GB2179369A

GB2179369A - Sintered aluminium alloy

Info

Publication number: GB2179369A
Application number: GB08619047A
Authority: GB
Inventors: Murdo Simon Maclean
Original assignee: UK Secretary of State for Trade and Industry
Current assignee: UK Secretary of State for Trade and Industry
Priority date: 1985-08-06
Filing date: 1986-08-05
Publication date: 1987-03-04
Also published as: GB8619047D0; GB2179369B; GB8519691D0

Abstract

A sintered aluminium alloy is formed from a green compact containing 4-12% by weight of an active ingredient consisting of one or more of iron, nickel and chromium, the balance being aluminium or aluminium based alloy containing at least 90% aluminium. The active ingredient has a particle size of not greater than 60 microns, the aluminium or aluminium alloy a particle size not greater than 120 microns, and the compact is formed with a compaction pressure in the range 60-120 MPa. The compact is sintered at a temperature which remains below the melting point of pure aluminium after initiation of an exothermic reaction between the aluminium and the active ingredient. The resulting alloy contains dispersed zones of intermetallic compounds in a matrix of aluminium or aluminium based alloy.

Description

SPECIFICATION Sintered aluminium alloys This invention relates to sintered aluminium alloys.

An industrial requirement exists for aluminium alloys which have high temperature strength and stability coupled with good wear resistance. The problem of satisfying both requirements in asingle material presents considerable difficulty.

Conventional aluminium alloys containing copper or magnesium exhibit age hardening, and although these alloys have good mechanical properties at relatively low temperatures, unfortunately they have low wear resistance. Furthermore, at temperatures in excess of 180"C these alloys average, resulting in detrioration of strength.

For the production of pistons, cylinder liners, or for other applications where a combination of high temperature strength and wear resistance is required, aluminium based casting alloys containing high levels of silicon, together with some copper and other ingredients are currently used.

However, the use of silicon has the adverse effect of lowering the melting point of the alloy by about 90"C, and, depending upon the other additions, this may be as much as 125"C below that of pure aluminium. This reduces the high temperature strength and increases the tendency to heat cracking.

In UK Patent No 1381145 there is disclosed a sintered aluminium alloy composition comprising 5 to 10% of iron, nickel or chromium together with 0.5 to 5% of silicon carbide. These alloys are said to exhibit good high temperature strength and wear resistance. However, the processing route required to realise these advantages in practice has been reported elsewhere as complex, and to require cold pressing, warm repressing, and subsequent hot forging to final shape. This prior patent contains no indication that any particular choice of particle sizes in the powder constituents might lead to any beneficial effect.

The problems inherent in the production of sintered aluminium alloys including iron, nickel or chromium arise from the fact that an intense exothermic reaction occurs during the sintering process. In the course of the reaction the aluminium melts, and there is an abrupt expansion of the sintering mass, and local weaknesses occur in the resultant alloy. The problems are well documented for the case of iron/aluminium, (with a preponderance of iron), in for example articles "Powder Metallurgy of Iron-Aluminium" by J S Sheasby in Volume 15 No 4, 1979, pages 301-305 of The International Journal of Powder Metallurgy and Powder Technology, and "Sintering Behaviour of Iron-Alloy Powder Mixes" by D J Lee and R M German in Volume 21 No 1, 1985, pages 9-20 of the same Journal.

Aluminium alloys having compositions similar to those of UK-A-13811145, but produced by processes other than Powder Metallurgy (sintering being a Powder Metallurgy process) are described in many publications. For example UK Patent Application 2088409A and US Patent 4347076 describe the production of alloys by the rapid cooling (typically 105 degrees C per second) of molten mixtures. This is a method requiring complicated and expensive equipment.

UK Patent 1498357 describes electrical conductors made by a method involving extrusion, and UK Patent 868769 describes an alloy produced by compression and extrusion "to produce a shearing effect" of a mixture. UK Patent 846,530 also describes an alloy produced by hotworking, it being a requirement of the claimed alloy that "the iron-containing constituent in the hot-worked article (is) present in the form of finely divided uniformly distributed insoluble particles having a maximum thickness of 0.4 micron". UK Patent 516474 describes a method of producing an abrasive article containing "abrasive grains, for example, diamonds, and a sintered bond consisting entirely of aluminium or an aluminium base alloy".The method claimed invoives the formation of a powdered mixture of abrasive, aluminium and a metal, pressurising to deform the metal particles, and then sintering.

The present invention seeks to make possible the provision of a sintered aluminium alloy having good high temperature strength, wear resistance, and a relatively simple production route.

Accordingly, in a first aspect, the invention provides a method of producing a sintered aluminium alloy comprising the steps of: (i) forming a green compact comprising a total of 4-12% by weight of an active ingredient in powder form consisting of one or more of iron, nickle and chromium and having a particle size not greater than 60 microns, the balance (other than incidental elements and impurities) being aluminium or an aluminium-based alloy containing at least 90% aluminium in the form of a powder having a particle size not greater than 120 microns, and (ii) sintering the compact at a temperature below the melting point of pure aluminium but sufficient to initiate an exothermic reaction between the aluminium and the active ingredient, whereby a sintered aluminium alloy is formed containing distinct dispersed zones of one or more intermetallic compounds in a matrix of aluminium or aluminium-based alloy.

It has been found that by keeping a strict control of the particle size of the active ingredient metal, of the compaction pressure, and of the rate of heating of the compact, the temperature at which the onset of the intense exothermal reaction, described by Sheasby in his article "Sintering Behaviour of Iron-Alloy Powder Mixes", occurs can be controlled to such that the overall temperature of the compact does not rise above the melting point of the pure aluminium.

It is believed that the intense exothermic reaction occurs around the individual particles of the active ingredient during sintering, leading to brief localised melting around these nucleating sites.

These zones contain a dispersed, relatively hard intermetallic compound leading to a considerable increase in strength, hardness and wear resistance.

The dispersed nature of the intermetallic compound provides a highly irregular outline which interlocks with the matrix material so as to provide an alloy which is resistant to crack propagation.

The particle sizes specified for the active ingredient are smaller than would normally be used for sintering. It has been found that this is necessary in order to produce the desired exothermic reaction during the sintering process.

Preferably the particle size of the active ingredient is less than 45 microns.

The compaction pressure at which the green compact is formed is preferably in the range 60-120 MPa, most preferably about 100 MPa. This compares with a conventional compaction pressure of about 1 50T250 MPa for sintering aluminium powder. The exothermic reaction is accompanied by some expansion, which will lead to cracking or blistering if the compaction pressure is too high.

The nature of the alloys produced in accordance with the invention is such as to provide a most suitable base for the production of metal matrix composites using the conventional press sinter technique.

Metal matrix composites usually comprise a metal or alloy base within which is contained a ceramic, carbide, or glossy addition in particulate or fibre form. The term ceramic as used herein includes such carbides, glassy or other equivalent materials. The level of these additions normally exceeds 10% by weight, and their size, shape and form may be varied to suit the particular material application. Their benefits include increased strength, modulus of elasticity, improved mass/strength ratios, etc. Various methods for the production of aluminium composites are known, including the casting of mixes or slurries, casting molten aluminium through ceramic fibres, co-spraying aluminium and ceramic powders, rolling, and forging mixtures of metal and ceramic powders.

The difficulty with the direct powder route is that the compactibility of the mixture decreases with increasing non-metallic additions, and a porous compact results. For example with 40% ceramic addition to aluminium powder, a porosity of about 15-20% can be expected even with compaction pressures as high as 600 MPa.

The process in accordance with the present invention can alleviate or overcome these problems to enable the production of dense aluminium matrix composites using a direct compact and sinter technique.

According to an advantageous feature of the invention, the green compact forms part of a blended mixture, up to 60% by weight, preferably no more than 40% by weight, of the mixture being of a powdered or fibrous ceramic material. In the case of a powdered ceramic material, the particle size should be in the range 20 to 200 microns.

The ceramic material has a beneficial moderating effect upon the exothermic reaction, in that its presence introduces a degree of porosity in the green compact resulting from the reduced compactibility noted hereinbefore. The presence of pores in the green compact provides for expansion of the liquid phase produced during the exothermic reaction. The porosity is thereby reduced, whiie at the same time the formation of cracks and blisters, which would otherwise result from the expansion, is inhibited. This leads to a product having high strength and integrity, and the presence of the hard non-metallic addition increases wear resistance.

When ceramic material is included in the composition, considerably higher compaction pressures may be used. For example, with a 40% by weight ceramic content, a compaction pressure of up to 600 MPa may be applied.

Alloys produced in accordance with the invention which contain ceramic material are particularly suitable as bearing materials, since they contain materials having three different hardness values, viz the ceramic material which is normally very hard at greater than 1000 VPN, the intermetallic compound at nominally say 600 VPN, and the relatively soft aluminium or aluminium alloy matrix material at say 45-80 VPN.

A ceramic material which has been found suitable is silicon carbide (SiC). Other ceramic materials such as other carbides, zirconia, magnesia, alumina or sialons may be effective.

In all cases the sintering parameters need to be controlled in order that the exothermic reaction can proceed as desired.

The aim in all cases is to bring the mixture to the point at which the intermetallic reaction takes place with a proportion of the mixture remaining below the melting point of the matrix material, ie 664"C in the case of pure aluminium, and lower in the case of an aluminium alloy.

This leads to the formation of the desired dispersed intermetallic compound in discrete zones within the matrix.

A typical sintering temperature would be in the range 600-640"C, and a typical rate of heating would be about 10-20"C/min, preferably 15 C/min.

A sintering temperature 615"C+5"C has been found suitable.

In the case of iron as the active ingredient, once the bulk temperature of the green compact reaches a level of 580 C nominal, the exothermic reaction may begin. The precise value of the temperature at which the reaction begins will depend upon the active ingredient used, and the nature and condition of the other powder ingredients.

The result of the exothermic reaction is to cause a brief localised melting, leading to the formation of a dispersed intermetallic dispersion results in a general overall increase in the mechanical properties such as hardness and strength.

In a further aspect, the invention provides a sintered aluminium alloy comprising a matrix of aluminium or aluminium alloy containing at least 90% aluminium, the matrix containing distinct zones of at least one intermetallic compound having an irregular dispersed form, the said intermetallic compound being formed of aluminium combined with an active ingredient consisting of at least one of iron, nickel or chromium, the active ingredient forming 4-12% by weight of the alloy composition.

The alloy may also contain up to 60% by weight, most desirably 10 to 40% by weight of ceramic material in particulate or fibrous form dispersed therein.

The ceramic material may be for example alumina, or carbides (such as silicon carbide) zirconia, magnesia, or sialons, although many other ceramic materials are suitable.

The invention will now be described for the purposes of illustration only, with reference to the following examples.

Example 1 An intimate blend was prepared comprising 10% by weight of an active ingredient in the form of a--10 micron iron powder, and the balance-i 25 micron aluminium powder. The mixture was compacted isostatically at a pressure of 15,000 Ib/sq in and subsequently placed in a sintering furnace. The temperature of the furnace was raised from ambient towards a set temperature of 620"C at a rate of 1O"C/min; the temperature rise of the compact is plotted to a time base in Fig. 1.

An exothermic reaction occurred within the compact when the bulk temperature thereof reached 580 C, as can be seen at 1 in Fig. 1. Sintering was continued until the exotherm was completed.

The resulting component was sound, and had the following mechanical properties: 0. 1% Proof Stress: 50 MPa Ultimate Tensile Strength: 108 MPa % elongation to failure: 9. 1 % Fig. 2 is a micrograph of a cut, polished and etched section which shows the microstructure of the component, to a magnification of 300.

As can be seen therein, the microstructure of the resulting specimen comprises a matrix 2 of aluminium represented by the light areas of the figure, containing a dispersed intermetallic phase 3 represented by the dark areas of the figure. The intermetallic phase has a highly irregular outline providing a highly keyed microstructure. The intermetallic phase 3 is of an aluminium/iron intermetallic compound, probably Al3 Fe.

Example 2 An intimate blend was prepared comprising 10% by weight of an active ingredient in the form of - 10 micron iron powder, 40% by weight of alumina (Al203) powder of 125 micron grit size, and the balance - 1 25 micron aluminium powder.

The mixture was compacted in a conventional die set using die wall lubrication (for convenience) and employing double action pressing. A pressure of 600 MPa was used, and the resulting green compact was sintered as described in Example 1.

An exothermic reaction was again produced, and the presence of the ceramic material resulted in an exotherm again with the bulk of the material at 580"C, but the plateau was of reduced duration in this case as compared with Example 1.

The resulting component was removed from the furnace after the exotherm was complete, and was found to be sound.

Fig. 3 is a micrograph of a cut, polished and etched surface which shows the microstructure of the component to a magnification of 50.

The microstructure of the resulting specimen comprises a matrix of aluminium containing a dispersed Al/Fe intermetallic phase similar to that of the component formed in Example 1, although the detail is less clear because the magnification is less. This is represented in Fig. 3 by the textured background areas 4. Also visible in Fig. 3 are individual ceramic particles represented by the dark areas 5. These ceramic particles form a strong mechanical interlock with the surround metallic material consequent upon localised melting.

Claims

1. A method of producing a sintered aluminium alloy, comprising the steps of: (i) forming a green compact comprising a total of 4-12% by weight of an active ingredient in powder form consisting of one or more of iron, nickel and chromium and having a particle size not greater than 60 microns, the balance (other than incidental elements and impurities) being aliminium or an aluminium-based alloy containing at least 90% aluminium in the forms of a powder having a particle size not greater than 120 microns, and (ii) sintering the compact at a temperature below the melting point of pure aluminium but sufficient to initiate an exothermic reaction between the aluminium and the active ingredient, whereby a sintered aluminium alloy is formed containing distinct dispersed zones of one or more intermetallic compounds in a matrix of aluminium or aluminium-based alloy.

2. A method according to claim 1 wherein the particle size of the active ingredient is less than 45 microns.

3. A method according to claim 1 or claim 2 wherein the green compact is formed at a composition pressure in the range 60-120 MPa.

4. A method according to claim 3 wherein the pressure is about 100 MPa.

5. A method according to any one preceding claim wherein the green compact forms part of a blended mixture, up to 60% by weight of the mixture being of a powdered or fibrous ceramic material.

6. A method according to claim 5 comprising a powder ceramic material having a particle size in the range 20 to 200 microns.

7. A method according to claim 5 or claim 6 wherein the mixture contains up to 40% by weight of the ceramic material.

8. A method according to any one of claims 5 to 7 wherein the mixture is formed at a compaction pressure in the range 80 to 600 MPa.

9. A method according to any one of claims 5 to 8 wherein the ceramic material is selected from the group comprising alumina, carbides, zirconia, magnesia, and sialons.

10. A method according to any one preceding claim wherein the sintering temperature is in the range 600-640"C.

11. A method according to claim 10 wherein the sintering temperature is 615 C+5 C.

12. A sintered aluminium alloy comprising a matrix of aluminium or aluminium alloy containing at least 90% aluminium, the matrix containing distinct zones of at least one intermetallic compound having an irregular dispersed form, the said intermetallic compound being formed of aluminium combined with an active ingredient consisting of at least one iron, nickel or chromium, the active ingredient forming 4-12% by weight of the alloy composition.

13. A sintered aluminium alloy according to claim 12 and containing up to 60% by weight of ceramic material.

14. A sintered aluminium alloy according to claim 12 and containing 10 to 40% by weight of ceramic material.

15. A sintered aluminium alloy according to claim 13 or claim 14 wherein the ceramic material is selected from the group comprising alumina, carbides, zirconia, magnesia, or sialons.

16. A method of producing a sintered aluminium alloy substantially as herein described in Example 1 or Example 2 hereof.

17. A sintered aluminium alloy produced substantially as hereinbefore described in Example 1 or Example 2 hereof.