WO2022081088A1

WO2022081088A1 - System and method of laser additive brazing

Info

Publication number: WO2022081088A1
Application number: PCT/SG2021/050611
Authority: WO
Inventors: Wei Zhou
Original assignee: Nanyang Technological University
Priority date: 2020-10-15
Filing date: 2021-10-11
Publication date: 2022-04-21
Also published as: CN116529014A

Abstract

A system and method of laser additive brazing. The method includes forming a melting zone spaced away from a target zone on a workpiece, and responsively adjusting a gap between the melting zone and the target plane according to a temperature at a reference surface. The system can be configured for performing on-site laser additive brazing outside a vacuum furnace, and is thus suitable for a wide range of applications, including repair and remanufacturing of large structures.

Description

SYSTEM AND METHOD OF LASER ADDITIVE BRAZING

The present application claims priority from the Singapore patent application no. 10202010248 S, the contents of which are incorporated herein in entirety by reference.

TECHNICAL FIELD

[0001] The present disclosure relates brazing, and more particularly to laser additive brazing for remanufacturing.

BACKGROUND

[0002] Conventional brazing includes torch brazing and induction brazing. The components to be joined are heated by a hot gas torch or by resistive heating using an induction coil. A filler metal that will melt at a lower temperature than the components is placed on or between the components to be joined. The components are deliberately heated to avoid having abrupt temperature gradients at different parts of the components while providing a temperature high enough for the filler metal to melt and flow in between the two components. When the melted filler cools and solidifies, it forms a bond between the two components. Conventional brazing is relatively time-consuming and energy-intensive, and is resorted to when welding is not possible. For instance, conventional brazing is carried out with the aircraft part placed within a vacuum furnace. The vacuum furnace needs to be evacuated to 10'⁶ Torr and heated up to a steady furnace temperature of over 1000 degree Celsius. Despite the large size of the component under repair or remanufacturing, conventional aerospace brazing is carried out in a vacuum furnace because a small amount of air can cause considerable oxidation and result in a joint quality that fails the exacting standards of the aerospace industry.

SUMMARY

[0003] In one aspect, the present disclosure provides a method of laser additive brazing a workpiece, the method comprising: providing a laser beam; delivering filler powder in a trajectory intersecting the laser beam at a melting zone, such that at least some of the filler powder forms a melt in the melting zone and such that the melt has a melt trajectory from the melting zone to a target zone on the workpiece; and varying a gap width between the melting zone and the target zone in response to a temperature sensed at the workpiece.

[0004] The method according to the above, wherein the filler powder is delivered via a brazing head, and wherein the melting zone is displaced by providing a relative displacement between the brazing head and the workpiece.

[0005] The method according to any of the above, wherein varying the gap width comprises increasing the gap width by displacing the melting zone away from the target zone in response to an increase in the temperature sensed at the workpiece.

[0006] The method according to any of the above, wherein the temperature is sensed at a reference surface on the workpiece, and wherein the reference surface is one selected from the target zone and a part of the workpiece spaced apart from the target zone.

[0007] The method according to any of the above, wherein the temperature sensed is kept below a reference temperature by displacing the melting zone away from the target zone.

[0008] The method according to any of the above, wherein the reference temperature corresponds to one of the following: a melting temperature or a recrystallization temperature of the workpiece and an ambient temperature.

[0009] The method according to any of the above, further comprising: providing a localized inert gas volume extending at least from the melting zone to the target zone.

[0010] The method according to any of the above, further comprising: providing a column of inert gas characterized by laminar flow, wherein the melt trajectory is in the column of inert gas.

[0011] The method according to any of the above, wherein the melt at any point of the melt trajectory is surrounded by the inert gas on all sides of the melt.

[0012] The method according to any of the above, wherein the melt trajectory is oblique relative to a vertical axis.

[0013] The method according to any of the above, wherein the melt is intermittently deposited on the target zone. Alternatively, the melt is continuously deposited on the target zone. Optionally, the filler powder may be characterized by a melting point similar to a melting point of the workpiece. Optionally, the filler powder may be selected from a material similar to a material of the workpiece.

[0014] A method of laser additive brazing a workpiece, the method comprising: determining multiple target zones along a contour of the workpiece; and moving a brazing head along the contour between performing the method according to any of the above at respective ones of the multiple target zones, wherein the brazing head is configured to provide the laser beam and deliver the filler, and wherein the gap width is iteratively varied at a selected one of the multiple target zones in response to a temperature sensed at the selected one of the multiple target zones.

[0015] The method according to any of the above, wherein the control system is configured to iteratively vary the gap width over a course of a processing time, and wherein the gap width is varied according to at least one parameter selected from the group consisting of a melt deposition rate, a laser power, the contour, and at least one heat transfer coefficient of the workpiece.

[0016] In another aspect, a system for laser additive brazing a workpiece, the system comprising: a temperature sensor configured to sense a temperature at a reference surface of the workpiece; a brazing head configured to provide a laser beam along a longitudinal axis, the brazing head being configured to deliver filler powder in a trajectory intersecting the laser beam at a melting zone, such that at least some of the filler powder forms a melt in the melting zone and such that the melt has a melt trajectory from the melting zone to a target zone on the workpiece, the melting zone and the target zone being spaced apart by a gap width; and a control system coupled to the temperature sensor and the brazing head, wherein the control system is configured to controllably vary a gap width according to any of the above.

[0017] The system according to any of the above, further comprising a robotic arm coupled with the control system and the brazing head, wherein the robotic arm is configured to displace the melting zone relative to the target zone by displacing the brazing head away from the workpiece. [0018] The system according to any of the above, wherein the control system is configured to increase the gap width in response to the temperature sensed at the workpiece, such that the temperature sensed is lower than a reference temperature.

[0019] The system according to any of the above, further comprising an inert gas supply configured to provide a localized inert gas volume such that the melt trajectory is in the localized inert gas volume.

[0020] The system according to any of the above, further comprising a filler material supply configured to deliver the filler powder.

[0021] The system according to any of the above, further comprising an infra-red sensor configured to obtain the temperature sensed at the workpiece.

[0022] The system according to any of the above, further comprising a 3D scanner configured to map a contour of the workpiece.

[0023] The system according to any of the above, wherein the brazing head comprises: a housing defining a laser channel with a first opening, the laser beam being configured to be directed through the laser channel and the first opening; a filler material channel disposed radially exterior of the laser channel such that the filler powder is deliverable in a trajectory intersecting the laser beam at a melt zone, wherein the trajectory extends from the melting zone as a melt trajectory; a reservoir disposed adjacent to and in fluid communication with the first opening; and an inert gas inlet configured to deliver an inert gas to the reservoir to form a localized inert gas volume extending from the first opening and throughout the melt trajectory.

BRIEF DESCRIPTION OF DRAWINGS

[0024] Fig. 1 is schematic representation of a system for laser brazing according to an embodiment;

[0025] Fig. 2 is a flowchart of a laser additive brazing method according to an embodiment of the present disclosure;

[0026] Fig. 3 is a flow chart schematically illustrating the method according to another embodiment of the present disclosure;

[0027] Fig. 4 is a schematic representation of a system for laser additive brazing according to another embodiment of the present disclosure;

[0028] Fig. 5 A is a partial schematic of the system of Fig. 4 in an operating state;

[0029] Fig. 5B is a more detailed view showing a melt trajectory across a gap;

[0030] Figs. 6A and 6B schematically illustrate the brazing head in different orientations according to the present method;

[0031] Fig. 7 is a schematic representation of a system for laser additive brazing according to yet another embodiment;

[0032] Fig. 8 illustrates a brazing head according to an embodiment;

[0033] Fig. 9 is a partial sectional view of the brazing head of Fig. 8; and

[0034] Fig. 10 is another sectional view of the brazing head of Fig. 8.

DETAILED DESCRIPTION

[0035] Reference throughout this specification to “one embodiment”, “another embodiment” or “an embodiment” (or the like) means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” or the like in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of embodiments. One skilled in the relevant art will recognize, that the various embodiments be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, some or all known structures, materials, or operations may not be shown or described in detail to avoid obfuscation.

[0036] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. As used herein, the singular ‘a’ and ‘an’ may be construed as including the plural “one or more” unless apparent from the context to be otherwise.

[0037] Terms such as “first” and “second” are used in the description and claims only for the sake of brevity and clarity, and do not necessarily imply a priority or order, unless required by the context. The terms "about" and "approximately" as applied to a stated numeric value encompasses the exact value and a reasonable variance as will be understood by one of ordinary skill in the art, and the terms “generally” and “substantially” are to be understood in a similar manner, unless otherwise specified.

[0038] The terms “laser additive brazing” and “laser brazing” are used interchangeably in the present disclosure, and refer to a process of the present disclosure in which a filler, such as a filler powder, is melted or made flowable by a laser beam and the filler is additively introduced to a target zone of a workpiece. The target zone may be an interface between two dissimilar materials or a discontinuity (e.g., crack, void, worn surfaces, etc.) in a workpiece of one or more workpieces (of similar or dissimilar materials). Laser additive brazing can be useful for enabling dissimilar materials to be joined together (e.g., in joining aluminum alloys to steels, or joining metals to ceramics). Laser additive brazing is also useful for relatively targeted work, such as repairs or remanufacturing.

[0039] A system 100 for laser additive brazing according to an embodiment of the present disclosure is shown in Fig. 1. The system (100) includes a brazing head (110) configured to direct a laser beam (212) to a melting zone (92) and to feed filler powder to the melting zone, according to a method (800, Fig. 2) of laser additive brazing. The brazing head is coupled to a robotic arm (510). The robotic arm is configured to support, position, and/or move the brazing head in response to commands from a control system (500). In an operating state of the system, the robotic arm is configured to maintain a position of the brazing head or to re-position the brazing head. In the operating state, the brazing head is positioned relative to a target zone (96) such that there is a gap (94), with a gap width, between the melting zone and the target zone.

[0040] The target zone (96) may be generally defined as the area on/in the workpiece where the filler is to be deposited. In some cases, the target zone is a discontinuity in the workpiece (90). In other cases, the target zone may be a joint to be formed between two or more parts (collectively referred to as a workpiece). The target zone may alternatively be defined as a unit deposition area, and determined in terms of a rate at which the melt is provided to the workpiece. A target plane (95) may be defined with respect to the target zone or the workpiece to provide a reference for defining a width of the gap (gap width) between the melting zone and the target zone. In the present disclosure, the terms “target zone” and “target plane” may be used interchangeably, and the terms “gap” and “gap width” may be used interchangeably, unless otherwise dictated by the context.

[0041] The system (100) includes a temperature sensor (610) configured to sense a temperature (810) at a reference surface (97) of the workpiece (90). In some examples, the reference surface is defined by a part of the target zone (96). In some other examples, the reference surface is defined by another part of the workpiece spaced apart from the target zone. The control system (500) is coupled to the temperature sensor (610) and the robotic arm (510). The control system is configured to control the robotic arm and to manipulate the brazing head in response to a temperature or a temperature change obtained from sensing the reference surface. The system can be configured to accommodate large workpieces or large structures, and be operable under ambient conditions or under room temperature and atmospheric pressure. Ambient temperature refers to normal room temperature without any deliberate modification or control. Atmospheric pressure refers to the air pressure in the workpiece surrounding without any deliberate modification or control. Deliberate control may include the use of a vacuum furnace, a heating chamber, a pressurized inert gas chamber, a chamber from which air has been partially evacuated and replaced by an inert gas, etc. The laser additive brazing method of the present disclosure may be performed outside controlled environment, i.e., without the use of a vacuum furnace or a vacuum chamber. Time and energy can be saved. Advantageously, the system (100) is suitable for use with a wide variety of workpieces. As an example, the workpiece can be substantially larger than a physical size of system and is disposed in an ambient environment, such an aircraft panel located in a hangar. The present method can be performed such that the temperature sensed at a reference surface is at or close to ambient temperature, in which the reference surface is spaced apart from the target zone and exposed to the atmosphere under atmospheric pressure.

[0042] In some examples, the control system (500) is configured to increase the gap width (94) in response to a temperature sensed at the reference surface (97). The control system may be configured to increase the gap width by displacing the melting zone (92) away from the target zone (96), in response to an increase in the temperature sensed at the workpiece. In some examples, the control system is configured to increase the gap width in response to the temperature sensed at the workpiece, such that the temperature sensed is lower than a reference temperature. In some examples, the control system is configured to determine a new gap width (820) based on at least one parameter, the at least one parameter including a temperature sensed at the reference surface or a temperature change at the reference surface. The control system may be configured to be responsive to a temperature of the target zone by displacing the melting zone to maintain the temperature of the reference surface below a reference temperature. The temperature sensed may be kept below a reference temperature by displacing the melting zone away from the target zone. The system (100) may be configured to perform the laser additive brazing process of iteratively adjusting the gap width (94) in response to a temperature sensed at the target zone (96); directing the laser beam (212) at the melting zone (92); melting the filler powder in the melting zone to form a melt (138); and depositing the melt on the target zone (96). The gap is configured to be adjustable in response to a temperature of the target zone such that the temperature is kept below a reference value. The control system may be configured to control the robotic arm such that the robotic arm displaces the brazing head away from the target plane, resulting in a displacement of the melting zone away from the target plane or the target zone, in response to a temperature of the reference surface being above a reference temperature. The control system may be configured such that, when a gap width is determined, commands are sent to the robotic arm to re-position the brazing head such that the melting zone and the target place are separated by an updated gap width. If the control system determines that a gap width between a melting zone and a target plane is within a range of acceptable gap width values, the brazing head may be held in a same position. Alternatively, if the control system determines that a gap width between a melting zone and a target plane is within a range of acceptable gap width values, and if the brazing head is to be moved to a new target zone, the brazing head may be moved to a new target zone while concurrently maintaining the current gap width. Alternatively, the control system may be configured to re-orientate the brazing head with the gap width changed or kept constant. In some examples, the control system is configured to determine a target gap width based on a temperature sensed at the reference surface and at least one other parameter. Examples of parameters from which the at least one other parameter may be selected include: a reference temperature characteristic of the workpiece material(s) and the filler, a rate of change of temperature at the reference surface, an orientation of the brazing head, a trajectory (1301) of the filler powder, a trajectory (1302) of the melt, and a physical geometry of the target zone.

[0043] In some embodiments, the control system is configured to adjust the gap, i.e., change the gap width in length along the longitudinal axis (120), such that a temperature sensed at the workpiece is driven towards a reference value until the temperature sensed is lower than the reference value (reference temperature). The gap may be iteratively adjusted by the control system, which constantly or periodically acquires the temperature of the target zone. The control system is configured to dynamically determine a relative displacement of the melting zone and the target plane, or to dynamically determine a desirable gap width, taking into consideration other parameters such as deposition rate, laser power, surface contour, etc. As an example, the reference value may correspond to a recrystallization temperature of a material of the workpiece, or a recrystallization temperature of a material of the target zone. The term “recrystallization temperature” refers to a temperature at which recrystallization or microstructural changes occur for a given material and processing conditions. The recrystallization temperature may not be a constant temperature and may be dependent upon factors such as duration of heating, whether the material is an alloy or an elemental metal, the amount of cold work to which the workpiece was previously subjected, the grain size, etc. As a non-limiting example given solely for the purpose of illustration, the reference value may be defined as a factor of the recrystallization temperature (e.g., 80% of the recrystallization temperature), or in another non-limiting example, the reference value may be defined as 15 °C (degree Celsius) lower than the recrystallization temperature. In yet another example, the reference value may be dynamically computed, taking into consideration one or more parameters such as, but not limited to, melt deposition rate, laser power, surface contour at the target zone, heat transfer coefficients, etc. In some embodiments, the target zone is maintained below the recrystallization temperature. As another non-limiting example, the reference temperature may be defined as a value corresponding to a melting temperature of a material at the target zone. As another nonlimiting example, the reference value may be defined as a factor of the melting point (e.g., 50% of the melting point of one of the materials at the target zone). In another non-limiting example, the reference value may be defined at 100 °C (degree Celsius) lower than the melting point. In another non -limiting example, the reference value may be defined at 100 °C (degree Celsius) higher than the melting point for laser deposition or cladding involving melting of the workpiece. In some examples, the reference temperature corresponds to an ambient temperature.

[0044] Fig. 4 illustrates another embodiment of the system (100) configured for a workpiece (90) that can be supported or stabilized by a stage (530). The stage may optionally be configured to move workpiece (90). The control system (500) may also include a motion control module (520) configured to control either or both of the robotic arm (510) and the stage (530) so as to control a relative motion or relative displacement between the brazing head and the target plane or target zone. That is, the control system is configured to control the gap width (94) between the melting zone and the target zone by controlling the relative displacement of the melting zone and the workpiece. The melting zone can be displaced by providing a relative displacement between the brazing head and the workpiece. The control system (500) may include a temperature acquisition module (540) configured to receive temperature measurements (620) from one or more temperature sensors (610). The control system (500) may include a laser control module (550) that is configured to control a laser source (210). In operation, the laser source (210) provides a laser beam (212) along a longitudinal axis (120) and toward the target zone (96). The control system (500) may be configured to control a filler powder supply module (310). The control system (500) may be configured to control a gas supply module (410). The control system may be in the form of one or more computing devices, controllers, or processors (560). In the present disclosure, the terms “control system”, “computing device”, “controller”, and “processor” are used interchangeably for the sake of brevity.

[0045] Referring to Figs. 5 A and 5B, a partial cross-section of the brazing head (110) shows a reservoir (144) defined therein. When the system (100) is in the operating state, a laser beam (212) is directed along the longitudinal axis (120) in the similarly longitudinally oriented channel (114). The laser beam is configured to exit the brazing head via a first opening (116) towards a target zone (96) of a workpiece (90). The laser beam is focused at a focal point (214) via a focusing system which includes a lens (150). In some embodiments, the focal point of the laser beam corresponds to the melting zone (92), i.e., the location of the melting zone may be defined by the position of the focal point. In some examples, the lens is fixed relative to the brazing head such that the focal point moves in tandem with the brazing head. When the robotic arm (510) moves the brazing head relative to the target plane (95), the position of the focal point and the corresponding position of the melting zone can be changed accordingly. In some examples, the gap (94) between the melting zone and the target zone may be adjusted by moving the lens or the focal point relative to the target zone. In other embodiments, the lens is moveable relative to the brazing head, such that the focal point is displaceable. The gap allows the target zone to slightly cool down prior to a next deposition of the melt (138). This helps to mitigate the formation of heat affected zones in the workpiece.

[0046] In operation, the laser beam is directed along the longitudinal axis towards a target zone or target plane. Concurrently, filler powder is projected or delivered along one or more trajectories (130) towards the laser beam or the longitudinal axis outside the brazing head, or ultimately towards the target zone or target plane. The trajectory (130) may be described in terms of a powder trajectory (1301) leading to the melting zone (92) and a melt trajectory (1302) extending from the melting zone (92) to the target zone (96) / target plane (95). The system (100) is configured such that a trajectory of the filler powder intersects the laser beam (212) or the longitudinal axis (120) at the melting zone. The control system is configured to control the brazing head such that the filler is at least partially melted to form a melt at the melting zone, and the melt is carried across the gap (gap width) in a melt trajectory to the target zone.

[0047] The melting zone (92) may be defined as a volumetric (three-dimensional) space in which the filler powder is heated and at least some of the filler powder is melted by a laser beam (212). The melting zone may define a volumetric size and a volumetric shape. The melting zone is shown schematically as a regular shape in the appended drawings, but in practice, the volumetric size and the volumetric shape of the melting zone is likely to be irregular and dynamically changing throughout the course of a laser additive brazing operation. In practical applications, a focal point (214) of the laser beam may be used as an estimated position corresponding to the position of the melting zone (92). In an actual repair or remanufacturing operation, the target zone on the workpiece is likely to include a cavity or a crack. For example, in the course of a laser additive brazing process, the target zone (96) changes in its physical geometry as the filler adds material to the target zone. A target plane (95) may be defined as a reference plane for defining the gap width (94) between the target zone and the melting zone. The target plane may be defined as a plane substantially normal to the longitudinal axis (120) and coincidental with a part of the target zone (96). The target zone refers to an area (not necessarily contiguous or flat) where the brazed joint is to be formed or where the filler melt is used to fill in a discontinuity in the workpiece or to join multiple workpieces, for example, as part of repair or remanufacturing operations.

[0048] During the process of laser additive brazing, the temperature of the workpiece (90) at the target zone (96) or in the vicinity of the target zone is also dynamically changing. As the melt (138) moves across the gap in a melt trajectory (1302) extending from the melting zone to the target plane or the target zone, the melt begins to cool such that the target zone is not subjected to as high a temperature as the melting zone (92). The target zone is not subjected to a continuous heating. The most significant source of heat energy at the target zone comes from the intermittent droplets of melt (138). As shown in Fig. 3, the present method (700) optionally includes intermittently depositing (or introducing at intervals) the melt in droplets to the target zone (740), for example, by providing a gap between the melting zone and the target zone. According to embodiments of the present disclosure, there is no need to carry out the laser additive brazing method within a vacuum furnace. The workpiece is thus not deliberately heated in its other parts beyond the target zone, enabling the rest of the workpiece to act as a heat sink and keep the target zone at a relatively low temperature such that heat affected zones-related (HAZ-related) issues are essentially circumvented. In one aspect, the target zone may be described as having intermittent periods of cooling (where heat conduction away from the target zone is more significant) and heating (as the melt is deposited in/on the target zone). Risks related to the formation heat affected zones (HAZ) on the workpiece can be mitigated. One resulting benefit is feasibility of selecting a higher power setting for the laser and the opportunity for operating at a higher throughput, since HAZ-related issues no longer dominate. Optionally, the laser beam may be maintained at a constant power throughout the operation.

[0049] The selection of the filler powder depends on the material of the workpieces. The filler powder may include filler material in a powder form with substantially homogenous grain size. Alternatively, the filler powder may have nonhomogeneous grain size. In some examples, the filler powder is selected from a material characterized by a melting point substantially lower than a melting point of the workpiece. According to other embodiments of the present disclosure, the filler powder can be selected from a material characterized by a melting point near to or similar to the melting point of the workpiece at the target zone. The control system is configured to determine an accepted limit or acceptable range of gap widths based on the melting points of the filler powder, the melting point of the workpiece at the target zone, and the temperature sensed at the reference surface. The control system is configured to responsively vary the relative displacement or positions of the brazing head and the stage such that the actual or current gap width is within the acceptable range of gap widths. The gap width may be varied prior to the melt being deposited on/in the target zone, or the gap width may be varied periodically or continuously throughout the laser additive brazing operation. The effect of heat affected zones (HAZ) in the target zone is less apparent in the present method when compared to conventional methods. At the same time, issues typically associated with abrupt temperature gradients are circumvented. The present method thus enables a greater choice of filler materials, including a choice of filler powders with melting points relatively close to melting points of the material(s) of the workpieces. The method of the present disclosure thus enables improved material or structural homogeneity in the finished product. In some examples of the present embodiment, the filler may be selected from a material similar to the material of the workpiece. The filler powder may be characterized by a melting point similar to a melting point of the workpiece. For example, the melting point of the filler powder may differ from the melting point of the workpiece by less than 100 °C. The filler powder may be selected from a material similar to a material of the workpiece.

[0050] According to some embodiments of the present disclosure, the method (700) of Fig. 3 optionally includes providing the filler powder in a controllable trajectory (750). The filler powder may be provided in a controllable trajectory (130) aided by a carrier gas, such as an inert gas. For the sake of clarity, Figs. 5 A and 5B do not show all possible trajectories. References in the present disclosure to a trajectory or one trajectory should be understood to apply to multiple trajectories. The filler powder is projected via a powder trajectory (1301) through/into the melting zone (92). At the melting zone, at least some of the filler powder is melted by the laser to form a plurality of melt or droplets of melt (138). The melt follows the melt trajectory (1302) and is eventually deposited in/on the target zone (96). The trajectory (1302) of the melt (between the melting zone and the target zone) may be substantially aligned with the longitudinal axis (120). For example, when the longitudinal axis is substantially a vertical axis, system may be configured to provide a melt trajectory that is substantially parallel to the vertical axis, for example, aided by gravity and/or a carrier gas. In other examples, as illustrated by Figs. 6A and 6B, the longitudinal axis may be angularly displaced (oblique) relative to the vertical, with the trajectories (130) correspondingly varied.

[0051] Optionally, the method according to the embodiment of Fig. 3 includes providing a melting zone (92) that is spaced apart from the target zone (96), in which the melting zone is wholly immersed or within a volume or a column (140) of inert gas (760). The control system (500) may be configured to provide a supply of inert gas at a flow rate to saturate the reservoir (144) in the brazing head (110), and to provide a laminar flow of the inert gas that forms a column (140) extending from the brazing head to the target zone. In operation, the melting zone is in/inside the column or volume of inert gas, as shown in Fig. 5B for example. The melting zone (92) is completely surrounded by the inert gas column (140). The gap (94) between the melting zone and the target zone is also entirely within the inert gas column such that the melt (138) is at all times surround on all sides by inert gas of the column of inert gas, even as the melt travels towards the target zone. The target zone (96) is disposed relative to the column of inert gas and the flow rate of the inert gas is configured such that the target zone is completely submerged under or covered by inert gas. In other words, the localized inert gas volume (140) is provided to concurrently cover both melting zone (92) and target zone (96), with the localized inert gas volume extending at least from the melting zone to the target zone. The inert gas is provided as a column or a volume, as opposed being provided as a thin shield or curtain. In other words, the inert gas is configured as a localized inert gas volume (140) in a proximity of the melting zone (92) and the target zone (96). Various gases may be selected. Non-limiting examples of inert gases include Argon gas for use with titanium workpieces, Nitrogen gas for use with stainless steel workpieces, etc.

[0052] In some examples, the system (100) is configured to provide the localized inert gas volume (140) prior to delivering the filler powder. Preferably, the system saturates the space between the brazing head and the target zone with an inert gas, displacing normal atmospheric gases (ambient air) from the possible trajectories (130) of the filler powder or the melt. The localized inert gas column would be surrounded by ambient air in cases where the method is performed outside of a vacuum chamber. Preferably, the localized inert gas volume is a laminar flow of the inert gas. The flow of inert gas may be continuously and controllably provided to the reservoir 144, with the flow rate controlled by the control system, such that a column of inert gas characterized by laminar flow is formed with the melt trajectory being in the column of inert gas or in a localized inert gas volume. The melt at any point of the melt trajectory is surrounded by the inert gas on all sides of the melt.

[0053] In response to an adjustment of the gap width, the control system is configured to adjust the delivery of the inert gas accordingly such that the melt (138) is in a flowing column of inert gas from its formation (by the melting of filler powder) to its deposition in/or the target zone on the workpiece. Optionally, a flow rate of the inert gas may be increased or decreased. In some examples where the trajectory of the powder filler and/or melt is predicted to be essentially downward, an inert gas having a density higher than ambient air may be selected. The localized gas volume may also serve to cool the target zone in mitigating HAZs, in addition to preventing or reducing oxidation.

[0054] Figs. 6A and 6B illustrate embodiments of the system (100) deployed in an on-site setting, that is, outside of a vacuum furnace. In the example of Fig. 6A, the target zone (96) is disposed on a side surface of workpiece (90). The filler powder is projected in a controllable trajectory (130). The trajectory (130) is projected through the melting zone (92) to be melted, across the gap (94) and extends to the target zone (96). As the target zone is orientated to face a generally horizontal direction, an axis (136) of the trajectory (130) is oblique relative to a vertical axis (98), forming an angle (138) therebetween. The control system (500) is configured to control the trajectory (130) based on an orientation of the target plane. As non-limiting examples, controlling the trajectory may include varying a position and/or an orientation of the brazing head by controlling the robotic arm, varying a velocity of the trajectory by controlling the filler material supply, varying the volumetric size of the inert gas volume by controlling the gas supply, etc. Further, controlling the trajectory may further take into consideration the relative position between the melting zone and the target zone, as well as external effect or interferences on the trajectory, such as gravitation effect or air resistance due to the inert gas volume. As the trajectory or path of the melt is substantially horizontal, the localized inert gas volume may be formed an inert gas with a similar density as that of the ambient air.

[0055] In the example of Fig. 6B, a target zone (96) is disposed on an under side of workpiece (90), making the target zone (96) relatively difficult to access for conventional apparatus. The filler powder is projected in a controllable trajectory (130), with the trajectory extending through the melting zone (92), across the gap (94), and continuing to the target zone (96). For the sake of brevity, the melt (138) is said to be deposited on the target zone (96) regardless of the orientation of the target zone. In this case, the target zone is orientated generally downward. The trajectory (130) is configured by the control system to be oblique relative to the vertical axis (98), forming an angle (138) therebetween. The control system (500) is configured to determine the trajectory (130) based on an orientation of the target plane. The localized inert gas volume may be formed using an inert gas with a similar or lower density compared to that of ambient air.

[0056] An embodiment of a portable system is described with reference to Fig. 7. The system (100) includes a temperature sensor (610). The system includes: an infra-red (IR) sensor (612) configured to measure the temperature of the target zone via an IR wave path (600); an IR filter (614) disposed along the IR wave path to filter spurious IR waves; an aperture (616) for varying the intensity of the IR wave path; and a laser filter (618) configured to filter undesirable radiations. For the sake of brevity, as used herein, the term “IR sensor” may refer to one or more devices (including combinations of devices) suitable for use in detecting or sensing temperature of the target zone, for example but not limited to, an IR camera, an IR temperature sensor, a thermocouple, etc. A mirror (216) may be provided to direct the laser beam (212) from the laser source (210) through the lens (150) to focus within the melting zone (92). In some embodiments, a 3D laser scanner (630) may be provided and configured to map or otherwise obtain a contour of the workpiece. The control system may be configured to determine multiple target zones along a contour of the workpiece to be repaired. The control system may be configured such that the brazing head is moved following the contour of the workpiece and between successive laser additive brazing at respective multiple target zones. The gap width may be iteratively varied at a selected one of the multiple target zones in response to a temperature sensed at the selected one of the multiple target zones. A different reference temperature may be determined for each of the multiple target zones if the conditions differ between the target zones. A different reference surface may be defined for each of the multiple target zones, for example, the respective target zone may also serve as a reference surface for temperature sensing. The control system may be configured to also take into consideration the physical contour at each of the multiple target zones, in addition to temperature measurements taken repeatedly over the course of a processing time, and to vary the gap width accordingly at each of the multiple target zones. The control system may be configured to iteratively vary the gap width over a course of a processing time according to at least one of the following: a melt deposition rate, a laser power, the contour, and at least one heat transfer coefficient of the workpiece.

[0057] An embodiment of a brazing head (110) will be described in greater detail with the aid of Figs. 8 to 10. The brazing head includes a housing (112) defining a laser channel (114) for passing a laser beam 212 therethrough. The laser channel has a first opening (116) and a second opening (118). The laser channel defines a longitudinal axis or a channel axis (120). The laser beam may be provided to the laser channel via the second opening, and the laser beam may be configured to exit the laser channel via the first opening. The laser channel may be configured such that the laser beam passes through at least a portion of the laser channel in a direction parallel to the channel axis (longitudinal axis). A focusing element, such as a lens (150), is disposed in the laser channel for focusing the laser beam. The laser beam is configured to focus at a focal point. A divider (160), such as a piece of protective glass transparent to the laser beam, may be disposed in the laser channel to hermetically separate the laser channel into at least two portions (144, 146), such that a reservoir (144) is provided next to the first opening (116). The divider is configured to allow the laser beam to pass through while providing a reservoir configured to accumulate a gas. In other words, the divider is configured to prevent fluid communication between the reservoir (144) and the rest of the laser channel (146).

[0058] The housing (112) further includes an inert gas inlet (142) which is in fluid communication with the reservoir (144), and in fluid communication with the laser channel (114). The inert gas inlet can be coupled to a gas supply for provision of an inert gas into the laser channel, and more specifically for the inert gas to be supplied to the reservoir. The reservoir is configured to enable the inert gas to first accumulate in the reservoir and to facilitate formation of a localized inert gas volume extending from the first opening and at least throughout the melt trajectory (1302), all the way to the target zone (96).

[0059] The brazing head (110) may further include a filler material channel (134) and at least one filler material inlet (132) in fluid communication with the filler material channel. The filler material channel is formed in the housing (112) and hermetically separated from the laser channel (114). The filler material channel may be configured as an annular channel disposed radially exterior of at least a part of the laser channel. A pair of filler material inlets may be provided on opposite sides of the housing, such that a vortex may be formed within the annular channel prior to exiting the brazing head. The vortex aids to proj ect filler powder to or through the melting zone (92) to form a melt that continues in its trajectory to be deposited onto the workpiece.

[0060] The present method and system enable laser brazing to be performed in ambient condition without the need to provide a controlled environment. That is, the present method can be used to perform on-site repair or remanufacturing. The system can be configured as a portable system that can be brought to the product or structure to be repaired, instead of having to break the product or structure into a workpiece that can fit within a vacuum furnace, and instead of bringing the workpiece to the vacuum furnace for brazing. In other words, the laser additive brazing may be performed at the location of the workpiece without the need for transporting the workpiece. Process portability has the unique advantage of the ability to perform laser additive brazing on large workpieces without the need for disassembly, transport, or reorientation. It may be appreciated that process portability lifts the limitation on workpiece sizes and workpiece locations. The present method and system open the way for laser additive brazing to be used in more situations, for example, on ship hulls at the shipyard, on pressure vessels in a nuclear power plant, or on aircraft bodies in a hangar. As described, the laser additive brazing process of the present disclosure may be used on target zones which oriented obliquely to the horizon. Workpiece reorientation is optional. Laser additive brazing can thus be performed on more complex contours such as airfoils which requires fine control, or on difficult to access areas.

[0061] All examples described herein, whether of apparatus, methods, materials, or products, are presented for the purpose of illustration and to aid understanding, and are not intended to be limiting or exhaustive. Various changes and modifications may be made by one of ordinary skill in the art without departing from the scope of the invention as claimed.

Claims

1. A method of laser additive brazing a workpiece, the method comprising: providing a laser beam; delivering filler powder in a trajectory intersecting the laser beam at a melting zone, such that at least some of the filler powder forms a melt in the melting zone and such that the melt has a melt trajectory from the melting zone to a target zone on the workpiece; and varying a gap width between the melting zone and the target zone in response to a temperature sensed at the workpiece.

2. The method according to claim 1, wherein the filler powder is delivered via a brazing head, and wherein the melting zone is displaced by providing a relative displacement between the brazing head and the workpiece.

3. The method according to claim 1 or claim 2, wherein varying the gap width comprises increasing the gap width by displacing the melting zone away from the target zone in response to an increase in the temperature sensed at the workpiece.

4. The method according to any one of claims 1 to 3, wherein the temperature is sensed at a reference surface on the workpiece, and wherein the reference surface is one selected from the target zone and a part of the workpiece spaced apart from the target zone.

5. The method according to claim 4, wherein the temperature sensed is kept below a reference temperature by displacing the melting zone away from the target zone.

6. The method according to claim 5, wherein the reference temperature corresponds to one of the following: a melting temperature or a recrystallization temperature of the workpiece and an ambient temperature.

7. The method according to any one of claims 1 to 6, further comprising: providing a localized inert gas volume extending at least from the melting zone to the target zone.

8. The method according to any one of claims 1 to 6, further comprising: providing a column of inert gas characterized by laminar flow, wherein the melt trajectory is in the column of inert gas.

9. The method according to claim 7 or claim 8, wherein the melt at any point of the melt trajectory is surrounded by the inert gas on all sides of the melt.

10. The method according to any one of claims 1 to 9, wherein the melt trajectory is oblique relative to a vertical axis.

11. The method according to any one of claims 1 to 9, wherein the melt is intermittently deposited on the target zone.

12. The method according to any one of claims 1 to 9, wherein the melt is continuously deposited on the target zone.

13. The method according to any one of claims 1 to 12, wherein the filler powder is characterized by a melting point similar to a melting point of the workpiece.

14. The method according to any one of claims 1 to 12, wherein the filler powder is selected from a material similar to a material of the workpiece.

15. A method of laser additive brazing a workpiece, the method comprising: determining multiple target zones along a contour of the workpiece; and moving a brazing head along the contour between performing the method according to any one of claims 1 to 10 at respective ones of the multiple target zones, wherein the brazing head is configured to provide the laser beam and deliver the filler, and wherein the gap width is iteratively varied at a selected one of the multiple target zones in response to a temperature sensed at the selected one of the multiple target zones.

16. The method according to claim 15, wherein the control system is configured to iteratively vary the gap width over a course of a processing time, and wherein the gap width is varied according to at least one parameter selected from the group consisting of a melt deposition rate, a laser power, the contour, and at least one heat transfer coefficient of the workpiece.

17. A system for laser additive brazing a workpiece, the system comprising: a temperature sensor configured to sense a temperature at a reference surface of the workpiece; a brazing head configured to provide a laser beam along a longitudinal axis, the brazing head being configured to deliver filler powder in a trajectory intersecting the laser beam at a melting zone, such that at least some of the filler powder forms a melt in the melting zone and such that the melt has a melt trajectory from the melting zone to a target zone on the workpiece, the melting zone and the target zone being spaced apart by a gap width; and a control system coupled to the temperature sensor and the brazing head, wherein the control system is configured to controllably vary the gap width according to the method of claim 1.

18. The system according to claim 17, further comprising a robotic arm coupled with the control system and the brazing head, wherein the robotic arm is configured to displace the melting zone relative to the target zone by displacing the brazing head away from the workpiece.

19. The system according to claim 17 or claim 18, wherein the control system is configured to increase the gap width in response to the temperature sensed at the workpiece, such that the temperature sensed is lower than a reference temperature.

20. The system according to any one of claims 17 to 19, further comprising an inert gas supply configured to provide a localized inert gas volume such that the melt trajectory is in the localized inert gas volume.

21. The system according to any one of claims 17 to 20, further comprising a filler material supply configured to deliver the filler powder.

22. The system according to any one of claims 17 to 21, further comprising an infra-red sensor configured to obtain the temperature sensed at the workpiece.

23. The system according to any one of claims 17 to 22, further comprising a 3D scanner configured to map a contour of the workpiece.

24. The system according to any one of claims 17 to 23, wherein the brazing head comprises: a housing defining a laser channel with a first opening, the laser beam being configured to be directed through the laser channel and the first opening; a filler material channel disposed radially exterior of the laser channel such that the filler powder is deliverable in a trajectory intersecting the laser beam at a melt zone, wherein the trajectory extends from the melting zone as a melt trajectory; a reservoir disposed adjacent to and in fluid communication with the first opening; and an inert gas inlet configured to deliver an inert gas to the reservoir to form a localized inert gas volume extending from the first opening and throughout the melt trajectory.