WO1995012473A1

WO1995012473A1 - Production of sprayed deposits

Info

Publication number: WO1995012473A1
Application number: PCT/GB1994/002386
Authority: WO
Inventors: Richard Michael Jordan; Alfred Richard Eric Singer; Gordon Idris Davies; Allen Dennis Roche
Original assignee: Sprayforming Developments Limited
Priority date: 1993-11-02
Filing date: 1994-10-31
Publication date: 1995-05-11
Also published as: GB9322565D0

Abstract

Sprays of different material compositions A and B are directed to form a layer of coincident deposition of both material compositions A and B. The deposition is controlled such that a gradient of the relevant proportion of material composition A to material composition B is produced across the deposited layer. The provision of a controllable gradient of the relative proportion of material composition A to material composition B across the deposited layer enables the physical properties of the deposit to alter gradually across the depth of the deposit. The spray deposition method is useful when producing a machine tool, mould or die.

Description

Production of Sprayed Deposits

The present invention relates to the production of sprayed deposits, and in particular to techniques for producing sprayed deposits using sprays of one or more metallic compositions.

EP-A-0270265 discloses the production of spray formed metal articles by building up a laminated structure comprising alternately deposited discrete layers of two or more metals. EP- A-0270265 specifically teaches that where overlapping sprays are utilised, it is necessary to operate the two sprays alternatively in order to achieve the desired laminated structure. EP-A- 0270265 also teaches that, in building up one of the layers comprising the laminated structure two or more sprays may be operated simultaneously to form a "composite" layer of the laminated structure. From the teaching of EP-A-0270265, such a composite layer would comprise constant proportions of the constituent materials throughout.

An improved process for producing spray formed deposits has now been devised which enables mechanical properties of the deposits to be closely controlled and varied through the deposit.

According to the invention, there is provided a method of producing a sprayed deposit, the method comprising directing sprays of different material compositions A and B respectively to form a layer formed by coincident deposition of both material composition A and material composition B, characterised in that deposition is controlled such that a gradient of the relative proportion of material composition A to material composition B is produced across the layer.

The provision of a controllable gradient of the relative proportion of material composition A to material composition B across the deposited layer provides a graded composition such that the physical properties of the deposited layer may alter gradually across the depth of the layer. For example, using the method according to the invention a metal product may be produced comprising a layer of two or more metals in which the boundary between two metals comprises a gradually graded composition from the composition of the first metal to the composition of the second metal. This inhibits possible delamination that might otherwise occur at a boundary between two different compositions.

In a wide range of products (particularly metallic products) it is desirable to have subsurface properties which differ from the surface properties. Examples of such instances are those where the surface coatings are used for corrosion protection, enhanced appearance, increased hardness and changed wear or friction properties. Prior art methods for achieving different properties within or at the surface of materials comprise surface coating or plating using an homogenous layer, laminating, diffusion techniques (e.g. nitriding or case hardening) and ion implantation techniques. All these prior art methods of treatment are applied after the first stage of manufacture and suffer from technical disadvantages such as limited penetration below the surface, delamination with time, or unacceptable cost.

The gradient variation of composition of the deposited layer provided by the present invention provides an intrinsic change of composition where there is no definite boundary between one composition and another. This allows gradual change of composition (and therefore properties) between different parts of the product being manufactured and also enables immiscible, insoluble, or otherwise incompatible materials to be combined in a composition.

The sprays of material compositions A and B may comprise, for example, liquid droplets, partially solidified droplets or fine solid particulate material. Desirably at least one of the sprays comprises a spray of metallic material which may be produced by any convenient means such as arc spray, oxyfuel spray, gas atomisation of a melt, plasma spray or other techniques. In certain embodiments both spray compositions A and B may be of metallic material; in other embodiments, one of spray composition material A or B may be a metal, with the other spray composition material being non-metallic.

In certain embodiments, a further spray having yet a further composition (such as material composition C, for example) , may be deposited coincident with deposition of material compositions A and B. The relative proportion of material composition C to the remainder of the material comprising the layer may be constant, or may be controlled to produce a gradient across the deposited layer. Fourth and further sprays of still further material compositions may be used in a similar manner where required.

It is preferred that the spray of material composition A and the spray of material composition B are arranged to overlap. They are preferably operated simultaneously to produce the layer of the deposit.

Desirably, deposition is controllable such that the gradient of the relative proportion of material composition A to material composition B is controlled. An approximately linear gradient may, for example, be relatively steep or relatively shallow giving, respectively, a relatively large or relatively small change in the relative proportions of material composition A and B per unit depth of deposited layer.

The sprays may be controllably scanned, rapidly, in a conventional manner to deposit over a wider area or contour the deposit. Alternatively, or in addition, the deposit may be moved relative to the sprays which may therefore be stationary.

The sprays depositing materials A and B can either be generated by identical spray processes or by different spray processes which may include atomisation of a melt, metallic arc, plasma spray or other thermal spray processes.

The deposited layer may be formed on a substrate or collector, which can either remain a part of the spray formed product, or alternatively be subsequently removed. For the spray forming of metals, with which the present invention is particularly concerned, an atomised spray (or sprays) of molten metal composition is directed at a cool or cold substrate or collector (usually of different chemical composition to the material of the spray) such that the molten particles splat and solidify to form a coherent solid deposit on cooling.

The sprays of material composition A and B may be controlled to be coincident over substantially the same area to form the deposited layer. It is preferred that the sprays are operated simultaneously, and are controllable to produce the required gradient within the deposited layer by varying over time the respective proportions of material compositions A and B being sprayed. The rate of change in the relative proportion of material compositions A to B being sprayed is preferably controllable, so as to determine the gradient of the proportion of material composition A to B in the spray deposited layer.

Alternatively, the sprays of material composition A and B may not be wholly coincident over the same area but may merely overlap and be operated simultaneously to form a coincident region. In this case, the gradient of the relative proportion of material composition A to composition B may be produced by moving the layer deposit (or substrate on which the layer deposit is formed) through the overlapping stationary sprays. The degree of overlap and fanning of the sprays may be controlled to vary the gradient of the proportion of material composition A to B in the spray deposited layer.

The invention will now be further described in a specific embodiment by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a schematic of a sprayed deposit being produced according to a preferred method according to the invention; and

Figure 2 is a schematic view of sprayed deposit being produced according to an alternative method according to the invention. Referring initially to Figure 1, an aluminium pattern of a round memorial coin (1) is attached to a 4-axis manipulator arm (2) operating inside a spray chamber (not shown) . The manipulator has x and y directions of motion and also rotation and tilting. A plasma gun (3) operating on N₂ + 10% H₂ into which is fed tungsten carbide powder at position (4) is directed towards the position where the pattern will be when it is introduced into the spray by the manipulator arm (2) . An arc spray gun (5) fed with two low carbon steel wires (6) and operating with N₂ is pointed such that the two spray footprints are coincident on the pattern. The gun positions and distances can be checked beforehand on a trial flat plate substrate to ensure coincidence.

The plasma spray is started producing a spray (7) and the pattern rapidly introduced by the manipulator arm (2) . The manipulator is operated rapidly in a manner to obtain uniform coverage of the working surface of the pattern (1) . After a few minutes of operation the arc spray gun is operated producing a spray (8) which coincides with the area of impingement of the plasma spray (7) on the pattern (1) . The current in the arc gun, and the wire feed rate, are gradually increased and therefore the feed of low carbon steel is increased and the power to the plasma gun is decreased together with the feed of tungsten carbide powder so as to give gradient or a gradation of composition across the deposit from 100% tungsten carbide at the working face of the replicated spray formed die (9) to 100% low carbon steel for the backing material of the die (10) .

In this particular case, because of the relatively high mass flows in the arc spray process, compared to the relatively low mass flows in the plasma spray process, the composition within the graded layer of the deposit very quickly increases to 90% low carbon steel and then gradually increases to 100% low carbon steel. A very ductile and high integrity bond is formed through the graded transition layer. hen sufficient backing low carbon steel has been sprayed the process is stopped, the pattern (1) with the die (10) attached is allowed to cool and the aluminium pattern is then removed by dissolving in NaOH solution.

By this means a hard tungsten carbide faced die can be produced from an easily machined aluminium pattern. The advantages are speed of replication, low usage of expensive tungsten carbide powder, cheapness of the backing material, ease of machining of the backing to fit any required bolster and excellent bonding of the die steel face to low carbon steel backing because of the graded composition.

In the second example, as shown in Figure 2, a laminated Cu/Ni-steel strip is produced by the use of two overlapping sprays. Two arc guns are directed vertically downwards towards a grit blasted flat steel substrate (11) at a temperature of 150°C travelling as indicated at a speed of 500mm/min. The first arc gun (12) is fed with twin wires of low carbon steel working at 1000 amps. The second arc gun (13) is fed with twin wires of 80/20 Cu/Ni also at a current of approximately 1000 amps.

The two arc guns are spaced longitudinally in relation to the direction of travel of the substrate and at a distance apart of 70mm, such that the sprays overlap. The spray from the arc guns is deflected by scanning jets of nitrogen (not shown) in a direction normal to the plane of the paper so that the two sprays are scanned rapidly at a speed of 20Hz across the strip (14) produced by deposition. The scanning action is used to ensure that the deposit is uniform in thickness across its width. The scanning procedure used is the subject of known technology.

When the two guns are operated simultaneously a laminated strip (14) is produced continuously having dimensions approximately 75mm width and 6mm thickness. Low carbon steel is deposited at position (15) followed by a graded composition having a gradient of linearly varying proportion (low carbon steel to Cu/Ni) from position (16) to (17) and pure Cu/Ni at position (18) . The mingling of the two overlapping sprays between positions (16) and (17) ensures that a graded mixture of steel and Cu/Ni is deposited and the extent of grading can be altered by adjusting the spacing of the two guns or the degree of fanning of the sprays. In the example quoted the laminated strip was composed of approximately 50% steel and 50% Cu/Ni with a graded composition zone 1mm in width the centre of which is indicated by the dotted line.

It is possible to make graded laminates of more than two different metals by adding further guns downstream of gun (13) with overlapping sprays.

All such laminates can be rolled subsequent to deposition to give greater accuracy of gauge, smaller thickness, higher density or hot or cold working. Such subsequent rolling may be carried out hot or cold, in-line or off-line.

In all the operations described above spray deposition can be carried out using a wide range of atomising and deposition procedures any one of which may be used in combination with one or more of the full range of procedures. The range of procedures includes the use of atomising from a melt, twin wire fed arc guns, single wire fed arc guns using a non-consumable electrode, plasma guns using either direct or transferred arc fed with either powder or wire or oxyfuel guns using either powder or wire. The deposition may be carried out substantially at atmospheric pressure or alternatively at reduced pressure which in the case of plasma is often advantageous because it gives a longer plasma flame, in addition to reducing oxidation and porosity.

In all cases where wire can be used as a feed the use of cored wire containing powder is permissible. Such cored wires can have a casing of metal or in certain instances a casing of polymer.

The ability to add powdered solids either by injection into the spray or by using cored wire enables high melting point ceramic and other powders to be used to produce a wide range of MMC-metal combinations with graded gradient composition.

Claims

CLAIMS :

1. A method of producing a sprayed deposit, the method comprising directing sprays of different material compositions A and B respectively to form a layer formed by coincident deposition of both material composition A and material composition B, characterised in that deposition is controlled such that a gradient of the relative proportion of material composition A to material composition B is produced across the layer.

2. A method of producing a sprayed deposit according to claim

1, wherein at least one of the sprays comprises a spray of metallic material.

3. A method of producing a sprayed deposit according to claim

2, wherein at least one metallic spray comprises a spray of molten metal.

4. A method of producing a sprayed deposit according to any preceding claim, which method comprises a further spray having yet a further composition being deposited coincident with deposition of material compositions A and B.

5. A method of producing a sprayed deposit according to claim 4, wherein the relative proportion of material having the yet further composition to the remainder of the material comprising the deposit is controlled to produce a gradient across the deposited layer.

6. A method of producing a sprayed deposit according to any preceding claim, wherein the spray of material composition A and the spray of material composition B are arranged to overlap.

7. A method of producing a sprayed deposit according to any preceding claim, wherein deposition is controllable such that the gradient of the relative proportion of material composition A to material composition B is controlled.

8. A method of producing a sprayed deposit according to any preceding claim, wherein one or both of the sprays are scanned.

9. A method of producing a sprayed deposit according to any preceding claim, wherein the deposit is moved relative to the sprays.

10. A method of producing a sprayed deposit according to any preceding claim, wherein the sprays of material composition A and B are controlled to be coincident over substantially the same area to form the deposited layer.

11. A method of producing a sprayed deposit according to any preceding claim, wherein the sprays are simultaneously controlled to produce the required gradient within the deposited layer by varying over time the respective proportions of material compositions A and B being sprayed.

12. A method of producing a sprayed deposit according to claim 11, wherein the rate of change in the relative proportion of material compositions A to B being sprayed is controlled so as to determine the gradient of the proportion of material composition A to B in the deposited layer.

13. A method of producing a machine tool, mould or die wherein a portion of the machine tool, mould or die is formed as a spray deposited layer according to the method of any preceding claim.

14. A method according to any preceding claim, wherein the spray deposited layer is applied as a coating to a preformed substrate or body.