.65 wt% manganese; and/or up to 0.85 wt% manganese or up to 0.8 wt% manganese.

9. A method according to any one of the preceding claims, wherein the structural aluminium alloy is a hypo-eutectic aluminium- silicon alloy comprising or consisting essentially of:

from 8.5 wt% to 11 wt% silicon;

more than 0.15 wt% and up to 0.6 wt% iron;

more than 0.6 wt% and up to 0.8 wt% manganese;

up to 0.25 wt% zinc;

up to 0.1 wt% magnesium;

up to 0.05 wt% strontium;

up to 0.01 wt% molybdenum;

up to 0.25 wt% of other alloying and/or refining elements including one or more of chromium, nickel, copper, lead, tin and titanium; and

aluminium and unavoidable impurities as the remaining balance.

10. A method according to any one of the preceding claims, wherein the ratio by weight of iron to manganese is no less than 1:5 and/or no more than 4:5.

11. A method according to any one of the preceding claims, wherein the ratio by weight of iron to manganese is no more than or no less than 1:2, 1:3 or 2:5.

12. A method according to any one of the preceding claims, wherein, as cast, the structural aluminium alloy has: a yield strength of 120 MPa or more or 130 MPa or more; and/or a tensile strength of 250 MPa or more; and/or an elongation of 7% or more.

13. A method according to any one of the preceding claims, wherein, after a heat treatment, the structural aluminium alloy has: a yield strength of 90 MPa or more; and/or a tensile strength of 175 MPa or more; and/or an elongation of 10% or 15% or more.

14. A method according to any one of the preceding claims, wherein the recycled aluminium comes from more than one source.

15. A method according to claim 14, comprising the step of mixing recycled aluminium from more than one source, in order to control and/or vary the composition of the recycled aluminium.

16. A method according to any one of the preceding claims, wherein the recycled aluminium comes from new production scrap and/or end of life scrap.

17. A method according to any one of the preceding claims comprising the preliminary step of measuring or determining the composition of the recycled aluminium.

18. A method according to any one of the preceding claims, wherein the recycled aluminium comprises up to 2 wt% iron or up to 1 wt% iron.

19. A method according to any one of the preceding claims, wherein the recycled aluminium is obtained from one or more sites remote from the location where the structural aluminium alloy is being manufactured.

20. A method of manufacture of a structural component or article comprising:

• providing a structural aluminium alloy made according to any one of claims 1 to 19; and

• casting, e.g. die casting, the structural component or article from the structural aluminium alloy.

21. A method according to claim 20 further comprising treating the cast component or article.

22. A method according to claim 21, wherein the cast component or article is annealed in accordance with a predetermined heat treatment schedule.

23. A structural component or article manufactured in accordance with a method according to any one of claims 19 to 22.

24. A component or article according to claim 23, wherein the component or article comprises a structural vehicle component such as a shock tower, e.g. a front shock tower, an engine mount bracket, a chassis component or a suspension system component.

25. A die casting tool or machine comprising a chamber containing a structural aluminium alloy, the chamber being in fluid communication with a mould cavity, wherein, in use, the structural aluminium alloy is urgable under pressure form the chamber to the mould cavity, wherein the structural aluminium alloy is manufactured in accordance with the method of any one of claims 1 to 19.