LASER CLADDING SYSTEM AND METHOD

A laser cladding system comprises a cladding head and a gas system. The cladding head extends along a primary axis, and comprises a mirror and a powder nozzle both situated at a distal end of the head. The powder nozzle directs weld material at a target point, and the mirror directs a beam of collimated light at the target point. The gas system comprises a high-speed gas nozzle and a gas knife. The high-speed gas nozzle produces a gas sheath coaxial with the beam in a region between the mirror and target point, shielding the mirror from debris and backspatter. The gas knife redirects the gas sheath away from the target point and redirects debris and molten backspatter away from an interior of the laser cladding head.

BACKGROUND

Laser powder deposition is commonly used in manufacture and repair methods in the aerospace industry, particularly for large, high-value castings that are too crack sensitive to be welded via conventional processes. Laser cladding heads spray pulverant towards an area to be joined or repaired, while a laser beam (typically received via an optical fiber line) creates a small melt pool on the surface of the workpiece. This melt pool captures and incorporates some of the powder. Lasers in such systems can be tightly focused, enabling precise deposition of powder in a target area (e.g. along a weld seam, or to repair a crack or inclusion arising during manufacture or operation of a workpiece).

Common weld heads consist of two concentric hollow cones separated by a small gap. Metal powder, carried by an inert gas such as helium, exits between these cones and is focused on a target point of the workpiece. A laser is simultaneously focused at the same target point through the center of the inner cone. In these systems, conical powder jets are disposed coaxially about the laser beam, and additional inert gas is often directed substantially along the same axis to the molten puddle at the target point to protect from oxidation while powder is deposited.

Coaxial laser cladding heads are typically quite long (e.g. 500 mm or more), and are consequently unsuitable for applications where obstructions near the target point block access, such as when performing a weld within a pipe or other confined space. Some laser cladding heads used for these kinds of applications instead emit both beam and powder sideways from the end of a long wand that can be inserted between obstructions to access the workpiece and target point.

SUMMARY

The present invention is directed towards a laser cladding system configured to receive weld material from a powder source and collimated light from a laser source. The system includes a laser cladding head and a gas system. The laser cladding head extends along a primary axis from a proximal end to a distal end, and includes a mirror and a powder nozzle. The mirror is situated at the distal end to direct the collimated light in a beam along an emission direction towards a target point separated from the mirror by a working distance. The powder nozzle is situated at the distal end to supply the weld material to the target point for melting by the beam. The gas system includes a high-speed gas nozzle disposed near the mirror, and a gas knife. The high speed gas nozzles are configured to produce a gas sheath coaxial with the beam in a region between the mirror and the target point, thereby shielding the mirror from debris and molten backspatter from the target point. The gas knife is disposed between the mirror and the target point, and redirects the gas sheath away from the target point.

DETAILED DESCRIPTION

The present invention includes a laser cladding system with laser cladding head containing a long focal length focal array at a proximal end, and an indexable nonfocal turning mirror at a distal end. The focal array focuses a laser beam along a primary axis extending between proximal and distal ends of the cladding head. The turning mirror redirects the laser beam towards a target point a working distance away from the primary axis. Temperature sensors situated at the turning mirror can be used to detect fault conditions, and the turning mirror can be indexed by rotating it to expose a new portion of the turning mirror to impingement by the laser beam, thereby allowing the same turning mirror to be reused several times, despite sensed faults. The turning mirror can be protected by a high-speed gas sheath that deflects debris and weld spatter away from the mirror. A gas knife reduces a risk that this high-speed gas sheath interferes with the weld, while additional jets of inert gas coaxial with powder jets prevent oxidation of the molten weld by reducing local oxygen concentration. The high precision welds enabled by this system necessitate high-resolution imaging. A borescope situated along the primary axis and directed towards the target point is used to provide an improved image.

FIG. 1is a schematic view of laser cladding system10with laser cladding head12. Laser cladding head12receives collimated light from laser source14(e.g. via an optical fiber line) and weld material from powder source16(e.g. via a pressurized tube), and directs both weld material and focused, collimated light at target point PTto perform welds and other repair operations. Laser cladding head12includes protective housing18, which extends along primary axis APfrom proximal end20(near laser source14and powder source16) and distal end22(near target point PT). Protective housing18encloses interior space24, which contains focal array26, laser beam28, turning mirror30, powder line32, powder nozzle34, and borescope38. Protective housing18can, for example, be a substantially cylindrical wand extending principally along primary axis AP, and having a comparatively small diameter to enable access to target points situated in narrow or obstructed confines, e.g. within a tube or other enclosed workpiece.

Focal array26is a lens, mirror, or cluster of optical elements with focal length f, that focuses collimated light from laser source14along primary axis AP. Collimated light forms beam28, which extends across the majority of the axial length of laser cladding head12and impinges on turning mirror30at impingement point PI. Turning mirror30is a nonfocal mirror that redirects beam28in emission direction DE, transverse to primary axis AP, towards target point PT. Turning mirror30can, in one embodiment, be a flat, gold-plated copper mirror. Turning mirror30is oriented along and rotatably anchored at rotational axis Ar. Impingement point PIon turning mirror30is separated from working point PTby working distance DT. By focusing beam28via focal array26, rather than at turning mirror30, the present invention is able to focus beam28over a long focal length f. In some embodiments, focal length f may be at least 400 mm or 450 mm. Focal length f is at least ten times working distance DT, and in some embodiments at least thirty times working distance DT. In at least some embodiments, working distance DTcan be less than 20 mm. The long focal length f compared to working distance DTallows beam28to be directed transverse to primary axis AP(e.g. for weld operations in obstructed areas) without hypersensitivity to variation in working distance DT. By pre-focusing beam28at focal array26, near the proximal end of laser cladding head12, focal length f can be substantially entire length of laser cladding head12, or even slightly longer. Increased focal length f correspondingly allows increased tolerance to working distance DT(i.e. ΔDT), since ΔDT∝f. Increased tolerance allows laser cladding system10to be substantially insensitive to minor variations in working distance DTthat occur during normal weld operations. Focal array26and turning mirror30are described in greater detail below with respect toFIG. 2.

Powder source16provides weld material in the form of powder or pulverant metal via powder line32. Weld material is supplied to target point PTvia powder nozzle34. Although only a single powder nozzle is illustrated in the present schematic view, embodiments of the present invention can include a plurality of distinct nozzles, e.g. symmetrically distributed with respect to target point PT. Beam28heats target material, forming a molten pool in the vicinity of target point PTthat incorporates weld material from powder nozzle34.

Laser cladding system10also includes borescope38. Borescope38includes borescope probe38, imaging head40, illumination source42, and borescope camera44, and is used to gather a high-resolution image of viewing area AVaround target point PT. Long focal length f permits reliably tight focus of laser beam28at target point PT, enabling high precision welds. Increase in the size of the impingement area of beam28on target point PTdue to the increased magnitude of focal length f can, in some embodiments, be compensated for by supplying collimated light from laser source14via a correspondingly lower-diameter optical fiber.

Borescope38facilitates high precision welds by providing the weld operator (whether human-controlled or automated) with correspondingly high resolution images of viewing area AV, surrounding target point PT. Borescope probe38extends parallel to primary axis AP, from imaging head40to borescope camera42. Imaging head40includes a mirror, prism, or array of mirrors and/or prisms directed at viewing area AV, as well as a protective cover separating the interior of borescope36from interior space24of protective housing18. In the illustrated embodiment, imaging head40is set back further than working distance DTfrom target point PT, so as to minimize debris and weld backspatter on imaging head40. Borescope probe38attaches to illumination source42and borescope camera44. Illumination source38supplies light (e.g. white) for imaging of viewing area AV, and can for example be an LED light source. Borescope camera44can for example be a charge-coupled device (CCD) camera used to guide laser cladding head12. In at least some embodiments protective housing18includes imaging window46, a transparent section of protective housing18between imaging head40and viewing area AV. In the illustrated embodiment, beam28passes through aperture48, a hole in protective housing18situated within imaging window48. Borescope36is described in greater detail below with respect toFIGS. 3 and 4, while imaging window46and aperture48are discussed in greater detail with respect toFIG. 5. In at least some embodiments borescope36is oriented in a viewing direction not parallel to the path of beam28between turning mirror30and target point PT. This non-parallel orientation prevents turning mirror30from occluding the view of borescope36, and additionally facilitates beam targeting by allowing the operator to gauge working distance DTbased on the location of the laser spot within viewing area AVIn one embodiment, the viewing angle of borescope36is angled at 15-25° with respect to the emission angle of beam28.

Laser cladding head12further includes gas system50, which draws gas from gas source52through gas lines54to protect turning mirror30from debris and weld backspatter, and to exclude oxygenated air from the molten weld in the vicinity of target point PTso as to avoid weld material oxidation.FIG. 1only provides a simplified view of a section of gas system50; gas system50is described in greater detail and in non-schematic form below with respect toFIGS. 5-7. In some embodiments weld material may be delivered via powder nozzle34using an independent gas source separate from gas system50. In general, powder nozzle34can use the same gas or gasses handled by gas system50to deliver weld material, or can use other inert gasses depending on specific need and cost. Gas system50can, in some embodiments, utilize several different gasses (e.g. Argon, Helium) to propel weld material, protect molten weld material from oxidation, and shield sensitive components (such as turning mirror30) from damage and fouling.

In at least some embodiments laser cladding head12includes temperature sensor56, a thermocouple or similar temperature sensor device situated within or adjacent turning mirror30. Temperature sensor56is used to detect an out-of-bounds temperature or rate of change of temperature at turning mirror30corresponding to an unacceptable maintenance condition due to fouling or damage. Fouling or damage reduce the reflectiveness of turning mirror30, causing an increase in its heating at impingement point PI. Temperature sensor56senses this temperature increase, e.g. as an increase in thermocouple voltage beyond a specified voltage threshold or faster than a specified rate, allowing laser cladding system10to recognize imminent failure conditions necessitating indexing or replacement of turning mirror30.

The present invention uses turning mirror30, a nonfocal optical element rather than a focal lens, to redirect axially-aligned beam28toward target point PT. Protective housing18and gas system50cooperate to protect turning mirror30from damage and fouling from its proximity to the weld operation at target point PT, which can for example be as close as 15-20 mm away. Aperture48exposes only a small portion of turning mirror30around impingement point PIto potential weld backspatter and debris, allowing the remainder of turning mirror30to stay clean and undamaged even when protective housing18and gas system50are insufficient to entirely protect turning mirror30. Turning mirror30can then be indexed by rotating about rotational axis Arto situate impingement point PIon a new, clean, undamaged portion of turning mirror30without causing any deviation in emission direction DEof beam28. In at least some embodiments, the narrow focus of beam28at impingement point PIallows turning mirror30to be indexed at least five times in this fashion before no additional unused portion of turning mirror30is available. The present invention reduces operating expense not only by allowing turning mirror30to be reused through indexing, but by only situating turning mirror30proximate to the weld, rather than a more costly focal element such as focal array26.

FIG. 2is a perspective view of an optical path within laser cladding head12, and illustrates focal array26, beam28, turning mirror30, rotational axis Ar, impingement point PI, target point PT, optical fiber100, collimator lens102, objective lens doublet104, and alignment mirrors106,108, and110.

Optical fiber100carries collimated light from laser source14to focal array26. Focal array26includes collimator lens102, objective lens doublet104, and alignment mirrors106,108, and110. Collimator lens102collimates the output of optical fiber100, and alignment mirrors106,108, and110cooperate to align the beam. Alignment mirrors106,108, and110can be responsible for different components of beam alignment, i.e. for alignment in orthogonal dimensions. Collectively, alignment mirrors106,108, and110align beam28along primary axis AP, thereby determining impingement point PIon turning mirror30and target point PI. In the illustrated embodiment, beam28is focused via objective lens doublet104. More generally, focal array26can comprise any optical element or collection of elements that cooperate to align beam28with primary axis APand focus beam28with focal length f. The full beam path of beam28from focal array26to target point PThas length substantially equal to focal length f, such that target point PTis aligned within a tolerance range ΔDTof the adjusted focal point of beam28. Tolerance range ΔDTcan, for example, be greater than 2 mm due to long focal length f.

Turning mirror30is rotatable about rotational axis Ar. Impingement point PIis offset from rotational axis Ar, such that for a given rotational alignment of turning mirror30, only a small angular subset of turning mirror30is exposed to beam28and to the weld via aperture48(seeFIG. 1). The size of turning mirror30determines the fractional angular subset of exposed mirror, with larger mirrors allowing a higher potential number of lifetime indexing operations.

As discussed above with respect toFIG. 1, borescope36includes borescope probe38, illumination source42, and borescope camera44. Borescope body200receives and redirects imaging light such as white light from illumination source42, and redirects this light along borescope probe38towards imaging head40. Borescope body200also attaches to camera adaptor202, which conditions the optical output of borescope head44for processing by borescope camera44. As noted above, borescope camera44can be a CCD camera or other electronic camera used to align laser cladding head12with a desired weld location.

As illustrated inFIG. 4, imaging head40includes borescope tip204and cover206. Borescope tip104is a mirrored input that defines illumination cone CI(the field of view of borescope36), and thereby viewing area AV. Borescope tip204is shielded by cover206, which protects borescope tip204and screens undesired wavelengths to improve image quality. and protect the borescope from thermal damage. Cover206can, for example, be formed of IR reflective glass.

Beam28can, for example have a width of 0.2-0.3 mm at target point PT. In some embodiments, beam28is capable of impinging on the target point at a width less than 0.25 mm. Borescope36provides an image of sufficiently high resolution to permit precise alignment of beam28at target point PT, e.g. a resolution dimension of one fifth the beam width at target point PT, or smaller. Compared to imaging via beam-path optics, borescope36also provides a wider view of the surroundings of target point PT, allowing target point TPto be more easily situated at a desired weld location.

FIG. 5is a transparent sectional view of laser cladding12illustrating a portion of a gas system50. In particularFIG. 5illustrates two symmetrically distributed powder nozzles34, high-speed gas line300, and coaxial nozzles304producing gas shield306in the vicinity of target point PT. Gas sheath302is supplied primarily by gas flow coaxial with beam28, injected into internal space24to protect lens array26and turning mirror30.

As noted above, powder nozzles34supply gas-driven jets of pulverant weld material. Powder nozzles34can, for example, carry pulverant weld material on a stream of Helium, Argon, or other inert gas or gasses. Power nozzles34supply weld material to target point PTfor incorporation in the molten weld. The weld can, however, give rise to both molten backspatter and flying debris in the form of uncaptured pulverant weld material. To prevent this material from impacting and damaging or fouling turning mirror30, gas system50includes high-speed gas line300, a high-speed gas outlet into interior space24(seeFIG. 1) that can escape solely through aperture48. As it escapes aperture48, high-speed gas line300produces gas sheath302, a pressurized gas jet that deflects debris and backspatter away from aperture48. Gas sheath302can be expelled from aperture48towards target point PT. The present invention can further include a gas knife (310; seeFIG. 6, discussed below) that redirects gas sheath302away from target point PT, thereby preventing gas sheath302from forming turbulent flow which may cause the weld being oxidized.

Coaxial nozzles304direct low-velocity inert gas towards the vicinity of target point PT, producing gas shield306. Coaxial nozzles304can, for example, carry argon gas from gas source52. Gas shield306displaces or excludes oxygen from the immediate vicinity of the weld near target point PT, thereby preventing oxidation of the molten weld material.

FIGS. 6 and 7are cross-sectional perspective views of laser cladding head12along axial cross-sections, and illustrate protective housing18, interior space24, turning mirror30, powder nozzle34, imaging head40(with borescope tip204and cover206as described with respect toFIG. 3), imaging window46with aperture48, and gas shield306all substantially as described above, as well as gas knife nozzle308.FIG. 6additionally illustrates gas sheath302, gas knife nozzle308, and gas knife310.

As shown inFIG. 5, gas sheath302exits internal space24via aperture48, coaxially with beam28. A common gas inlet supplies high speed nozzle300and gas knife nozzle308, while low-speed gas is passed near turning mirror30to provide protection and cooling. Gas flow within internal space24forms sheath flow302as it exits orifice48.FIGS. 6 and 7further illustrate the release of gas shield306coaxially with powder nozzle34.

Gas knife nozzle308is routed through turning mirror30from behind, and directs inert gas, e.g. of the same type and from the same reservoir as high-speed nozzle300, as gas knife310. Gas knife310is a gas jet released laterally across viewing window46, transverse to gas sheath302, and serves to redirect gas sheath302away from target point PTso as to avoid interfering with welding. Gas knife310also serves to redirect any weld spatter and/or powder away from internal space24so as to prevent contamination of turning mirror30. Laser cladding head12is capable of performing weld tasks in confined or obstructed areas. Due to the long focal length f of focal array26, the weld beam of cladding head12is largely insensitive to small changes in working distance. Temperature probe56ensures that turning mirror30is indexed or replaced whenever damage or fouling impedes its performance, and gas system50protects turning mirror30without interfering with the weld at target point PT. Gas system50additionally supplies the weld location with inert gas to prevent the weld from oxidizing.

Discussion of Possible Embodiments

A laser cladding system configured to receive weld material from a powder source and collimated light from a laser source, the laser cladding system comprising: a laser cladding head extending along a primary axis from a proximal end to a distal end, the laser cladding head comprising: a mirror situated at the distal end to direct the collimated light in a beam along an emission direction towards a target point separated from the mirror by a working distance; and a powder nozzle situated at the distal end to supply the weld material to the target point for melting by the beam; and a gas system comprising: a high-speed gas nozzle disposed near the mirror, and configured to produce a gas sheath coaxial with the beam in a region between the mirror and the target point, thereby shielding the mirror from debris and molten backspatter from the target point; and a gas knife disposed, between the mirror and the target point, such that the gas knife redirects the gas sheath away from the target point and redirects debris and molten backspatter away from an interior of the laser cladding head.

A further embodiment of the foregoing laser cladding system, wherein the gas knife is a gas jet disposed substantially transverse to the gas sheath.

A further embodiment of the foregoing laser cladding system, wherein the gas knife is a gas jet disposed substantially parallel to the turning mirror.

A further embodiment of the foregoing laser cladding system, wherein the gas subsystem further comprises a low-speed inert gas nozzle directed towards the target point such that oxygen density in the vicinity of the target point is reduced.

A further embodiment of the foregoing laser cladding system, wherein the low-speed inert gas nozzle is situated coaxially about the powder jet.

A further embodiment of the foregoing laser cladding system, wherein the mirror is a nonfocal mirror, and wherein the laser cladding head further comprises a focal array at the proximal end oriented to focus the collimated light substantially along the primary axis, impinging on the mirror at an impingement location.

A further embodiment of the foregoing laser cladding system, wherein the focal array has a focal length at least thirty times greater than the working distance.

A further embodiment of the foregoing laser cladding system, wherein the nonfocal mirror is indexable at least five times by rotating the mirror such that the impingement location changes without altering the emission direction.

A further embodiment of the foregoing laser cladding system, further comprising: a boroscope disposed along the primary axis of the laser cladding head, and having an imaging head at the distal end directed toward the target point.

A further embodiment of the foregoing laser cladding system, wherein the imaging head is separated from the target point by more than the working distance.

A further embodiment of the foregoing laser cladding system, wherein the beam has a width at the target point that is at least five times greater than a resolution of the borescope.

A method of operating a laser cladding system, the method comprising: focusing a laser beam along a primary axis using a focal array; supplying weld material via a powder nozzle to a target location offset from the primary axis by a working distance; redirecting the laser beam in an emission direction transverse to the primary axis via a nonfocal turning mirror, towards the target point; and deflecting debris and molten backspatter away from the turning mirror via a high-speed gas sheath.

A further embodiment of the foregoing method, wherein the high-speed gas sheath is coaxial with the laser beam in a region between the turning mirror and the target point.

A further embodiment of the foregoing method, further comprising: redirecting the high-speed gas sheath away from the target point via a gas knife situated between turning mirror and the target point.

A further embodiment of the foregoing method, further comprising: directing a stream of inert gas transverse to the primary axis, to cover the target point, thereby excluding oxygen from the vicinity of the target point.

A further embodiment of the foregoing method, wherein the stream of inert gas is supplied via a flow coaxial to the powder nozzle.

A further embodiment of the foregoing method, further comprising: sensing failure conditions at the turning mirror indicating fouling or damage to the turning mirror.

A further embodiment of the foregoing method, wherein sensing failure conditions comprises detecting any out-of-bounds increase in temperature at the turning mirror.

A further embodiment of the foregoing method, further comprising: indexing the turning mirror to change which portion of the turning mirror the laser beam impinges upon, without changing the emission direction.

A further embodiment of the foregoing method, further comprising: performing at least one action from the group consisting of replacing, repairing, and cleaning the turning mirror after it has been indexed at least five times.

SUMMATION

Any relative terms or terms of degree used herein, such as “substantially”, “essentially”, “generally”, “approximately” and the like, should be interpreted in accordance with and subject to any applicable definitions or limits expressly stated herein. In all instances, any relative terms or terms of degree used herein should be interpreted to broadly encompass any relevant disclosed embodiments as well as such ranges or variations as would be understood by a person of ordinary skill in the art in view of the entirety of the present disclosure, such as to encompass ordinary manufacturing tolerance variations, incidental alignment variations, alignment or shape variations induced by thermal, rotational or vibrational operational conditions, and the like.