This invention relates to sintered aluminium nickel alloys, and in particular to such alloys containing ceramic materials.

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 a single 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 overage, resulting in deterioration 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 1331145 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.

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 10 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 hot-working, 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 involves the formation of a powdered mixture of abrasive, aluminium and a metal, pressurising to deform the metal particles, and then sintering. The present Applicant's Patent Application GB2179369 A describes a method of producing a sintered aluminium alloy having good high temperature strength, wear resistance, and a relatively simple production route.

The Applicant has now found an improved method of producing a sintered aluminium nickel alloy.

According to the present invention a method of producing a sintered aluminium nickel alloy includes the steps of oxidising the surface of particles of nickel powder,mixing the surface oxidised particles with aluminium powder, and sintering the mixture.

In an extension of this method particles of ceramic powder are coated with nickel, the surface of which is oxidised, the resultant powder being mixed with aluminium powder and sintered.

The aluminium in he aluminium powder may already be in an alloyed form.

Some embodiments of the invention will now be described, by way o_f example only.

Aluminium Nickel Patents Wear Resistant Material Activated Sintering

1. This application is based upon prior patent application GBT A - 2179369.

2. The basis of this invention is the utilisation of the exothermal reaction generated between the elemental Aluminium and Nickel powders, contained in a powder metallurgy compaction (either die or isostatically) to produce localised melting and thus create a liquid phase sinter. The claims are similar

As per earlier patent application.

3. The earlier application, primarily based on the Aluminium - Irβn system had one specific drawback -

It resulted in a strong exotherm and higher localised temps. When used with pre-alloy aluminium powders, or elemental alloy powders, the alloy element preferentially diffused to the high temperature, localised molten zone thus denuding the matrix ofit's alloy content and creating a segregated structure in the reduction of copper magnesium and silicon then within the matrix had the result of reducing the hardness andmechanical properties that should be obtained on heat treatment had segregation not occurred.

4. This problem can be largely overcome by the use of Nickel as an active alloying addition.

The use of Nickel as an elemental alloying element results in a. An exotherm starting about 5 0°C b. A less intense exotherm than that obtained with, iron c. Lower localised temperatures d. A liquid phase sinter is still obtained e. An intermetallic AlNi is obtained which improves strength and f. minimal segregation in the aluminium alloy material eg 6001 or 2016 alloys g. results in an alloy which will fully respond to convential heat treatments eg quench and age

h. densffication' luring sintering.

i. shorter sintering cycles.

5. The base matrix powder may be: a. Pure aluminium powder b. Prealloyed powder to compositions to alloys 6061 or 2014 any suitable alloy of particle size less than 125 microns, preferably less than 45 microns c. Elemental alloys to 6061 or 2014 compositions or any compositions or any suitable composition.

6. The Nickel Powder as the reactive material may be of the types

Carbonyl Nickel type 123 " type 254 " type 255 " type HDNP

7. Percentage of Nickel powder blended with the Aluminium, or Aluminium alloy powder (elemental or prealloy) should be within the range 4 to 16% by weight.

8. The intensity of the exothermal reaction increased with - a. increasing nickel additions b. decreasing particle size of Nickel powder. c. increased surface yield on Nickel powder selective oxidation

The particle size may be within th range of 1.5 to 20 micron and surface oxide levels between 500 and 2000 ppm

9. It was established that 10% HDNP Nickel in 45 micron aluminium powder did not produce an exotherm

By selective oxidation of the powder to an oxide level of 1750 ppm an exotherm was generated at a higher threshold of 570°C

SUBSTITUTE SHEET

Selective oxidation may be used to intensify the exotherm of any of the Nickel types, or generate an exotherm in specific conditions.

10. The use of Nickel allows a modified approach to the inclusion of wearresistant particles - ceramics carbides borides nitrides oranysuitable addition. The material must be capable of being Nickel coated using either the Sherrif Process or by the dissipation of Nickel Carbonyl gas in a tower or chamber.

11. The powder material usually in excess of 10 microns particle size may be coated with Nickel to a thickness between 3 and 15 microns.

12. Additions of this nickel coated powder may be made to elemental aluminium or aluminium alloy powders (prealloy or elemental). Sufficient nickel must be present to induce an exotherm. eg 10 % by weight or greater.

13. After convential composition, the mixture should be sintered at 610°C nominally to produce an exothermal reaction.

14. In practice, when an exotherm has been found difficult to generate, selective oxidation has been found to produce conditions compatible to the formation of an exotherm

15. When small additions of the ceramic or wear resistant coating are being made eg - less than 10% to encourage the exotherm, additional Nickel powder may be added to bring the total nickel content to around 1 or greater.

16. The result of the nickel coating orv the hard particles is to ai produce an exotherm and localised h) melting around each particle thus locking into the matrix c. create a liquid phase sintex with subsequent densi- fication I claim that by using Nickel powder on an elemental alloy addition in Aluminium or Aluminium alloys powder (prealloys or elemental ) I can

SUBSTITUTE SH

a. produce enhanced liquid phase sinter b. an increase in density c. an increase in strength.and ductility d. reduced segregation (compared with the Al - Fe system) e. produce Al Ni as a hard wear resistant dispersive phase.

By Nickel plating the wear resistant ceramic particles, carbides borides etc I claim that a. these are locked into the matrix by a liquid phase reaction around their outer surface b. Result in a wear resistant material of high modules and reduced thermal conductivity c. Uniquely increased density on sintering, close to theoretical

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