GB2059443A - Process for making multi- layered strip - Google Patents

Abstract

A method for producing metal strip from metal powder comprises compacting together at least two contiguous layers derived from a slurry of metal powder in a film forming cellulose derivative and heat treating the compacted layers, the metal in at least two of which are of different composition. The layers are compacted together by rolling and then sintered.

Description

SPECIFICATION Process for making multi layered strip This invention relates to the production of metal strip from metal powder and is directed to the production of such metal strip having at least two bonded layers of different composition.

Conventionally, multiple layer strip is produced by suitably bonding together two or more relatively thick slabs of the metal each having the thickness and composition required for the layers of the strip and progressively reducing the combined thickness of the contiguous bonded slabs by hot or cold rolling, a heat treatment where required being interposed between working operations. This method for producing strip in addition to being costly requires a high level of care and displays a relatively high reject rate which adds further to the cost of strip having the required characteristics. It is accordingly an object in the present invention to produce an alternative improved route for producing such strip.

The present invention is based upon the discovery that multi-layer strip displaying a high degree of bonding between layers can be produced by compacting layers derived from a slurry of metal powder in a film forming cellulose derivative, with none of the layers prior to bonding being processed beyond the stage of an individually dried and possibly compacted slurry film. The degree of bonding so produced is important in applications where strain and particularly heat-induced strain exists such as in bi-metallic thermostats.

The present invention, accordingly in its broadest aspect, provides a method for producing metal strip from metal powder by compacting together at least two contiguous layers derived from a slurry of metal powder in a film forming cellulose derivative and heat treating the compacted layers, the metal in at least two of which are of different composition.

Preferably the heat treatment is a sintering operation effective to provide bonding between the metal particles in each layer and at the interface between layers.

The composition of the metal in any layer may be determined by incorporating into the cellulose derivative powders of different metals with or without any non-metallic additive capable of modifying metal characteristics, the combination of different metals producing the required composition.

Alternatively the compositions of metal in any layer may be determined by incorporating into the cellulose derivative metal powder which already is of the composition required and which may be produced by any of the means such as alloying well known in the art. If necessary, the metal composition of any layer may be produced by a combination of powders of individual metal and alloys.

In a preferred embodiment of the invention, a metal layer is produced by depositing onto a moving support, a slurry of the metal powder of powders and the film forming cellulose derivative and subsequently removing the film or layer from the support for compaction with one or more contiguous layers.

The film or layer so produced may be compacted with a similar contiguous layer of dried film.

Alternatively however, the slurry may be deposited on an already dried film or layer for subsequent drying of the deposited slurry and consequent compacting. in a further arrangement the slurry may be deposited upon a previously deposited film or layer which has been individually compacted and which possibly has been subjected to a sintering operation.

Suitably the cellulose derivative is methyl cellulose; in this case, an aqueous slurry is deposited upon a moving support which is heated to promote gelling of the methyl cellulose; gelling which occurs at a temperature in excess of about 400C. conveniently is followed by drying to remove water and produces a self supporting film or layer referred to as "flexistrip". The flexistrip can be removed from the moving support with relative ease for subsequent compaction.

In one embodiment of the invention, the self supporting layerofflexistrip is coiled in conventional manner so that it can be subsequently uncoiled for feeding into the nip of the compaction mill rolls. To produce for example a bi-metallic strip, that is to say strip containing two layers of different metal composition, the layers of flexistrips of appropriate metal composition may be derived from two coils previously produced and arranged simultaneously to uncoil into the compaction mill. In the case where strip containing more than two layers is required, the number of coils of flexistrip feeding layers into the nip of the mill rolls for compaction must be correspondingly increased.

In an alternative embodiment of the invention, a self supporting layer of flexistrip removed from the moving support may be directly applied into the nip of the mill rolls for compaction with and bonding to an adjacent contiguous layer. In this case the contiguous layer with which it is compacted may also be derived directly from a second moving support operating substantially in synchronism with the first, or from a coil of flexistrip previously produced. Where strip incorporating more than two layers is required, further layers may both be derived from either a moving support or from a coil or from a suitable combination of support and coil.

Embodiments of the invention will now be particularly described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic side view of apparatus for producing bi-metallic strip by one method according to the present invention.

Figure 2 is a schematic side view of apparatus for producing the bi-metallic strip of Figure 1 by an alternative method.

Figure 3 is a schematic side view of apparatus for producing the strip of Figure 1 by a further method; and Figure 4 is a schematic side view of apparatus for producing multi-metallic strip comprising three layers of different metal composition.

Referring to Figure 1 this illustrates apparatus for producing strip comprising two layers of metal of different composition. Such strip may comprise bi-metallic strip of well known type conveniently used in the production of thermostats and like heat sensitive devices in which differential expansion of the layer on sensing heat produces mechanical movement.

Typical bi-metallic strip for such heat sensitive applications have metal compositions in adjacent layers as detailed in Table I; these however are merely examples of a wider range of composition combinations which can be produced by the apparatus of the invention.

Alternatively, the apparatus may be used to produce strip in which the layers have a thickness and composition providing differing resistance to corrosion or different thermal conductivities, so as to be capable of use in holloware and the like. The metal composition of such strip is also detailed in Table 1.

In the apparatus shown in Figure 1 when used for producing bi-metallic strip, a slurry 4 is retained in a vessel at a station indicated generally at 2. The slurry which may be that disclosed in our copending U.K. Patent Application No. 43852/75 conveniently is based upon multiples of 300 g of methyl cellulose treated with glyoxal as a solubility inhibitor together with 12 litres of water optionally containing suitable slurrying and wetting agent. Incorporated in the aqueous methyl cellulose is 35 Kg of a suitable fine metal powder typically of below 80 B.S. mesh, the particles having a composition by weight of 22% Ni and 3% Chromium the balance being iron except for incidental impurities.The concentration of the metal powder in the aqueous slurry is approximately 75% by weight, although lower or higher concentrations may be used according to the mechanical and thermal properties which are required.

The metal powder may be produced by any conventional means, for example, by atomising the appropriate alloy melt; it is intended to produce in the bi-metallic strip a layer of metal of the powder composition detailed; however, the metal composition in the layer may alternatively be produced by incorporating into the aqueous methyl cellulose, a mix of unalloyed metal powders.

At station 2 the slurry 4 is transferred by way of train of rollers 6, 8 onto a coating roller 10 arranged uniformly to deposit slurry to a selected thickness and width onto the region 12 of a continuous belt 14 of inert metal such as stainless steel looped around drums 1 6 and 1 8. Other means of slurry deposition for example curtain coating or extrusion may however be employed.Drive applied to at least one of the drums feeds the belt through a drying oven 20 which is effective initially to raise the temperature of the deposited slurry layer to above 450C to induce gelling of the methyl cellulose to form a film and subsequently to drive water from the gelled slurry; the gelled slurry film emerges from the furnace as a flexible and self supporting strip 21 which can be continuously peeled off from the polished surface of belt 14 which conveniently is pretreated to ensure easy release. The flexible and self supporting strip 21 peeled off the exit end of belt 14 at drum 18 is referred to as 'flexistrip'.

Disposed adjacent the exit end of strip 14 at drum 1 8 is a coil 23 of flexible and self supporting flexistrip 24 which has previously been produced by a coating line such as that shown in Figure 1 and which has been similarly stripped off the belt prior te coiling in conventional manner. Strip 24 is produced in like manner to that of strio 21; however the composition of the metal powder used to produce strip 24 is generally of different composition and possibly of different thickness to that of strip 21; however in this embodiment, the metal strip 24 is 36% by weight of nickel with the remainder being iron except for incidental impurities.

At station 22, flexistrip 21 derived directly from the coating plant together with flexistrip 24 derived from the previously produced coil 23 are simultaneously fed, in contiguous face to face relation into the nip between a pair of rolls 25, 26 effective to produce the first stage of compaction together of individual particles in strips 21 and 24 as well as simultaneous compaction together of the strips.

While the adjustment of the mill rolls would depend upon the thickness of the strips 21 and 24, as well as the concentration of metal powder in the slurry, a pressure effective to produce about 60% reduction of gauge is acceptable for the metal compositions detailed.

After the first stage of compaction and bonding at the rolls, 25, 26 the strip is fed through sintering furnace 30 by way of inlet and outlet guide rolls 32 and 34 respectively.

The sintering furnace 30 is generally of the belt or roller hearth type containing an atmosphere which is non-oxidising to the materials being processed. In the sinterfurnace which has a temperature plateau determined by the metals in the layers and is in this embodiment about 11 500C the methyl cellulose in the compacted and bonded flexistrips 21,23 becomes fugitive while the metal particles in the layers as well as the layers themselves become further bonded.

The strip leaving the sinter furnace 30 by way of guide rolls 34 is now effectively bi-metallic strip having a strip density of approximately 90% of full density and having two bonded layers of metal respectively having the composition of the metal powder incorporated in the aqueous slurries at the coating stations.

After leaving the sintering furnace the bi-metallic strip 35 may be subject to further compaction at a mill (not shown) and to a further sintering or heat treating operation to produce a material of full density with the mechanical thermal corrosion and wear resistant characteristics required.

In an alternative embodiment of the invention, the bi-metallic strip of Figure 1 may be produced by the apparatus shown in Figure 2 in which like parts have like numerals. In this embodiment flexistrip 21 and the flexistrip 211 are produced simultaneously on separate coating lines indicated generally at 42 and 44. The coating lines are substantially identical to one another and to the coating line shown in Figure 1.

Thus in coating line 42 and44 the holding tanks 4 and 41 contains a slurry of the composition detailed in relation to the coating line of Figure 1.

Flexistrip 21 and 211 respectively produced on lines 42 and 44 are simultaneously applied in contiguous face to face relation into the nip between mill rolls 25, 26 to produce the first degree of compaction and bonding in like manner to that shown in Figure 1. Strip emerging from the mill rolls are as in the embodiment of Figure 1 applied to sinter furnace 30 for further bonding.

In the embodiment of Figure 3 a coating line such as line 42 or line 44 of Figure 2e is used initially to produce flexistrip 21 which incorporates metal powder of the appropriate composition and which is directly coiled into coil 36 directly after removal from the exit end of support belt 14.

Similarly line 42 or 44 is used to produce flexistrip 211 which is also of the appropriate metal composition and which is similarly coiled to form coil 38.

As shown in Figure 3 coils 36 and 38 disposed in suitable juxtaposition are arranged to simultaneously feed flexistrip 21 and 211 into the nip between rolls 25 and 26 for compaction and bonding in like manner to that of Figures 1 and 2. As in Figures 1 and 2 compacted and bonded strip emerging from the mill rolls 25, 26 are fed through a sinter furnace 30 to produce the bi-metallic strip which may be further processed.

Figure 4 illustrates apparatus for producing strip comprising three contiguous bonded layer of metal of different composition. Typical strip of this kind for use in thermostats comprises one outer layer of 22% by weight of nickel, 3% chromium with the balance being iron except for incidental impurities.

The opposite outer layer comprises 36% by weight of nickel with the balance being iron, with the intermediate layer being of nickel alone. One alternative composition for three layer strip also is shown by way of example in Table I, which also details one other typical composition and use of strip having three layers. As shown in Figure 4 the flexistrip comprising the intermediate layer 50 is produced from a slurry of aqueous methyl cellulose and nickel powder, on a continuous coating line substantially as that shown in Figures 1 to 3. Outer layers 52 and 54 previously produced on a similar continuous coating line and previously coiled on to coils 56 and 58 respectively are uncoiled and simultaneously applied to the nip between mill rolls 25 and 26 so that they lie on opposite faces of the nickel flexistrip derived directly from the exit end of support belt 14.The mill rolls 25, 26 accordingly not only produce compaction of the metal powder in the flexistrips 50, 52 and 54 but also produced bonding of the outer layers or flexistrips 52 and 54 on to the intermediate layer 50. Flexistrips 52 and 54 have previously incorporated powder of the composition recited for the outer layers of the multi-metallic strip proposed.

As in the previous embodiments compacted and bonded multi-metallic strip emerging from the mill rolls passes through sinter furnace 30 for further bonding before being further processed as in the previous embodiments.

It will be appreciated that while the invention has been described with reference to bi-metallic and multi-metallic strip incorporating layers of metal of specific composition, metals of other composition may equally be produced by modifying and selecting the powders which are incorporated in the slurries from which the intermediate flexistrip is produced. Table 1 lists the wide range of composition of bimetallic and multi-metallic strip which are of commercial interest, for thermostats and like heat sensitive devices and which may be produced by the method of the invention.

It will also be appreciated that while methyl cellulose has been described as a film forming cellulose derivative capable of producing a self supporting and flexible green strip, other cellulose derivatives having similar properties may equally be employed. As in the case of methyl cellulose these may incorporate anti-foaming agents and the like.

In the embodiments described, the individual flexistrips have been introduced into the nip of the first compaction rolls. While this is the preferred method, the contiguous relationship of the layers in the metal strip produced by the method of the invention, may alternatively be produced by building up the layers upon a metal flexistrip substrate by further deposition upon it of a slurry containing the appropriate metal powder in aqueous methyl cellulose.

In all the methods disclosed for producing multi-layer metal strip, solid reinforcing material such as wires tapes or mesh may be suitably interposed between the layers before the first compaction. The method of the invention may for example be used to produce high strength sheet material for aeronautical applications, comprising aluminium layers having high strength steel wires bonded between them.

TABLE 1. EXAMPLES OF MULTIPLE LAYER MATERIALS APPLICATIONS. No. of layers Typical compositions Applications Bimetals (1) 36Ni 64Fe / 22 Ni 3Cr 75Fe Thermostats, temperature actuated 2 layers controls. (2) 36Ni 64Fe / 25 Ni 8.5Cr 66.5 Fe (3) 42 Ni 58Fe / 22 Ni 3Cr 75Fe Clad materials (1) Austenitic stainless steel one side, ferritic Sheet form, holloware etc. 2 layers stainless steel other side. corrosion resistance different on each side (2) Stainless steel/mild steel. Tri-metals 22Ni 3Cr 75Fe/Ni/36Ni 64Fe Thermostats, etc. 3 layers 22 Ni 3Cr 75Fe/Cu/36Ni 64Fe Clad materials Mild steel core with stainless steel, copper, nickel, Chemical reactor vessels, pressure 3 layers etc. outer layers. vessels, etc.

Claims (14)

1. A method for producing metal strip from metal powder comprising compacting together at least two contiguous layers derived from a slurry of metal powder in a film forming cellulose derivative and heat treating the compacted layers, the metal in at least two of which are of different composition.

2. A method as claimed in claim 1 wherein the heat treatment is a sintering operation.

3. A method as claimed in claim 2 wherein the sintering operation is at a temperature selected to provide bonding between metal particles in each layer and at the interface between layers.

4. A method as claimed in any preceding claim wherein the cellulose derivative is methyl cellulose.

5. A method as claimed in claim 4 wherein the methyl cellulose forms an aqueous slurry.

6. A method as claimed in claim 5 wherein the slurry is dried to remove water and produce a selfsupporting film or layer.

7. A method as claimed in claim 6 wherein the self-supporting strip is coiled for subsequent uncoiling and compaction with an adjacent contiguous layer.

8. A method as claimed in claim 7 wherein the adjacent contiguous layer is derived from a coil or is derived directly after the drying of a film forming slurry.

9. A method as claimed in any one of claims 1 to 7 wherein adjacent contiguous layers are derived directly from the drying of a film forming slurry.

10. A method as claimed in any one of claims 5 to 9 wherein the aqueous methyl cellulose is initially heated to induce jelling prior to drying.

11. A method as claimed in any one of claims 5 to 10 wherein the aqueous methyl cellulose is deposited by extrusion, roller coating or coil coating onto a temporary moving support.

1 2. A method as claimed in any preceding claim wherein differences in the metal composition of any two layers are produced by powders of different metal or by powders of metal alloys.

1 3. A method as claimed in any preceding claim wherein the bonded layers are compacted after sintering.

14. A method of producing metal strip from metal powder substantially as hereinbefore described with reference to any one of Figures 1 to 4 of the accompanying drawings.

1 5. Metal strip produced from metal powder by the method as claimed in any one preceding claim.