Composite Metal

Abstract

A metal composite comprising a milled and compacted mixture of powdered aluminium or aluminium alloy and ceramic particles, wherein, on loading of the aluminium with the ceramic particles, the ceramic particles are of an average size of between 0.85 μm and 0.6 μm.

Description

The present invention relates to a composite metal.

It is known that metal alloys exhibit differing properties in accordance with the different constitution of the alloys. Further it is known that non-metallic constituents can have significant effects. Small amounts of carbon changes soft iron into strong and tough steel, although we believe that the mechanism in the case of steel is different from that of the present invention.

We have combined much larger quantities and sizes of ceramic particles, compared with carbon inclusions in steel, into aluminium alloys and achieved significant increases in strength, without compromise to ductility and machinability.

Our method of forming our composite aluminium alloys is essentially as described and claimed in U.S. Pat. No 4,749,545, namely preparing metal matrix composites comprising a hard material selected from silicon carbides, silicon nitrides, silicon oxides, boron carbides, boron nitrides and boron oxides, and a lightweight component selected from aluminium, magnesium and alloys of either, the method comprising:

• • intimately mixing using a high energy milling technique a powder of the hard material and either aluminium or magnesium in its powder form to produce a uniform powder mixture and
  • compacting the powder mixture at elevated temperatures.

Broadly we load aluminium alloy with 3 μm, i.e. 3micron or 3 micrometre, particles of silicon carbide, normally at 25% or 40% by volume. We achieve good strength to weight ratios whilst retaining machinability and ductility, which enables forging of parts from our composite aluminium alloys.

As recorded in U.S. Pat. No. 6,398,843, we have also proposed the use of ceramic particles an order of magnitude smaller, that is of up to 0.4 μm. However we experience difficulty with loadings higher than 10% in that it is difficult to distribute the ceramic particles evenly throughout the aluminium. Further, the particles tend to agglomerate, resulting in weak spots in the finished composite metal.

We have now unexpectedly discovered that we can load aluminium alloys with particles not much larger than 0.4 μm to at least some of the same loadings that we use with 3 μm particles.

The object of the present invention is to provide metal composite.

According to the invention there is provided a metal composite comprising a milled and compacted mixture of powdered aluminium or aluminium alloy and ceramic particles, wherein the ceramic particles are of an average size of between 1.0 μm and 0.5 μm.

Preferably the particles will be between 0.85 μm and 0.6 μm and in particularly between 0.75 μm and 0.65 μm. The single most preferred particle size is 0.7 μm.

We anticipate that the invention will be operable with pure aluminium and with aluminium alloys having single or joint alloy additions of Cu, Mg, Mn, Li, Zn, Si, Zr, Cr, Fe, Ni, Ti. We prefer to use medium strength alloys, in particular aluminium alloys including Cu, Mg, Mn. Where enhanced corrosion resistance (achieved by limiting Cu content) and/or enhanced ductility is required, as with relatively high ceramic particle loading, we prefer to use low strength alloys, in particular aluminium alloys including Mg, Si some copper.

In particular we prefer AA2124 as such a medium strength matrix alloy and AA6061 as such a low strength matrix alloy. These have the compositions shown in the following Table 1:

TABLE 1
Preferred Matrix Alloy Compositions
	(Weight %)	AA2124	AA6061
	Cu	3.6-4.9	0.15-0.40
	Mg	1.2-1.8	0.8-1.2
	Si	0.20 max	0.4-0.8
	Fe	0.30 max	0.70 max
	Zn	0.25 max	—
	Mn	0.4-0.9	—
	Cr	0.1	0.04-0.35

Further we anticipate that the invention will be operable with silicon carbide, boron carbide, aluminium oxide and other ceramics based on metal carbides, oxides or nitrides. Silicon carbide is our preferred ceramic on economic grounds. Again, we anticipate that the invention will be operable for volume percentage loading of ceramic in the aluminium or aluminium alloy of between 15% and 50% and preferably 18% and 40%. The most preferred volume percentages are 18%, 25% and 40%.

Where we specify a ceramic by diameter, we expect that in accordance with industry standards, the size distribution will be Gaussian with upper and lower quartiles at 125% and 75% of the quoted particle size.

To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:

FIG. 1 is a chart showing our existing and proposed loadings and the loading that would be expected if the same correlation were to be applied to use of ceramic particles sized in accordance with the invention;

FIG. 2 are stress-strain plots for the matrix alloy without any reinforcement, our conventional 3.0 μm ceramic particle loaded aluminium alloy composite and 0.7 μm ceramic particle loaded metal composite;

FIG. 3 is a plot of elongation to failure against ceramic particle loading in volume %.

Referring to referring to FIG. 1, our current aluminium alloy composites use from 18% to 40% by volume of 3 μm particles. This is shown at I in FIG. 1. We have also proposed use of up to 0.4 μm particles and have been able to produce composites with good ceramic distribution and without agglomerations, but in practice we have to limit the ceramic content to no more than 10% silicon-carbide by volume. This is shown at II in FIG. 1. Our experience suggests that loading with more than 10% of 0.4 μm particles leads to lack of homogeneity and performance degradation, in particular to occurrence of unpredictable weak spots in the alloy.

Our new aluminium alloy composite preferably uses 0.7 μm particles. Assuming a straight line correlation, we would have expected to have been able to use only up to 13.5% of such particles, as shown at III. In fact we have been able to use 40%, shown at IV, which is unexpected. Indeed it appears that we can use the same loadings by volume as with particles, despite the particle size being much closer to 0.4 μm with it loading limitation than 3 μm.

Referring to FIG. 2, the stress/strain curve are shown for:

• • 1. Our preferred matrix alloy AA2124 alone,
  • 2. The same matrix alloy loaded conventionally with 25% of 3.0 μm ceramic particles,
  • 3. The same matrix alloy with the improved loading of 25% of 0.7 μm ceramic particles.

Not only is the improved composite metal much stronger than the matrix alloy alone, but also it has 30% improvement in yield strength Y, or elastic limit, just past the limit of proportionality, compared with our conventional composite metal for the same loading of larger particles.

In the greater portion of the elastic region, the two composite metals behave similarly, but in the plastic region, whilst the shapes of the curves are similar, the 0.7 μm ceramic particles deforms at higher stress corresponding to the 30% improvement in yield strength.

The density, Young's modulus, strain to fail, yield strength and ultimate strengths of plain alloy and the 3.0 μm and 0.7 μm, at 18 and 25 volume % loaded composite aluminium alloys are as follows in Table 2 for as-compacted billet:

TABLE 2
Comparison of Properties of Existing and Improved Metal Composites - as compacted billet.
	Baseline	3.0 μm	0.7 μm	3.0 μm	0.7 μm
	Alloy	SiC—18% vol %	SiC—18% vol %	SiC—25% vol %	SiC—25% vol %
Property Comparison	2124	2124/SiC/18p	2124/SiC/18p	2124/SiC/25p	2124/SiC/25p
	(T4)	as-compacted	as-compacted	as-compacted	as-compacted
		billet (T4)	billet (T4)	billet (T4)	billet (T4)
Density (g/cc)	2.77	2.85	2.85	2.88	2.88
Tensile Modulus	72	100	100	115	115
(GPa)
Strain to Failure	12	4	4	2	2
(%)
0.2% Yield Stress	325	420	490	460	600
(MPa)
Ultimate Tensile	470	555	600	580	680
Strength (MPa)

From Table 2, it will be noted that the properties of the 0.7 μm particle composites alloys are for each of the properties not only greatly improved over the plain alloy (with the exception of the expected decrease in strain to failure) but also either the same as or improved with respect to the properties of the 3.0 μm particle composites alloys.

Again the density, Young's modulus, strain to fail, yield strength and ultimate strengths of of plain alloy and the 3.0 μm and 0.7 μm, at 18% loaded composite aluminium alloys are as follows in Table 3 for extruded bar:

TABLE 3
Comparison of Properties of Existing and
Improved Metal Composites - as extruded.
	Baseline	3.0 μm	0.7 μm
	Alloy	SiC—18% vol %	SiC—18% vol %
Property Comparison	2124	2124/SiC/18p	2124/SiC/18p
	(T4)	Extrusion (T4)	Extrusion (T4)
Density (g/cc)	2.77	2.85	2.85
Tensile Modulus	72	100	100
(GPa)
Strain to Failure	12	7	7
(%)
0.2% Yield Strength	325	420	480
(MPa)
Ultimate Tensile	470	620	680
Strength (Mpa)

Again Table 3 shows the same or improved properties through use of 0.7 μm particle size.

Looked at differently, from Table 2 and Table 3 it can be seen that there is a significant advantage to strength using in the 0.7 μm ceramic reinforcement and that this is achieved without the detriment to the strain to failure, as might be expected.

Turning to FIG. 3, the strain to failure or elongation is shown as a function of loading of the matrix metal with different sizes of ceramic particles. The plots 4,5 for 3.0 μm and 0.7 μm are very similar and distinct from the 0.4 μm particle size plot 6. It is surprising in view of the distinction between the 3.0 μm and 0.4 μm plots that the 0.7 μm plot tracks 3.0 μm plot so closely.

It is our experience that our composite aluminium alloys having the mechanical properties shown in FIG. 2 are readily machinable, but tool wear may be evident and increase the cost of manufacture. Therefore we expect the composites of the invention to be more readily machinable with less tool wear offering a significant economic advantage.

In summary of the properties of the specific aluminium alloy composites described above, use of 0.7 μm silicon carbide particle reinforcement directly in place of 3.0 μm reinforcement in AA2124 achieves enhanced results, which is surprising in view of your earlier experience with 0.4 μm silicon carbide particle reinforcement.

Claims

1. A metal composite comprising a milled and compacted mixture of powdered aluminium or aluminium alloy and ceramic particles, wherein, on loading of the aluminium with the ceramic particles, the ceramic particles are of an average size of between 0.85 μm and 0.6 μm.

2. A metal composite as claimed in claim 1, wherein the ceramic particles are of an average size of between 0.75 μm and 0.65 μm.

3. A metal composite as claimed in claim 1, wherein the ceramic particles are of 0.7 μm in size.

4. A metal composite as claimed claim 1, wherein the aluminium is pure aluminium.

5. A metal composite as claimed in claim 1, wherein the aluminium alloy is one having single or joint alloy additions of Cu, Mg, Mn, Li, Zn, Si, Zr, Cr, Fe, Ni, Ti.

6. A metal composite as claimed in claim 6, wherein the aluminium alloy is a medium strength alloy including Cu, Mg and Mn.

7. A metal composite as claimed in claim 7, wherein the medium strength alloy is AA2124,

8. A metal composite as claimed in claim 6, wherein the aluminium alloy is a low strength alloy including Mg, Si and Cu.

9. A metal composite as claimed in claim 6, wherein the low strength alloy is AA6061.

10. A metal composite as claimed in claim 1, wherein the ceramic particles of silicon carbide, boron carbide or aluminium oxide.

11. A metal composite as claimed in claim 1, wherein the volume percentage loading of ceramic particles in the aluminium or aluminium alloy is between 15% and 50%.

12. A metal composite as claimed in claim 1, wherein the volume percentage loading of ceramic particles in the aluminium or aluminium alloy is between 18% and 40%.

13. A metal composite as claimed in claim 1, wherein the volume percentage loading of ceramic particles in the aluminium or aluminium alloy is 18%, 25% or 40%.

14. (canceled)