Patent Publication Number: US-2020300818-A1

Title: High-frequency oscillatory plastic deformation based solid-state material deposition for metal surface repair

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 62/821,228, filed Mar. 20, 2019, the entirety of which is herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The presently disclosed subject matter relates in some embodiments to methods and systems for repairing surface defects (e.g., surface cracks) in metallic structures or components. 
     BACKGROUND 
     During service or during manufacturing, surface defects (e.g., most commonly in the form of surface cracks) often form on the surface of metallic components, often due to cyclic loading thereof during use. If not repaired, such surface cracks will inevitably spread and/or grow. It is well known that the presence of surface cracks in metallic structures or components, whether caused due to metal fatigue or otherwise due to some acute cause, such as physical damage from an impact, can, and ultimately will if not timely repaired, result in catastrophic failure of such metallic components. Several techniques exist at present to repair such cracks, including fusion welding methods like Tungsten Inert Gas (TIG) welding. These processes use heat energy, usually generated by an electric arc, to melt filler material and fill the crack. Other, more recent, repair methods, including laser direct metal deposition (LDMD), Laser Engineered Net Shape (LENS), and Cold Spray techniques, have also been used for repairing surface cracks in metallic components. 
     All such known repair processes utilize heat energy to create a melt pool of the provided filler metal at the location of the crack. After the filler material has been melted and the melt-pool has infiltrated the crack, the filler material rapidly solidifies to permanently fill the surface crack, thereby repairing the surface crack. The heat energy that enables melt-pool formation, however, also results in a large heat-affected zone in the undamaged portion of the metallic component near (e.g., adjacent to and/or in the immediate vicinity of) the repaired region. The presence of this heat-affected zone alters the microstructure of the metal of the metallic component itself in the repaired region. This heat-affected zone can result in the metallic structure having different physical properties in the repaired region than elsewhere in the metallic component, which can cause the repaired metallic component to have different characteristics from a metallic component that has not undergone crack repair. As such, a need exists for new methods and systems for repairing a surface defect in a metallic substrate or component without generating a significant amount of heat within the metallic component during the repair process. 
     SUMMARY 
     To prevent such heat-induced changes in microstructure of the metallic component adjacent the repaired region, methods and systems using acoustic energy to deform and deposit voxels of a filler material within such surface defects are disclosed. The methods and systems disclosed herein eliminate the aforementioned issues in the final product associated with thermal history and solidification induced during such known repair methods and systems. The methods and systems disclosed herein utilize a solid state, room temperature technique in which high-frequency, small amplitude local shear strain is used to achieve energy-efficient volumetric conformation of a metallic filler material (e.g., a wire-shaped filament) within such a surface defect. Once the surface defect is filled, such methods and systems induce metallurgical bonding between the filler material and the metallic substrate at the surface of the surface defect at which the filler material makes contact and/or to which the filler material conforms. 
     This two-fold effect is similar in effect to what heating and melting a filler metal does, but in the new methods and systems disclosed herein, no heat is applied to either the filler material or the metallic substrate, and both the filler material and the surface of the surface defect remain solid (e.g., remain substantially at room temperature) throughout the time when the surface defect is being repaired. Additionally, the use of high-frequency, small amplitude oscillatory shear strain softens the filler material, allowing the filler material to “flow” into, and conform to, the internal surfaces and contours of such surface defects to which the filler material is applied. The methods and systems disclosed herein are further advantageous over the prior art, in that they are highly energy efficient compared with presently known fusion welding or laser-based techniques noted hereinabove. Additionally, the methods and systems disclosed herein eliminate safety and health hazards presented by melt-fusion based repair processes known according to the prior art. 
     The methods and systems disclosed herein also enable large-scale materials exchange (e.g., in the form of inter-metallic diffusion) at the interface between the filler material and the internal surfaces of the surface defect, thereby enabling metallurgical bonding between the filler material and the metallic component being repaired at the bondline formed. Since the methods and system use no heat energy and causes negligible temperature rise (e.g., less than about 10° C., or at least a temperature rise that does not cause a change in the microstructure of the metallic component), the microstructure of the metal of the substrate in the vicinity of the repaired region remains unaffected by the repair process. The methods and systems disclosed herein can be used to repair metallic substrates, structures, components, and the like in any of a wide variety of industries, including, for example, aerospace, maritime, automotive, and even including small-scale fabrication endeavors. 
     The methods and systems disclosed herein use two solid-state physical phenomena that result from the interaction of metals with high frequency acoustic energy. According to the first phenomena, acoustic energy causes metal to soften, resulting in lower stresses required during deformation of the filler material. According to the second phenomena, acoustic energy results in inter-metallic diffusion, causing bonding of the deformed filler material to the inner surface or contours of the surface defect against which the filler material is applied. The first phenomenon of softening causes the deformed metal to conform to the shape of the surface defect (e.g., a crack), while, according to the second phenomena, acoustic energy-enabled diffusion causes bonding of the filler material to the internal surfaces of the surface defect, thereby permanently filling the surface defect. Both of these solid-state physical phenomena induce only a negligible rise in temperature of the metal being repaired and no supplemental or auxiliary heat energy is applied during the repair process. The elimination of the use of heat energy results in a substantially unaltered microstructure of the metal in region(s) of the metallic substrate that have been repaired. 
     This summary lists several embodiments of the presently disclosed subject matter, and in many cases lists variations and permutations of these embodiments. This summary is merely exemplary of the numerous and varied embodiments. Mention of one or more representative features of a given embodiment is likewise exemplary. Such an embodiment can typically exist with or without the feature(s) mentioned; likewise, those features can be applied to other embodiments of the presently disclosed subject matter, whether listed in this summary or not. To avoid excessive repetition, this summary does not list or suggest all possible combinations of such features. 
     According to an example embodiment, a system for repairing a surface defect in a metallic substrate is provided, the system comprising: a transducer configured to generate acoustic energy; and an acoustic energy coupling tool connected to the transducer and configured to receive the acoustic energy from the transducer; wherein the acoustic energy coupling tool is configured for oscillatory movement at a frequency corresponding to a frequency of the acoustic energy generated by the transducer to deform a filler material that is positioned in and/or over the surface defect and underneath the acoustic energy coupling tool, the acoustic energy coupling tool being configured such that the oscillatory movement thereof conforms the filler material to at least a portion of an internal surface of the surface defect; and wherein the acoustic energy coupling tool is configured to irradiate the filler material with the acoustic energy at a same time as when the filler material is being conformed to at least the portion of the internal surface of the surface defect by the acoustic energy coupling tool. 
     In some embodiments of the system, the acoustic energy coupling tool is configured, by irradiating the filler material with the acoustic energy, to cause the filler material to soften and causes inter-metallic diffusion between the filler material and one or more internal surfaces of the surface defect against which the filler material is conformed by the acoustic energy coupling tool, thereby bonding the filler material to the substrate within the surface defect. 
     In some embodiments of the system, the acoustic energy coupling tool is movable, relative to the metallic substrate, to deposit the filler material as one or more successive layers formed within the surface defect to repair the surface defect and produce a repaired region of the metallic substrate that has a microstructure that is integrated with the microstructure of the metallic substrate. 
     In some embodiments, the system comprises a horn that couples the transducer to the acoustic energy coupling tool, the acoustic energy being transmitted from the transducer to the acoustic energy coupling tool via the horn. 
     In some embodiments of the system, the filler material is a filament having a generally annular cross-sectional shape. 
     In some embodiments of the system, the filler material and the metallic substrate comprise a same metal or metal alloy. 
     In some embodiments of the system, oscillating the acoustic energy coupling tool to deform and irradiate the filler material induces no heat gain, or negligible heat gain, in the filler material and/or the metallic substrate. 
     In some embodiments of the system, a microstructure of the metallic substrate is substantially unaltered during repair of the surface defect. 
     In some embodiments of the system, a frequency and/or amplitude of acoustic energy and/or a placement of the filler material within the surface defect is selected to minimize voids within a repaired region of the metallic substrate. 
     In some embodiments of the system, the acoustic energy coupling tool has a hardness greater than a hardness of the filler material and/or the metallic substrate. According to another example embodiment, a method of repairing a surface defect in a metallic substrate is provided, the method comprising: coupling a transducer to an acoustic energy coupling tool; arranging the acoustic energy coupling tool over a portion of the surface defect to be repaired; feeding a filler material underneath the acoustic energy coupling tool and/or at least partially within the surface defect; generating acoustic energy via the transducer to cause an oscillatory movement of the acoustic energy coupling tool at a frequency corresponding to a frequency of the acoustic energy generated by the transducer; impacting the filler material positioned underneath the acoustic energy coupling tool and/or at least partially within the surface defect with the acoustic energy coupling tool to deform the filler material so that the filler material conforms to at least a portion of an internal surface of the surface defect; irradiating the filler material with the acoustic energy at a same time as when the filler material is being deformed to conform to at least the portion of the internal surface of the surface defect by the acoustic energy coupling tool; and filling at least a portion of the surface defect with the filler material. 
     In some embodiments of the method, irradiating the filler material with the acoustic energy causes the filler material to soften and causes inter-metallic diffusion between the filler material and one or more internal surfaces of the surface defect against which the filler material is conformed by the acoustic energy coupling tool, thereby bonding the filler material to the substrate within the surface defect. 
     In some embodiments, the method comprises moving the acoustic energy coupling tool relative to the metallic substrate to deposit the filler material as one or more successive layers formed within the surface defect to repair the surface defect and produce a repaired region of the metallic substrate that has a microstructure that is integrated with the microstructure of the metallic substrate. 
     In some embodiments, the method comprises coupling the transducer to the acoustic energy coupling tool via a horn and transmitting the acoustic energy from the transducer to the acoustic energy coupling tool via the horn. 
     In some embodiments of the method, the filler material has a generally annular cross-sectional shape. 
     In some embodiments of the method, the filler material and the metallic substrate comprise a same metal or metal alloy. 
     In some embodiments of the method, the oscillatory movement of the acoustic energy coupling tool that causes the acoustic energy coupling tool to impact the filler material to deform and irradiate the filler material within the surface defect induces no heat gain, or negligible heat gain, in the filler material and/or the metallic substrate. 
     In some embodiments of the method, a microstructure of the metallic substrate is substantially unaltered during repair of the surface defect. 
     In some embodiments of the method, a frequency and/or amplitude of acoustic energy and/or a placement of the filler material within the surface defect is selected to minimize voids within a repaired region of the metallic substrate. 
     In some embodiments of the method, the acoustic energy coupling tool has a hardness greater than a hardness of the filler material and/or the metallic substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows an exemplary embodiment of a system of the presently disclosed subject matter for using acoustic energy to repair a crack in the surface of a metallic component. 
         FIG. 2A  is a top view of the exemplary embodiment of the acoustic energy coupling tool shown in the system of  FIG. 1 . 
         FIGS. 2B and 2C  show exemplary dimensions of the acoustic energy coupling tool shown in the system of  FIG. 1 . 
         FIG. 3A  is a cross-sectional view of a substrate with an artificially created surface crack formed therein. 
         FIG. 3B  is a cross-sectional Scanning Electron Microscopy (SEM) image of the surface crack after the repair process is completed. 
         FIG. 3C  is a detailed cross-sectional view of the region indicated in  FIG. 3B , showing the microstructure of the metal at the interface between the substrate and the repaired region. 
         FIGS. 4A and 4B  are respective cross-sectional views of two repaired samples having voids in the repaired region. 
     
    
    
     DETAILED DESCRIPTION 
     The presently disclosed subject matter relates to methods and systems for using acoustic energy to repair a surface crack in a metallic component without the need to apply heat energy to the metallic component during the repair.  FIG. 1  shows an example embodiment of a system, generally designated  100 , for applying acoustic energy to repair a crack, generally designated  20 , in the surface of a substrate  10 , such as the metallic component shown therein. In the example embodiment shown, the system  100  comprises an acoustic energy coupling tool  120  that is connected, via a stainless steel horn  140  in the embodiment shown, to a piezo-electric transducer  160  that vibrates at, at least in this example embodiment, a 60 KHz frequency, or otherwise produces ultrasonic acoustic energy. In some other embodiments, different excitation frequencies may be generated by the transducer  160  and transferred to the acoustic energy coupling tool  120 , whether or not via a horn  140 . In any such embodiments, the particular excitation frequency is based on the material being used as the filler material  15 , the material of the substrate (e.g., substrate  10 ) being repaired, or any other considerations, without deviating from the scope of the disclosure herein. Similarly, different materials for the horn  140  and different types of transducers  160  from the example embodiments disclosed herein may be used without deviating from the scope of the present disclosure. 
     The method of using the system to repair a surface crack in a metallic substrate comprises positioning the acoustic energy coupling tool  120  over the surface crack  20 , for example, by attaching the acoustic energy coupling tool  120  to a desktop gantry platform. The transducer  160  is energized at a specified frequency, 60 KHz in the example embodiment disclosed herein, and the oscillations of the transducer  160  are transmitted in the form of acoustic energy to the acoustic energy coupling tool  120  via the horn  140 . A filler material  15 , which is a filament feed made of solid aluminum in the example embodiment shown and described herein, is fed under the vibrating acoustic energy coupling tool  120 . While other cross-sectional shapes for the filler material  15  may be used in other embodiments, the un-deformed filament of the filler material  15  has a generally annular cross-sectional shape in the embodiment shown. 
     The acoustic energy coupling tool  120  moves, as a result of the vibrations at the excitation frequency from the transducer  160 , in a substantially vertical direction, generally designated O, to compress the filler material  15  within the surface crack  20  and, simultaneously, irradiates (e.g., transmits) acoustic energy at the excitation frequency into the filler material  15  as the filler material  15  is being compressed within the surface crack  20  to fill the surface crack  20 . In some embodiments, the filler material  15  is therefore deformed by the oscillatory movements of the acoustic energy coupling tool  120  to have a shape that is substantially similar to the cross-sectional shape of the surface crack  20 . In some embodiments, the surface crack  20  may have a cross-sectional area that is larger than a cross-sectional area of the filler material  15 , in which case it is generally advantageous to apply multiple consecutive layers of the filler material  15  within the surface crack  20 , until the filler material  15  within the surface crack  20  has substantially a same height as the outer edges of the surface crack  20  that define an outer surface of the metallic substrate  10 . 
     The irradiation of the filler material  15  with acoustic energy via the acoustic energy coupling tool  120  causes the portion of the filler material  15  directly under the tip of the acoustic energy coupling tool  120  to soften, thereby simultaneously causing the filler material to conform to the shape of the surface crack  20  due to the vertical compression and/or lateral expansion of the filler material  15  within the surface crack  20  caused by the vertical motion of the acoustic energy coupling tool  120 . At the same time, by using the acoustic energy coupling tool  120  to irradiate the filler material  15  with the acoustic energy as the filler material  15  is compressed within the surface crack  20 , inter-metallic diffusion occurs between the substrate  10 , at the internal surfaces and/or contours of the surface crack, and the filler material  15 , thereby bonding the deformed filler material  15  to the internal surfaces of the surface crack  20  against which the filler material  15  is being compressively applied. This results in a voxel of the filler material  15  being deposited within and/or on the crack surface  20 . 
     The steps of the method are repeated until a “run” of the filler material  15  is deposited over the entire length, or a portion thereof, of the surface crack  20 . Several such “runs” can be deposited sequentially on top of each other, as necessary based on the depth of the surface crack  20 , to completely fill up the surface crack  20 . It has been observed that the acoustic energy density of 493.61 J/m 3  provides the best conformance and bonding of the filler material  15  to the shape of the inner surface of the surface crack  20  in the example embodiment shown in  FIG. 1 . 
     It is advantageous for the acoustic energy coupling tool  120  to have a comparatively sharp tip, such that a width of the surface of the tip that makes contact with the filler material  15  is smaller (e.g., narrower) than the size (e.g., the width, which can be measured, for example, at the base or at the outer surface of the surface crack  20 ) of the surface crack  20  being repaired, so that the acoustic energy coupling tool  120  is able to adequately compress the filler material  15  within the surface crack  20  to substantially entirely fill the surface crack  20 , so that the substrate  10  will have a same thickness (e.g., allowing for process tolerance variations) in the repaired region  30  as in the immediately adjacent portions of the substrate  10 . 
       FIGS. 2A-2C  show a top view and exemplary dimensions of an example embodiment of the acoustic energy coupling tool  120  suitable for use in repairing a surface crack  20  in a substrate  10 , for example, in a substrate  10  made of aluminum using a filler material  15  made of aluminum. The acoustic energy coupling tool  120  has a body  125  that is a generally longitudinally extending member with a D-shaped cross-sectional area that tapers in a generally conically-shaped manner to a pointed tip, generally designated  130 . The tip  125  physically impacts and compresses the filler material  15  within the surface crack and/or irradiates the filler material  15  such that the filler material  15  conforms to, and bonds with, the internal surfaces and/or contours of the surface crack  20 . In the embodiment shown, the width of the body  125  is greater (e.g., wider) than a depth of the tapering portion that defines the tip  130 . The disclosed geometry of the tip  130  is advantageous in that it is capable of inducing sufficient compression in the filler material  15 , while at the same time also allowing for the tip  130  to reach and/or access smaller features. 
     To validate the suitability of the methods and systems disclosed herein in repairing surface cracks  20  in a substrate  10  in the form of a metallic component, empirical testing was performed. Surface cracks were formed in the substrates  10  formed from one or more aluminum plates and a solid aluminum filament was used as the filler material  15 . During the testing, the piezo transducer  160  was connected to the acoustic energy coupling tool  120  by the stainless steel horn  140  and energized to produce an oscillatory vibration and/or movement at 60 KHz, such that the acoustic energy coupling tool  120  oscillated at a substantially similar frequency (e.g., at about 60 KHz). Oscillation of the acoustic energy coupling tool  120  can be in the axial (e.g., vertical) and/or lateral (e.g., horizontal) directions of the acoustic energy coupling tool  120 , or in combinations thereof, but in plane with the filler material  15  and the substrate  10  workpiece (e.g., aligned with the direction of extension of the surface crack). The filler material  15 , in the form of a solid aluminum filament, is progressively fed into and/or directly on top of (e.g., over) the surface crack  20  and under the tip  130  of the acoustic energy coupling tool  120 , which irradiates the filler material  15  with the acoustic energy generated by the piezo transducer to compress the filler material  15  into the surface crack  20  and also to promote inter-metallic diffusion between the filler material  15  and the inner surface of the surface crack  20 , thereby bonding the filler material  15  with the internal surfaces of the surface crack  20  (e.g., to the substrate) to fill, at least partially, the surface crack  20  and form the repaired region  30 . 
     In some embodiments, the substrate  10  having the surface crack  20  can be held in a fixed position while the acoustic energy coupling tool  120  moves in the direction T along the length of the surface crack  20  to compress and/or bond the filler material  15  within and along the length of the surface crack  20 . The movement and vertical position of the acoustic energy coupling tool  120  can be fully or partially automated or, in some embodiments, can even be manually controlled (e.g., configured to be hand-held by a user, or otherwise capable of being manually controlled). In some other embodiments, the acoustic energy coupling tool  120  can be held stationary while the substrate having the surface crack is mobile (e.g., movable) thereunder. Any combination of mobile/stationary components of the system  100  is contemplated. 
     To determine that no microstructure change occurred in the vicinity of the repair of the surface crack, Electron Backscatter Diffraction (EBSD) analysis was performed in the repaired region  30  of the surface crack  20  to validate the methods and systems disclosed herein. 
     In  FIG. 3A , an optical image of the cross-section of a substrate  10  made of aluminum with an artificially-created surface crack  20  formed therein is shown. The upper bounds of the surface crack  20  are shown schematically by the broken line connecting the outer edges of the substrate  10  on opposite sides of the surface crack  10 . To repair this surface crack  20 , the method was utilized three times to successively deposit the filler material  15 , in the form of an aluminum filament, within the surface crack  20  to form three discrete layers of material (e.g., a first layer  30 A, then a second layer  30 B, then a third layer  30 C) within the surface crack  20  to completely fill the surface crack  20 . The result of this successive deposition method of the filler material  15  within the surface crack  20  completely fills the previously-defined surface crack  20  with the same material (e.g., aluminum) as the material of the substrate  10  (e.g., aluminum). 
     The filler material  15  and the substrate  10  may be a metal, metal alloy, or any suitable material.  FIG. 3B  shows a Scanning Electron Microscopy (SEM) image of a cross-sectional view of the repaired sample, as described herein with respect to  FIG. 3A . The three successively deposited layers ( 30 A,  30 B,  30 C) of the filler material  15  define a repaired region (e.g.,  30 ,  FIG. 1 ) and can be discerned upon close inspection, yet it is clearly visible from the image that the filler material  15  is deformed, such that the filler material  15  conforms to the shape of the inner surface  12  of the surface crack  20 . As discussed elsewhere herein, the acoustic softening phenomenon aids in softening the filler material  15 , which can be in the form of a wire, so that the filler material  15  conforms to the internal shape and/or contours of the surface crack  20 .  FIG. 3C  is a detailed view of the area indicated in  FIG. 3B , showing the microstructure of the substrate  10  and filler material  15  at the inner surface  12  of the surface crack  20 , where an interface (e.g., bondline) between the substrate  10  and the filler material  15  is formed at the repaired region  30 . As shown, the metallic microstructure of the substrate  10  at and/or adjacent to the interface between the substrate  10  and the filler material  15  does not show any appreciable change after the repair has been completed, relative to the metallic microstructure of the substrate  10  away from the interface between the substrate  10  and the filler material  15 , according to the methods and systems disclosed herein. The unaltered microstructure of the substrate  10  at the interface between the substrate  10  and the filler material  15  provides a significant advantage over the heat energy-based surface repair processes currently known and utilized in the prior art. 
     In  FIG. 4A , a plurality of layers of filler material have been successively deposited to fill the surface crack, thereby defining a repaired region  30 . In this embodiment, a plurality of external layers  35  are applied successively over the outer surface of both the substrate  10  and the repaired region  30 . One or more of these external layers  35  can be provided and may cover only the repaired region  30 , all of the repaired region  30  and a portion of the substrate  10  that is immediately adjacent (e.g., extending 50% or less of the width of the surface crack  20 ) to the surface crack  20 , or over substantially all of (e.g., at least 75%, at least 90%, at least 95%, or at least 99%) the outer surface of the substrate  10 .  FIG. 4B  shows an example embodiment in which five layers ( 30 A through  30 E) of filler material have been successively deposited. The layers  30 A through  30 E contact each other at boundary lines  32  and/or the substrate  10  at the inner surface  12  thereof. 
       FIGS. 4A and 4B  also show examples of repaired substrates  10  that have voids  40  (e.g., air pockets, or regions in which the deformed filler material  15  is not present) in the repaired region  30  of the substrate  10 . These voids are a result of improper positioning of the filler material  15  and/or acoustic energy density from the piezo transducer  160 . These voids  40  result in a repaired region  30  that is weaker than would otherwise be anticipated of a repaired substrate and can result in premature material failure. Through proper application of the methods and use of such systems, it is possible to minimize, if not entirely eliminate, the presence of such undesirable voids in the repaired region  30  of the substrate  10 . 
     Examples of applications in which the methods and systems disclosed herein may be implemented include, by way of non-limiting example, a machine that can perform surface repairs on metal components; a robotic arm with a surface repair tool head based on the methods and system disclosed herein to perform in-place/in-situ repair of components in service; a method and corresponding machine or system that uses surface vibrations to both detect surface defects and then repair the defects detected; and a method and corresponding machine or system that controls the microstructure of the metal at the interface between the filler material and the metallic substrate within the repaired region by varying the amount of vibratory shear strain energy applied during the repair. 
     While the subject matter has been described herein with reference to specific aspects, features, and illustrative embodiments, it will be appreciated that the utility of the subject matter is not thus limited, but rather extends to and encompasses numerous other variations, modifications and alternative embodiments, as will suggest themselves to those of ordinary skill in the field of the present subject matter, based on the disclosure herein. For example, such barriers may be used as an enclosure for patios, driveways, driveway entrances, fences, docks, and the like. 
     Various combinations and sub-combinations of the structures and features described herein are contemplated and will be apparent to a skilled person having knowledge of this disclosure. Any of the various features and elements as disclosed herein can be combined with one or more other disclosed features and elements unless indicated to the contrary herein. Correspondingly, the subject matter as hereinafter claimed is intended to be broadly construed and interpreted, as including all such variations, modifications and alternative embodiments, within its scope and including equivalents of the claims.