Process for producing a preform using cold spray

Abstract

A process for producing a preform by cold spray deposition, the process comprising: providing a starter substrate about a preform axis of rotation, the starter substrate having at least one axial end having a substantially flat deposition surface; rotating the starter substrate about the preform axis of rotation; depositing material onto the deposition surface of the starter substrate using cold spray deposition to form a product deposition surface, the cold spray deposition process including a cold spray applicator through which the material is sprayed onto the deposition surface; successively depositing material onto a respective top product deposition surface using cold spray deposition to form successive deposition layers of the material; and moving at least one of: the cold spray applicator; or the starter substrate and preform product, relative to the other in an axial direction along the preform axis of rotation to maintain a constant distance between the cold spray applicator and the top product deposition surface, thereby forming a preform product of a selected length, wherein the cold spray applicator is moved in a plane perpendicular to the preform axis of rotation so as to deposit material as a substantially flat surface on each respective deposition surface of the starter substrate or product deposition surface of the preform product.

Claims

1. A process for producing a round preform by cold spray deposition, the process comprising: providing a starter substrate about a preform axis of rotation, the starter substrate having at least one axial end having a substantially flat deposition surface; rotating the starter substrate about the preform axis of rotation; depositing material onto the deposition surface of the starter substrate using cold spray deposition to form a product deposition surface, the cold spray deposition process including a cold spray applicator through which the material is sprayed onto the deposition surface; successively depositing material onto a respective top product deposition surface using cold spray deposition to form successive deposition layers of the material; and moving at least one of: the cold spray applicator; or the starter substrate and preform product, relative to the other in an axial direction along the preform axis of rotation to maintain a constant distance between the cold spray applicator and the top product deposition surface, thereby forming a round preform product about the preform axis of rotation of a selected length, wherein the cold spray applicator is moved in a plane perpendicular to the preform axis of rotation so as to deposit material as a flat surface on each respective deposition surface of the starter substrate or product deposition surface of the preform product, wherein the flat surface of deposit material is maintained through controlled movement of cold spray applicator in both the axial direction and in the plane perpendicular to the preform axis of rotation, wherein movement of the cold spray applicator is controlled so that the instantaneous velocity of the cold spray applicator relative to the deposit surface is inversely proportional to radial distance the cold spray applicator is to the preform axis of rotation, and wherein the deposited material comprises a metal or alloy thereof.

2. The process according to claim 1, wherein the controlled movement comprises a linear cyclical motion between at least two points.

3. The process according to claim 2, wherein the controlled movement comprises a linear cyclical motion between two points, point A and point B, selected from at least one of: point A is at an edge of the preform product, and point B is close to, or at the centre of the preform product; or point A and point B are at an edge of the preform product.

4. The process according to claim 1, wherein the movement of the spray applicator is configured to have a radial offset from a parallel path running through the preform axis of rotation.

5. The process according to claim 1, wherein the controlled movement comprises a linear cyclical motion between four points, points A, B, C and D.

6. The process according to claim 5, wherein points A, B, C and D define the vertices of a regular polygon, and the controlled movement comprises linear movement traces the polygon shape between the respective points.

7. The process according to claim 1, wherein movement of the cold spray applicator in both the axial direction and in the plane perpendicular to the preform axis of rotation is controlled by a multi-axis robot arm.

8. The process according to claim 1, wherein the cold spray applicator includes a nozzle having an exit opening through which deposit material is sprayed, the nozzle directing the sprayed deposit material in a desired direction.

9. The process according to claim 8, wherein the nozzle is aligned to or parallel to the axis of preform rotation during movement.

10. The process according to claim 8, wherein the nozzle is directed to an angle, towards the centre of the axis of preform rotation when at or near an outer edge of the preform product.

11. The process according to claim 1, further comprising the step of: removing the preform product from the starter substrate.

12. The process according to claim 1, wherein the starter substrate comprises at least one of: a substrate with matching material properties; or a substrate made of dissimilar material.

13. The process according to claim 1, wherein the starter substrate comprises a starter preform.

14. The process according to claim 13, wherein the starter preform is made by a cold spray method.

15. The process according to claim 1, wherein the starter substrate has at least the same diameter as the preform product.

16. The process according claim 1, wherein the axial end surface of the starter substrate comprises a radially flat surface relative to the preform axis of rotation.

17. The process according to claim 1, wherein the starter substrate is held about the preform axis of rotation using a mounting arrangement which includes clamp or chuck.

18. The process according to claim 1, wherein the deposited material comprises a metal, an alloy thereof, or a metal composite, or a combination thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein:

(2) FIG. 1 is a schematic diagram of one embodiment of the cold spray process of the present invention at start up.

(3) FIG. 2 is a schematic diagram of one embodiment of the cold spray process shown in FIG. 1 with a preform product deposited onto a starter substrate.

(4) FIG. 3 is (A) a schematic of cold spray deposition pattern used to form a preform using two points according to an embodiment of the present invention; and (B) a plot of the instantaneous nozzle velocity when moving in that pattern.

(5) FIG. 4 is (A) a further schematic of cold spray deposition pattern used to form a preform using two points according to an embodiment of the present invention; and (B) a plot of the instantaneous nozzle velocity when moving in that pattern.

(6) FIG. 5 is (A) a schematic of cold spray deposition pattern used to form a preform using four points according to an embodiment of the present invention; and (B) a plot of the instantaneous nozzle velocity when moving in that pattern.

(7) FIG. 6 provides a photograph of a Ti-6Al-4V preform attached to a starter substrate made using a spray method according to the present invention.

(8) FIG. 7 provides a photograph of a titanium alloy Ti-6Al-4V preform made using a spray method according to the present invention. The starter substrate has been cut off from the bottom of the preform and the top surface machined.

(9) FIG. 8 is an optical micrograph of a pure titanium preform.

DETAILED DESCRIPTION

(10) The present invention provides a process for forming a preform such as a disk, bar, rod, cone or the like of material using cold spray technology.

(11) Cold spraying is a known process that has been used for applying coatings to surfaces. In general terms, the process involves feeding (metallic and/or non-metallic) particles into a high pressure gas flow stream which is then passed through a converging/diverging nozzle that causes the gas stream to be accelerated to supersonic velocities, or feeding particles into a supersonic gas stream after the nozzle throat. The particles are then directed to a surface to be deposited. The process is carried out at relatively low temperatures, below the melting point of the substrate and the particles to be deposited, with a coating being formed as a result of particle impingement on the substrate surface. The process takes place at relatively low temperature thereby allowing thermodynamic, thermal and/or chemical effects, on the surface being coated and the particles making up the coating, to be reduced or avoided. This means that the original structure and properties of the particles can be preserved without phase transformations or the like that might otherwise be associated with high temperature coating processes such as plasma, HVOF, arc, gas-flame spraying or other thermal spraying processes. The underlying principles, apparatus and methodology of cold spraying are described, for example, in U.S. Pat. No. 5,302,414 the contents of which should be understood to be incorporated into this specification by this reference.

(12) In the present invention, cold spray technology is used to build up a preform structure on the axial end surface of a starter substrate. The starter substrate can then be removed to produce a primary preform product.

(13) FIG. 1 illustrates the basic schematic of one apparatus 100 for forming a preform according to the present invention. In this embodiment an initiating substrate, in the form of a starter substrate 130 is initially used to provide a surface on to which the product preform 132 (FIG. 2) is sprayed. The illustrated starter substrate 130 is a round bar having an outer diameter which is about the same as the desired outer diameter of the preform 132 being produced. However, it should be appreciated that the starter substrate could be of any suitable shape, configuration or diameter, and in particular of a diameter which is at least the same as the diameter of the preform product 132 being produced. The starter substrate 130 includes an axial deposition end 135 having a substantially flat deposition surface 136 on which the cold spray material is deposited during operation.

(14) The starter substrate 130 is mounted and held about a preform axis of rotation X-X within the apparatus 100 using a mounting arrangement 134. Whilst not shown in detail in FIG. 1 or 2, this mounting arrangement 134 could be any suitable clamp or chuck type arrangement, a number of which are currently commercially available. In exemplary embodiments, the starter substrate 130 is held about the preform axis of rotation X-X chuck, preferably a feed-through chuck. Whilst not illustrated, the mounting arrangement 134 can also include one or more rests, bearings or rollers on which the starter substrate 130 and/or product preform 132 can engage, bear or otherwise be supported during operation of the apparatus 100.

(15) At least part of the mounting arrangement 134 is operatively driven about the preform axis of rotation X-X which in turn drives rotation of the starter substrate 130 about the axis X-X in the direction of arrow R. A number of suitable rotation arrangements are possible, including but not limited to drive wheels, turntables, lathe arrangements or the like. In one embodiment, the starter substrate 130 can be locked in place using a chuck attached to a lathe and the lathe used to rotate the chuck.

(16) Once the starter substrate 130 is mounted in the mounting arrangement, the starter substrate 130 is rotated about the preform axis of rotation X-X. A cold spray applicator, in this case cold spray gun 140, is used to spray a desired material onto the deposition surface 136 of the starter substrate 130. As can be appreciated, the cold spray gun 140 includes a nozzle 142 through which material is sprayed and directed in a spray stream 144 onto the deposition surface 136. The cold spray gun 140 supplies a source of inert carrier gas and material feed particles to nozzle 142. The cold spray gun 140 and attached nozzle 142 is likely to be of conventional form and, in general terms, the basis of the equipment is as described and illustrated in U.S. Pat. No. 5,302,414. The material particles are entrained in the carrier gas and the carrier gas and particles are accelerated to supersonic velocities. Accordingly, the spray 144 exiting the nozzle 142 comprises a jet of carrier gas and entrained material particles.

(17) The cold spray gun 140 and associated cold spray system may be operated using any of the gases that are common with this process, for example nitrogen or air. Helium is sometimes used because it provides greater particle acceleration. For example, acceptable results can be obtained with titanium and its alloys using nitrogen. However, if possible reaction with the particles is a concern, then argon may be a useful alternative.

(18) The cold spray gun 140 is controlled to move about a three dimensional axis (each of the X, Y and Z axis) by robotic arm 146. However, it should be appreciated that the cold spray gun 140 could be moved by any suitable means, including a linear actuator or other means. Prior to spray application, the end 148 of the nozzle 142 is brought to a suitable deposition distance D from the deposition surface 136. This deposition distance is preferably 10 to 50 mm, more preferably 20 to 30 mm (depending on the cold spray gun 140) in order to provide a desired deposition pattern on the deposition surface 136.

(19) Spraying of materials particles from a nozzle 142 is commenced when the nozzle 142 is positioned the required deposition distance D from the deposition surface 136. The robotic arm 146 is used to move the cold spray gun 140 and nozzle 142 radially (about the X and Y axes shown in FIGS. 1 and 2) relative to the preform axis of the rotation X-X to cold spray material onto the deposition surface 136 of the starter substrate 130. In this case, rotation of the starter substrate 130 combined with radial movement of the nozzle 142 is responsible for build up of a deposition on the deposition surface 136 of the starter substrate 130. As shown in FIG. 2, a number of spray patterns can be used to form each deposition layer 137 of material forming the product preform 132. An example of some suitable spray patterns is described in more detail below.

(20) The cold spray gun 140 and nozzle 142 are used to spray a first deposition layer on the deposition surface 136 of the starter substrate 130. The particles sprayed on the deposition surface 136 bond onto a portion of the deposition surface 136. The position of the starter substrate 130 is moved relative to the nozzle 144 along the preform axis of rotation X-X by either moving the starter substrate along the axis X-X or the nozzle 142 or both, in order to maintain a constant distance D between the end of the nozzle 148 and the top spray layer 137 of the axial deposition end 135. The spray gun 140 is then operated to deposit another layer of material onto the top spray layer 137 of material on the axial deposition end 135 thus extending the length of the product preform 132.

(21) In some embodiments, the starter substrate 130 and product preform 132 is fed slowly through a feed-through chuck, in the lengthwise direction along axis X-X, away from the cold spray nozzle 142 so that as the preform grows a constant distance is maintained between the nozzle end 148 and the flat surface of the preform (deposition surface 136). In other embodiments, the spray gun 140 and nozzle 142 are moved in the lengthwise direction along axis X-X, away from the axial deposition end 135 of the product preform 132 and starter substrate 130. In yet other embodiments, a combination of the above two movements is used.

(22) The movement of the preform 132 in the direction of arrow S (FIG. 2) and/or the spray gun 140 in the direction of arrow T (FIG. 2) is performed continuously at a slow rate that is equivalent to the rate of particles required to build up each layer of the product preform 132. In this manner, the product preform 132 is formed continuously and can be formed in any desired length.

(23) The freshly deposited material should constantly maintain a substantially flat surface during each cold spray deposition on the top layer 137 of material on axial deposition end 135 in order that the product preform 132 grows at a constant rate over the entire cross-sectional area. This flat surface is maintained using the spray patterns and method described below.

(24) When the desired length of formed preform 132 is reached, the starter substrate 130 is removed from the remainder of the formed preform 132. Separation of the preform 132 from the starter substrate 130 may be achieved by any suitable means, including mechanical such as cutting, cleaving, breaking, fracturing, shearing, breaking or the like, or by other means including dissolving, melting, evaporating or the like of the starter substrate.

(25) As noted above, the product preform 132 should grow at a constant rate over the entire cross-sectional area in order for the freshly deposited material to maintain a flat surface during each cold spray deposition on the top layer 137 of material on axial deposition end 135. This flat surface is maintained using the spray patterns in which the amount of time spent by the cold spray nozzle 142 at any radial distance from the preform axis of rotation X-X is proportional to the radial distance from the nozzle 142 (taken as the radial center along axis N-N (FIGS. 1 and 2) of the nozzle 142) to the preform axis of rotation X-X. In these spray patterns, the feed rate of powder/particles through the nozzle 142 is substantially constant and the speed of rotation of the starter substrate and attached product preform is substantially constant.

(26) This above condition may be met by an infinite number of different spray methods. The following three spray patterns provide non-limiting examples of spray patterns which can meet the above conditions. However, it should be appreciated that the present invention should not be limited to these spray patterns, and that a variety of other spray patterns are possible. In each example, movement of the nozzle 142 can be controlled by a multi-axis robot arm.

(27) Spray Method 1

(28) As shown in FIG. 3(A), in spray method 1, the nozzle 142 is moved back-and-forth between two points, Point A and Point B1. Point A is at the edge of preform 132, and Point B1 is close to, or at the centre of the preform 132. The instantaneous velocity of the nozzle 142 moving across the end 135 is controlled to be inversely proportional to the distance from the end 143 of the nozzle 142 to the preform axis of rotation X-X. As shown in FIG. 3(B), the velocity of the nozzle 142 is therefore higher near Point B1, relative to the velocity of the nozzle near Point A.

(29) Spray Method 2

(30) As shown in FIG. 4(A), in spray method 2, the nozzle 142 is moved back-and-forth between two points, Point A and Point B2. Both Point A and Point B2 are at the edge of the preform 132, usually on opposite sides. The instantaneous velocity of the nozzle 142 moving across the end 135 is controlled to be inversely proportional to the distance from the nozzle 142 to the preform axis of rotation X-X. As shown in FIG. 4(B), while moving from Point A towards Point B2 or from Point B2 towards A the velocity of the nozzle 142 initially increases, reaching a maximum at the point closest to the preform axis of rotation X-X (Point C, which is equidistant from Point A and Point B), and then decreases.

(31) Spray Method 3

(32) As shown in FIG. 5(A), in spray method 3, four points are used, Point A, B, C and D, and the nozzle 142 traces a rectangular path between them. Point A and B are on opposite edges of the preform 132 to Point C and D. There is a small distance, for example 0.5 to 10 mm, separating Point A from Point B, and an equally small distance separating Point C from Point D. In moving from Point A to Point B, and likewise from Point C to Point D, the instantaneous velocity of the nozzle 142 moving across the end 135 is controlled to be inversely proportional to the distance from the end 143 of the nozzle 142 to the preform axis of rotation X-X. A relatively fast nozzle movement can be used in moving from Point B to Point C and in moving from Point D to Point A.

(33) It should be appreciated that strictly speaking, if the instantaneous nozzle velocity is inversely proportional to distance to the preform axis of rotation X-X, the nozzle 142 could only ever cross the preform axis of rotation X-X with an infinite velocity. In practice, it may be found acceptable to clip the maximum velocity so that the deposition rate at the centre of the preform 132 is not significantly greater than at greater diameters. In some embodiments, it may be preferable to prevent the nozzle 142 from crossing the preform axis of rotation X-X by offsetting the path of nozzle 142 movement by a small distance, for example 0.5 to 10 mm as shown in FIGS. 3(A), 4(A) and 5(A). Spray beams generally exhibit some degree of divergence which principally depends on the nozzle design. For example, a nozzle 142 with circular cross-section produces a circular spot pattern on the substrate surface. Accordingly, particles at the edge of the spray beam 144 should therefore fill in the central part of the preform 132.

(34) The nozzle 142 is normally aligned parallel or approximately parallel to the preform axis of rotation X-X. In some embodiments, it may also be necessary to change the angle of the nozzle 142 with respect to the preform axis of rotation X-X each time the nozzle 142 approaches the edge 150 (FIGS. 3 and 4) of the preform 132. Here, the cold spray nozzle 142 is turned so that it is angled inwards, towards the preform axis of rotation X-X (and the centre of the preform 132). This technique is used to control the growth of the edges 150 of the preform 132 so that it maintains a constant diameter.

(35) Spray Method 4

(36) Whilst not illustrated, a fourth spray method comprises movement of the nozzle 142 in a spiral pattern while the starter substrate 130 is rotating about the preform axis X-X. In this embodiment, the nozzle 142 can in some embodiments be moved by the robot at a substantially constant velocity.

(37) Spray Method 5

(38) Any of spray methods 1, 2 or 3 and other additional methods could be modified so that instead of the nozzle velocity being inversely proportional to distance to the preform axis of rotation X-X, the rotational speed of the starter substrate 130 and product preform 132 about the preform axis of rotation X-X is varied as a function of the nozzle 142 radial distance from the axis of rotation X-X. As can be appreciated, this also varies the instantaneous velocity between the nozzle end 148 and deposition surface 136. In such an embodiment, the speed of movement of the nozzle 142 as moved by a robot could be kept substantially constant.

EXAMPLES

(39) The description of embodiments of the invention in the following examples is in the context of producing a round titanium alloy preform from titanium alloy particles. However, it will be appreciated that the invention enables production of preform of various metals and alloys thereof and the description should not be interpreted as limiting the embodiments to producing titanium alloy preform only.

Example 1

(40) The apparatus 100 described and illustrated above was used to make a Ti-6Al-4V alloy preform. The cold spray system and conditions used were as follows: Cold spray equipment: CGT Kinetiks 4000 system Robot arm for controlling movement of cold spray gun: ABB IRB2600 Number of supersonic nozzles: one Rotational mounting: a lathe with swivel head Lathe speed 1000 rpm Stand-off: 30 mm Spray angle: Normal to the surface at all times Gas: nitrogen Gas stagnation temperature: 800 C. Gas stagnation pressure: 3.5 MPa Powder feed rate: 21.4 g/min Robot traverse speed range: 7-163 mm/s

(41) The feedstock powder was Ti-6Al-4V manufactured by gas atomization. The starter substrate was an aluminium disc.

(42) The Ti-6Al-4V preform was made using spray method 3 as described above. In producing the preform, the distance D between the end 144 of the nozzle 142 of the spray gun 140 and the top layer 137 of the end 135 was maintained by slowly moving the spray gun 140 backwards in the direction of arrow T (FIG. 2) during spraying away from the starter substrate by 0.3 mm for each repeat of the path shown in FIG. 5 so as to allow for growth of the deposit. Once spray deposition was ended, the starter preform was cut off the end of the produced round disk.

(43) FIG. 6 shows a photograph of the Ti-6Al-4V preform and starter substrate after spraying with the aluminium starter substrate attached.

Example 2

(44) The apparatus 100 described and illustrated above was used to make a Ti-6Al-4V alloy preform. The cold spray system and conditions used were as follows: Cold spray equipment: Plasma Giken PCS-1000 Robot arm for controlling movement of cold spray gun: ABB IRB4600 Number of supersonic nozzles: one Rotational mounting: a lathe with swivel head Lathe speed 500 rpm Stand-off: 20 mm Spray angle: Normal to the surface at all times Gas: nitrogen Gas stagnation temperature: 900 C. Gas stagnation pressure: 5.0 MPa Powder feed rate: 41.3 g/min Robot traverse speed range: 2-63 mm/s

(45) The feedstock powder was Ti-6Al-4V manufactured by gas atomization. The starter substrate was an aluminium disc.

(46) Similar to Example 1, a Ti-6Al-4V preform was made using spray method 3 as described above. In producing the preform, the distance D between the end 144 of the nozzle 142 of the spray gun 140 and the top layer 137 of the end 135 was maintained by slowly moving the spray gun 140 backwards in the direction of arrow T (FIG. 2) during spraying away from the starter substrate by 1.0 mm for each repeat of the path shown in FIG. 5 so as to allow for growth of the deposit.

(47) Following cold spray the titanium deposit was removed from the aluminium starter disc by parting off in a lathe. The rough material on the surface was removed by machining leaving the shape shown in FIG. 7. From the machined face of this preform (FIG. 7) it is evident that a solid, metallic preform had been made.

Example 3

(48) The apparatus described and illustrated above was used to make a further short pure titanium preform. The apparatus and spray conditions were the same as in Example 1 except for the following: Lathe speed 500 rpm Powder feed rate: 13.9 g/min Robot traverse speed range: 2-80 mm/s The nozzle was moved away from the starter substrate by 0.7 mm for each repeat of the path shown in FIG. 5 so as to allow for growth of the deposit.

(49) In this example, the feedstock powder was commercial purity titanium powder manufactured by the hydride-dehydride process. Again, a disc-shaped titanium preform was made having a similar configuration as the preforms shown in FIGS. 6 and 7.

(50) Following cold spray the titanium deposit was removed from the aluminium starter by parting off in a lathe. The rough material on the surface was removed by machining, leaving a disc 73.9 mm in diameter and 8.6 mm thick. A slice was then cut from this disc and the slice then further cross-sectioned, cold mounted in epoxy resin and polished using standard metallographic techniques.

(51) FIG. 8 shows the unetched microstructure from a photograph taken using an optical microscope. Pores could be seen between particles (black in FIG. 8). The concentration and distribution of pores was very uniform throughout the disc. The porosity was measured at a series of radial distances from the centre of the disc, by digital image analysis of micrographs such as FIG. 8. At each distance, measurements were taken from five micrographs in order to obtain a statistical average. The results, given in Table 1, show that the range of porosity was 4.6-7.0% throughout.

(52) TABLE-US-00001 TABLE 1 Porosity Measurements for representative Ti preform sample Distance from axis of rotation (mm) Measured porosity (%) 0 5.3 0.2 7 4.6 0.2 12 4.6 0.1 20 6.6 0.3 27 5.9 0.2 34 7.0 0.1

Example 4

(53) The apparatus 100 described and illustrated above was used to make a copper, disc-shaped preform. Pure, <200 mesh copper powder was used as the feedstock. The starter substrate was an aluminium disc. The cold spray system and conditions used were identical to Example 1 except for the following: Lathe speed 500 rpm; Gas stagnation temperature: 600 C.; Gas stagnation pressure: 3.5 MPa; Powder feed rate: 52.4 g/min; Robot traverse speed range: 2-60 mm/s.

(54) From weight measurements of the powder feeder directly before and after spray, it was determined that 885 g of powder was used. The weight added to the starter disc by the copper deposit was 823 g. From these two values, it can be concluded that the deposition efficiency was 93.1%.

(55) Following cold spray, a round disc with diameter 82.3 mm and thickness 11.7 mm was machined. The weight of the disc was 551.43 g, giving a density of 8.86 g/cm.sup.3, or 98.9% of the theoretical density of copper.

(56) Whilst the examples and accompanying description only show preforms having a circular cross-section, it should be appreciated, that an asymmetrical round shape such as an oval shape could be produced by synchronising the rotational movement of the starter substrate and formed preform product with the lateral movement of the spray nozzle. Similarly, it should be appreciated that a void or hollow could also be introduced into the billet by introducing a no-deposit area or zone in the spray pattern of the cold spray applicator, where no material is deposited.

(57) Similarly, whilst the examples and accompanying description only show preforms having a substantially constant cross-section, it should be appreciated that the preform can also be formed with variable or non-constant diameter such as a cone shapes, cone section, or shapes with a step or taper (large diameter to smaller diameter).

(58) Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

(59) Where the terms comprise, comprises, comprised or comprising are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.