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(11) | EP 0 227 352 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Metal matrix composites |
(57) A composite material comprising a refractory, inorganic filler material (e.g. SiC
or Al₂O₃) in a matrix of aluminium or an aluminium-containing alloy is made by incorporating
the filler material into the aluminium or aluminium-containing alloy at a temperature
above its liquidus, e.g. by vacuum stirring, followed by cooling to solidify the composite.
In order to improve the wettability of the filler by the molten metal, the filler
has a bismuth additive, for example in the form of a bismuth compound such as bismuth
oxide, dispersed therein. This may be done by intimately mixing elemental bismuth
with particulate filler followed by oxidation of the bismuth to bismuth oxide. e.g.
by heating in air.
[0001] This invention relates to a method of making metal matrix composites where the metal is aluminium or an alloy thereof.
[0002] Metal matrix composites comprising a refractory inorganic filler material such as silicon carbide or alumina in a matrix of aluminium or an alloy thereof are useful engineering materials, for example where high strength and low density are required as in the motor vehicle and aerospace industries. However, molten Al or Al alloy cannot wet the solid filler material sufficiently for such composites to be made satisfactorily by stirring the filler into the molten metal.
[0003] It has now been found that this problem may be ameliorated by using a particular additive with the filler. Thus, the invention provides a method of making a solid composite material comprising a refractory, inorganic filler material in a matrix of aluminium or an aluminium-containing alloy such as an aluminium base alloy, which comprises
(i) preparing a uniform dispersion of a bismuth additive in the filler material;
(ii) incorporating the dispersion into the aluminium or aluminium-containing alloy, the aluminium or aluminium-containing alloy being at a temperature above its liquidus; and
(iii) cooling the product of step (ii) to produce a solid composite material.
[0004] It has been found, in specific examples of this method, that the molten Al or alloy takes up at least twice the amount of filler by weight when the bismuth additive is present than when it is absent, and further that the filler is more uniformly dispersed in the matrix when a bismuth additive is used.
[0005] The filler material may be a material known in the art for enhancing the properties of aluminium and aluminium base alloys. Examples are refractory carbides such as silicon carbide and refractory oxides such as alumina.
[0006] Preferably the bismuth additive is a bismuth compound such as bismuth oxide when the dispersion may be prepared in step (i) by intimately mixing fine particle elemental bismuth with particulate filler material, for example of particle size in the range of 5 µm to 100 µm wherein the weight of bismuth is preferably less than 10% of the weight of the filler, followed by oxidising the elemental bismuth to bismuth oxide (Bi₂O₃), e.g. by heating in air at 500°C to 1000°C. Heating in air causes the bismuth to burn and generate smoke thereby facilitating production of a fine oxide dispersion.
[0007] It may however be possible to prepare the dispersion in step (i) in other ways, for example by mixing the filler material with an aqueous solution of a decomposable bismuth salt such as bismuth nitrate followed by heating to decompose the salt to bismuth oxide.
[0008] Step (ii) may be carried out by the known technique of vacuum stirring wherein the dispersion of step (i) is stirred into the molten aluminium or aluminium base alloy in vacuo. The aluminium or aluminium base alloy may be at a temperature considerably above its liquidus, for example at 150°C above its liquidus.
[0009] The presence of bismuth compounds is known to embrittle aluminium and aluminium base alloys at elevated temperature. It may, therefore, be necessary to add a material capable of reacting with surplus bismuth additive, i.e. a bismuth "getter", between steps (ii) and (iii). Examples of such getters are magnesium and manganese. However, a bismuth getter may not be necessary in all cases; for example, an aluminium base alloy, if used, may contain a getter as a component thereof and hence provide at sufficient quantity of the getter.
[0010] To produce a finished shaped article, the composite material produced in step (iii) may, for example, be pressure die cast, squeeze cast, chill cast or wrought. However, a shaped composite may be produced directly in step (iii) by cooling the product of step (ii) in a mould.
[0011] The composite produced by this invention may have a range of compositions subject to product and/or processing requirements and restrictions.
EXAMPLE
[0013] Step (i) Elemental bismuth in the form of fine filings was added to fused and crushed alumina (approximately 9 µm particle size), wherein the weight of bismuth was 2% of that of the alumina, and thoroughly mixed by barrelling. The resulting mixture was then heated in air at 550°C for 2 hours to oxidise the bismuth to bismuth oxide thereby producing a uniform dispersion of bismuth oxide in alumina.
[0014] Step (ii) The above dispersion was stirred into an aluminium base alloy (LM6, containing ≃ 10.5% Si) in vacuo for 2 hours in the temperature range of 740°C to 770°C.
[0015] Step (iii) The molten product of step (ii) was cooled to the freezing point of the alloy in about half an hour.
[0016] In the resulting material, the alloy had taken up about 20% of its weight of alumina which was fairly uniformly distributed therein on a macro scale. The material was capable of being melted and chill cast and pressure die cast without undue difficulty. In particular, a sample of the material was heated to 850°C and poured through a 10 mm hole under a head of approximately 20 mm to show that it was sufficiently fluid when molten to be cast. The material was then heated to 850°C and cast into a cast iron mould. Sections of the resulting casting were examined microscopically where it was observed that the alumina filler material had been retained and that its distribution in the matrix was, if anything, more uniform than in the composite material product of step (iii).
1. A method of making a solid composite material comprising a refractory, inorganic
filler material in a matrix of aluminium or an aluminium-containing alloy, which comprises
(a) incorporating the filler material into the aluminium or aluminium-containing alloy, the aluminium or aluminium-containing alloy being at a temperature above its liquidus; and
(b) cooling the product of step (a) to produce a solid composite material,
characterised in that the filler material has a bismuth additive uniformly dispersed therein.
2. A method according to claim 1 wherein the bismuth additive is bismuth oxide.
3. A method according to claim 2 wherein the bismuth oxide is dispersed in the filler
material by intimately mixing elemental bismuth therewith, followed by oxidising the
elemental bismuth to bismuth oxide.
4. A method according to claim 3 wherein the bismuth is oxidised by heating it in
air.
5. A method according to claim 4 wherein the temperature of the heating is in the
range of 500°C to 1000°C.
6. A method according to any of the preceding claims wherein the filler material has
a particle size in the range of 5 µm to 100 µm.
7. A method according to any of the preceding claims wherein step (a) comprises vacuum
stirring.
8. A method according to any of the preceding claims wherein the filler material is
silicon carbide or alumina.
9. A method according to any of the preceding claims wherein the solid composite material
is melted and cast to give a shaped article.
10. A method according to any of the preceding claims wherein the matrix is of an
aluminium-base alloy.