GB2174717A - Improvements in or relating to the production of composite materials - Google Patents

Abstract

Composite material comprising a lead-containing copper-based alloy on steel is produced by depositing a stream of copper alloy particles at least some of which are partially liquid, on to a ferrous substrate within a chamber the ferrous substrate being at a temperature between 500 DEG C and the solidus temperature of the copper alloy matrix being deposited, the chamber further containing a gas atmosphere having reducing properties and wherein the copper-based alloy contains a maximum of 0.15% by weight of phosphorus and the gas atmosphere has a dew point of not higher than -20 DEG C.

Description

SPECIFICATION Improvements in or relating to the production of composite materials The present invention relates to the production of alloys and in particulartothe production of copper alloys bonded on to a steel substrate.

Copper alloys containing alloying additions of, for example, lead andtin are frequently used for bearing applications. Usuallythe copper alloys are bonded to a backing substrate material such as steel. Methods of depositing alloys on to the backing material include sintering of a powder layer to the substrate, casting of molten metal and spraying.

The sintering route is expensive since the copper alloy needs first to be converted to a powder which then itself needs processing to obtain the correct size fractions etc. The powder is then sintered to the backing under a protective atmosphere, rolled to compactthe porous layer, re-sintered and then probably size rolled. Material utilisation is relatively low, energy consumption is high and capital investment in plant and machinery is also high. The sintering route is, however, flexible in that thick layers of alloy may be applied and the same sintering plant may be used to produce material with both thin strip in coil form and thick plate in lengths for the steel backing. The casting route has its drawbacks since it is also expensive and only applicable to coating flat or concave surfaces.It is not possible, for example, to coatthe outside of a round tube or semi-circular half-shell and similarly it is not possible or is very difficu It to use the same plant for both coil strip and plate lengths.

Metal spraying has been used in the pastforthe deposition of metal at relatively high rates. Brooks, in GB 1 ,379,261,describes the deposition of metal and alloys to manufacture shaped articles by directing a stream of atomised molten metal into a shaped mould and then subsequently working the shaped mass after removal from the mould. Singer, in GB 1,359,486, describes a method for the production of alloys deposited onto a backing strip with particular referenceto aluminium alloys having high levels of lead therein. Molten aluminium-lead alloy or molten alumi nium and molten lead separately are allowed tofall from a crucible under gravity and are then atomised by gas jets, the resulting particles ofthe metal impinging on a backing strip.It is intended that a composite strip should result from this process and in one paragraph the text mentions that the method is equally applicable to producing copper-lead alloys on a backing. This, however, has not proved to be the case. It has been found that in attempts to produce a composite material comprising a copper based alloy strongly bonded onto a steel backing it has not been possible to effect a sufficiently strong bond. Material has been produced and test samples for bond evaluation have been manufactured, the bond tests being carried out using standardised procedures. The results obtained from these tests have in many instances been found to be only about halfthose values obtained on material produced by the other conventional methods of powder sintering and strip casting referred to above.

The ability to produce strong bonds between copper-based lead-containing alloys and steel by spray deposition of atomised metal would be advantageous because of the relatively high deposition rates, and hence reduced production costs, which may be achieved by the process and also the additional ability to easily coat rounded or otherwise convex surfaces.

It has now been discovered that the conditions under which the process is operated and the impurity levels of the copper alloys themselves are critcial to the strength ofthe bond formed between the alloy and the steel. More particularly it has been found that the bond strength is dependent upon the oxygen and hence the moisture content of the protective gas atmosophere within the process chamber and on the phosphorus content of the copper alloy.

According to the present invention a processforthe production of composite material comprising a lead containing copper-based alloy on a ferrous substrate comprises depositing a stream of copper alloy particles, at least some of which are partially liquid, on to a ferrous substrate within a chamber, the ferrous substrate being at a temperature between 500"C and the solidus temperature of the copper alloy matrix being deposited, the chamberfurther containing a gas atmosphere having reducing properties and wherein the copper-based alloy contains a maximum of 0.15% by weight of phosphorus andthe gas atmosphere has adewpointofnothigherthan -20"C.

It has been found that where the copper-based alloy has a phosphorus content exceeding 0.15% by weight then the formation of a brittle phase corresponding to FE3P may occur nearthe alloy/steel interface. This phase is not necessarily observable underthe optical microscope but has been identified by electron microprobe analysis. Preferably the phosphorus content may be maintained below 0.10% or more preferably below 0.05%.

Furthermore it has been discovered that if the dew pointofthe gas atmosphere is higherthan -20Cthe oxides which form on a steel substrate are not reducable at the temperatures and underthe gas atmospheres normally employed.

The gas atmosphere may comprise pure hydrogen but for economic and safety reasons it is preferred that gas with a lower hydrogen content is used such as, for example, a mixture of hydrogen and nitrogen with a hydrogen content below 20% and preferably below 10% by volume The oxygen content of the gas may preferably be below 376 p.p.m. and more preferably may be below 100 p.p.m.

The dew point of the gas atmosphere may preferably be below -30 C since for practical reasons not only the thermodynamics of the reactions involving the oxides of iron must be considered but also the kinetics.

Thetemperature of 500"C isthe minimum tempera- ture of the substrate steel at the point of deposition of the copper based alloy on to the steel. The solidus of the copper alloy matrix has been specified as the uppertemperature because the solidustemperature forthealloyasawholewould bethesolidification temperature of pure lead or lead containing a propor tion of,forexample, tin which would be a temperature of 327"C or below.

Generally speaking, ifthe conditions within the deposition chamber are sufficently g ood to produce a clean oxide-free surface on the steel then the copper alloy itself will be clean and oxide-free during atomisation and subsequent deposition.

Preferably the steel substrate should reach a temperature within the range700"Cto 850"C during pre-heating priorto deposition as the higher temperature also improves the kinetics ofthe oxide reduction reaction. The steel temperature must be belowthe matrix solidus temperature ofthe alloy being deposited in orderthatthe alloy particles solidify. After pre-heating and oxide reduction the steel may be allowed to cool to a somewhat lowertemperature prior to deposition.

The metal particles after atomisation and immediately before deposition may be a mixture of solid and liquid particles with a proportion being within the liquidus/solidus range oftemperatures. Generally the smaller particles within the size range distribution may besolid andthe largerparticles liquid orpartiaily liquid.

In order that the invention may be more fully understood examples will now be described by way of illustration only with reference to the accompanying drawing which shows a schematic sectional view of an apparatusforcarrying outthe process ofthe invention.

Referring now to the drawing and where the apparatus comprises a chamber 10 having at its top an annular manifold 11 which further includes jets 12 arranged to direct gas radially inwardly and downwardly. The manifold 11 is connected to a source of high pressure gas 13 (not shown) bya pipe 14. Above the chamberl0 is metal melting and temperature control apparatus indicated generally at 15. Within the apparatus 15 is a crucible 16 having at its lower end an orifice 17 and nozzle insert 18. The nozzle 18 has its outer hole coaxial with the manifold 11 and point of impingement 19 ofthe jets 12. A stopper rod/valve 20 is provided within the crucible 16, the stopper rod/valve 20 also being connected to drive and position control means 21 (not shown).A gas inlet pipe 22 is provided in the wall ofthe chamber 10 which itself has a gas/overspray outlet 23 connected to means 24 (notshown) forseparating the gas and oversprayand means 25 (notshown) fordrying and recycling the gas. A steel strip 26 which is to be coated with a copper-based alloy 27 contained within the crucible 16 passes firstthrough cleaning and degreas- ing apparatus 28 (notshown),then through grit blasting orotherabrading apparatus 29 and then into the entry orifice 30 of a pre-heating muffle 31 Within the muffle 31 is an induction heating coil 32 with associated known temperature sensors 33 (not shown) which provide signals to control apparatus 34 (not shown) to vary the power inputto the coil 32 in a known manner.The strip 26 passes through the chamber 10 and outthrough cooling apparatus 35 comprising high-velocity gas impingement cooling jets 36.

In operationthecopper-based alloy 27 is melted and raised to a desired temperature the tapered end ofthe stopper rod being engaged with the inlet hole ofthe nozzle 18. The chamber 10, muffle 31 and cooling apparatus 35 are purged of air with the gas introduced via the inlet pipe 22. To coat the strip 26 the alloy 27 is allowedtofallin astream37undergravityfromthe nozzle 18, the flow rate being controlled by the position ofthe rod 20. The falling stream 37 is then atomised by gas streams 38 from the jets 12. The gas steams 38 also direct the resulting metal spray 39 downwardly towards the strip 26 which receives a coating 40 ofthe alloy 27.

Example 1 In one experiment steel ofthickness 6 mm was linished and pre-heated to 7800C. Copper alloy of approximate composition Cu80-Sn 1 0-Pb1 0 and having a phosphorus content of 0.03 wt% was deposited on the steel. The gas atmosphere within the chamber comprised 97 vol% nitrogen -3 vol% hydrogen and having a dew point of -20"C.

Sample bearings prepared from the strip so produced gave fatigue results which were at least equal to similar bearings made from strip produced by the powdersintering route.

Metallurgical examination of microsections ofthe strip produced revealed a dense coherent oxide-free structure with a uniform distribution offine lead particles.

Example 2 Steel ofthe same thickness as in Example 1 was grit-blasted and pre-heated to 81 0C. Alloy ofsimilar composition to that of Example 1 but having a phosphorus content of 0.005% by weight was deposited on the steel. The gas atmosphere comprised 96 vol% nitrogen -4% hydrogen having a dew point of -40"C.

Mechanical tests conducted on the strip so pro duced gavethe results shown in theTable below.The results are shown compared with those obtained on material produced bythe powder sintering route.

TABLE Corrosion-Weight Bond Tensile Loss in Oit at Process Strength Strength 120 C Powder 155-170 MPa 215-260 MPa 10.0 mg Sintering Route Spraying 163-175 MPa 220-260 MPa 2.2 mg Route It may be seen from the above results thatthe bond strength ofthe spray-deposited material is at least equal to that produced by the powder sintering route.

Furthermorethe corrosion weight loss in oil ofthe sprayed material is much less than in the sintered material. It was further observed in microsections of the material that the lead phase was extremely fine, the lead particle size typically being about 0.01 mm diameter compared with 0.02 mm forthe sintered structure of similar composition.

Example3 Steel, again of 6mm thickness, was linished and pre-heated to 840 C. Alloy of approximate composi tion Cu72.5-Pb26-Sn1.5 and having a phosphorus content of 0.02% was sprayed on to the steel. The gas atmosphere comprised 92vol% nitrogen -8vol% hydrogen having a dew point of -30 C.

Bond testis on the strip so produced revealed a bond strength at least equal to that produced on sintered material.

Example 4 In a modification of the apparatus shown in the drawing the steel strip workpiece 26 was replaced by a cylindrical steel tube of 35mm diameter and of 2mm wall thickness. The outside diameter of the tube was grit-blasted and pre-heated to 780 C. Alloy of approximate composition Cu80-Sn10-Pb10 and having a phosphorus content of 0.005% was deposited onto the outside ofthetube. The gas atmosphere was 96 vol% nitrogen -4vol% hydrogen and having a dew point of -35"C. During deposition the tube was rotated to achieve a uniform thickness of alloy on the outside.

Metallurgical examination of microsections taken from the coated tube revealed a dense, coherent, oxide-free structure with a uniform distribution of fine lead particles. Bonded strength measurements gave results within the ranges indicated in the Table above relating to measurements on flat strip.

It will be appreciated that the steel substrate may be heated to a much greatertemperaturethan indicated above during the pre-heating and de-oxidising stages so long as it is allowed to cool to a temperature below the alloy matrix solidus priorto deposition of the alloy. In heating the steel to a highertemperature during pre-heating the kinetics of the oxide reduction phase are made more favourable and hence the reduction reaction proceeds more rapidly.

Where steel strip upto about 300 mm isto be coated it may be necessary to use a scanning type atomising nozzle arrangement in orderto give adequate coverage ofthe strip width. An example of such a scanning nozzle is described in GB 1,455,862.

Itwill befurtherappreciatedthatthefatigue strength of bearings is dependent inter alia upon the quality of the bond between the steel backing and the alloy lining and the integrity ofthe lining itself. In the process of the present invention it has been found that the bond quality is directly affected by the phosphorus content of the copper alloy and the moisture content of the atmosphere underwhich the process is operated. It will be further appreciated that many modifications may be made to the other parameters of the process described without departing from the essence ofthe invention. For example, the mannerofatomising the molten metal may be by any known method and the methods of heating or cooling the strip mayvary so long as the moisture content ofthe gas atmosphere within the chamber is not deleteriously affected.

Claims (8)

1. Aprocessforthe production of composite material comprising a lead-containing copper-based alloy on a ferrous substrate comprises depositing a stream of copper alloy particles, at least some of which are partially liquid, on to a ferrous substrate within a chamberthe ferrous substrate being at a temperature between 500C and the solidus tempera- ture of the copper alloy matrix being deposited, the chamberfurthercontaining a gas atmosphere having reducing properties and wherein the copper-based alloy contains a maximum of 0.15% by weight of phosphorus and the gas atmosphere has a dew point of not higherthan -200C.

2. A process according to Claim 1 and wherein the phosphorus content is less than 0.10% by weight.

3. A process according to Claim 1 and wherein the phosphorus content is less than 0.05% by weight.

4. A process according to any one preceding claim and wherein the dew point ofthe gas atmos phereisnothigherthan -30"C.

5. A process according to any one preceding claim and wherein the dew point ofthe gas is not higherthan -40 C.

6. A process according to any one of the preced- ing claim and wherein the gas atmosphere contains hydrogen.

7. A process according to any one preceding claim and wherein the ferrous substrate is pre-heated to a highertemperaturethanthetemperature at which deposition of the copper-based alloy on to the strip occurs.

8. A process substantially as hereinbefore described with reference to the drawing and any one of the examples.