GB2209345A - Making aluminium metal-refractory powder composite by milling - Google Patents

Abstract

A mixture consisting of at least 50% by volume of aluminium metal particles and correspondingly less than 50% by volume of refractory material is milled until at least some of the refractory particles are below 1 micron in diameter and at least some are embedded in the metal particles. The particulate composite material so produced may be consolidated to form a solid composite product.

Description

ALUMINIUM MATRIX COMPOSITES This invention relates to a method of making composite materials based on aluminium whose properties are improved, compared to aluminium metal, by a dispersion of refractory particles. The invention involves powder metallurgy, and includes, as a new material in its own right, the particulate composite material produced as an intermediate product. The invention takes advantage of two known phenomena: i) Dispersion strengthening. This phenomenon arises when the dispersed particles are fine (typically 0.05 to 2 microns and preferably 0.1 to 1 micron) and relatively closely spaced. The dispersion strengthening effect arises from the fact that dislocations are forced to go round the particles during deformation. The properties of the dispersoid particles themselves do not contribute to any significant degree to the overall strength of the material.

ii) Composite strengthening. For this phenomenon, the particles are typically 5 to 20 microns in size, i.e. an order of magnitude greater than in dispersion strengthened materials, and the volume fraction of such particles is typically in the range 10% to 40 k. At such particle sizes and volume fractions, the material strength is not governed by dispersion hardening. Rather it is governed in a simplistic sense by the rule of mixtures: P, Pp Vp + Pm m P indicates the property measured V indicates the volume fraction subscripts c, p and m refer to the composite, particle and metal respectively.

Thus to get improved properties in the composite one requires a high volume fraction of particles with a much improved property compared to the metal. Thus the properties of the particles do contribute to the strength of the composite. It is advantageous to have the particulate as fine as possible since large particles promote low fracture toughness and low ductility.

Various attempts have been made to prepare aluminium matrix composite materials. Infiltration of liquid metal into particulate material is possible, but is limited to very high volume fraction composites which have characteristics more akin to ceramics than to metal matrix materials. Molten metal casting routes are only effective with relatively coarse dispersoid particles. Similarly powder metallurgy routes have only been achieved using comparatively coarse particulate strengthening materials. This is because intimate mixtures cannot be achieved due to the relative coarseness of the metal powder.

US Patent 3,816,080 describes composites of aluminium metal dispersion strengthened by means of aluminium oxide. These composites are made by high energy milling of aluminium powder, the particles of which carry a thin oxide coating, to such an extent that the coating is broken up and the resulting particles are mechanically alloyed, i.e. homogeneously distributed in a matrix of the metal, the particles themselves being less than 0.2 microns diameter. It is mentioned that a fine powder of aluminium oxide, or even of a refractory oxide other than alumina, may be added to the powder charge. The method is expensive on both equipment and power, due to the need for prolonged high energy milling. The product is a thermodynamically stable dipsersion-strengthened Alalloy which does not show composite-strengthened characteristics.

In one aspect, this invention provides a method of making a composite material which method comprises providing a mixture of more than 50% by volume of aluminium metal particles and correspondingly less than 50% by volume of particles of refractory material, and milling the mixture to reduce the refractory material particle size until at least some of the particles are below one micron in diameter and at least some are embedded in the metal particles. The resulting particulate composite material may be consolidated to form a solid composite product.

In another aspect, the invention provides a particulate composite material consisting essentially of more than 50 h by volume of aluminium metal particles and correspondingly less than 50% of refractory material in the form of particles at least some of which are below 1 micron in diameter and at least some of which are embedded in the metal particles.

The term "particle" is used herein to denote a comminuted solid, and without reference to particle size. Many of the particles with which this invention is concerned are not even approximately spherical.

The term "particle diameter" refers to the diameter of a sphere of equivalent volume to the particle considered. The term "aluminium" includes both the pure metal and alloys in which aluminium is the major component.

The starting material for the method comprises more than 50% by volume, and preferably, from 60 to 90% by volume, of aluminium metal particles. Although the size of these particles is not critical, their average diameter will typically be in the range 10 to 75 microns. If particles coarser than about 75 microns diameter are used, prolonged milling may be required in order to homogeneously disperse the refractory material. At the other end of the scale, it is both difficult and dangerous to make aluminium particles below about 10 microns diameter.

The refractory material needs to be solid, insoluble in aluminium and inert to aluminium under the processing conditions employed. Suitable materials include refractory nitrides, borides, carbides and oxides of various metals and semi-metals. These are frequently ceramic materials, and some are known as refractory hard metals.

This invention relies at least partly on the known composite strengthening phenomenon described above.

As a result, the best refractory material to use in any particular circumstance will depend on the desired properties of the composite. For example, to improve wear resistance, one might use a refractory oxide such as alumina or zirconia, whereas for a high modulus material silicon carbide or boron carbide might be preferred.

The volume fraction of the added refractory particles in the mixture is less than 50% and is preferably from to 10% to 40 h. Below 10%, the amount of composite strengthening achieved is rather unlikely to be useful. The composite strengthening increases with the volume fraction, but the composite also becomes more brittle. At volume fractions of refractory particles above about 40%, the composite is likely to be too brittle to be useful for many purposes. These figures are in addition to the alumina particles present, typically at a concentration of a few percent, as a result of milling aluminium metal particles having a coating of oxide.

Although there are no critical limits, the starting refractory particles generally have an average diameter greater than 1 micron. It is merely necessary that the refractory particles be broken down to a smaller average diameter during milling.

Milling may be effected using conventional equipment such as a ball mill. High energy equipment such as a stirred ball mill, a shaker mill, a vibratory ball mill or a planetary ball mill may be used to reduce milling time, but are not generally necessary.

It is not necessary, though it may be preferred, to exclude oxygen. To prevent the metal particles from welding together, a lubricant such as stearic acid may be included.

Milling is continued for long enough to achieve three effects. First, the refractory material particle size is reduced.

Second, at least some of the milled refractory material is in the form of particles having diameters below 1 micron. This fraction should be at least 25%, and may be higher, up to 75, although (depending on the nature of the refractory material) that might require prolonged milling. This sub-micron fraction imparts dispersion strengthening properties to the resulting composite. More preferably, there is present 25 - 50 /0 of the refractory material having particle diameters below 0.5 microns, for example from 0.05 microns to 0.2 microns.

Third, milling is continued until a substantial proportion, e.g. at least 50% and preferably more than 95%, of the refractory material particles are embedded in the metal particles. Depending on the nature of the milling operation, the metal particles may be reduced in size thereby, or merely distorted, e.g.

flattened. If the refractory material particles are homogeneously embedded in the surface, or the interior, of the metal particles, that helps to ensure that they are rather homogeneously dispersed in the compacted product. By contrast, a simple powder mixture of aluminium particles with fine refractory material does not give rise to a particularly homogeneous product on compaction.

The resulting particulate composite material can be compacted in various ways, as is well known to workers in the field. The loose powder may be introduced into an aluminium can, which is degassed and thereafter extruded. Or the powder can be compacted, hot, warm or cold into a non porous block, which can then be extruded or rolled or forged. If desired, the resulting solid non-porous wrought product may be worked or heat-treated in known manner to improve its properties.

The resulting solid product contains a substantially uniform dispersion of the refractory material particles. Preferably a volume fraction of at least 5% (more preferably about 10%) is in the form of particles having diameters below I micron, preferably below 0.5 microns, this corresponding to 25 - 50% of the total of refractory particles. This volume fraction imparts dispersion strengthening properties to the composite, and may have the affect of improving its strength and/or fracture toughness.

The remaining coarser fraction of refractory material affects the properties of the product by virtue of the composite strengthening phenomenon referred to above. Particular changes in properties result from the use of particular refractories.

Silicon carbide and boron carbide can give products having a high modulus. Refractory oxides such as alumina and zirconia can improve wear resistance.

Ceramics, which have low or even negative coefficients of thermal expansion, can be used to prepare products showing reduced dimensional changes on heating. All the composites, irrespective of the nature of the refractory material, are likely to show enhanced performamce at elevated temperature. For example: a) High modulus materials for aerospace applications may be prepared by the addition of 20% to 40% by volume of high modulus materials such as silicon carbide or boron carbide, to aluminium alloys particularly high strength alloys such as 7075, 2014 or 8090.

b) Temperature resistant materials with good wear resistance may be prepared by the addition of refractory oxides such as aluminia or zirconia to alloys such as 88Al-12Si for automotive engine parts.

The following examples illustrate the invention.

Example 1 160g of Al - 4.8wt% Cr - l.4wt% Zr - 1.4wit Nm air atomised powder which was sieved to 75 microns in diameter and 409 (i.e. 2Owt%) of zirconium S-oxide (average particle size 11.86 microns with 4.1% less than 5 microns in diameter) were high energy ball milled for 18 hours in a toluene slurry containing 29 of stearic acid. The resultant material was dried and examined in a scanning electron microscope. The aluminium powder particles were coated with fine zirconium - S-oxide particles. There was no evidence that any of the refractory oxide was not incorporated into the aluminium powder particles. The zirconium S-oxide particles on the surface of the powder particles were substantially sub-micron in size i.e.

had been broken down during processing from their original size of rev 11.86 microns. The volume fraction of zirconium S-oxide on the aluminium powder was substantially less than that added to the mixture.

Since no evidence of loose zirconium S-oxide was visible the remaining portion of the zirconium S-oxide had been incorporated within the aluminium powder.

The powder mixture was dried, placed in an aluminium can, degassed and extruded to a round bar.

The fine dispersion of zirconium S-oxide in the aluminium matrix was retained during fabrication.

The micro hardness of the extrusion was 142VPN which was unchanged after elevated temperature exposure at 4000C for periods exceeding 24 hours. Thus such materials are suitable for use in high temperature environments e.g. automotive and aerospace engine components.

If the ball milling is carried out under substantially identical conditions but using a 2Owt% addition of zirconium S-oxide of average particles diameter 1 micron i.e. similar to that produced during the milling of the coarser zirconium S-oxide as described above, then a substantial proportion of zirconium S-oxide does not become incorporated into the aluminium powder which is then unsuitable for further processing.

Example 2 A similar experiment to the above was carried out using a 14 by weight addition of zirconia to 99.7% pure aluminium. The mixing conditions were identical to those above, however the dried powder was fabricated directely to 1 mm sheet material by roll compaction.

The tensile properties of the resulting sheet in the annealed temper were compared with 1060 grade aluminium in the work hardened H16 temper.

0.22 PS TS El Composite 142 MPa 167 MPa 1.1 /0 1060-H16 83 MPa min 97 MPa min - 131 MPa max 1% The composite sheet, even in the annealed temper showed a 0.2% PS 71% higher than 1060 min and TS 27% greater than 1060 mixture when in the work hardened H16 temper.

Claims (10)

1. A method of making a composite material which method comprises providing a mixture of more than 50% by volume of aluminium metal particles and correspondingly less than 50% by volume of particles of refractory material, and milling the mixture to reduce the refractory material particle size until at least 5 volume percent are below 1 micron in diameter and a substantial proportion are embedded in the metal particles.

2. A method as claimed in claim 1, comprising the additional step of compacting the resulting particulate material to form a solid non-porous composite.

3. A method as claimed in claim 2, comprising the additional step of extruding, forging or rolling the solid non-porous composite.

4. A method as claimed in any one of claims 1 to 3, wherein the initial mixture comprises from 60% to 90% by volume of aluminium metal particles and correspondingly from 40% to 10% by volume of particles of refractory material.

5. A method as claimed in any one of claims 1 to 4, wherein milling is continued until the composite contains from 1% to 10 ío by volume of refractory material particles having diameters below 1 micron.

6. A method as claimed in any one of claims 1 to 5, wherein the starting refractory particles have an average diameter greater than 1 micron.

7. A method as claimed in any one of claims 1 to 6, wherein milling is continued until from 25% to 75% of the refractory material is in the form of particles having diameters below 1 micron.

8. A method as claimed in claim 7, wherein milling is continued until from 25% to 50% of the refractory material is in the form of particles having diameters below 0.5 microns.

9. A method as claimed in any one of claims 1 to 8, wherein milling is continued until more than 95% of the refractory material particles are embedded in the metal particles.

10. A particulate composite material consisting essentially of more than 50% by volume of aluminium metal particles and correspondingly less than 50% by volume of refractory material in the form of particles at least some of which are below 1 micron in diameter and at least some of which are embedded in the metal particles.