GB2264719A - Spraying onto rotating substrates; coating internal tubular surfaces using exothermic mixture; centrifugal force - Google Patents

Abstract

A method of providing a coating on a substrate comprises spraying particulate material via spraying means onto the substrate while rotating the substrate at a sufficiently high speed that the particulate material is initially held on the substrate by centrifugal force. The substrate may be planar but is preferably tubular. As illustrated in Fig 4 thermal spray nozzles along a support 9 deposit metal and ceramic powder onto the internal surface of the rotating tube 7. In a modification, a planar substrate and spray nozzles are rotated at substantially the same angular velocity (20, 15, respectively Fig 8). The coating material may be an exothermic reaction mixture which is subsequently ignited. Holes may be provided in the substrate so that portions of the sprayed material extend therein to provide a mechanical lock with the substrate. A number of individual planar substrates (29, Fig 10) may be mounted about the circumference of a support with a nozzle fixed at the centre (30, Fig 10). Centrifugal force is also used to i) coat internal surfaces by packing an exothermic medium mixture in a tube, rotating and subsequently igniting (Fig 11); ii) weld components together using a ceramic/metal powder deposited in a V-shaped joint (Figs 12a, 12b); and iii) to form a moulded product by rotating a mould containing a thermally softened material (Figs 1 to 3b). In addition centrifugal force is used in a number of different applications using a variety of materials including forming glass articles; manufacturing bread; removing fat from meat products; impregnating wood products; and forcing lubricants into bearings. <IMAGE>

Description

Manufacture and Processing of components The invention relates to methods of manufacturing and processing components by the use of centrifugal force, for example cylindrical, hollow symmetrical, or symmetrical individual, components.

The basic theory for a revolving cylindrical component can best be understood from the following: According to Newton's first law of motion: 'Every body continues in its state of rest, or of uniform motion in a straight line, unless compelled by impressed forces to change that state'.

Since all the particles which make up the cylindrical component move in a circle, instead of a straight line, it follows that every particle must be continuously acted upon by some force whose line of action is towards the centre of the cylinder. This force is termed the centripetal force.

According to Newton's third law of motion; 'To every action there is an equal and opposite reaction'. Hence there must be a force equal to the centripetal force, but acting in the opposite direction, this equal and opposite force is termed the centrifugal force.

The inherent phenomenon of the separation of a liquid from solids of greater volume (not necessarily of greater density) e.g. water spun from a towel (via drain holes comparatively smaller in diameter than the size of the solids) in a washing machine spin dryer, the centrifugal separation of oil from machine swarf, and the fairground centrifuges which press people against the inner walls of large rotating drums, indicate well known uses of centrifugal force.

Centrifugal casting techniques are also well known and have been used to produce dense castings, largely free from porosity and of improved microstructure. Excessive centrifugal force, however, can lead to segregation of different constituent parts of certain monolithic alloy systems, especially metal-matrix-composite materials. This inherent segregation effect between dense and less dense constituent parts has been reported in the literature (Advances in Materials Technology: MONITOR Metal-matrixcomposites, Issue No. 17 February 1990) and depending on the relative densities of the particulate and the matrix material can be used to advantage to manufacture 'enriched' composite external or internal surfaces from composite melts. Both horizontal and vertical centrifugal techniques are in common use.Centrifugal castings i.e. where casting cavities or multiple castings are arranged equally and radially about the centre axis of rotation is also a variant technique in common use. Generally speeds of rotation are used that create a centrifugal force ranging from 75 to 120 times the force of gravity. The mould wall adjacent the casting is usually water cooled to aid solidification and prevent the mould from thermally weakening. Typically, centrifugal cast parts have a homogeneous microstructure and high degree of metallurgical cleanliness.

In accordance with a first aspect of the present invention, a method of providing a coating on a substrate comprises spraying particulate material via spraying means onto the substrate while rotating the substrate at a sufficiently high speed that the particulate material is held on the substrate by centrifugal force.

The first aspect of the invention termed "centrifugal powder fuse cladding enables powder or other particulate material to be applied in any temporal sequence, to a component, for example internally to a cylindrical component and held in place while it is fused and consolidated under the effect of a suitable centrifugal force. Typical powders would include metal, glass and plastic etc.

The invention enables particulate and/or whisker reinforced metal-matrix-composite or non-metal composites to be produced in-situ e.g. by the controlled addition of ceramic particulate reinforcement to a monolithic localised melt region in any temporal sequence while under the effect of a centrifugal force. Examples of suitable ceramic reinforcement are silicon carbide - SIC, Alumina - Al203, Boron Nitride - BN, and compounds of ceramics i.e. SIALON Si3N4/ALN/Al203 and CORUNDUM - Al203 + Si02. The rate of particle transfer through the matrix is dependent on the process conditions; material properties, melt temperature, centrifugal force, and (controlled) the cooling rate.

Process parameters with respect to powder fuse techniques can be adjusted to produce a through-thickness uniform dispersion of particulate or be adjusted to form enriched external or internal circumferential surfaces. A protective or inert gas may be used as necessary to form a protective shield or as a carrier gas for ceramic particle additions etc. A supplementary heating source, or cooling facility, or a vacuum chamber may also be used as necessary.

The features which the inventors have recognised are that loose powder particles under suitable conditions will behave in a quasi-fluid manner, and that loose powder particles subjected to a centrifugal force will tend to flow to a uniform depth and be held in place. Materials which can behave in a quasi-fluid manner i.e metal-matrixcomposites, inter-metallics, plastics, glass, glass-fibre, wax, rubber solutions, confectionery (chocolate), powders ceramic, industrial or natural diamond, cement wood chip, paper pulp etc., are particularly suited to the present invention.

The invention enables internally clad components to be manufactured from materials normally regarded as difficult to bond.

Preferably, relative traverse movement is caused between the substrate and spraying means in a direction substantially parallel to the axis of rotation.

A suitable heat source can be applied to fuse the powder particles to the substrate without the powder being displaced or disturbed. Examples are oxy-fuel or laser heat sources, non-consumable arcs or plasmas, or induction heating.

Depending on the temporal sequence the powder may be incrementaly fused to previously deposited material, effectively fixing discreet additions of reinforcement material throughout the matrix material, i.e. that a semi solid transition layer exists between the molten inner surface extending from the inner diameter and an inwardly advancing solidification front extending from the outer diameter which anchors the reinforcement material.

Preferably, the material is centrifugally forced into prepared longitudinal or circumferential dovetail serrations or radially produced holes to form a clad layer mechanically locked to the component.

The invention enables lubrication retaining bearings to be manufactured whereby solid lubricant (molybdenum disulphide, graphite, PFTE etc) is forced into bronze or other structural material in which holes or grooves are machined to form combination bearings or where oil is forced into porous bearings material while under a centrifugal force at ambient or elevated temperature conditions.

With thermal spraying processes, i.e. flame, arc, plasma, high velocity oxy fuel (HVOF) spray processes etc., adhesion of the deposit to the substrate is considered to be largely mechanical (i.e with limited metallurgical bond) with some porosity always present. A method of improving the integrity of the bond between the deposit and the substrate and to improve the integrity of the deposit itself is well known and has resulted in the development of a two stage spray fusion technique. This latter technique relies on a fusing operation which is carried out after applying the first stage. In addition a metal shot compacting spray technique to minimise porosity and improve integrity has also been developed and widely reported in the literature.

The process of centrifugal spray deposition (CSD) has been reported in the literature (A Future for Spray Forming by Singer R., Spray Forming Developments Ltd, The First International Conference on Spray forming Swansea 1990, Section 1, Paper 2, 15 pages). This CSD process consists of atomising(breaking down to small droplets) a stream of molten metal by means of a rotating disc, or spinner. A stream of metal from a melt is poured on to and spun off the spinner as a horizontal spray to impact (flung out by centrifugal force) a circular mould which surrounds it. The mould can be moved vertically or rotated but does not develop a centrifugal force itself.

The invention of centrifugal thermal spray cladding and centrifugal powder fuse cladding differ from the latter Two Stage Fuse and Metal Shot Compacting and CSD techniques in that the component is rotated to develop sufficient centrifugal force to improve deposit integrity, minimise over spray losses and retain metal powder in place while it is fused (without necessarily making any physical contact).

The above invention of a combined thermal spray with centrifugal force, i.e. a method of applying a force to consolidate the internal clad layer of a cylindrical component, enables products of improved integrity to be manufactured.

The industrial application of continuous fibre reinforced metal-matrix-composite and resin coated glass or ceramic fibres have to some extent been hampered by the unreliability, cost and limitations of manufacturing processes. Current methods of filament winding are limiting to techniques that wrap around a rotating mandrel ie. externally applied. While the simultaneous external filament winding of fibre and plastic matrix composites is relatively easy, with metals this is more difficult.

Continuous fibre reinforced metals are usually produced by metal sprayed onto the fibres during winding then consolidation under pressure at high temperature, typically by Hot Isostatic Pressing (HIP).

The circumferential stress in the wall of a cylindrical pressure vessel is twice the longitudinal stress ie. cylinders tend to split longitudinal when taken beyond their design limits.

Circumferential reinforced ceramic fibre can, because of the anisotropic properties (increased tensile strength in the direction of the fibres) of the material, strengthen cylindrical pressure vessels to withstand a greater hoop bursting stress than that possible when using monolithic reinforced materials.

Accordingly, a centrifugal force can be used to internally spin/wind monofilament fibres (which may or may not be pre-coated with the matrix material) or co-wound ceramic fibres. The fibres will be pressed against the internal wall of a cylindrical component or mould (or part filled mould) by the centrifugal force ready for infiltration of the matrix material. By thermal spray, powder fuse, or thermit melt technique to produce internally clad components and to join individual parts to manufacture said components. If necessary, an axial traversing feed system will enable a helical winding technique to apply various thicknesses of fibre layers and accommodate certain internal cavities.

By selecting the direction and position of the fibres it is possible to produce optimum strength characteristics ie. tubes internally wound at a helical angle almost square to the longitudinal axis of the tube give maximum bursting strength. Tubes wound at a helical angle inclined more toward the longitudinal axis of the tube provide more resistance to bending.

Examples of applications of the thermal spray, powder fuse, or thermit melt techniques for centrifugal internally wound continuous fibre reinforced composite components include: torpedoes, rocket missiles, rocket engines, brake drums and high speed rotating equipment - turbo fans, turbine fans, marine propellers and aeroengine components.

Exothermit welding has been in use since 1900. The exothermit welding process relies on an exothermic chemical reaction e.g.between aluminium and iron oxide when ignited.

The reaction, once started, will continue throughout the rest of the mass and produce superheated liquid steel at temperatures approaching 2500 degrees C. Almost twice as hot as the melting temperature of the base steel to be welded, a liquid/solid phase bond occurs at the edge forming the joint.

Exothermit welding can also be used for non-ferrous materials i.e the reduction of copper oxide by aluminium to produce super heated liquid copper.

Typically the exothermit reaction results in twice as much slag as deposited metal. During normal exothermit welding conditions, the greater density deposit material sinks under the effect of gravity while the lesser density slag rises above, leaving a 2/3rd volume of aluminium oxide slag on top of 1/3 volume of deposit material.

A detracting inherent feature which has limited the development of the exothermit welding process is the relatively high levels of porosity in the deposited material under normal gravity conditions.

In accordance with a second aspect of the present invention, a method of providing a coating on an internal surface of a tubular substrate comprises providing a thermit material within the internal volume defined by the substrate, igniting the material, and rotating the substrate and material at a sufficient angular velocity that coating material is urged against the substrate by centrifugal force and any slag produced by the thermit reaction separates radially inwardly from the remainder of the material.

The development of the centrifugal/exothermit technique provides typically in excess of 100g (lg is the force due to gravity, 200 times gravity is the maximum safe practice for steel moulds at the outer diameter - much greater force can be applied if stronger mould materials are used). The natural tendency for a centrifugal force to segregate dense constituents from less dense constituents will help separate the aluminium oxide slag from the exothermit deposited material and produce relatively clean, homogenous microstructures.

Thermit powder can be packed in annular, tubular or cylindrical containers and placed into a close fitting centrifugal mould or cylindrical component and ignited when under the effect of a suitable centrifugal force to provide an improved cladding or joining technique. The tubular exothermit container may have various cross sections and may be shaped to suit 'V' or 'U' prepared butt joints or shaped to suit internal clad regions of cylindrical components.

In accordance with a third aspect of the invention, a method of joining components comprises rotating the components while supplying weld material to a joint zone defined by the components whereby the weld material experiences centrifugal force.

In this aspect of the invention, two or more individual parts can be joined to manufacture one cylindrical component by the application of molten (or highly plasticised) material while under a centrifugal force to form a metallurgically bonded metal joint or thermoset plastic or thermoplastic joint.

In accordance with a fourth aspect of the invention, a method of forming a product comprises rotating a mould and material to be moulded located in the mould whereby centrifugal force urges the material against and into the form of the mould.

This aspect of the present invention provides improved means of forming and shaping a circular hollow symmetrical component, cylinder, or tube, or individual strips of material, to produce a larger diameter component by rotating the same, at a sufficient peripheral speed (below the limiting peripheral velocity - revolving ring bursting stress of the work holding device) by imposing a centrifugal force on a normally previously thermally softened region of the said component. In some cases, the addition of large reusable ceramic granules or metal may also be introduced, during rotation, to increase the effect of the centrifugal force acting to form the component.

In accordance with a fifth aspect of the present invention, a method of treating a product comprises subjecting the product to centrifugal force while carrying out a process on the product.

This aspect of the invention enables concrete products (tubes, blocks etc) to be manufactured while under a centrifugal force to provide improved density products with enhanced freeze/thaw durability. In addition glass fibrereinforced composite cement structural products can also be produced by internally helically winding fibre reinforcement. Engineering bricks or roof tiles of improved quality can also be produced - fired while under a centrifugal force. The combined pressure and heat providing a denser more cohesive product. Likewise grinding wheels can be manufactured by centrifugally compacting and sintering ie corundum material.

This aspect of the invention also enables centrifugal pressure cooking ie the cooking or baking of individual or tubular loaves, cakes, food processing etc while under a centrifugal force. Typically, the temperature and centrifugal force can be independently varied to control the degree of the leven or unleven condition - density and texture of the bread etc.

Furthermore this aspect of the invention enables the gradation of material properties in rubber or foamable plastics (ie micro cellular polypropylene or polyethylene etc) which solidify while under a centrifugal force.

Comparatively dense layers are produced during high centrifugal force conditions and comparatively less dense layers during comparatively low centrifugal force conditions.

The invention enables dense clad layers or billets of monolithic, composite, or intermetallic compound (e.g.

Titanium aluminides(Ti3Al), Iron aluminides(FeAl), Nickel aluminides (NiAl), Niobium aluminides (NbAl3)) to be produced using centrifuge techniques whereby the material is applied (by a thermal spray or atomised spray of thermit etc) while the spraying means and substrate undergo the same angular velocity.

Compressed wood chip and paper pulp products can also be manufactured, (cured with or without additional heating). Wood preservatives and paint can also be impregnated into timber products.

Examples of applications of this centrifugal forming technique include: seamless pressure vessels, accumulator bodies, gas cylinders, aero-space shock absorbers (landing gear), fire extinguishers, and ornamental metal work.

Examples of application of in-situ manufactured MMC by centrifugal force include the internal surface of 'wet' piston liners, and wear chutes and the external surface roller bodies, diamond rock drills and pistons.

Examples of application of the centrifugal mechanically clad technique include, steel backed internal bearings for marine propeller shafts, typically produced using lead, tin, aluminium, and copper base bearing alloys, tin/lead, aluminium/tin bearing alloys and steel backed ceramic engine cylinder liners.

Examples of applications of centrifugal impregnation include forcing solid lubricant or oil into grooves or voids to form combination bearings or oilite bearings.

Examples of applications of the centrifugal casting and exothermit joining techniques include the manufacture of cutter blocks, and joints for aero-engine turbine fans.

Examples of applications of centrifugal 'thermal spray' and 'powder fuse' techniques include internal bearings, internal liners etc.

Examples of applications of centrifugal compacting of concrete products include, chimney stacks and prefabricated concrete shuttering which becomes an integral part of reinforced concrete columns ie the exposed external surface of the column providing enhanced environmental durability.

Aspects of the invention enable the recycling of glass, for example using thermal heaters and centrifugal force to manufacture large diameter glass products.

Examples of applications of centrifugal compacting and sintering include house building and engineering bricks etc and grinding wheels.

Examples of applications of centrifugal cooking and baking include food processing under high pressure, removal of fat from meat products. The manufacture of bread tubular loafs etc.

Examples of application of centrifugal gradation of rubber and formable plastic include automotive tyres whereby a hard wearing rubber composite tread material and wire or nylon reinforcement are initially applied.

Subsequently followed by more flexible side wall material.

In another example solid inner tyres (puncture proof) can be manufactured to the required foam density and thereby maintain material properties.

Other components which may be manufactured by the invention include clad brake drums, propellers, wear tiles, wear chutes, torpedo bodies, accumulators, rocket engines, high speed rotating components, aero-engine components, turbine fans blades internal bearings, cutter blocks, engine cylinder liners, masts, light weight gun barrels, clad pipes, concrete products, bricks and sintered products.

The invention includes horizontal, vertical or angled centrifugal force and centrifuge techniques.

Brief description of the drawings Some examples of methods according to the invention will now be described with reference to the accompanying drawings, in which: Figures 1, 2, 3a and 3b illustrate schematically a centrifuge moulding process; Figure 4 is a longitudinal section and an end view of an internal tube cladding process; Figure 5 is a view similar to Figure 4 but showing a modified process; Figure 6 is an end view of a tube which has been internally clad using a method of the type shown in Figure 4 and Figure 5; Figure 7 and Figure 8 are a plan and side view of another cladding arrangement for cladding planar elements; Figure 9 illustrates the use of keying apertures when cladding a substrate; Figure 10 illustrates a further thermal spray cladding process; Figure 11 illustrates schematically a process using thermit material; and, Figures 12a and 12b illustrate 2 different methods for joining components.

Figure 1 is a schematic longitudinal section of a stationary or slowly rotating tubular component 1 which is being thermally softened 2 by for example induction, resistance, or flame heating. The thermally softened component is placed in a mould 3 which is rotated at relatively high speed such that centrifugal force causes the softened portion of the tubular component to distort into the shape of the mould as shown in Figure 2.

Figures 3a and 3b illustrate a mould 5 which, during the heating process, is offset along the component (Figure 3b). Once the portion of the component 6 has been heated, the mould 5 is slid to the left, as seen in the drawing, and then the component together with the mould is rotated at high speed so that the component takes up the mould shape, as seen in Figure 3a.

In a further example (not shown) the component can be heated internally in which case this could be done with the mould in place.

Figure 4 illustrates the cladding of an inside face of a tube 7. A number of thermal spray nozzles 8 are mounted along a support 9 in communication with a particulate material supply tube 10. The tube 10 is surrounded by a ceramic heat shield 11. The tube 7 is rotated at high speed to generate a high centrifugal force while particulate material (eg powder) is sprayed from the nozzles 8. At the same time the tube 10 is traversed to and fro in the direction of arrows 12 so that the entire inner face of the tube 7 is clad. In practice, a final clad layer 13 will be made up of many discrete layers as shown in Figure 6.

The particulate material preferably includes ceramic particles so that the clad layer 13 is formed of a metal matrix composite. Monolithic clad layers can also be formed.

Figure 5 illustrates a modification of the Figure 4 example in which the nozzle mounting also carries a number of heating torches 14. In addition, instead of thermal spray nozzles 8, the mounting carries powder nozzles 8'.

Again, the tube 7 is rotated at high speed and the powder will be held on the inner face of the tube under centrifugal force. The subsequent passage of the heating torches will cause the powder to fuse to the tube.

In the examples shown in Figures 4 and 5, the thermal spray nozzles and powder nozzles remained rotationally stationary while the tube rotated. In the example shown in Figures 7 and 8, a pair of spray nozzles 15 are provided connected to a supply of particulate material 16, for example ceramic particulate added to a metal spray. A substrate support 17 is mounted to the support 18 of the nozzles 15 so as to rotate with the nozzles 15 at substantially the same angular velocity and direction under the control of a motor 19. Each arm of the support 17 holds a planar substrate 20 which will remain stationary relative to the nozzles 15.

In use, the assembly is rotated at relatively high speed and the particulate material is ejected from the nozzles 15 which may be thermal spray guns or nozzles through which the material is ejected by centrifugal force and is caused to lodge on the substrates 20 as a cladding layer 21. Particulate material is held on the substrate 20 by the centrifugal force generated on rotating the support 17 while the spray itself is provided with additional energy as a result of the spray nozzles 15 rotating.

A flux may be provided on the substrate to wet the substrate before the application of the denser powder particulate which displaces the flux.

The particulate material in the funnel shaped supply 22 is a thermit mix which would be ignited after causing rotation of the spray nozzles 15. The rotation direction of the nozzles 15 could be in the opposite direction to the substrate 20 in the case of a tubular substrate so as to increase tangential impact.

Figure 9 shows how the previous cladding examples can be further modified by providing holes 23 in the substrate 20 mounted on the support 17 so that portions of the clad layer 21 extend into the holes 23 to provide a mechanical lock with the substrate.

Figure 10 illustrates another variation of the thermal spray technique in which a number of individual, planar substrates 29 are mounted about the circumference of a support 28. A nozzle 30 is fixed in the position shown while the support 28 is rotated at high speed. This results in the cladding layer being formed on each of the substrates 29 and demonstrates again that the invention is not limited to the cladding of tubular substrates.

Figure 11 illustrates another way in which thermit powder can be used to internally clad a tube. In this case, the thermit powder is packed in an annular, tubular member 31 formed, for example, by inner and outer sleeves of thin steel sheet. This assembly is then placed inside the tube 32 to be clad, the thermit powder ignited, and hole rotated at very high speed. As described above, this causes the thermit material in a relatively pure form to clad the internal surface of the tube 32 while slag and other and unwanted components are drawn radially inwardly and can be removed.

Figure 12a illustrates the joining of two tubes 33,34.

The tubes 33,34 are butted together to define a V-shaped joint region 35, and are placed in a mould 36. Weld material, for example ceramic powder/metal powder is then deposited in the V-shaped joint using a nozzle of the form shown in Figure 4 (but not shown in Figure 12a) while the mould 36 and tubes are rotated at high speed. This causes the weld material to join the two tubes together which are then removed from the mould.

Figure 12b illustrates a modified form of the Figure 12a example in which dovetailed blades 37 are joined to tubular parts 38.

Claims (18)

Claims

1. A method of providing a coating on a substrate, the method comprising spraying particulate material via spraying means onto the substrate while rotating the substrate at a sufficiently high speed that the particulate material is held on the substrate by centrifugal force.

2. A method according claim 1, wherein the spraying means is also rotated.

3. A method according to claim 2, wherein the substrate and spraying means rotate at substantially the same angular velocity.

4. A method according to claim 3, wherein the substrate is planar.

5. A method according to any of the preceding claims, further comprising heating the coated particulate material to fuse the material onto the substrate.

6. A method according to any of the preceding claims, further comprising causing relative traverse movement between the substrate and spraying means in a direction substantially parallel to the axis of rotation.

7. A method according to any of the preceding claims, wherein the particulate material is a ceramic.

8. A method according to any of the preceding claims, wherein the centrifugal force is greater than lg, preferably greater than 10g, more preferably greater than 50g, most preferably greater than 100g.

9. A method according to any of the preceding claims, wherein the particulate material includes re-usable ceramic or metal balls to further augment the centrifugal force.

10. A method according to any of the preceding claims, wherein the particulate material includes ceramic particulate or whisker reinforcement, for forming a metal matrix composite.

11. A method according to any of the preceding claims, wherein the centrifugal force is used to apply and hold metal, glass or plastic powder in place for subsequent fusing and consolidation.

12. A method of providing a coating on an internal surface of a tubular substrate, the method comprising providing a thermit material within the internal volume defined by the substrate, igniting the material, and rotating the substrate and material at a sufficient angular velocity that coating material is urged against the substrate by centrifugal force and any slag produced by the thermit reaction separates radially inwardly from the remainder of the material.

13. A method according to claim 12, wherein the material is held in an elongate, annular container within the substrate.

14. A method of treating a product comprising subjecting the product to centrifugal force while carrying out a physical or chemical process on the product.

15. A method according to claim 14, wherein a combined centrifugal force and applied heat is used to pressure cook and bake the product.

16. A method of joining components, the method comprising rotating the components while supplying weld material to a joint zone defined by the components whereby the weld material experiences centrifugal force.

17. A method according to claim 16, wherein the weld material comprises a molten or liquid material.

18. A method of forming a product, the method comprising rotating a mould and material to be moulded located in the mould whereby centrifugal force urges the material against and into the form of the mould.