Method and apparatus for surface enhancement

Systems and methods for generating beneficial residual stresses in a target material by generating cavitation shock waves through the use of a cavitation intensification conditioner. Shock waves emanate through the target material from collapsing cavitation voids in a liquid jet to generate residual stresses without significantly deforming the surface of the target material. A high pressure liquid is accelerated through a submerged peening nozzle to generate a high-speed liquid cavitating jet that is further intensified and controlled by use of the cavitation intensification conditioner.

FIELD OF THE INVENTION

The present invention relates to generally to systems and methods of surface enhancement, and more particularly, to systems and methods of utilizing high pressure liquid jets that induce cavitation to perform one or a combination of surface enhancement processes on materials (“target materials”).

BACKGROUND OF THE INVENTION

The most common method of generating compressive residual stress in the surface of a material is shot peening, where small particles or balls (shot) are impacted against the target material to deform the surface. The shot is typically propelled with compressed air using automated equipment to move the peening nozzle over the surface of the part to be peened. The shot, frequently steel or ceramic, is usually accelerated to 50-100 m/s by the compressed air and strikes the surface with enough energy to deform the top layer of material beyond its elastic limit.

This plastically deformed surface induces residual compressive stresses in the material as the material underneath, which is not plastically deformed, tries to push the plastically deformed material back into its original volume. This “pushing” is the compressive stress that is a beneficial material property.

Variations on this method include striking the surface with particles spun off from a rotating wheel, low plasticity burnishing with a ball that is hydraulically pressed into the surface as it rolls across the part, and laser shock peening (LSP).

Cavitation peening is another method that involves shooting a high-pressure liquid jet against the target material in such a manner that cavitation bubbles collapse and shock waves pass into the material. Cavitation peening is generally performed with the liquid jet and the target material both submerged in a liquid. The shock waves generate compressive residual stresses in the target material similar to the other methods described above. However, cavitation peening has traditionally presented several shortcomings, such as limited stress depth, limited stress intensity and limited process rates, and has been known to cause damage to the surface of the peened material.

Examples of cleaning or stripping methods may include removal of scale, oxides, chrome coatings, thermal barrier coatings, or others. Examples of surface roughening applications include roughening metals or ceramics to create a desirable bonding surface geometry for coatings or bonding agents.

DESCRIPTION OF THE INVENTION

Methods of inducing residual compressive stresses in materials are desired in order to improve properties such as resistance to fatigue failure and stress corrosion cracking. Further, methods are needed to clean, strip coatings from, or roughen surfaces in difficult applications. High-speed methods of performing the above-mentioned processes without damaging the processed target material are needed as an improvement over current methods.

The inventors of the present invention have recognized that all of the aforementioned methods have various shortcomings and limitations. Some or all of these shortcomings and limitations are remedied by the embodiments of the present invention discussed below. What follows is a discussion of some of the recognized shortcomings of past peening methods.

Conventional shot peening only produces relatively shallow compressive stresses, typically less than 0.25 mm deep. It also has the considerable drawback of roughening up the surface to be peened, thereby causing a limitation to the improvement in fatigue life.

Low plasticity burnishing is generally limited to accessible geometry that will provide access for the rolling ball and hydraulic actuators. Ultrasonic peening, such as described in U.S. Pat. No. 7,276,824, is faced with similar limitations.

Laser shock peening is comparatively slow and expensive. The equipment typically costs millions of dollars per station. The materials that can be processed using this method are limited, and this method is difficult to deploy under water. It is also difficult to apply laser peening to confined spaces, such as inside of small-diameter tubes or cavities.

Cavitation peening is lower cost than laser shock peening but has traditionally been more expensive than conventional peening, due in part to long process times. The residual stresses generated using cavitation peening can be deeper than conventional peening. U.S. Pat. No. 5,778,713 describes a cavitation peening method that shoots the liquid jet directly at the target material to perform peening. However, that invention is stated to be suitable for metal materials only and the direct impingement of the liquid jet requires utilization of a fine resolution raster pattern to cover the surface with the small jet footprint, requiring a significant amount of process time. The direct impingement method can also cause surface damage by erosion caused by the high velocity liquid jet that acts upon the surface of the material, thus limiting the available developed stress intensity. This is particularly true if the process time is long enough to provide the desired stress intensity and depth. U.S. Pat. No. 6,855,208 requires elevated ambient pressure to provide the desired performance.

U.S. Pat. No. 6,345,083 requires the use of an energy wasting deflecting element to peen along the side of the jet.

Japanese Patent 06-047665 utilizes a large enclosing shroud to generate turbulence, which is stated to improved performance.

Conventional cleaning and coating removal methods often involve the undesired use of chemicals or destructive mechanical methods. Some of the above-mentioned references utilize cavitation and mention surface cleaning, however the required direct impingement of the high velocity liquid jets cause damage to the substrate material when tough coatings are to be removed due to erosion by the high velocity liquid jet. U.S. Pat. No. 5,086,974 is a direct impingement cavitating liquid jet. However, the energy level of the jet must be limited so as not to damage the processed material and the processing rate and performance is limited.

It should be noted that methods such as burnishing, laser shock peening, or lower pressure cavitation peening (which requires higher liquid flow rates) could be difficult or impossible to deploy in many applications due to the tool loading or support equipment that is required.

Embodiments of the present invention overcome many of these difficulties by utilizing a submerged pressurized liquid jet to perform cavitation peening. Illustrated embodiments of the current invention utilize a peening head design and operating parameters that significantly improve performance and process flexibility. Specifically, as discussed in further detail below, embodiments include a peening head having a cavitation intensification conditioner (or “conditioner”) coupled thereto oriented substantially parallel to the cavitation or liquid jet. This conditioner acts to create a low-pressure region between the cavitation jet and the conditioner, thus increasing the cavitation intensity in and around the jet. This allows the use of cavitation for peening in a broader range of applications and at reduced cost and with improved results.

Embodiments of the present invention also support higher processing rates due to the increased cavitation intensity that they generate. The increased peening intensity allows higher processing rates because the systems and methods generate residual stresses of a given level faster than other cavitation peening methods and systems.

Further, embodiments of the present invention create more intense cavitation jets and make it possible to generate deeper and more intense residual stresses compared to other cavitation peening methods. Embodiments of the present invention have been shown to be capable of peening metals, as well as other materials such as ceramics, glass, composites, and plastics. Similarly, tougher coatings can be removed at high rates where past practices fail.

One of the benefits of the embodiments disclosed herein is that excellent results can be provided with the cavitation jet oriented at any angle relative to the surface of the material being peened (i.e., the “target material”). The jet can even be oriented parallel or tangent to a surface being peened without actually striking the surface with the jet, but still results in improved results over other cavitation peening methods. The benefit of this feature is that high residual stress magnitudes and depths can be obtained without damaging the surface of the target material.

FIG. 1is a schematic block diagram of a cavitation or peening system10in accordance with an embodiment of the present invention. The system10comprises a high pressure liquid pump12that is provided to generate liquid pressures that are preferably between 15,000 psi to 200,000 psi, or higher. A rigid or flexible high-pressure liquid conduit14is provided to couple pressurized liquid16from the pump12to an input port of a peening head21comprising a liquid nozzle22. The liquid16may comprise liquid water, cryogenic liquid, liquid rust inhibitor, or other suitable liquid. As an example, the pump12may be a KMT Waterjet Streamline V, a Flow International 20X pump, or another suitable pump.

The nozzle22(or a plurality of nozzles) is mounted to a robotic manipulator24configured to provide relative motion between the nozzle22and a target material40(e.g., the portion thereof to be processed). The nozzle22and the target material40are submerged in a tank44of liquid46. The relative motion between the nozzle22and the target material40is designed such that a high-pressure liquid or cavitation jet50passes proximate to or in contact with a surface42of the target material40in areas that are desired to be processed. The robotic manipulator24may be coupled to a computer control unit48configured to preprogram and control the movement of the nozzle22in a plurality of dimensions and to control the starting and stopping of the process (e.g., by controlling the operation of the pump12, etc.) using pre-programmed instructions.

Alternatively, the target material40may be mounted on the robotic manipulator24to provide the relative motion with the nozzle22being stationary. A further alternative is that both the nozzle22and the target material40are mounted on separate robotic manipulators24to provide the relative motion. Additionally, the nozzle22could also be held by a person and pointed at the surface42of the target material40, wherein the operator manually moves the nozzle22to process a desired area of the material. As an example, the robotic manipulator24may be a Flying Bridge available from Flow International, Inc., a PAR Vector CNC, or other suitable robotic manipulator. An additional alternative is that, if only a small area is to be processed in one operation, processing may be performed with little or no relative motion between the nozzle22and the target material40.

Another example of a robotic motion device is a remotely operated vehicle. The robotic motion device can be pre-programmed or may be operated manually to create the desired relative motion between the nozzle22and the material40so that a cavitation footprint54(seeFIGS. 3A-3B) covers the area to be processed. There may also be tooling to hold the processed material40or to mount the robotic motion device24.

As shown inFIGS. 1,7A, and7B, the peening head21includes a cavitation intensification conditioner56coupled to the nozzle22near a distal end58(or exit portion) thereof whereat the liquid exits the nozzle to produce a liquid or cavitation jet50and extending outwardly from the end58in a direction substantially parallel to the cavitation jet50. The conditioner is adjacent to or “surrounds” at least a portion but not the entire circumference of the cavitation jet50. The conditioner56acts to create a low-pressure region59between the jet50and a jet-facing surface57of the conditioner, thus increasing the cavitation intensity in and around the jet. The conditioner56is shaped to be substantially parallel to the jet50so that the conditioner guides the additional cavitation (or “cavitation cloud”) toward the surface42of the target material40to enhance the cavitation peening process. Although the conditioner56is shown positioned on an opposite side of the jet50from the surface42, the conditioner56may be oriented on any side of the jet50(e.g., between the jet and the surface42(FIG. 4), on a side of the jet, etc.).

As shown inFIGS. 8,9, and10(discussed below), the conditioner56may be designed using a range of shapes (see conditioners90,92, and94). The finish of the jet-facing surface57of the conditioner56has an effect on the properties of the cavitation jet50and can be used to further improve performance. For example, the jet-facing surface57may include one or more enhancements61(seeFIG. 7B). The enhancements61may be machined ridges, knurling, holes, slots, or other surface finish types. Depending on the specific application, the intensification conditioner56may be from 0.25 inches to 10 inches or longer in length Lc(seeFIG. 7B), measured from the distal end58of the nozzle22. Generally, higher cavitation jet flow rates may utilize a longer intensification conditioner56, but the length Lcmay also depend on ambient pressure, the desired results, etc. The conditioner56may be positioned at a distance Dcfrom the jet50(seeFIG. 1). The distance Dcmay be between approximately 0 inches (e.g., nearly touching the jet50) to up to 2.00 inches (5.08 centimeters), with the distance Dcoften being related (e.g., proportional) to the flow rate of the jet50.

FIG. 2illustrates a perspective view of the peening head21configured to direct the liquid jet50in a direction substantially parallel to the surface42of the material40at a stand-off distance D. In this example, the nozzle22moves in the direction of the arrow58creating a processed area60of the surface42of the material40. In this example, the cavitation jet50is substantially parallel to the surface42of the material40and the jet is operated at a standoff distance D of approximately 0.010 inches (0.0254 cm) to 2.00 inches (5.08 centimeters) away from the surface of the material.

As shown inFIGS. 3A and 3B, embodiments of the present invention also support significantly higher processing rates due to the much larger cavitation footprint54on the surface42of the target material40and the higher power capacity when the jet50is substantially parallel to the surface42of the material40. The parallel flow of the cavitation jet50over the surface42creates the elongated footprint54that has a width W that is greater than the cross-sectional diameter of the cavitation jet and a length L that corresponds to the portion of the cavitation jet that passes over the surface42with sufficient energy to process the surface. This is in contrast to a direct impingement cavitation jet footprint that will normally have a diameter of about 1 mm (e.g., approximately the cross-sectional diameter of the cavitation jet). The substantially parallel orientation of the cavitation jet50is can increase the processing rate by a factor of 100 times in many cases because the cavitation footprint54of the parallel oriented jet50can be 100 or more times the area of the diameter of the cavitation jet.

Further, the non-contact jet50allows the use of a higher pressure, higher velocity, more intense cavitation jet, without damaging the surface42by direct contact of the high velocity cavitation jet against the material40. Because there is little danger of damaging the material40, embodiments of the present invention allow intense cavitation peening and result in improved residual stress results compared to direct impingement peening. A unit-less example of a stress-depth curve45that can be generated using the peening system10is shown inFIG. 11. The methods disclosed herein are operative to peen metals as well as other materials such as ceramics, glass, composites, and plastics. Similarly, tougher coatings can be removed using the methods disclosed herein where past practice methods fail.

When roughening surfaces, embodiments of the invention utilizing the parallel oriented jet50may be used to provide extremely well controlled consistent finishes for the surface42because the finish is created by action of cavitation only and is not influenced by cavitation jet erosion. Because the cavitation jet50does not contact the surface42, high-energy cavitation jets can be utilized without danger of erosion caused by the jets.

Embodiments of the present invention are easily deployed because the nozzle22can be small, lightweight, and in some embodiments (ultra-high pressure/low flow rate embodiments), the reaction load on the manipulator24or processed material40is relatively very low. One benefit of the invention is that the system10is operative to, with a single tool, perform one or a combination of processes including cleaning material surfaces, removing coatings from materials, roughening material surfaces, and/or generating beneficial compressive residual stresses or reducing tensile residual stresses in materials.

As discussed above, some embodiments of the present invention use the high-pressure cavitation jet50to generate cavitation that peens materials, thereby creating beneficial compressive residual stresses. The process relies on shock waves induced by cavitation bubbles collapsing on the surface42of the material40to be peened, instead of deformation of the surface. The process may be performed with the nozzle22and conditioner56, cavitation jet50, and the processed material40submerged in the tank44of liquid46(seeFIG. 1). The liquid46in the tank may be, for example, water, oil, various liquids in solution with other liquids, liquids with dissolved solids added, or other liquids.

As shown inFIG. 4, in some embodiments the nozzle22may be positioned to orient the cavitation jet50at a shallow angle α relative to the surface42of the material40, rather than substantially parallel therewith. For example, the angle α may be approximately 0 degrees to 10 degrees. As will be appreciated, a higher flow rate jet50may be used if the jet is positioned farther away from the material40.

As shown inFIG. 5, in some embodiments the nozzle22may be positioned to orient the cavitation jet50at a substantially right angle relative to the surface42of the material.

FIGS. 6A and 6Billustrate use of the system10to process an exterior curved surface76of a cylindrically shaped material74. InFIG. 6A, the jet50is oriented substantially tangent to the curve of the surface76. InFIG. 6B, the jet50is oriented substantially along a longitudinal axis of the cylindrically shaped material74. As indicated by the arrow78inFIG. 6B, the nozzle22may rotate in a circular path to direct the jet50along the surface76of the material74offset from the surface74, maintaining a standoff distance D throughout the rotation. It should be appreciated that the jet50may be also positioned at an angle to the longitudinal axis of the material74in other embodiments (seeFIGS. 4 and 5). For irregular surfaces, in some embodiments the jet50may be oriented substantially parallel to the mean of the surface, or within substantially 10 degrees from the mean of the surface. This orientation maximizes the cavitation footprint of the jet50and maximizes the process rate, while preventing damage to the surface of the material caused by a direct impingement of a high-pressure cavitation jet.

If the jet50is oriented off-parallel to the surface42of the material40as shown inFIG. 4, the jet will strike the surface42at a contact point64at the angle α and flow over the surface42. The particular footprint is dependent on the pressure, type of nozzle, type of liquid, orientation angle α, type of material40, and other factors. When the jet50is oriented at an angle α to strike the surface42of the material40, the distance from the nozzle22to the contact point64where the jet strikes the surface42may be referred to a jet distance DJ. The distance DJmay be approximately 0.25 inches (0.635 cm) to 10 inches (25.4 cm) or more, depending on the application and jet flow rate. Generally, it has been found that the conditioner56is most effective when it is spaced apart from the target surface42by at least 0.25 inches (0.635 cm).

The nozzle22and jet50can be passed over the material40to cover large areas, or alternatively, can be operated momentarily at a stationary location over the material to process a limited area. In the latter case, the jet50can then be turned off and moved to another location and operated a multiple of times to provide the desired coverage.

This invention can be used on shapes ranging from simple flat or cylindrical materials, to complex shapes such as gears, turbines, or nuclear reactor core components.

Examples of liquids that may be used as the peening liquid16may include water, oil, liquid rust inhibitor, a solution of one liquid containing other liquid, or a solution of a liquid containing dissolved solids. The liquid16may be supplied to the nozzle22at pumped pressures of 15,000 to 200,000 psi, or higher. A non-limiting example nozzle22may have an orifice-opening diameter of between approximately 0.003 inches (0.00762 cm) and 0.25 inches (0.635 cm). The cavitation jet50can be operated when the surrounding liquid46(seeFIG. 1) is at ambient atmospheric pressure or when the ambient pressure is elevated.

FIGS. 8,9, and10illustrate three cavitation intensification conditioners90,92, and94, respectfully, having differing shapes. It should be appreciated that the conditioners90,92, and94are shown as non-limiting examples of shapes for the conditioners. In each example, the conditioners90,92, and94are coupled to and extend outwardly from the distal end58of the nozzle22in a direction substantially parallel with the cavitation jet50. The conditioners90,92, and94may be selectively or permanently coupled to the nozzle22, or may be integrally formed therewith as a single component. Generally, the conditioners90,92, and94act to restrict the flow of liquid46(seeFIG. 1) surrounding the jet50by an amount sufficient to create additional cavitation between the conditioners and the jet, but do not restrict the flow so much that additional cavitation does not occur. As an example, a cylindrical tube having a diameter that is slightly larger than the diameter of the jet50would most likely restrict the flow between the jet and an inner wall of the cylindrical by an amount such that increased cavitation would not occur. In addition to facilitating the production of additional cavitation, the conditioners90,92, and94also act as a guide to direct the intensified cavitation cloud toward the target surface42to increase the effectiveness of the peening process.

The conditioner90ofFIG. 8is formed in the shape of half a hollow cylinder. The conditioner90includes a jet-facing surface91positioned a predetermined distance from the cavitation jet50. Since the conditioner90is disposed on only one side of the jet50and does not completely surround the jet, the flow of liquid around the jet is not overly restricted. As discussed above with reference to the cavitation conditioner56, the finish of the jet-facing surface91of the conditioner90may include one or more enhancements (see the enhancements61on the surface57shown inFIG. 7B).

The conditioner92ofFIG. 9is formed in a shape having an inverted “V” or “chevron” cross-section. The conditioner92also has a jet-facing surface93, which may in some embodiments include one or more enhancements.

The conditioner94ofFIG. 10is formed in the shape of a hollow cylinder having lengthwise apertures or through-slots that extend the entire length thereof. This is achieved by providing four spaced-apart, elongated projections94A,94B,94C, and94D having concave cross-sections and being disposed substantially concentrically around the cavitation jet50. Each of the projections94A-D includes a jet-facing surface95, which may in some embodiments include one or more enhancements. The gaps between each of the projections94A-94D allow for sufficient liquid to flow between the jet50and the surfaces95to facilitate intensified cavitation. The projections94A-D extend substantially parallel to the jet50so that the additional cavitation is directed toward the surface42of the target material40.

FIG. 12is a perspective view illustrating a method of processing the target material40using the peening system10ofFIG. 1wherein the cavitation jet50is oriented parallel to the surface42of the target material and does not strike the surface. In this embodiment, the nozzle22is coupled to a shroud80having an interior portion82configured for receiving a liquid (not shown for clarity) from a liquid conduit88coupled to the shroud. The shroud80is open at the bottom exposing a shrouded portion86of the surface42of the material40to the liquid. Thus, the jet50and at least a portion of the shrouded portion86of the target material40are submerged in a liquid. In operation, the nozzle22and shroud80may be moved over the surface42to process the material40as desired. This method may be beneficial in applications where it is not feasible to submerge the entire target material40into the liquid tank44.

Other peening methods rely on the use of elevated ambient pressure in the liquid surrounding the cavitation jet and target material. Embodiments of the present invention reduce the requirement to pressurize the surrounding liquid bath, depending on the application. This is a benefit over other cavitation peening methods because it simplifies the cost and complexity of the equipment needed to perform the process. This is because in some embodiments, a pressure vessel that peening would otherwise need to be performed within is either not needed, or at least a pressure vessel with reduced pressure rating requirements may be used. Further, generally it is not feasible to peen many large components inside of a pressure vessel due to cost.

In applications where an elevated ambient pressure is inherent, such as in nuclear reactor vessels, the elevated ambient pressure does nothing to damage performance, but can increase performance somewhat. Embodiments of the present invention make changes in water depth (and therefore ambient pressure) while performing cavitation peening much less of an impediment on the process. In other words, the process performance does not change as parts are peened at different water depths (e.g., in a submerged reactor vessel).

Because the conditioners disclosed herein can generate more intense cavitation, when desired they can be used to generate roughened surfaces faster than conventional methods. When roughening surfaces at shallow impingement angles (seeFIG. 4), or without striking the surface with the jet (seeFIG. 2), embodiments may be used to provide extremely well-controlled, consistent finishes because the finishes are created by action of cavitation only and are not influenced by cavitation jet erosion.

While not required, an option that may improve residual stress magnitude and depth in some applications using precipitation hardening stainless steels or other heat treatable materials is to peen before heat-treating, and again after heat treating. This is beneficial in materials that are not stress relieved during a heat treatment process, such as PH15-5 or Custom 465 stainless steel. Peening before heat treatment (or otherwise termed “aging”) provides good depth penetration because of the low strength of the target material. The magnitude of the residual may be, however, limited due to the low yield strength of the material. Peening again after heat treatment may provide increased residual stress magnitude due to the increased yield strength after the heat treatment.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that, based upon the teachings herein, changes and modifications may be made without departing from this invention and its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of this invention. Furthermore, it is to be understood that the invention is solely defined by the appended claims. It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.).