EP 0 746 633 B1 discloses a method for the production of an aluminium component. This method comprises the steps of producing a first powder which is a near-eutectic aluminium-silicon based alloy containing from 9 to 13 wt% silicon ; producing a second powder which is a hypereutectic aluminium- silicon based alloy ; producing a powder mixture comprising desired proportions of the first powder, and the second powder, compacting the powder mixture, and sintering the compacted powder. In this method, the first powder and the second powder are in relative proportions to one another lying in the range from 25: 75% to 75: 25%. This document explains that the method gives the advantages that aluminium components, which are suitable for some wear resistance and structural applications, can be more easily produced by a near net-shape compaction-and-sinter powder metallurgy route.

As explained in EP 0 746 633 B1, the term"near-eutectic"aluminium- silicon based alloy refers to an alloy with 9 to 13 wt% of silicon. The position of the eutectic point is influenced by any additional alloying elements present which may be included to confer improved properties. A hypereutectic aluminium-silicon alloy is defined as comprising more than 13 wt% of silicon.

While the method of EP 0 746 633 B1 is successful in producing components suitable for many applications, it is found that, in some cases, such components do not have the mechanical properties that are desirable in some applications. Accordingly, it is an object of the present invention to provide a method for the production of an aluminium component which enables an aluminium component having improved mechanical properties to be manufactured.

The invention provides a method for the production of an aluminium component by a powder metallurgy route, the method comprising the steps of producing a first powder which is a near-eutectic aluminium-silicon based alloy containing from 9 to 13 wt% silicon; and producing a second powder which is a hypereutectic aluminium-silicon based alloy ; characterised in that the method also comprises producing a powder mixture comprising desired proportions of the first powder, the second powder, and zinc or zinc-based powder, the first powder and the second powder being in relative proportions to one another lying in the range from 25: 75% to 75: 25%, and the zinc forming at least 0.5 wt% of the powder mixture, the method also comprising compacting the powder mixture and sintering the compacted powder.

In a method according to the invention, it is found that components produced have improved mechanical properties, which are conveniently measured as increased transverse rupture strength. Furthermore, it is surprisingly found that the addition of zinc powder in the proportion indicated does not cause swelling of the component during sintering as happens when zinc is. added to elemental blends. Indeed, it is found that shrinkage occurs to a comparable extent with components manufactured by the method of EP 0 746 633 B1, shrinkage being desirable to a limited extent particularly where the component is to be re-pressed to achieve correct/desired finished dimensions.

Herein, the terms"near-eutectic"and"hypereutectic"are used in the same sense as in EP 0 746 633 B1. Furthermore, as disclosed in that document the alloys may contain additional alloying elements and the powder mixture may also contain additions such as a fugitive lubricant wax (for example atomised ethylenebisstearamide) to aid pressing, sintering aids etc. In a method according to the invention, it is also possible to include in the powder mixture 1 to 5 wt% of a solid lubricant such as graphite, mica, talc or tungsten disulphide. It is also possible to include similar quantities of other particles to increase wear resistance, eg inorganic carbides.

Where zinc-based powder is used in a method according to the invention, it may be a zinc powder containing iron in solid solution, eg up to 5 wt%. In some cases, residue from galvanising operations provides a source of suitable powder.

Preferably, in a method according to the invention, the zinc forms 1 to 8 wt% (more preferably 2 to 6 wt%) of the powder mixture. It is found that zinc levels below 0.5 wt% have little effect on the mechanical properties while levels above about 8 wt% have a detrimental effect on the surface quality of the component.

Preferably, in a method according to the invention, the zinc or zinc- based powder has a particle size in the range 30 to 150 microns, as it is found that larger particle sizes have a detrimental effect on the surface quality of the component.

Preferably, in order to give improvements in compressibility, the method also comprises annealing the first powder and/or the second powder prior to producing the powder mixture.

The invention enables manufacture of an aluminium component made ,. * by a powder metallurgy route, the aluminium component having a structure comprising at least two interpenetrating reticular structures derived from the original powder particles, said at least two reticular structures including a first structure derived from a first powder comprising a near-eutectic aluminium- silicon based material containing from 9 to 13 wt% silicon, and a second structure derived from a second powder comprising a hypereutectic aluminium- silicon based material, said aluminium component having relative proportions of said first and said second structures lying in the range from 25: 75% to 75: 25%, the aluminium component also comprising at least 0.5 wt% of zinc derived from the original powder particles.

A component according to the invention may, for example, be a bearing cap for a camshaft or a bushing.

There now follows a detailed description of an illustrative method in accordance with the invention, and of a component made by the illustrative method which is illustrative of the invention in its component aspects.

The illustrative method is a method for the production of an aluminium component by a powder metallurgy route. The illustrative method comprises the steps of producing a first powder which is a near-eutectic aluminium-silicon based alloy containing about 11 wt% silicon, specifically the alloy contained 10.23. wt% silicon, 1.04 wt% copper, 0.05 wt% magnesium, 0.16 wt% iron, 0.04 wt% manganese, and the balance substantially aluminium. The oxygen content of the first powder was 0.147 wt%. The illustrative method also comprises producing a second powder which is a hypereutectic aluminium- silicon based alloy, specifically this alloy had a composition of 17.7 wt% silicon, 4.20 wt% copper, 0.55 wt% magnesium, 0.35 wt% iron, 0.23 wt% manganese, and the balance substantially aluminium. The oxygen content of the second powder was 0.115 wt%. The first and the second powders were produced by conventional air atomising.

Next, in the illustrative method, the first and the second powder were annealed by heating in a protected atmosphere.

"The illustrative method also comprises producing a powder mixture comprising desired proportions of the first powder, the second powder, and zinc or zinc-based powder. In the powder mixture, the first powder and the second powder were in equal proportions, each forming 47.5 wt% of the mixture. The zinc or zinc-based powder has an average particle size of 100 microns and forms 5 wt% of the powder mixture.

The illustrative method also comprises compacting the powder mixture and sintering the compacted powder, thereby forming an aluminium component. It is believed that, in the illustrative method, the zinc acts to break- up oxide layers on the aluminium particles.

The aluminium component produced by the illustrative method has a structure comprising two interpenetrating reticular structures derived from the original alloy powder particles. The two reticular structures include a first structure derived from the first alloy powder and a second structure derived from the second alloy powder. The aluminium component also comprises 5 wt% of zinc derived from the original powder particles.

The aluminium component made by the illustrative method was found to have increased transverse rupture strength compared to a component made by the same method but without zinc in the powder mixture which was 50 wt% of the first alloy powder and 50 wt% of the second alloy powder. Specifically, the transverse rupture strength was approximately 200 MPa as compared with approximately 140 MPa. The average radial shrinkage was found to be substantially equal in both components.

A component made by the illustrative method can be impregnated with oil to make it self-lubricating.

Figure 1 shows a graph plotting the transverse rupture strength in MPa (Y axis) against zinc content in the powder mixture with the remainder of the mixture being formed from equal proportions of the first and the second alloy powders. Figure 1 shows that a significant increase in transverse rupture strength is achieved with a 0.5% or more zinc addition which allows rupture of oxide layers, and that, at approximately 6% zinc addition, the increase in transverse rupture strength levels off.

Figure 2 shows a graph plotting the transverse rupture strength in MPa (Y axis) against average zinc particle size in microns, the zinc content being 5 wt% as in the illustrative method. It can be seen that all particle sizes below 30 microns there is little effect on the transverse rupture strength and above 150 microns the strength levels off.

Figure 3 shows a graph plotting pressed density in grammes/cc (Y axis) against pressing load in MPa. The upper curve represents the powder mixture used in the illustrative method (with the first and the second powder having been annealed) and the lower curve represents a similar method to the illustrative method but omitting annealing of the powders (ie the first and the second powder were as atomised).

The tensile properties of the component made by the illustrative method were tested before and after re-pressing at 460 MPa. Before repressing tensile strength was found to be 99.03 MPa and after re-pressing 123.89 MPa. The comparable figures for ductility were 0.76% and 0.32%.

There now follows a table illustrating the tensile properties as sintered and as re-pressed for components manufactured by the illustrative method (samples 7 and 8) and by variations of the illustrative method with different percentages of zinc and different proportions of the first and the second powders. Sample % Zinc Ratio Alloy Repressed, Tensile Ductility, Identity 1 : 2 460 MPa Strength, % MPa 1 3.5 40:60 No 81.03 1.11 2 3.5 40:60 Yes 118.73 0.34 3 3. 5 50 : 50 No 89. 00 0. 96 4 3. 5 50 : 50 Yes 123. 33 0. 33 5 40 : 60 No 84. 65 0. 83 6 5 40 : 60 Yes 114. 16 0. 29 7 5 50 : 50 No 99. 03 0. 76 8 5 50 : 50 Yes 123. 89'0. 32 5 60 : 40 No 89. 87 054 10 5 60 : 40 Yes 107. 96 0. 28