METHOD AND APPARATUS FOR CLADDING A SURFACE OF AN ARTICLE

Abstract

This invention relates to a method, system and apparatus for cladding a surface of an articles subject to corrosive, erosive or abrasive wear, such as impact or grinding tools. The method includes providing a supply of stock material and feeding the stock material towards a portion of the surface of the article via a dedicated feed source. A dedicated heat source heats the fed stock material and the portion of the surface of the article such that the heated stock material and the portion of the surface at least partially melt. Upon removal of the heat, the molten feedstock and the surface portion form a bonded coating layer on at least a portion of the surface of the article, thereby protecting that part of the assembly against wear.

Claims

1. A method of cladding a surface of an article, the method including the steps of: providing a supply of feedstock material; feeding the feedstock material towards a portion of the surface of the article via a dedicated feed source; and heating the portion of the surface of the article via a dedicated heat source such that the fed feedstock material and the portion of the surface at least partially melt, whereby, upon removal of the heat, the molten feedstock and the surface portion form a bonded coating layer deposited by way of overlapping beads on the surface of the article, wherein each deposited bead is overlapped with the previously deposited bead to an extent of greater than 50% of the respective bead width.

2. A method according to claim 1, wherein the deposited beads are overlapped to an extent within the range of approximately 50% to approximately 95% of the bead width.

3. A method according to claim 2, wherein the deposited beads are overlapped to an extent within the range of approximately 75% to approximately 95% of a bead width.

4. A method according to any one of the preceding claims, including the step of providing a selectively programmable control means configured for controlling movement of the feed and heat sources relative to the article.

5. A method according to claim 4, wherein the control means is configured to cause the beads of the bonded coating layer to be deposited at a surface speed rate of at least 500 m/min.

6. A method according to claim 4, wherein the control means is configured to cause the beads of the bonded coating layer to be deposited at a surface speed rate of at least 2000 m/min.

7. A method according to claim 4, wherein the control means is configured to cause the beads of the bonded coating layer to be deposited at a surface speed rate of at least 4000 m/min.

8. A method according to any one of the preceding claims, wherein the heat source is in the form of a laser.

9. A method according to claim 8, including the step of setting the power output of the laser to be within the range of approximately 3 kW to approximately 20 kW.

10. A method according to claim 8, including the step of setting the power output of the laser to be within the range of approximately 4 kW to approximately 10 kW.

11. A method according to any one of claims 8 to 10, including the step of providing an optical focusing mechanism for directing and focusing a laser beam from the laser onto the desired portion of the surface of the article.

12. A method of cladding a surface of an article according to any one of the preceding claims, including the step of depositing a multilayer, functionally graded coating layer onto the surface of the article.

13. A method of cladding a surface of an article according to claim 12, including the step of providing the feed mechanism with at least two feed nozzles for depositing a double graded coating layer in a single pass, wherein the feed mechanism comprises a first feed nozzle adapted to deposit a first feedstock material directly onto the surface of the article to form a primary sub-layer of the coating, and a second feed nozzle adapted to deposit a second feedstock material adapted to deposit the second feedstock material onto the first sub-layer to form a secondary sub-layer of the coating.

14. A method of cladding a surface of an article according to claim 12 or claim 13, including the step of depositing the multilayer coating in a single pass across the surface of the article.

15. A method of cladding a surface of an article according to any one of the preceding claims, wherein the stock material is in the form of a metal matrix composite (MMC).

16. A method of cladding a surface of an article according to claim 15, wherein the MMC has a composition including a matrix material and a reinforcing material dispersed within the matrix material.

17. A method of cladding a surface of an article according to claim 16, wherein the matrix material is wear resistant and formed of a self fluxing alloy.

18. A method of cladding a surface of an article according to claim 16, wherein the reinforcing material is a particulate, granular, powdered, or fibrous material.

19. A method of cladding a surface of an article according to claim 17 or claim 18, wherein the wear resistant matrix material is selected from the group, including: nickel, cobalt and iron.

20. A method of cladding a surface of an article according to any one of claims 16 to 19, wherein the reinforcing material is selected from the group including: tungsten carbide, titanium carbide, chromium carbide, niobium carbide, silicon carbide, vanadium carbide and boron carbide.

21. A method of cladding a surface of an article according to any one of claims 16 to 20, wherein the MMC comprises approximately 5 to 90 percent by weight of binder phase matrix material, and from 10 to 95 percent by weight of hard phase particulate reinforcing material.

22. A method of cladding a surface of an article according to any one of claims 16 to 21, wherein the bonded coating layer has a substantially even distribution of the hard phase particles within the matrix material, across the entire coating layer on the surface of the article.

23. A method of cladding a surface of an article according to any one of claims 16 to 22, wherein the MMC layer contains 68 w % WC, and 32 w % NiBSi alloy.

24. A system for cladding a surface of an article, the system including: a feed mechanism associated with a supply of feedstock material, the feed mechanism being configured for feeding the feedstock material towards a portion of the surface of the article; and a heat source for heating the portion of the surface of the article such that the fed feedstock material and the portion of the surface at least partially melt, whereby, upon removal of the heat, the molten feedstock and the surface portion cooperate to form overlapping beads to define a bonded coating layer on the surface of the article, wherein each bead of the coating layer is overlapped with the previously deposited bead to an extent of greater than 50% of the respective bead width.

25. A system according to claim 24, wherein the feed mechanism and the heat source are adapted to deposit the coating layers such that the deposited beads are overlapped to an extent within the range of approximately 40% to approximately 95% of the bead width.

26. A system according to claim 24 or claim 25, wherein the control means is configured to cause the beads of the bonded coating layer to be deposited at a rate of at least 500 m/min.

27. A system according to any one of claims 24 to 26, wherein the feed mechanism is a gravity feeding mechanism or a pressurised feeding mechanism.

28. A system according to any one of claims 24 to 27, wherein the feed mechanism includes at least one feed nozzle, the at least one feed nozzle being operatively associated with a reservoir of the stock material.

29. A system according to any one of claims 24 to 28, wherein the feed mechanism is adapted to enable the deposition of a multi-layer, functionally graded coating layer in a single pass across the surface of the article.

30. A system according to claim 29, wherein the feed mechanism has at least two feed nozzles for depositing a double graded coating layer in a single pass, wherein the feed mechanism comprises a first feed nozzle adapted to deposit a first feedstock material directly onto the surface of the article to form a primary sub-layer of the coating, and a second feed nozzle adapted to deposit a second feedstock material adapted to deposit the second feedstock material onto the first sub-layer to form a secondary sub-layer of the coating.

31. A system according to claim 30, wherein the first feedstock material is contained in a first reservoir, the first feed nozzle being in communication with, and operatively associated with, the first reservoir such that the first feedstock material can be fed through the first feed nozzle.

32. A system according to claim 30 or claim 31, wherein the second feedstock material is contained in a second reservoir or container, the second feed nozzle being in communication with, and operatively associated with, the second reservoir such that the second feedstock material can be fed through the second feed nozzle.

33. A system according to claim 30, wherein the first and second nozzles are both in communication with a single reservoir of feedstock material, whereby the same stock material is used for both the primary and secondary sub-layers.

34. A system according to any one of claims 24 to 33, wherein the heat source is in the form of a laser adapted to emit a laser beam, the laser being configured in use with a predetermined energy rating for simultaneously heating and melting the portion of the surface of the article and separately delivered feedstock to form a melt pool.

35. A system according to claim 34, including an optical focusing mechanism operatively associated with the laser for directing and focusing the laser beam onto the desired portion of the surface of the article.

36. A system according to any one of claims 24 to 35, including a workstation to which the article is releasably mountable, the feed mechanism, heat source and workstation being configured to enable relative movement therebetween to provide a desired position, orientation and spacing between the feed mechanism, heat source and article.

37. A system according to any one of claims 24 to 36, including a control means operatively associated with the workstation for controlling movement of the workstation, and thereby the relative position and/or orientation with respect to the feed mechanism and heat source.

38. A system according to any one of claims 24 to 37, wherein the stock material is in the form of a metal matrix composite (MMC), wherein the MMC has a composition including a matrix material and a reinforcing material dispersed within the matrix material.

39. A system according to claim 38 adapted to deposit the bonded coating layer such that there is a substantially even distribution of the reinforcing material within the matrix material throughout the coating layer.

40. A system according to any one of claims 24 to 39, wherein the article is one of a cutting tool, impacting tool, drilling tool and grinding tool.

41. A down the hole hammer drill assembly, including: a casing, a top sub releasably engagable with an operative upper end of the casing; a drive chuck releasably engagable with an operative lower end of the casing; and a drill bit releasably engagable with an operative lower end of the front (or drive) chuck; wherein, at least one of the casing, top sub, front chuck, and drill bit (collectively the “parts” of the assembly) is at least partially coated with a protective coating layer, thereby protecting that part of the assembly against wear.

42. A drill assembly according to claim 41, wherein the casing is a cylindrical tubular member with the protective coating applied to an exterior surface thereof.

43. A drill assembly according to claim 42, wherein the exterior surface of the casing has at least one zone in which an auxiliary protective coating layer is applied, thereby enhancing the ability of the coating to protect against wear in that zone.

44. A drill assembly according to claim 43, wherein the exterior surface of the casing has two dedicated zones in which an auxiliary protective coating layer is applied, the two zones being spaced apart and positioned so as to be substantially aligned in use with jaws of a clamping mechanism of a drilling machine.

45. A drill assembly according to claim 44, wherein the auxiliary protective coating is applied to a greater thickness relative to the thickness of the coating applied to other regions of the exterior surface of the casing.

46. A drill assembly according to any one of claims 43 to 45, wherein the auxiliary protective coating is applied in one or more bands, each band extending circumferentially around the casing.

47. A drill assembly according to claim 44 or claim 45, wherein the two dedicated zones include a first zone in which three circumferential bands of auxiliary protective coating are applied.

48. A drill assembly according to claim 47, wherein the two dedicated zones include a second zone in which three circumferential bands of auxiliary protective coating are applied.

49. A drill assembly according to any one of claims 41 to 48, wherein a first pair of mating surfaces is provided between the top sub and the casing, a second pair of mating surfaces is provided between the casing and the front or drive chuck, and a third pair of mating surfaces is provided between the front or drive chuck and the drill bit.

50. A drill assembly according to any one of claims 41 to 49, in which the top sub has a spigot adapted to be releasably received in the casing via the upper end, and a boss defining a leading end mating surface adapted to abut the upper mating surface of the casing, thereby limiting the extent to which the spigot is received in the casing; wherein, the protective coating is applied to an exterior surface of the boss.

51. A drill assembly according to claim 50, wherein an operatively lower end of the boss of the top sub has a bevelled profile such that a lower mating surface of the boss has a smaller diameter than a diameter of an upper mating surface of the casing.

52. A drill assembly according to claim 51, wherein an operatively lower end of the casing has a bevelled profile such that a lower mating surface of the casing has a smaller diameter relative to a diameter of an upper mating surface of the drive chuck.

53. A drill assembly according to any one of claims 41 to 52, in which the drive chuck has a spigot adapted to be releasably received in the casing via the lower end, and a boss defining a leading end mating surface adapted to abut an upper mating surface of the drill bit; wherein, the protective coating is applied to an exterior surface of the boss.

54. A drill assembly according to claim 53, wherein the boss of the drive chuck has a tapered outer surface profile such that its leading end mating surface is smaller than an upper mating surface of the drill bit.

55. A drill assembly according to any one of claims 41 to 54, in which the drill bit has a spindle adapted to be releasably received in a lower opening in the drive chuck, and a drill boss defining the upper mating surface of the drill bit and adapted to abut the lower mating surface of the boss of the drive chuck; wherein, the protective coating is applied to an exterior surface of the boss.

56. A drill assembly according to any one of claims 41 to 55, wherein the protective coating layer is formed by way of a laser deposited metal matrix composite (MMC).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0118] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: —

[0119] FIG. 1 is a schematic representation of a system for laser cladding an article with a coating layer in accordance with the present invention;

[0120] FIG. 2 is a schematic representation of laser clad layer, having a bead overlap of approximately 50%;

[0121] FIG. 3 is a schematic representation of a laser clad layer, having a bead overlap of approximately 85%;

[0122] FIG. 4 is a schematic representation of a functionally graded laser deposition layer, having a bead overlap of approximately 85%;

[0123] FIG. 5 is an exploded diagrammatic perspective view of a DTH hammer drill, having a laser cladding layer with properties, and applied, according to the present invention;

[0124] FIG. 6 shows an assembled diagrammatic perspective view of an embodiment of a modified DTH hammer drill according to the present invention;

[0125] FIG. 7 is an exploded diagrammatic perspective view of the DTH hammer drill of FIG. 6;

[0126] FIG. 8 is an enlarged diagrammatic partial view of the DTH hammer drill of FIG. 6, showing the assembly of the casing and top sub of the drill;

[0127] FIG. 9 is an enlarged diagrammatic view of the top sub of the DTH hammer drill of FIG. 6, showing the laser clad layer coated area and a bevelled leading end;

[0128] FIG. 10 is an enlarged diagrammatic partial view of the casing of the DTH hammer drill of FIG. 6, showing additional laser clad layer coated areas;

[0129] FIG. 11 is an enlarged diagrammatic partial view of the DTH hammer drill of FIG. 6, showing the assembly of the casing, the drive chuck, and the drill bit;

[0130] FIG. 12 is an enlarged diagrammatic partial view of the drill bit of the DTH hammer drill of FIG. 6, showing the laser clad layer coated area; and

[0131] FIG. 13 is a sectional side view of another embodiment of a DTH hammer drill, showing the top sub, control tube, casing, piston, guide bush, front chuck, and drill bit.

PREFERRED EMBODIMENTS OF THE INVENTION

[0132] Referring to the drawings, the invention in a first aspect provides a method and apparatus for cladding a surface of an article. The method is particularly suited to cladding the outside surface of metallic objects such as, for example, cutting, impacting, drilling and grinding tools. Such tools can be used, for example, in portable power driven percussive tools with fluid-pressurised drive, electromotor drive or electromagnetic drive for mining or quarrying applications. The following description will be made with reference to one particular use of the cladding method for depositing a wear resistant coating layer on a down the hole (“DTH”) hammer drill, as shown in FIGS. 5 to 13. It should be appreciated, however, that the cladding method is not limited to this particular application, which is provided by way of illustrative example only to highlight the benefits of the present invention.

[0133] Referring initially to FIG. 1, an apparatus or system 1 is provided for cladding an outer surface 2 of an article 3. The apparatus 1 includes a feed mechanism in the form of a feed nozzle 4 operatively connected, via suitable piping or ducting (not shown), to a supply of powered or granular feedstock material 5 held in a reservoir or container (not shown). The feed nozzle 4 is configured for feeding the stock material 5 towards a target portion of the outer surface 2 of the article 3.

[0134] The system 1 further includes a heat source in the form of a laser 6 for heating the fed stock material 5 and the target portion of the surface 2 of the article 3. The laser 6 preferably has a control mechanism such that the laser can be selectively controlled to produce a desired output power suitable for the particular material of the article 3 to be coated.

[0135] The laser 6 is set to produce the desired power output such that the heated stock material and the target portion of the surface at least partially melt, whereby, upon removal of the heat, the molten feedstock and the surface portion fuse (or otherwise cooperate or interact) to form a bonded coating layer on the surface of the article.

[0136] The article 1 is arranged so as to be movable relative to the feed and heat sources (4, 6). To achieve this relative movement, the article is releasably mounted to a workstation (not shown). For generally cylindrical articles, such as the component parts of the DTH hammer drill, the workstation comprises a rotatable mounting assembly (not shown) in which component parts can be mounted such that rotation of the mounting assembly causes a corresponding rotation of the article or component part 3. The article 3 is typically mounted to rotate about its longitudinal axis. The mounting assembly is preferably driven by a suitable drive means, such as an electric motor.

[0137] In some embodiments such as that shown in FIG. 1, the article 1 may have a generally planar or flat surface, and the workstation may be configured to enable linear motion of the workstation, thereby causing a corresponding movement of the article mounted thereto. For example, the workstation may be adapted for movement along each of the Cartesian coordinate axes (i.e. x-, y-, z-axes). However, for ease of reference, the following description will be made with reference to a rotatable mounting assembly in which the article is rotated during the cladding process.

[0138] The feed and heat sources (4, 6) are also selectively movable relative to the workstation and article 3 to provide the desired position, orientation and spacing between the respective components. In particular, the article 3, the feed nozzle 4 and the heat source 6 are all configured to move in a predetermined manner during the cladding process such that the feedstock material is deposited on the surface 2 of the article 3 so as to extend along a desired path or pattern. For cylindrical components, the feedstock material is preferably deposited on the surface of the article in a circumferentially overlapping, spirally directed path, wherein successive passes (or beads) of the feedstock material overlap the previous (or immediately adjacent) bead.

[0139] The movement of the workstation, feed nozzle 4 and laser 6 is controlled by a control mechanism in the form of a computer numerically controlled (CNC) unit. The CNC unit is adapted to enable the workstation to be rotated at a desired speed and the feed and heat sources (4, 6) to simultaneously track along the longitudinal axis of the workstation/article. The CNC unit may also be configured to control the feed rate of the feed nozzle 4, and the power output of the laser 6.

[0140] It will be appreciated that the feedstock material may be selected so as to have predetermined chemical properties to facilitate coalescing, fusing, mixing and/or bonding with the base material of the component or article to be coated. The feedstock material is preferably a powdered metallic material, advantageously adapted to form a strong metallurgically bonded wear resistant coating layer on the surface of the metallic article, following the laser cladding process.

[0141] In the illustrated embodiment, the preferred feedstock material is in the form of a metal matrix composite (MMC), having characteristics adapted to provide high abrasive and erosive wear resistance properties in the coating layer.

[0142] The composition of the MMC includes a matrix material and a reinforcing material dispersed within the matrix material. The MMC comprises approximately 5 to 90 percent by weight of matrix material (i.e. binder phase), and from 10 to 95 percent by weight of reinforcing material (i.e. carbide phase).

[0143] The matrix material is in the form of a powdered self fluxing alloy, with particles sized within the range of approximately 15 μm to 200 μm. For example, the matrix material may be selected from the group, including but not limited to, nickel, cobalt and iron, preferably containing additions of boron or silicon.

[0144] Nickel, cobalt or iron based self fluxing alloys are preferred for the matrix material, due to their lower melting temperatures and associated lower reactive influence on the carbide particles, as well as having excellent wetting characteristics with the carbide particles and the base metal.

[0145] The carbide phase reinforcing material is also a particulate material and preferably selected from the group including, but not limited to, tungsten carbide, titanium carbide, chromium carbide, niobium carbide, silicon carbide, vanadium carbide and boron carbide. The reinforcing material is preferably formed of particles having a size within the range of approximately 1 μm to 350 μm, more preferably approximately 5 μm to 200 μm.

[0146] Tungsten carbide is preferred as the hard phase particle due to its high hardness, high melting point, and low coefficient of thermal expansion. Tungsten carbide is also advantageous as it exhibits good wettability with molten metals. One drawback of tungsten carbide relates to the fact that it has a low heat of formation, making it easily dissolved by molten metals. It is therefore important that the tungsten carbide be subjected to the minimum degree of heat energy possible.

[0147] The MMC advantageously enables the simultaneous cladding of a matrix alloy and hard-phase particles, yielding a composite microstructure in which the hard-phase particles retain their integrity in a ductile matrix. In one preferred composition, the metal matrix composite (MMC) layer is formed of tungsten carbide, in a nickel based self fluxing alloy matrix, and is deposited with a laser heat source.

[0148] In the illustrated embodiment, the hard phase particles and the matrix alloy can be fed separately to the feed nozzle 4, via separate inlet pipes 7, as shown in FIG. 1. In other embodiments, the hard phase particles and the matrix alloy can be blended together to form a composition with desired weight percentages before being delivered to the feed/cladding nozzle 4 of the feed mechanism.

[0149] It has been found that, in certain preferred embodiments, feeding the hard phase particles and the matrix alloy separately can advantageously produce an MMC deposit with a substantially more even distribution of hard phase particles across the entire coating layer. This provides significant advantages in terms of improved ability to withstand abrasive and/or erosive wear conditions over existing cladding techniques. It has been observed that, in MMC coating layers, the ductile matrix material is worn away first. Once there is insufficient matrix encapsulating the hard phase particles, these particles are then subsequently worn away. In particular, it has been observed that those areas within an MMC deposited layer having an unevenly lower percentage of hard phase particles will have a higher rate of wear compared to those areas having a relatively higher percentage or concentration of hard phase particles. Thus, the preferred embodiments of the present invention enabling laser clad wear resistance coating layers to be formed with a substantially more even distribution of hard phase particles across the entire coating surface provide a significant advantage to the functionality of the coating layer described herein.

[0150] In an effort to provide a more even distribution of hard phase particles within a laser clad MMC layer, the heat source (laser), in combination with the feed source, is adapted to provide a substantially continuous, steady, even flow of stock material to the melt pool. The CNC unit, or independent control and/or sensing means, is provided to continuously monitor and, if necessary, adjust the input parameters to either or both of the heat and feed sources such that the desired continuous, steady, even flow of stock material is achieved and maintained over the coating cycle.

[0151] Further advantages arising from the process parameters and MMC composition described herein include the ability to produce MMC layers, deposited via laser cladding, with beneficial properties, including but not limited to: reduction of total laser heat input, smaller mean carbide particle sizes, higher percentage of entrained carbide, lower decarburisation, and dissolution of the carbide, lower dilution with base material, and smaller heat affected zones.

[0152] In one preferred embodiment, the MMC layer may contain, for example, 68 w % WC, and 32 w % NiBSi alloy. It has been observed in trials that an MMC having these exemplary properties, when used in combination with predetermined laser cladding parameters, produces coating layers having dramatically improved properties and characteristics in terms of wear performance.

[0153] Referring to FIG. 1, the feed nozzle 4 is arranged upstream of the laser 6 and is adapted to feed the feedstock material 5 along an axis or plane which is inclined relative to the surface of the article (e.g. the feed nozzle feeds the stock material along an axis within the range of 0 to 90 degrees, relative to the surface of the article). The feed nozzle can be adapted to feed the feedstock material in front of the laser beam as represented whereby the powder feedstock material travels through the laser beam, or in other preferred embodiments can be adapted behind the laser beam. It has been observed that with the deposition MMC layers it is beneficial that the feed nozzle be positioned behind the laser beam and the powder material is fed directly into the molten pool generated by the laser beam and does not pass through the laser beam.

[0154] The laser 6 is adapted to emit a laser beam 8, via an optical focusing mechanism, in a direction substantially orthogonal to the surface of the article. The optical focusing mechanism is in the form of a set of lenses 9 and is operatively associated with the laser for directing and focusing the laser beam onto the desired target area or portion of the surface 2 of the article 3. For example, the laser may have a minimum beam spot diameter in the range of approximately 2 mm to 20 mm.

[0155] The laser 6 is preferably selected from the group, including but not limited to, CO.sub.2 lasers, Nd:YAG lasers, Nd:YVO.sub.4 lasers, diode pumped with Nd:YAG lasers, diode lasers, disc lasers, and fibre lasers.

[0156] As foreshadowed, the laser 6 has control or adjustment means for selectively adjusting, controlling and setting the power output of the laser. The laser preferably has a power output within the range of approximately 3 kW to 20 kW. It will of course be appreciated by those skilled in the art that the invention is not limited to applications with a laser operating within the specified power range, but rather may be selected so as to have the necessary power requirements for the intended cladding application.

[0157] However, it has been observed that the present invention does provide particular advantages in terms of enabling the use of lower laser power output relative to total layer material flow. For example, the present cladding method advantageously enables a faster laser clad deposition rate for a layer of a predetermined thickness at a given laser power (higher material flow rates, and faster laser scanning speed).

[0158] The laser 6 melts the surface 2 of the article 3 to a predetermined depth, thereby forming a molten bonding zone on the surface of the article. The feedstock material 5 is simultaneously melted. The predetermined chemical properties of the MMC are selected such that the molten feedstock and substrate metal in the bonding zone coalesce within the melt pool, to form a metallurgically bonded wear resistant layer on the surface of the metal base or substrate.

[0159] The ability to control the depth to which the surface of the article melts, as described, reduces the dilution of the feedstock with the substrate metal within the molten bonding zone, thereby substantially maintaining the initial and intended material properties of the feedstock upon formation of the wear resistant coating layer. The material properties of the feedstock include, for example, its composition and hardness. In this context, geometric dilution is defined as the ratio of the clad depth in the substrate to the total clad height. It is possible to achieve dilution rates of less than 5% with the laser cladding process described herein, subject to suitably accurate control of the laser parameters within a narrow processing range. However, dilution rates within a range of approximately 5% and 10% are more typical and readily obtainable.

[0160] As is described in further detail below, the use of a lower specific heat energy arises through the use of relatively higher heat source travel speeds. In particular, it is the increased scanning speed, which necessitates the requirement of multiple passes (reduced pitch/increased overlap) to achieve the desired thickness, that advantageously enables the size of the melt pool to be reduced. The increased scanning speed also lowers the conductive losses in the base material, thereby enabling the lower power required for a given material feed rate. Consequently, and advantageously, this enables the use of an increased material feed rate and heat source scanning speed for the same laser power.

[0161] Referring to FIG. 4, the feed mechanism 4 of this embodiment is adapted to enable the deposition of functionally graded layers in a single pass or step across the surface of the article. The feed mechanism has two feed nozzles (4A, 4B) for depositing a double graded coating layer in a single pass.

[0162] The feed mechanism 4 of FIG. 4 comprises a first feed nozzle 4A adapted to deposit a first feedstock material 5A, and a second feed nozzle 4B adapted to deposit a second feedstock material 5B. The first nozzle 4A is adapted to deposit the first feedstock material 5A directly onto the surface 2 of the article 1 to form a primary sub-layer 10 of the coating, while the second nozzle 4B is adapted to deposit the second feedstock material 5B onto the first sub-layer to form a secondary sub-layer 11 of the coating.

[0163] By incorporating more than one feed nozzle and delivering different feedstock materials to the melt pool, via the respective nozzles, the deposited coating structures can be advantageously tailored to suit the requirements of particular applications (e.g. desired wear resistance properties). The thickness of each of the sub-layers of the coating may be controlled by the respective feed rate of the associated nozzle, and may be the same thickness as each other, or vary with respect to the thickness of the other sub-layer. Furthermore, it will be appreciated that the ability to deposit functionally graded layers in a single pass advantageously eliminates the requirement for a second pass, giving rise to improvements in efficiency of the process as a whole, with associated labour, time and cost savings.

[0164] Referring to FIGS. 2 to 4, to achieve area coverage of the coating layer across a desired area of the surface 2 of the article 3, the deposited beads are overlapped with the previously deposited bead. The deposited beads may be overlapped to an extent within the range of approximately 40% to 60% of the bead width, with the desired thickness/height of the layer achieved with each bead. It has been found that a decreased pitch/increased bead overlap within the range of 75% to 95%, when coupled with an increased laser heat source travel speed within the range of 4,000 mm/min to 40,000 mm/min, provides particular advantages in terms of improvements to cladding properties (including improved wear resistance) of the coating layer.

[0165] For functionally graded laser deposition layers, as shown in FIG. 4, the coating may have a bead overlap greater than 50%. In FIG. 4, the functionally graded laser deposition layer has a bead overlap of approximately 85%.

[0166] The heat source (laser) 6 is configured such that a bead or track of coating is deposited or applied to the surface at a predetermined travel speed, being the speed at which the laser (and thus melt pool) travels with respect to the surface 2 of the article 3; that is, the speed of movement of the laser 6 along or over the surface of the article. For example, with a laser power output in the range of 4 kW to 10 kW, travel speeds may typically be set to within the range of 500 mm/min to 2,000 mm/min. Surprisingly and advantageously, however, it has been found that improved cladding properties (including improved wear resistance) can be achieved by increasing the travel speed of the laser. It has been found that an increased travel speed within the range of 4,000 mm/min to 40,000 mm/min, whilst simultaneously decreasing pitch/increasing bead overlap (e.g. to within the range of 75% to 95%), provides particular advantages in terms of improvements to cladding properties (including improved wear resistance) of the coating layer. The higher travel speeds of the laser described herein are beneficial in producing a coating layer with substantially greater even distribution of hard phase particles with the coating layer, substantially lower dilution with the base material and smaller heat effected zones.

[0167] Referring to FIGS. 5 to 13, an embodiment of a pneumatically actuated down the hole (“DTH”) hammer drill 12 is shown. The DTH drill 12 has particular wear-prone components and regions which have been treated with the wear resistant coatings, based on formulations and techniques as described herein.

[0168] The coating is selectively applied to the DTH drill 12 at discrete positions along the length of the drill. Referring to FIG. 13, the DTH drill 12 has a top sub 13, a control tube 14, a casing 15, a piston 16, a guide bush 17, a front chuck 18, and a drill bit 19.

[0169] In FIGS. 5 and 6, the DTH drill has the wear resistant coating applied to predetermined regions of the top sub 13, the casing 15, the front chuck 18, and the drill bit 19.

[0170] In the embodiment of FIG. 5, the coating is applied substantially uniformly to each of those parts, zones, positions or regions of the DTH drill to which the coating is applied.

[0171] By contrast, in the embodiment of FIG. 6, the coating is applied to the same parts as is done in the embodiment of FIG. 5, but the coating in certain areas has a greater thickness relative to the thickness of the coating applied on other parts of the drill. For example, the thickness of the coating at a particular position, or on a particular part of the drill, may be determined based upon the extent of erosive or abrasive movement or conditions to which that part of the drill is expected to be subjected during its working life. That is, a greater coating thickness may be employed on those areas of the drill which are expected to be subjected to harsher working conditions.

[0172] In FIG. 6, the casing 15 of the DTH drill 12 has two zones 20 in which an auxiliary coating 21, for enhancing the wear resistance of the casing in those zones, is deposited. In the embodiment of FIG. 6, the two zones 20 of auxiliary coating 21 are positioned so as to correspond with a clamp (not shown) of an associated drilling machine (not shown) in which the DTH drill 12 is clamped, in use. Thus, the auxiliary coatings 21 enhance the ability of the casing 15 to withstand wear arising from sliding contact and/or twisting movement of the casing against the jaws of the clamp.

[0173] Each zone 20 of auxiliary coating 21 comprises three bands of coating. Each band is configured to extend in an uninterrupted manner, circumferentially around the casing. In the illustrated embodiment, the bands are of equal width and evenly spaced apart by a predetermined distance (e.g. the band width may be equal to the width of the spacing between bands).

[0174] A first zone of the auxiliary coating is arranged at or adjacent an operatively upper end of the casing such that the first zone is positioned to be substantially in line with the location of the jaws of an upper clamp of the drilling machine. A second zone of the auxiliary coating is arranged at or adjacent an operatively lower end of the casing such that the second zone is positioned to be substantially in line with the location of the jaws of the lower clamp of the drilling machine.

[0175] It has been found that the longevity of the MMC wear resistant coating on the leading edges of the parts of the DTH hammer drill assembly can be significantly improved by making the respective parts such that a leading edge of each part of the assembly has a smaller diameter, relative to the size of the mating surface of the immediately adjacent part in the assembly.

[0176] In the illustrated embodiments, as most easily seen in FIG. 7, the leading edge of the top sub 13 is bevelled to provide the difference in diameters between the mating surfaces of the top sub 13 and casing 15 (see also FIGS. 8 and 9). Similarly, the leading edge of the casing 15 is bevelled to provide the difference in diameters between the mating surfaces of the casing 15 and front chuck 18 (FIG. 10). An outer surface of the front chuck 18 is tapered to provide the difference in diameters between the mating surfaces of the front chuck 18 and drill bit 19 (FIG. 12).

[0177] By way of example, the following process parameters are provided to illustrate the potential advantages of the invention. In one exemplary embodiment of the cladding method, the movement apparatus is configured to move the surface of the article with respect to the laser beam at a travel speed of 5,000 mm/min. The heat and feed sources are configured to provide an overlap of approximately 85%. By increasing the travel speed and increasing the overlap of each bead with the previous bead, the clad height per pass and the overall melt pool size are dramatically reduced, thereby minimising the effect of small inconsistencies in powder feeding as well as reducing the effects of melt pool stirring.

[0178] Under these parameters, the mode of bonding with the base metal also changes. Instead of the laser beam directly melting and mixing the base material and powder (as is required with standard laser cladding process parameters), the laser heats the base material to a temperature that allows the boron and silicon within the matrix material, to act as fluxing and deoxidising elements so as to allow the lower melting point self fluxing alloy to wet and fuse with the base metal and produce a metallurgical bond, via an intergranular alloying/cohesion mechanism, similar to that achieved with the spray and fusing process.

[0179] In experimental trials conducted with these parameters, it was found that during deposition, it was beneficial to reduce the laser power down to 5,000 watts. For a 100 mm diameter test piece, an increased rotational speed of 15.91 rpm (up from a standard parameter setting of 4.77 rpm) reduced the heat conduction losses into the base material, giving rise to the unexpected requirement of having to reduce the laser power needed to achieve the desired melting of the previously deposited layer and heating of the base material to effect a well bonded and homogenous wear resistant layer. It was also found that a thicker layer was deposited for the same total powder feed rate of 74 gpm.

[0180] Without limiting the scope or efficacy of the invention to any particular theoretical proposition, it is hypothesised that the elongation of the melt pool, due to the increased travel speed, increases the catchment efficiency of the injected powder. On the basis of these trials, it has also been found that increasing travel speed decreases the heat conductive losses in the base metal and therefore the deposited layer retains a higher temperature as it returns to be irradiated by the laser beam for the subsequent deposit of material. Lower laser power is therefore required to achieve the desired melting. As such, the new process is vastly more efficient than existing laser cladding processes.

[0181] Metallographic examination of sections of the deposit revealed microstructures with even distribution of WC particles, no cracks, very little porosity, discernibly lower heat effects to the WC particles, and a geometric dilution that was so small as to be unmeasurable.

[0182] It is further hypothesised that higher travel speeds, increased powder feed rates, and/or a higher retained volume % of WC could be achieved.

[0183] Using a 6 kW CO.sub.2 laser and maintaining the desired MMC layer thickness, the following parameters were applied: — [0184] Laser power=5,500 watts [0185] Laser spot diameter=4 mm [0186] Travel speed=183.33 mm/sec-11,000 mm/min [0187] Pitch/overlap=0.6 mm/85% [0188] Clad height=1.2 mm [0189] Total powder feed rate=110 gpm [0190] Volume of retained WC=63% [0191] 183.33 mm/sec travel speed×4 mm spot size=733 mm.sup.2/sec [0192] 5,500 watts laser power divided by 733 mm.sup.2/sec, gives a [0193] Specific Energy=7.50 joules/mm.sup.2/sec [0194] The base metal is directly subjected to a specific laser energy of [0195] 7.50 divided by (4 mm divided by 0.6)=1.125 joules/mm.sup.2/sec. [0196] Deposit thickness per pass=180 microns

[0197] Under these exemplary parameters, a deposit rate increase of 76% was obtained compared to the standard laser cladding parameters.

[0198] Metallographic examination of sections of the layer deposited using the above aforementioned parameters revealed microstructures with an even distribution of WC particles, no cracks, very little porosity, discernible lower heat effects to the WC particles, and a geometric dilution that was not measurable.

[0199] Thinner layers can easily be achieved with this method by an increase in travel speed. For example increasing the travel speed to 366.6 mm/sec-22,000 mm/min and, keeping all other parameters the same, a deposit thickness of 0.6 mm was achieved (deposit thickness per pass of 90 microns).

[0200] Factors that influence and result in the improved abrasive and erosive wear performance of the laser clad MMC layer include: the overall percentage of entrained hard particles in the deposited layer, the size and shape of the entrained hard particles, the distribution of hard particles within the deposited layer, and the chemistry and hardness of the entrained hard particles. It is therefore advantageous to laser clad an MMC layer by the manner described herein so as to provide a high percentage of entrained hard particles that are evenly distributed within the deposited layer, and that the entrained hard particles are of a chemistry, size and hardness that are suitable for the application.

[0201] Factors that influence and result in the improved overall percentage of the entrained hard particles within a laser clad MMC layer and the distribution of the entrained hard particles within the deposited layer include: the accuracy of the process by which the hard particles are mixed with the matrix material before being delivered to the melt pool, the quality and accuracy of the feed mechanism, the specific energy applied by the laser, and the nature and extent of melt pool stirring due to temperature gradients and convective flow within the melt pool.

[0202] Test results on several samples have demonstrated, via XFM scans, that exemplary embodiments of the laser clad coating with a tungsten carbide (WC) in a nickel matrix significantly reduce undesired iron migration into the clad layer compared to existing cladding techniques, regardless of the WC concentration.

[0203] It will be appreciated that the invention in its various aspects and preferred embodiments provides a number of advantages. The invention was developed in part to improve the erosion resistance of laser clad MMC deposits and, in various preferred embodiments, provides one or more of the following advantages in that context: no measurable geometric dilution with the base material; increased efficiency of deposition; lower heat effects to hard phase particles; the ability to increase retained hard phase particle percentages with no increase in porosity or cracking; the ability to deposit smaller sized hard phase particles; dramatically improved distribution of hard phase particles; smoother as deposited surface finish; reduced residual stress formation within the deposited layer; less distortion; the ability to apply thin coatings at very fast travel speeds (enabling the process to compete cost-effectively with traditional methods such as hard chrome plating for the deposition of protective layers); lower laser energy input into the base metal; smaller heat affected zones; and the ability to deposit functionally graded layers in a single step. Based on a substantially improved coating methodology, the invention also provides a DTH hammer drill with dramatically improved wear resistance characteristics.

[0204] In these and other respects, the invention represents a practical and commercially significant improvement over the prior art.

[0205] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. It should also be understood that the various aspects and embodiments of the invention as described can be implemented either independently, or in conjunction with all viable permutations and combinations of other aspects and embodiments. All such permutations and combinations should be regarded as having been herein disclosed.