GB2522716A - Method of manufacture - Google Patents

Abstract

A method of manufacturing an article by casting the article from a hypo-eutectic aluminium-silicon alloy comprising (by weight): 8.5-11 % silicon, 0.6-0.85 % manganese, >0.15-0.6 % iron, up to 0.25 % zinc, up to 0.1 % magnesium, not more than 0.05 % strontium, not more than 0.001 % molybdenum, up to 0.25 % other alloying and/or refining elements including one or more of Cr, Ni, Cu, Pb, Sn, Ti, with the balance being aluminium and impurities, and then annealing the cast alloy at a temperature in the range 300-400 0C for 15-35 minutes. The Fe:Mn ratio preferably lies in the range 1:5 to 4:5. The alloy can be cast into an ingot or into a vehicle component such as a front shock tower or an engine mount bracket. The article can have a yield strength of 90 MPa or more, a tensile strength of 175 MPa or more and an elongation of 8 % or more.

Description

Method of Manufacture The present invention relates to the manufacture of aluminium components or articles by casting, in particu'ar die casting.

In the automotive industry, it is known to manufacture vehicle structural components by die casting aluminium alloys. By using aluminiuni 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 component depends on the chemical composition and structure of the aluminium alloy used as well as the machine setting and the process selected, Ductility i'nay be particuarIy important, particulady in components of complex design, e.g. a component with a complex geometry and thin walls. Thus, a die casting alloy should typically have a high elongation to fracture in the cast state.

In addition, the alloy should typically be easfly 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 I 1.0 wt% sihcon. 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 to 30 ppm sodium and/or I to 30 ppm calcium for permanent refinement, An A1Si9Mn pressure die casting alloy within the scope of US 6,824,737 is produced and sold commercially by Rheinfelden as Castasil®-37, Aluminium casting alloys, such as the alloy disclosed in US 6,824,737, typically may be relatively expensive and consequently may not be cost-competitive with other materia's, 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.

In order to produce a component such as an automotive structural component with the required mechanical properties for its intended use and service life, it may be necessary to further treat, e.g. by annealing, the component after it have been cast.

Such further treatment can strongly influence the mechanical properties of the final S component. Accordingly, it is generally desired to optimise any such further treatment for a given component type and/or material, A first aspect of the invention provides a method of manufacture of an article or component comprising: casting an intermediate article or component from a hvpo-eutectic aluminium-silicon alloy comprising morc than 0.15 wt% irom and annealing the intermediate article or component in accordance with a predetermined heat treatment scheduk.

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

Typically, annealing may improve the life cycle fatigue of the 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 intermediate article or component at a predetermined annealing temperature for a predetermined period of time, The predetermined heat treatment schedule may comprise annealing the intermediate article or component at a substantially constant annealing temperature.

In an embodiment, the annealing temperature may be selected to be 300°C or more and/or no more than 400°C. For example. the annealing temperature may be at least 350°C and/or up to 385°C. The annealing temperature may be at least 365°C or no more than 365°C.

In an embodiment, the predetermined period of time may be at least 15 minutes or at least 20 minutes. The predetermined period of time may be no more than 35 minutes or no more than 30 minutes. The predetermined period of time may be 25 minutes.

S The aluminium-silicon aHoy contains a relatively high proportion of iron. Typicafly, in prior art aluminium casting 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 folLnd that the strength of the alloy may be improved by having a higher iron content, In an embodiment, the aluminium-silicon alloy may comprise no more than 0.6 wt'3'o 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 additionafly. the aluminium-silicon 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 aluminium-silicon alloy may comprise from 0.16 wt% to 0.3 wt% iron.

The aluminium-silicon alloy may comprise no more thal1 0.01 wt% molybdenum or no more than 0,00 1 wt% molybdenum, in an embodiment, the aluminium-silicon alloy may be substantially free of molybdenum. Conveniently, no additional molybdenum may need to be added to the aHoy during manufacture, This may be economically advantageous, since molybdenum is a relatively expensive alloying addition.

The aluminium-silicon 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 aluminium-silicon 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-euteetic aluminium-silicon alloy having a mierostrueture and mechanical properties suitable for die casting of components.

The aluminium-silicon alloy may comprise at least 8.5 wt% silicon, e.g. 9 wt% silicon or more. The aluminium-silicon alloy may comprise up to 11 wt?/o silicon. e.g. 10,5 wt% silicon or less.

S The aluminium-silicon aHoy may comprise at least 0,6 wt% manganese or at least 0,65 wt% manganese. Additionally or alternatively, the aluminium-silicon alloy may comprise up to 0.85 wt% manganese or up to 0.8 wt% manganese.

The aluminium-silicon alloy may comprise up to 0,1 wt% magnesium.

The aluminium-silicon alloy may comprise no more than 0,025 wt% of zinc, Typically, the aluminium-silicon alloy may comprise only trace amounts of zinc.

The aluminium-silicon alloy may comprise up to 0.25 wt% of other alloying and/or refining clements 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, kad, tin and titanium together may make up no more than 0,05 wt%, preferably no more than 0.04 wt%. of the aluminium-silicon alloy, Typically, the remaining balance of the aluminium-silicon alloy may be made up of aluminium and unavoidable impurities.

In an embodiment, the hypo-eutectic aluminium-silicon alloy may comprise from 0.16 wt% to 0,3 wt% iron and from 0,6 wt% to 0.8 wt% manganese.

In an embodiment, the hypo-eutectic aluminium-silicon alloy may comprise or consist 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.

Tn an embodiment, the ratio by weight of iron to manganese may be no tess 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 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 aluminium-silicon aHoy may contain a r&atively high proportion of iron and other impurities, increased use of aluminium from secondary sources. e.g. scrap, during manufacture may be enabled, Significant cost and environmental benefits may be realised by making IL5C of secondary aluminium during the manufacture of the aluminium-silicon aHoy.

Conveniently, the intermediate article or component may be made by die casting, e.g. high pressure die casting (HPDC).

The article or component may comprise an ingot for subsequent processing or a structural or non-structural component, e.g. for a vehicle.

The structur& component may comprise a shock tower. e.g. a front shock tower, or an engine mount bracket, Advantageously, a front shock tower or an engine mount bracket made according to the method of manufacture of the invention may be used in place of a front shock tower or an engine mount made from a typically relatively more expensive speciality casting aHoy such as CastasiLk-37.

In an embodiment, the article or component 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 8% or more.

Tn an embodiment, the article or component may have: a yield strength of I 10 MPa or more: and/or a tensile strcngth of 185 MPa or more; and/or an elongation of 10% or more.

A second aspect of the invention provides an article or component manufactured according to the method of the first aspect of the invention.

In order that the invention can be well understood, it will now be described by way of example only with reference to the accompanying drawings in which: Figurc 1 is an analysis of variancc (ANOVA) boxplot of mcasurcd yicld strength for sanipks taken from a front shock tower made in accordance with the invention; Figure 2 is an ANOVA boxplot of measured tensile strength for samples taken from a front shock tower made in accordance with the invention; Figure 3 is an ANOVA boxpot of measured dongation for samples taken from a front shock tower made in accordance with the invention; Figure 4 is a graph showing fatigue damage curves for samples taken from a front shock tower made in accordance with the invention; Figure 5 is a graph showing (s-N) for samples taken from a front shock tower made in accordance with the invention; Figure 6 is a graph comparing a combined fatigue damage curve for heat treated samples taken from a front shock tower made in accordance with the invention with a fatigue damage curve for a sample made from heat treated Castasilg)-37; and Figure 7 is a graph comparing a combined cyclic stress-strain curve for heat treated samp'es taken from a front shock tower made in accordance with the invention with a cyclic stress-strain curve for a sample made from heat treated Castasil®-37.

S Tensile testing was carried out on 12 "plate" samples taken from front shock towers die cast from a hypo-eutectic aluminium-silicon alloy comprising more than 0.15 wt% iron.

Three of the samples were as-cast (AC). Three of the samples were annealed at 350°C for 25 minutes (HT1). Three of the samples were annealed at 365°C for 25 minutes (HT2). Three of the samples werc annealed at 385°C for 25 minutes (HT3).

Table I below summarises the results of the tensfle tests carried out on the samples, AC HT1 HT2 HT3 R01(MPa) Mean 128 109 103 98 B10 122 106 100 93 Rm(MPa) Mean 278 221 210 199 B10 272 218 206 197 Aj%) Mean 11.3 12.5 16,9 18,0 B0 9.3 8,2 14 15,4

Table 1

The measured mean yield strength (measured as 0,2% proof stress. R02) of the as-cast (AC) alloy was 128 MPa. with a lower confidence bound B10 of 122 MPa, The measured mean yield strengths of the heat-trcated alloys HT1. HT2. HT3 were 109 MPa 103 MPa and 98 MPa respectively. Lower bounds B10 for yield strengths of the HT I, HT2 and HT3 alloys were 106 MPa. 100 MPa and 93 MPa respectively.

The measured mean tensile strength (R1) of the AC alloy was 278 MPa, with a lower confidence bound B10 272 MPa. The measured mean tensile strengths of the HTI, HT2 and HT3 aHoys were 221 MPa. 210 MPa and 199 MPa respectively, Lower bounds B10 for tensile strengths of the HT1, HT2 and HT3 alloys were 218 MPa, 206 MPa and 197 MPa.

The measured mean extension (Ar) of the AC alloy was 11.3%, with a lower confidence bound B1 of 9.3%. The measured extensions of the HTI, HT2 and HT3 alloys were 12,5%, 16.9% and 18.0% respectively. Lower bounds B10 for extensions of the HTI. HT2 and HT3 afloys were 8.2%, 14% and 15.4%.

The lower bound tensile properties of the as-east (AC) samples (R02 = 122 MPa. Rni = 272 MPa and A = 9.3%) compared favourably with the minima (R02 = 120 MPa, R111 = 280 MPa and A = 10%) quoted for as-east Castasil®-37 by its producer, Rheinfelden. Accordingly, these data suggest that the as-cast alloy could be used as a viable alternative to Castasil®-37 in at least some instances, In general, ductility increased with annealing temperature. However, there was no significant difference between the ductility of the AC samples and the HT I samples (i.e. annealed at 350°C for 25 minutes), Also, there was no significant difference between the ductility of the HT2 samples (treated at 365°C for 25 minutes) and the HT3 samples (treated at 3 85°C for 25 minutes).

Figure 1 is an analysis of variance (ANOVA) boxplot, which shows that the measured yield strength of the four sample types, AC, HT1, HT2 and HT3. are statistically different from each other.

Figure 2 is an ANOVA boxplot, which shows that the measured tensile strength of the four sample types, AC, HT1. HT2 and HT3. are statistically different from each other.

Figure 3 is an ANOVA boxplot for the measured elongation for the four sample types, AC. HT1. HT2 and HT3. The data sets for AC and HT1 constitute one population, while the data sets for HT2 and HT3 constitute a separate, statistically different population.

The results indicate that annealing the alloy at 350°C for 25 minutes (HTI) may not significantly increase the ductility of the alloy. In contrast, there is a significant increase in ductility when the alloy is heat treated at 365°C or 380°C for 25 minutes.

Raising the annealing temperature by 15°C from 350°C to 365°C had a significant effect on ductility; a 35% increase in mean measured elongation (mean A for HTI = 12.5°%; mean A for HT2 = 16.9%) was observed. Raising the annealing temperature by 15°C from 365°C to 380°C had a less significant effect on ductility.

The optimum heat treatment schedule (annealing temperature(s) and time) may vary S for each aHoy composition and each article or component being manufactured, Life cycle fatigue tests have been carried out on samples from front shock towers made from the same alloys as discussed above in relation to the tensile tests. 11 as-cast (AC) samples were tested. In addition, 10 HTI samp'es. 6 HT2 samples and 8 HT3 samples were tested.

Figure 4 is a graph showing fatigue damage (a-N) curves for samples taken from a front shock tower made in accordance with the invention, Four curves are p'otted on the graph: a first curve 1 for as-cast (AC) alloy; a second curve 2 for HT1 alloy; a third curve 3 for HT2 alloy; and a fourth curve 4 for HT3 alloy.

Figure 5 is a graph showing cyclic stress-strain (G-s) curves for samples taken from a front shock tower made in accordance with the invention, Four curves are plotted on the graph: a first curve S for as-cast (AC) alloy; a second curve 6 for HT1 alloy; a third curve 7 for HT2 alloy: and a fourth curve 8 for I-1T3 alloy.

The fatigue damage (a-N) data sets for the heat treated alloys. i.e. the HTI, HT2 and HT3 data sets, were combined into a single, combined data set. In Figure 6, a fatigue damage (s-N) curve 9 for the combined data set is shown, along with a fatigue-damage (a-N) curve 10 for heat treated Castasil®-37. The raw data points for the combined data set are indicated by triangles. The raw data points for the Castasil®-37 data set are indicated by circles.

The cyclic stress-strain (a-c) data sets for the heat treated alloys, i.e. the HTI. HT2 and HT3 data sets, were combined into a sing'e, combined data set. In Figure 7, a cyclic stress-strain (c-c) curve 11 for the combined data set is shown, along with a cyclic stress-strain (c-c) curve 12 for heat treated CastasilK-37. The raw data points for the combined data set are indicated by triangles. The raw data points for the Castasil®-37 data set are indicated by circles, As can be seen from Figures 6 and 7, the life cycle fatigue properties of the example heat treated afloys HTI. HT2 and HT3 of the invention are comparable with those of heat treated Castasil®-37, Accordingly, these results suggest that the heat treated alloys HTI, HT2 and HT3 cou'd be used as a viable afternative to heat treated Castasil®-37 in at least some instances.

Table 2 below summarises the calculated life cycle fatigue parameters for the as-cast (AC) alloy, thc EIT1 alloy, thc I-1T2 alloy, thc I-1T3 alloy, thc combincd data sct (Combined HT) and heat treated Castasil®-37 (Castasfl®-37).

B s1' c K' n' E (MPa) (MPa) (MPa) AC 597 -01200 1.1171 -0.8850 349 0.0698 75541 HT1 546 -0,1540 0.2901 -0.5610 394 0.1942 74756 HT2 544 -0,1640 0.3736 -0.6030 313 0.1714 74515 HT3 423 -0.1400 0.2184 -0.5470 302 0.1662 75555 Combincd 480 -01460 0.2594 -0.5580 336 0.1775 74962

HT

Castasil(!-342 -0.1140 0.2635 -0.5890 347 0.1704 76062 37 HT

Table 2

The data shown in Table 2 also indicate that the heat treated aHoys HTI, HT2 and HT3 could be used as a viable alternative to heat treated Castasil®-37 in at least some instances.

A scrics of rig tests wcrc carricd out on front shock towers madc from: hcat trcatcd Castasil®-37; a heat-treated alloy according to the invention; and an as-cast aHoy of the same composition as the heat-treated alloy according to the invention, The rig tests measured the static strength of the front shock towers. The resufts are summarised in Table 3 below.

Heat treated Heat treated As-east Castasil®-37 alloy alloy "Modu'us" (kNmm") 21.9 20.7 21,5 Mean maximum load (kN) 13 I,S 124.4 1 2,7 Mean displacement at maximum load (mm) 17.6 13,4 12,2

Table 3

The results presented in Table 3 indicated that the front shock towers made from the heat treated aHoy have comparable static strength properties to the front shock towers made from heat treated Castasil®-37, Accordingly, front shock towers made from the heat treated alloy could be a viable alternative to front shock towers made from heat treated CastasilR)-37.

In addition to being applicable to the manufacture, typically by high pressure die casting, of structural automotive components (e.g. and part of a vehicle chassis, shock towers, engine mount brackets and/or suspension system components), it is envisaged that the method of manufacture according to the present invention may be apphcabe to the manufacture of other, non-structural components such as powertrain components and body parts, e.g. panels. Accordingly, manufacture and assembly of a IS 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 aHoy may allow 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 aiuminium-sihcon alloy may be manufactured using a'uminium from secondary sources. For exampk, secondary aluminium may be blended with primary aluniinium 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.

Claims (7)

1. Claims 1, A method of manufacture of an article or component comprising: casting an intermediate artic'e or component from a hvpo-eutectic &uminium-silicon alloy comprising more than 0,15 wt% iron; and annealing the intermediate article or component in accordance with a predetermined heat treatment schedule.
2. 2, A method according to claim 1. wherein the intermediate article or component is annealed at an annealing temperature of 300°C or more and/or no more than 400°C.
3. 3. A method according to claim 2 wherein the annealing temperature is at least 350°C and/or up to 385°C.
4. 4. A method according to claim 2 or claim 3. wherein the annealing temperature is at least 365°C or no more than 365°C.
5. 5, A method according to any one of the preceding claims, wherein the intermediate article or component is annealed for a predetermined period of timc of: at least 15 minutes or at least 20 minutes: and/or no more than 35 minutes or no more than 30 minutes.
6. 6. A method according to any one of the preceding claims wherein the aluminium-silicon aHoy comprises no more than 0,6 wt% iron, no more than 0.55 wt% iron, no more than 0,45 wt°/o iron, no more than 0,35 wt% iron or no more than 0,3 wt% iron.
7. 7. A method according to claim 6. wherein the aluminium-silicon alloy comprises from 0,I6wt%to 0.3 wt% iron, 8, A method according to any one of the preceding claims, wherein the aluminium-silicon alloy comprises no more than 0.01 wt% molybdenum or no more than 0.001 wt% m&vbdenurn or is substantiaflv free of molybdenum.9. A method according to any one of the preceding claims, wherein the aluminium-silicon aHoy comprises no more than 0,05 wt% strontium, no more than 0,01 wt% strontium or no more than 0.001 wt% strontium.10, A method according to any one of the preceding claims, wherein the aluminium-silicon 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.11. A method of manufacture according to any one of the preceding claims, wherein, in the aluminium-silicon alloy, the ratio by weight of iron to manganese is no less than 1:5 and/or no more than 4:5, 12, A method according to any one of the preceding claims, wherein the intermediate article or component is made by die casting.13. A method according to any one of the preceding claims, wherein the artide or component comprises an ingot for subsequent processing or a structnr& or non-structural component. e.g. for a vehicle.14. A method according to claim 13, wherein the structural component comprises a shock tower, e.g. a front shock tower, or an engine mount bracket.15. A method according to any one of the preceding claims, wherein the article or component has: a yie'd strength of 90 MPa or more: and/or a tensile strength of 175 MPa or more; and/or an elongation of 8% or morc.16. A method according to any one of claims 1 to 14, whercin the article or component has: a yield strength of 110 MPa or more; and/or a tensile strength of 185 MPa or more: and/or an elongation of 10% or more, 17. An article or component manufactured according to the method of any one of claims 1 to 16.18, A method of manufacture substantiaHy as described herein with reference to the accompanying drawings.19, An article or component substantiafly as described herein with reference to the accompanying drawings