Background of the Invention Rivets are a preferred fastener for use in assembling aluminum and titanium airframe components because they provide a joint having a high shear strength. This strength arises from the interference fit between a rivet and the components it joins. Commercial aircraft incorporate millions of riveted joints.

Riveting is the largest single cost operation associated with airframe manufacturing. In addition, rivets are also the fastener of choice for use in producing automobile and truck bodies, mobile homes, and light rail vehicles.

The conventional impact riveting process is time consuming and causes repetitive stress health problems related to operator handling of impact tools. This process often requires two operators, positioned on opposite sides of the work piece, further increasing costs and limiting application of this process for joining components in structures having limited access. Hydraulic squeeze riveting technology avoids the repetitive stress injury associated with impact riveting, but this process typically employs large hydraulic tools, which are not readily adapted to join components in complex structures or to meet constantly changing productions needs. Achieving a reliable interference fit while riveting components requires match-drilled holes made with precise dimensional control.

A riveting process that does not require operators on both sides of the pieces being joined is referred to as blind hole fastening. Blind hole fastening has the attraction of potential tremendous labor cost savings. Existing blind hole fasteners are typically variations of some type of pull-through swaging process or

use threaded expandable fasteners. These systems are all relatively complex, involving tapered sleeve mechanisms. The structural performance of multi- component fasteners of the type often used for blind hole joints is inherently inferior to a single piece fastener. There are currently no structural, blind-hole fasteners in widespread use for structural airframe manufacturing.

It would be desirable to develop a simple method for riveting, which uses compact, easy to manipulate tools, do not use multi-component fasteners, do not subject the operator to significant vibrational force, and which can be used for setting blind hole fasteners. Currently, such methods are not provided in the prior art.

While rivets are currently the preferred type of fastener for assembling aluminum and titanium airframe components, riveting is labor intensive and costly. Such a labor intensive process is poorly suited to high volume manufacturing applications. Aluminum welding is appropriate for some components, but this process requires skilled workers, careful quality control, and post processing to relieve stresses in the welded assembly. Explosive bonding of metal structures has been investigated as a means of directly bonding aluminum alloys. This process involves the detonation of high explosive sheets against plates of materials to be bonded. The high velocity impact of the two plates generates extremely high stresses at the material interface, allowing the materials to diffusion bond and interlock mechanically. This process is not suited for high volume continuous manufacturing of structures such as airframes and vehicles.

It would be desirable to develop an economical alternative to conventional riveting and the use of hydraulic riveting tools for joining two components that does not require the skill, or careful attention to quality control, and the post processing required for aluminum welding, and which is also well suited for high volume continuous manufacturing. Currently, such alternatives do not appear evident in the prior art.

Another prior art process for joining two components, which can be employed as an alternative to riveting, is to provide an interlocking clinch fastener between two components, such as two sheets of metal. Mechanical clinch forming has become accepted practice as an alternative to tack welding in a number of manufacturing applications, including the production of appliances, automotive, and general sheet metal fabrication (e. g., in the production of duct work and electrical fixtures). Mechanical clinch fastening systems that use a punch and die are well known and in common use.

Disadvantages of these prior art clinch forming methods are that they are limited to only being usable with highly ductile, relatively thin sheet steel and some aluminum alloys. Thicker steel alloys, structural aluminum, and titanium cannot be joined using existing clinch forming methods. Furthermore, the stiff, precisely aligned punch and die combination is not suited for large structures.

The associated tooling generally comprises a very large, heavy, and expensive structure. The process cycle time is relatively slow, and only one size clinch may be formed by a specific tool. The process is limited to forming a circular clinch, which may not always be the desired shape. Finally, punch wear and breakage is a problem with mechanical clinch forming.

It would be desirable to provide a clinch forming method that does not suffer from the above-noted disadvantages. The method should preferably be suited to high volume and flexible manufacturing of large aluminum structures, such as appliances, ductwork, automobiles, trucks, recreational vehicles, and trailer homes; and should provide stronger joints than existing prior art techniques.

Mechanical cold-working of holes is currently used by aircraft manufacturers to improve the fatigue resistance of critical structures that are joined with rivets or other fasteners. Mechanical cold working of holes involves applying a radial force to the inside surface of the hole. The residual compressive stress resulting from plastic radial strain around cold-worked holes is known to result in an improved fatigue life for the hole. The conventional process of mechanical cold working of rivet holes includes the following steps. After the holes are drilled, a special gauge is inserted to verify hole dimensions. A disposable sleeve is placed in the hole and a hydraulically-actuated mandrel is drawn through the hole. The hydraulic pressure causes the mandrel diameter to expand while the mandrel is in the hole. A second inspection is performed to verify that the diameter is still within tolerance and that the proper expansion and desired cold working has occurred. It will be apparent that these steps comprise a time consuming, labor-intensive process.

It would be desirable to provide a method for the mechanical cold working of holes that is much less labor intensive. This new method should preferably be capable of forming and cold working holes in aluminum in a single step.

Over the past 20 years, surface impact treatment has emerged as an important technique to enhance the fatigue life of metal components by introducing a residual compressive stress in a thin surface layer. In particular, shot peening can significantly enhance the fatigue life of welded structures by

eliminating the residual tensile stress that occurs due to thermal contraction at the weld toe. Shot peening is often used for surface impact treatment (as opposed to cleaning) and is carried out by directing a stream of small (typically under lmm) steel shot at a metal surface. One inherent disadvantage of this process is that it must be carried out in a confined space, because the shot ricochet wildly from the work piece in many directions. In addition, the process generates considerable metal dust and noise. As a result, shot peening is carried out as a batch process on assembled structures. In order to ensure a good surface finish, the shot must be clean, round, and free from broken material. Shot peening at the high velocities required for surface cold working results in breakage of a significant fraction of the shot; the broken shot must be removed and replaced at significant cost.

It would thus be desirable to develop a method for introducing a residual compressive stress in a thin surface layer of a piece, which can be carried out as a continual process, rather than a batch process, and which does not include the disadvantages of ricocheting or broken shot. Such a method can be integrated with robotic welding to provide weld stress relief and cold work following the welding operation. The prior art does not disclose such a method.

Summary of the Invention Each of the above-described processes for expansion fastening, pulse bonding, clinch formation fastening, cold working holes, and cold working metal surfaces can be carried out with an ultra high-pressure fluid pulse. Such a pulse can be generated by actuating a fast opening valve to release water or other similar fluids contained in a pressure vessel and compressed to ultra high- pressures of 100 MPa or more (such as disclosed in commonly assigned U. S.

Patent No. 5,000,516); however, other valve configurations suitable for producing such a pulse are contemplated to implement the present invention. The creation of ultra high-pressure fluid pulses requires a valve that opens very quickly to ensure that the leading edge of the pulse is traveling at high velocity before it impacts the work piece or enters a tapered nozzle for further acceleration.

In accord with the present invention, a method is defined for using an ultra high-pressure fluid pulse to join a plurality of components. The method employs an ultra high-pressure fluid pulse generator having an outlet adapted to direct the ultra high-pressure fluid pulse at a surface of one of the plurality of components to be joined. The components are positioned in a desired configuration, and the ultra high-pressure fluid pulse generator is disposed so that the ultra high-pressure fluid pulse is directed at the surface of one of the components. The ultra high-pressure fluid pulse is generated and applied to the surface of one of the components,

causing a localized deformation of at least the one component to join and fasten the plurality of components together.

In one application of this method, the one component comprises a fastener having a cavity into which the ultra high-pressure fluid pulse is applied. The other components (e. g., two stacked metal sheets) include aligned orifices in which the fastener is disposed during the step of positioning the plurality of components. The ultra high-pressure fluid pulse causes the fastener to expand outwardly into an interference fit within the orifices to join and fasten the plurality of components together.

When generating the ultra high-pressure fluid pulse, a valve in the ultra high-pressure fluid pulse generator is rapidly opened to release the ultra high- pressure fluid into a channel directed toward the surface of one of the plurality of components. The ultra high-pressure fluid pulse generator preferably consists of a pressure vessel filled with a fluid at a pressure of greater than 100 MPa. The fluid is preferably water or a mixture that includes water.

Also provided in the ultra high-pressure fluid pulse generator is a nozzle through which the ultra high-pressure fluid pulse is directed toward the surface of one of the plurality of components. A length of the nozzle determines a duration of the ultra high-pressure fluid pulse.

In another application of this method, the ultra high-pressure fluid pulse causes a plastic deformation at an interface between at least two of the plurality of components. A structure is preferably provided to support the plurality of components. A magnitude of a shock pressure of the ultra high-pressure fluid pulse used in this application is preferably greater than about 1 GPa. In one embodiment, at the interface between the components, they each have a surface that is substantially smooth. In another embodiment, a surface of at least one of the two components is textured to facilitate bonding the components. As a further variation, at the interface, a surface of at least one of the components can include a cavity, so that an impulsive loading applied to the interface above the cavity by the ultra high-pressure fluid pulse forms a tack joint. In yet another embodiment, at the interface between the components, a surface of at least one of the components includes a channel, so that an impulsive loading applied above the channel by the ultra high-pressure fluid pulse forms a seam.

Yet another embodiment includes the step of positioning an intermediate material at the interface between the components, to enhance bonding between them in response to the ultra high-pressure fluid pulse. If the intermediate material is sufficiently hard, the ultra high-pressure fluid pulse interpenetrates

surfaces of the at least two components, forming a mechanical joint. If the intermediate material is a powder, upon being subjected to an impulsive loading produced by the ultra high-pressure fluid pulse, the surfaces of the components melt, bonding the surfaces together. The powder may comprise ceramic particles, or metal particles in a different embodiment.

Another embodiment of this method requires that a cavity be disposed behind a rear component, opposite a point where the ultra high-pressure fluid pulse is applied to the surface of the top component. This ultra high-pressure fluid pulse forces the plurality of components into the cavity to form an interlocking clinch fastening. Preferably, a shock pressure produced by the ultra high-pressure fluid pulse has a magnitude greater than 1 GPa. It is also preferred that the ultra high-pressure fluid pulse be applied to an area on the surface that is not greater than that of an inlet into the cavity to encourage thinning of a center of the interlocking clinch fastening and formation of a thick-walled interlock, with minimum shear in a load bearing section of the interlocking clinch fastening. The cavity can be formed in a supporting structure that underlies the plurality of components being joined. Or, the cavity can be in a die that is mounted on the supporting structure.

Another aspect of the present invention is directed to a method for using an ultra high-pressure fluid pulse to cold-work metal by introducing a residual compressive stress in the metal, thereby increasing an expected fatigue life of the metal. Still another aspect of the invention relates to apparatus for applying an ultra high-pressure fluid pulse to a component to produce a localized plastic deformation. This apparatus includes a discharge valve that is rapidly selectively moved from a closed to an open position to enable a pressurized fluid in a high- pressure chamber formed within a housing of the valve to travel down a fluid channel in the nozzle to an outlet port formed at a distal end of the nozzle. This ultra high-pressure fluid pulse is employed generally as described above.

Brief Description of the Drawing Figures The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: FIGURE I is a schematic view showing an ultra high-pressure fluid pulse being applied to a hollow fastener;

FIGURE 2 is a schematic view showing a hollow fastener that has been expanded to join two metal sheets by the application of an ultra high-pressure fluid pulse; FIGURE 3 is a schematic view showing an ultra high-pressure fluid pulse being applied to inject a pin into a hollow fastener; FIGURE 4 is a schematic view showing a hollow fastener that has been expanded and filled by the pin injected due to the application of the ultra high- pressure fluid pulse shown in FIGURE 3, for use in joining two metal sheets; FIGURE 5 is a schematic view of an apparatus for applying an ultra high- pressure fluid pulse that is being used to expand a hollow fastener to join two metal sheets; FIGURES 6A and 6B are schematic views illustrating use of an ultra high- pressure fluid pulse to bond two metal sheets by plastically deforming the metal comprising the sheets; FIGURES 7A and 7B are schematic views illustrating texturing of the surface of one of two metal sheets to enhance the bond formed after an ultra high- pressure fluid pulse has caused the metals to plastically deform; FIGURES 8A and 8B are schematic views showing the texturing of the surface of one of two metal sheets to enhance the bond formed after the application of an ultra high-pressure fluid pulse has caused the metals to plastically deform; FIGURE 9 is a schematic view illustrating the use of an intermediate material sandwiched between two metal sheets to enhance the bond formed by an ultra high-pressure fluid pulse; FIGURES 10A and 10B are schematic views showing an ultra high- pressure fluid pulse being used to bond two metal sheets by plastically deforming the metal sheets to form a clinch fastening; FIGURE 11 is a schematic view of an ultra high-pressure fluid pulse directed toward two metal sheets that are placed over a cavity; FIGURE 12 is a schematic view of an ultra high-pressure fluid pulse immediately after it has impacted two metal sheets, causing the sheets to begin to plastically deform into a cavity; FIGURE 13 is a schematic view of an ultra high-pressure fluid pulse that has impacted two metal sheets, causing a final plastic deformation of the sheets; FIGURES 14A and 14B are schematic views of an ultra high-pressure fluid pulse being used to cold work a hole in a metal sheet to increase the fatigue life of the hole;

FIGURE 15 is a schematic view of an ultra high-pressure fluid pulse being used to form a hole in a metal sheet, the pulse also cold working the hole immediately after it is formed; and FIGURES 16A and 16B are schematic views showing a tapered nozzle that is used to accelerate a fluid packet to increase a magnitude of a pressure pulse.

Description of the Preferred Embodiment Using Ultra High-pressure fluid pulses to Expand Hollow Fasteners One aspect of the present invention is a method for joining two materials by using an ultra high-pressure fluid pulse to expand a hollow fastener. FIGURE 1 illustrates an ultra high-pressure fluid pulse being used to expand a hollow fastener to join two sheets of metal. A nozzle 10 of an ultra high-pressure fluid pulse generating tool is shown positioned over a hollow fastener 14. Hollow fastener 14 includes a relatively small diameter cavity 15, which extends part way through the body of the fastener, toward a bottom 16. Hollow fastener 14 has a head 52. A distal end 46 of nozzle 10 incorporates a cavity 47 that is adapted to seat over the head 52 of hollow fastener 14. An ultra high-pressure fluid pulse 12 is shown advancing down a constant diameter fluid channel 54 in nozzle 10, towards hollow fastener 14. Cavity 47 both directs ultra high-pressure fluid pulse 12 into cavity 15 in hollow fastener 14 (substantial leakage would tend to reduce the shock pressure applied to cavity 15) and also prevents head 52 of hollow fastener 14 from expanding.

Hollow fastener 14 is disposed in an orifice 22 that has been formed, e. g., by a drilling or a die punching operation, in both a metal sheet 18 and a metal sheet 20. In this Figure, an ultra high-pressure fluid pulse 12 is shown advancing through nozzle 10 into cavity 15 within hollow fastener 14.

FIGURE 2 shows the result of ultra high-pressure fluid pulse 12 impacting the bottom 16 of hollow fastener 14. Hollow fastener 14 is inelastically expanded outwardly within orifice 22, so that hollow fastener 14 interferes with metal sheet 18 and metal sheet 20. The force causing the inelastic expansion of hollow fastener 14 is created when ultra high-pressure fluid pulse 12 impacts the bottom 16 of cavity 15 in hollow fastener 14. The sudden arrest of the ultra high- pressure fluid pulse creates a shock pressure pulse that inelastically expands hollow fastener 14. Because the diameter of cavity 15 is small, the impact force on the fastener is relatively small, and the fastener joint can be formed without supporting the back side of the components being joined.

FIGURE 3 illustrates an embodiment of the present invention in which a soft metal or plastic pin 17 is initially disposed within a discharge nozzle 10a.

Pin 17 is formed of a material that is chosen to have a yield strength substantially less than the force produced by the high pressure fluid pulse. Pin 17 has a diameter substantially equal to the diameters of nozzle exit 46 and cavity 15 in the fastener. FIGURE 3 shows that nozzle 10a includes a tapered fluid channel 54a. This tapered fluid channel accelerates high-pressure fluid pulse 12, thereby increasing the shock pressure applied to drive pin 17 into hollow fastener 14. A non tapered nozzle may be used, so long as the magnitude of the fluid pressure pulse is great enough to cause the inelastic expansion of pin 17 within cavity 15.

FIGURE 4 illustrates how ultra high-pressure fluid pulse 12 has forced pin 17 into cavity 15 of hollow fastener 14. Pin 17 expands hydrodynamically, since its yield strength is much smaller than the for generated by the impact shock pressure that results when pin 17 impacts bottom 16 of cavity 15 in hollow fastener 14. The length of pin 17 is chosen so that the volume of pin 17 after its expansion equals the desired expanded volume of cavity 15. This feature results in a solid fastener (i. e., a fastener having a cavity that is now filled) that is more corrosion and fatigue resistant than a hollow fastener. If hollow fastener 14 is formed of aluminum or an aluminum alloy, pin 17 preferably also comprises aluminum or an aluminum alloy.

The pressure, P, required to expand a thick-walled cylinder made of an elastic-perfectly plastic material is: Pi = up In K, where orp is the plastic flow stress and K is the ratio of the outer to inner diameter of the cylinder. For the purposes of this discussion, the plastic flow stress in work-hardening alloys is approximated as yield stress at 10% strain.

For joining aluminum airframes a rivet alloy, such as 7050, is suitable, while a titanium rivet alloy, such as Ti-6AI-4V, would be used for fastening titanium. A relatively soft 2000-series alloy, such as 2024, would be appropriate for fabricating truss-head fasteners used to join composite panels to metallic frames. Table 1, which follows, lists plastic flow stresses for these three high- performance aerospace alloys, with the capacity for up to 10% ductile strain.

Table 1 includes expansion pressures for a diameter ratio of 10, which corresponds to the expansion of a 1 mm diameter cavity in a 9.5 mm diameter cylinder; a pressure of 2.3 times the plastic flow stress is required. At this strain

level, the fastener should expand to fill a 10.5-mm diameter orifice in an interference fit. Typical expansion of a conventional rivet is only 1%, so the present invention offers the ability to fasten materials with an expanded rivet, with much larger dimensional-tolerances for the orifice in which the rivet is expanded.

Table 1 also includes the impact force, Fi, on the fastener, which is equal to the product of the impulse pressure and the transverse cross sectional area of the cavity.

Since the cavity diameter is relatively small, the impact force is two orders of magnitude smaller than the 100 kN force required for conventional rivet upset.

Table 1 Material #p(#p=10%), MPa P (K=10), Mpa Fi (dol mm), N 2024 Aluminum T3 420 966 760 7050 Aluminum 550 1265 992 Ti-6A1-4V 1034 2378 1864 A typical hydraulic pulse generator can produce a 300 MPa jet. The shock pressure for a fluid jet that is suddenly arrested in a rigid tube can be determined from the Rankine-Hugoniot equation, which relates initial particle velocity, uO, to impact shock pressure, p : where pO is the fluid density ahead of the shock, c, is the sound speed in the unshocked fluid and p is the fluid density behind the shock front. Solving this equation requires an equation of state for the fluid. The Tate equation of state for water is a useful approximation at high pressures: where a= 0. 13986.

The discharge velocity from a compressed-water-pulse generator is related to the discharge pressure, Pd, by Bernoulli's equation for a free jet discharge: A relatively small discharge pressure is capable of generating extremely high shock pressures. For example, a 300 MPa pulse generator can generate over 2 GPa water shock pressure, which is capable of easily expanding rivets made of any alloy listed in Table 1.

Ultra High-Pressure Pulse Generating Tool FIGURE 5 illustrates one preferred embodiment of apparatus usable to create ultra high-pressure fluid pulse 12. An ultra high-pressure fluid pulse generating tool 24 is shown positioned over hollow fastener 14, which is in turn positioned within orifice 22 formed within metal sheet 18 and metal sheet 20.

Tapered fluid channel 54a extends longitudinally through the center of the end of ultra high-pressure fluid pulse generating tool 24 and into tapered nozzle 10a, and a nozzle inlet 50 is in fluid communication with an outlet 48 of an ultra high- pressure fluid chamber 26. This ultra high-pressure fluid chamber is disposed internally within the ultra high-pressure fluid pulse generating tool. The diameters of outlet 48 and nozzle inlet 50 are identical. The diameter of fluid channel 54a is tapered to decrease toward nozzle outlet 46, thereby increasing the velocity of ultra high-pressure fluid pulse 12, and increasing the magnitude of the shock pressure created when ultra high-pressure fluid pulse 12 impacts the bottom 16 of cavity 15 in hollow fastener 14.

A relatively small diameter drain channel 44, which extends radially outward from above outlet 48, is preferably connected to a vacuum chamber (not shown), so that the fluid channel may be drained of fluid between successive pressure pulses. Draining fluid channel 54a between pulses is important to achieve consistent pulses of the desired magnitude. If fluid channel 54a is not drained, ultra high-pressure fluid pulse 12 will be reduced in magnitude.

A discharge poppet valve 30 is held closed by hydraulic fluid pressure applied to a poppet vent cavity 32. Discharge poppet valve 30 is seated on a valve seat 42 in a closed position of the discharge poppet valve, so that fluid from an ultra high-pressure fluid chamber 26 is blocked from entering fluid channel 54a.

Ultra high-pressure fluid chamber 26 is in fluid communication with a high- pressure source (not shown) via a radially outwardly extending inlet port 28.

When discharge poppet valve 30 is closed, the pressure source will continue to pump fluid into sealed ultra high-pressure fluid chamber 26, further pressurizing the working fluid. At ultra-high-pressures, generally considered to be greater than 100 MPa, water is slightly compressible. For example, water is compressed about 10% at a pressure of 300 MPa. Water is the preferred fluid for use in the present invention, but it is also contemplated that oil, an oil/water mixture, and other fluids may alternatively be used. The pressure of the fluid in ultra high-pressure chamber between pulses preferably exceeds 100 MPa.

Discharge poppet valve 30 has a greater diameter than valve seat 42, which permits the fluid pressure in ultra high-pressure fluid chamber 26 to rapidly

move discharge poppet valve 30 away from valve seat 42 when the hydraulic pressure applied to discharge poppet valve 30 at poppet vent cavity 32 is removed.

This type of fast opening valve arrangement is disclosed in detail in commonly held U. S. Patent No. 5,000,516, the disclosure and drawings of which are hereby specifically incorporated herein by reference. Use of a fast opening valve in an ultra high-pressure fluid pulse generating tool is critical to creating an ultra high- pressure fluid pulse.

Discharge poppet valve 30 is free to move axially inside a sleeve 34.

Sleeve 34 is connected to a vent poppet valve 36, which is coupled to a piston 40 that is driven to hold the vent poppet valve closed by hydraulic fluid pressure applied to the piston in a vent chamber 38. The hydraulic fluid in poppet vent cavity 32 is released when a solenoid-actuated valve (not shown) vents the hydraulic fluid pressure from vent chamber 38, allowing piston 40 to move vent poppet valve 36 to its open position. Once vent poppet valve 36 is open, discharge poppet valve 30 becomes unbalanced. Since the pressure in ultra high- pressure fluid chamber 26 is extremely high, discharge poppet valve 30 opens . rapidly. As discharge poppet valve 30 opens, ultra high-pressure fluid pulse 12 is released from ultra high-pressure fluid chamber 26 and advances down tapered fluid channel 54a into hollow fastener 14. This cycle is repeated to generate successive ultra high-pressure fluid pulses.

It is also contemplated that different valve configurations can be used to generate ultra high-pressure fluid pulses in accord with the present invention.

Any valve employed for this purpose must be capable of rapidly opening and closing. While FIGURE 5 illustrates nozzle outlet 46 as including cavity 47, ultra high-pressure fluid pulse generating tool 24 may be used for other purposes, including pulse bonding, clinch forming and cold working in which cavity 47 is not needed. For these other applications, the nozzle outlet is preferably formed to have a relatively flat end (see for example, a nozzle outlet 46a as shown in FIGURES 6A, 6B, 8A, 8B, 10A, 10B, 11-13,14A, 14B, 15,16A, and 16B).

It should be noted that an ultra high-pressure fluid pulse generating tool may be configured with a straight fluid channel (i. e., fluid channel 54) or with a tapered fluid channel (i. e., tapered fluid channel 54a) in the nozzle. A tapered fluid channel accelerates ultra high-pressure fluid pulse 12. The need to incorporate a tapered fluid channel in the nozzle depends on the shock pressure magnitude required for a particular application. The shock pressure magnitude required for a given application depends on the material that the components to which the shock pressure is being applied are comprised. In general, for example,

aluminum requires a lower shock pressure than titanium. The shock pressure magnitude is also dependent upon the position of the ultra high-pressure generating tool relative to the components being worked with the fluid pressure pulse. If the outlet of the nozzle substantially abuts the component being worked, the ultra high-pressure fluid pulse acts is confined in nozzle and cannot readily leak out at the interface with the component, which has the effect of maintaining the shock pressure compared to an ultra high-pressure fluid pulse that is applied to the component as a free jet, i. e., with the nozzle spaced apart from the component.

To determine whether a tapered fluid channel is required for a particular application, it is necessary to know: (1) the shock pressure required to work the particular material comprising a component that is to be worked; (2) the magnitude of the ultra high-pressure fluid pulse as it enters the fluid channel in the nozzle; and, (3) whether the nozzle outlet will abut the component to which the ultra high-pressure fluid pulse is being applied. In each of the embodiments discussed herein, for a specific fluid channel, an exemplary shock pressure is noted for successfully applying the invention to components of a specific material, such as aluminum.

Pulse Bonding Using Ultra High-pressure fluid pulses Another application of the present invention is directed to joining two materials using an ultra high-pressure fluid pulse to plastically deform the materials, thereby creating a bond at the interface between the materials.

FIGURES 6A and 6B illustrate an ultra high-pressure fluid pulse generating tool 24 being used to apply ultra high-pressure fluid pulse 12 to a metal sheet 18a and a metal sheet 20a, which are supported by a support structure 74. Ultra high- pressure fluid pulse tool 24 as depicted in FIGURES 6A and 6B, directs an ultra high-pressure fluid pulse 12 against the surface of a substantially flat work piece (as opposed to a hollow fastener as depicted in FIGURE 5). For this application, the ultra high-pressure generating tool includes a nozzle outlet 46a that seats substantially flush against metal sheet 18a. Any offset between the nozzle outlet and this surface should be substantially smaller than the nozzle outlet diameter.

Tapered nozzle 10a and tapered fluid channel 54a are depicted in FIGURES 6A and 6B to illustrate how the magnitude of the pressure pulse is amplified to impart sufficient shock pressure against metal sheets 18a and 20a to create a bond between the two components. The nozzle and fluid channel may alternatively be straight instead of being tapered, if the straight fluid path is capable of providing the required shock pressure. An analysis of the required shock pressure is provided below.

Note that the valve arrangement depicted in tool 24 is but one preferred embodiment of a fast opening valve that releases fluid from an ultra high-pressure chamber. high-pressure fluid pulse The method for pulse bonding two materials together requires that the pressure pulse generated by the ultra high-pressure fluid pulse generating tool have a stagnation pressure of at least 100 MPa so that the shock pressure created upon impact of pulse 54 against metal sheet 18a has a magnitude of at least 500 Mpa. This magnitude is sufficient to plastically deform the metallic materials to create a bond between them.

FIGURES 7A and 7B illustrate an embodiment of the pulse bonding method in which one of the surfaces of the materials to be bonded has been machined to improve the bonding process. As shown in FIGURE 7A, metal sheet 18a has a smooth lower surface in contact with a metal sheet 20b. In contrast, the surface of metal sheet 20b has small surface groove and channel features 76 formed therein. FIGURE 7B illustrates the inter-locking deformation of the surface groove and channel features 76 after the ultra high-pressure fluid pulse has been applied by ultra high-pressure fluid pulse generating tool 24. It will be apparent that providing surface groove and channel features 76 substantially enhances the mechanical interlock between metal sheets 18a and 20b that results from application of the ultra high-pressure fluid pulses to the surface of metal sheet 18a.

FIGURES 8A and 8B illustrate another embodiment in which a surface feature cavity 80 has been formed in the surface of a metal sheet 20c to further improve the mechanical interlock in the pulse bonding method. In these Figures, nozzle outlet 46a is shown abutted or seated substantially flush against metal sheet 18a. At the interface between metal sheet 18a and metal sheet 20c, metal sheet 18a has a smooth surface, while metal sheet 20c has surface feature cavity 80. FIGURE 8B illustrates the deformation and bonding of the metal sheets caused by ultra high-pressure fluid pulse 12. Material from metal sheet 18a has been forced into the surface feature cavity 80 of metal sheet 20c by ultra high- pressure fluid pulse 12, creating an enhanced mechanical interlock between the two sheets. In this manner a tack joint may be formed. If the surface feature formed on metal sheet 20c is a channel instead of a cavity, a seam is formed by application of successive pressure pulses, rather than a tack joint.

FIGURE 9 illustrates an embodiment in which a patch of an intermediate material 82 has been placed between metal sheet 18a and metal sheet 20a prior to application of the ultra high-pressure fluid pulse. Preferably, intermediate material 82 is a very hard material having a serrated surfaces. As metal sheet 18a

and metal sheet 20a are forced together by ultra high-pressure fluid pulse 12, intermediate material 82 will interpenetrate both metal sheet 18a and metal sheet 20a, thereby interlocking the two sheets. Alternately, intermediate material 82 may be a material, which undergoes substantial compression at the high level of stress induced by the ultra high-pressure fluid pulse. A ceramic or metal powder undergoing a high stress, high strain deformation will experience substantial heating, resulting in local softening and melting of the adjacent metal component surfaces. Local softening and melting of these surfaces will result in localized welding and enhanced mechanical interlock between the metal sheets.

The stress distribution beneath a uniform pressure, P, distributed over a circular area of radius a, can be determined by superimposing the Boussinesq solution for stress beneath a point load. In this case, the maximum shear stress has a magnitude of C = 0.33P at z = 0.6a.

Plastic yielding will initiate when the shear stress is about half the plastic flow strength, ap, which is taken as the average of the yield strength and the ultimate strength, and occurs when Psi = 1.5 up. According to the Tresca yield criterion, the boundary of the plastic strain region will extend from the surface to a depth of 2.5a at a mean pressure P, sp = S crp. At intermediate pressures, the plastic flow is localized beneath the surface. Values for these plastic flow limit pressures are listed in Table 2 for a range of aluminum alloys. The plastic flow limit pressures represent a range of pressures where plastic flow occurs beneath the surface, but should not extend to the surface. In a preferred embodiment of the invention, the diameter of the opening of nozzle outlet 46a is between one and two times the thickness of metal sheet 18a in order to ensure that deformation is maximized near the interface between sheet 18a and sheet 20a.

Table 2 Material (0. 2%) * MPa ma _vz, MPa Elongation% 2024 Aluminum-T3 414 621 2070 18 Annealed 131 196 655 22 5083 Aluminum-H116 272 408 1360 16 Annealed 217 326 1085 22 6061 Aluminum-T6 293 440 1465 15 Annealed 90 135 450 25 7050 Aluminum-T76 507 761 2535 8 7075 Aluminum-T6 534 801 2670 11 Annealed 165 248 825 17 The shock pressure for a fluid jet that is suddenly arrested in a rigid tube has been discussed in detail above. As noted previously, a relatively small

discharge pressure is capable of generating extremely high shock pressures. The shock pressure would have a duration related to the travel time of the shock wave, which is determined by the shock wave speed and nozzle length. The shock wave speed in water at this pressure is around 2000 m/s, which is relatively slow compared to the speed of sound in aluminum or other metals. A preferred nozzle design will have length of about 100 mm, resulting in a pulse duration of about 100 « us. This duration is still relatively long compared to the time required for sound to travel through the work piece. As long as the nozzle is longer than the stack of component being joined, the stress in the work piece should be essentially constant during the pulse.

Preferably a pulse generator operating at a pressures of 300 MPa will be used for joining components with this method. Such a device would generate over 2 GPa shock pressure pulse, which is capable of causing plastic shear in all of the aluminum alloys listed in Table 2. The presence of surface features on one of the components will cause localized stress risers leading to the deformation of the components at lower pressures. Since the shock pressures are high enough to cause plastic shear in intact material, these pressures will cause complete closure between the two components followed by additional plastic deformation.

Clinch Formation Using Ultra High Pressure Pulses Still another aspect of the present invention is directed to a method for joining two materials by using an ultra high-pressure fluid pulse to plastically deform the materials to create a clinch fastening. FIGURES 10A and 10B illustrate ultra high-pressure fluid pulse generating tool 24a being used to create a clinch-type fastening. The nozzle and fluid channel within the nozzle are not tapered in this exemplary embodiment of the tool, because the shock pressure employed for clinch fastening is sufficiently low that a tapered nozzle and tapered fluid channel will probably not be needed to increase the magnitude of the shock pressure produced by the fluid pulse. However, if desired, a tapered nozzle and tapered fluid channel may be used. Also, in this embodiment of the tool, nozzle outlet 46a abuts or seats flush against the surface of metal sheet 18a, or at a standoff distance above the surface that is much smaller than the diameter of the nozzle outlet.

In FIGURES 10A and 10B, metal sheet 18a and metal sheet 20a are shown disposed on a support structure 74a. In this embodiment, support structure 74a incorporates a die 100, which includes a cavity 102. Nozzle 10 of ultra high- pressure fluid pulse generating tool 24a is placed directly over cavity 102 with metal sheets 18a and 20a disposed between cavity 102 and nozzle 10. Ultra high-

pressure fluid pulse 12 advances down fluid channel 54 and impacts metal sheets 18a and 20a, producing a shock pressure that forces the sheets into cavity 102, as illustrated in FIGURE 10B.

FIGURES 11, 12, and 13 show the interaction between the shock pressure produced by ultra high-pressure fluid pulse 12, metal sheets 18a and 20a, and cavity 102. FIGURE 11 illustrates ultra high-pressure fluid pulse 12 advancing through fluid channel 54 in nozzle 10 towards metal sheets 18a and 20a.

FIGURE 12 shows the initial effect of ultra high-pressure fluid pulse 12 impacting metal sheets 18a and 20a. As shown, metal sheets 18a and 20a are being forced into cavity 102. Sheets 18a and 20a plastically deform until impacting a bottom 106 of cavity 102. FIGURE 13 illustrates the final configurations of metal sheets 18a and 20a following this plastic deformation. After metal sheet 20a impacts the bottom 106 of cavity 102, the material of metal sheet 20a is further thinned. Hydraulic pressure expands the bottom of the clinch until it fills the cavity. This expansion insures that the upper metal sheet 18a wraps underneath the lower metal sheet 20a, forming a mechanical interlock 108. The thickness of the upper material at the interlock area determines the shear strength of the joint.

In an alternate embodiment, cavity 102 may be formed directly into support structure 74a instead of in die 100.

The ultra high-pressure fluid pulse clinch forming process allows the formation of a significantly thicker shoulder interlock 108 and therefore, greater joint strength than a mechanical clinch fastening. The strain in the shoulder area is also much smaller than is produced with mechanical punch techniques, so that the technique can be applied to materials with lower ductility; particularly structural aluminum.

Preferably, the diameter of fluid channel 54 is smaller than the diameter of cavity 102 to ensure that the center of the clinch is thinned and forced downwardly, while the shoulders of the clinch remain relatively thick. A smaller nozzle outlet diameter also reduces the overall load on the work piece. A different nozzle having a different fluid channel diameter may be used for different clinch size fastenings and shapes, with the same ultra high-pressure fluid pulse generating tool 24a The cavity 102 may be circular or non circular. Non- circular or asymmetric clinch bonds provide greater resistance to joint rotation than circular clinch joints.

The stagnation pressure for a fluid jet, which is suddenly arrested in a rigid tube is determined by the Rankine Hugoniot shock equations discussed previously. The ultra high-pressure fluid pulse generator tool should be capable

of generating a dynamic pressure pulse with a stagnation pressure of about half the charge pressure. A preferable tool designed for operation at 300 MPa would therefore produce a 150 MPa pulse with a peak velocity of 548 m/s. The shock pressure spike produced by this tool will have an amplitude of about 1100 MPa.

The clinch forming fastening process involves the finite strain of a thick plate with complex boundary conditions. A first order estimate of the pressures required to form the clinch can be obtained by evaluating the surface pressure required to induce plastic flow in an infinite half space as discussed previously.

Values for limit pressures are listed in Table 2 for a range of aluminum alloys.

The limit pressures provide a rough upper limit of the pressure required to initiate forming of a stack of plates with a thickness of 1.2 to 5 times the pressurized circle diameter.

A comparison of the plastic flow pressures listed in Table 2 indicates that an 1100 MPa water-hammer pressure spike, produced by the preferable 300 MPa pulse generator would be sufficient to initiate plastic deformation of clinches in a stack of material with a thickness comparable to or greater than the clinch diameter. Once deformation initiates, the center of the clinch will thin, leading to accelerated deformation and clinch formation.

Cold Working Holes Using Ultra High-pressure fluid pulses In accord with the present invention, an ultra high-pressure fluid pulse can also be used for cold working orifices in a metal material by causing a plastic radial strain around the orifice, thus producing a residual compressive stress that increases the fatigue life of the orifice. FIGURES 14A and 14B illustrate ultra high-pressure fluid pulse generating tool 24a in use for cold working an orifice 22a in metal sheet 18. Note that in the illustrated embodiment, the nozzle and the fluid channel extending through the nozzle are not tapered, because the shock pressure required for cold working an orifice will probably not require that the magnitude of the shock pulse be increased. However, if desired (or necessary due to other parameters), a tapered nozzle and tapered fluid channel may be used.

Nozzle outlet 46a is abutted or seated substantially flush against metal sheet 18, or at a slight standoff.

In this application of an ultra high-pressure fluid pulse, metal sheet 18 is placed on a support structure 74b. Support structure 74b includes a blind hole 124. Metal sheet 18 has an orifice 22a and is positioned on support structure 74b so that the orifice is located directly over blind hole 124 in support structure 74b. Ultra high-pressure fluid pulse generating tool 24 is positioned so that nozzle 10 is located directly over orifice 22a and blind hole 124.

When discharge poppet valve 30 is in its closed position, fluid channel 54 is empty, but when discharge poppet valve 30 is lifted to its open position, ultra high-pressure fluid pulse 12 enters fluid channel 54 and advances down nozzle 10, passing through orifice 22a in metal sheet 18 and into blind hole 124. The sudden arrest of the ultra high-pressure fluid pulse at the bottom of blind hole 124 results in a shock pressure pulse that acts upon the orifice 22a, producing a plastic radial strain around the orifice.

The equations for determining the stagnation pressure for a fluid jet that is suddenly arrested have been discussed in detail above. Ultra high-pressure fluid pulse generating tool 24a is able to create a 300 MPa pulse. The velocity of the pulse is preferably at least 775 m/s, and the shock pressure spike will have an amplitude of about 1100 MPa. The maximum tensile load around an internally pressurized orifice in an infinite solid is equal to the internal pressure. The yield strength for a range of metal alloys is listed below in Table 3. A shock pressure of 1100 MPa will cause permanent plastic strain in the region surrounding an orifice in all of the alloys listed in Table 3, resulting in improved fatigue life for the orifice.

Table 3 Material dozy (02%), MPa 2024 Aluminum-T3 345 5083 Aluminum-H116 228 6061 Aluminum-T6 276 7050 Aluminum 483 7075 Aluminum-T6 503 A569 Mild Steel Sheet 310 A715 Grade 70, High Strength Steel Sheet 550 FIGURE 15 illustrates an embodiment in which an ultra high-pressure fluid pulse may be used to punch out a slug 126 to both form an orifice 128 in metal sheet 18a, as well as cold working the orifice in a single step. As shown, nozzle 10 and fluid channel 54 are used instead of the tapered embodiment, since the shock pressure (about 1100 Mpa) employed for forming and cold working an orifice should not be sufficiently great to require the use of a tapered nozzle and tapered fluid channel to increase the magnitude of the shock pulse. However, a tapered nozzle and tapered fluid channel may be used. This procedure is also

implemented by abutting or seating nozzle outlet 46a substantially flush against metal sheet 18a, or at a slight standoff.

Metal sheet 18a is positioned over support structure 74c, which incorporates blind hole 124a. Ultra high-pressure fluid pulse 12 is used to punch slug 126 out of the metal sheet, thereby forming orifice 128 in the sheet. Ultra high-pressure fluid pulse 12 continues to advance metal slug 126 until metal slug 126 impacts the bottom of blind hole 124a. Blind hole 124a as shown extends through support structure 74c. A moveable block 125 is shown at the bottom of blind hole 124 and can be shifted away from the blind hole to allow slug 126 to be removed. It is contemplated that other methods, such as a spring device at the bottom of blind hole 124a, may also be used to remove slug 126.

The arrest of ultra high-pressure fluid pulse 12 when the metal slug impacts the bottom of the blind hole produces a shock pressure pulse that imparts a compressive radial strain on the orifice formed in metal sheet 18a.

The hydraulic pressure required to punch an orifice in a sheet is simply related to the material yield strength, as indicated by: P-2tay/d h (5) where dh is the orifice diameter and t is the sheet thickness. The pressure required to punch a 0.5 inch diameter orifice in 0.1 inch thick mild steel sheet is about 124 MPa. The pressure required to form the orifice is substantially less than the jet stagnation pressure.

Cold Working Metal Surfaces Using Ultra High-Pressure Fluid Pulses Still another aspect of the present invention duplicates the cold working effect that shot peening has on metal surfaces. The advantages of the surface impact treatment of metal parts by shot peening is well established. Shot peening occurs when steel or glass balls impact the surface at speeds that are sufficient to cause yielding in a surface layer of the material comprising the surface. The yielded metal is constrained by elasticity of the underlying material, so that a compressive residual stress with a magnitude approximately equal to the plastic flow stress occurs in the surface material. The distribution of stress beneath a stagnating jet has been discussed in detail previously. Table 4 shows the hydraulic pressure Pp required to induce deformation from the surface to a depth of twice the pressurized area diameter in a variety of metal alloys. The table provides a range of pressures that should induce plastic flow at a depth equal to approximately twice the pressurized area. The pressures listed in Table 4 for plastic flow of metal beneath a stagnating jet range from 1140 to 2750 MPa. These pressures are much higher than

the stagnation pressure generated by conventional ultra-high-pressure pumps, which operate below 400 MPa. It is not feasible to generate a continuous water jet at pressures over 1 GPa, because of limitations of the materials from which pumps are fabricated, however, these pressures can be produced using an accelerated pulsed jet approach, as described above.

Table 4 Material (rp, Mpa Psp MPa 2024 Aluminum-T3 345 1725 5083 Aluminum-H116 228 1140 6061 Aluminum-T6 276 1380 7050 Aluminum 483 2415 7075 Aluminum-T6 503 2515 A569 Mild Steel Sheet 310 1550 A715 Grade 70, High 550 2750 Strength Steel Sheet

The high-pressures required can be generated by abutting the discharge orifice of an ultra high-pressure fluid pulse generator against the component as discussed previously. This configuration may not be convenient when the component has a complex shape or adjacent to corners. The pulse may also be accelerated to produce a free jet with a stagnation pressure in excess of 1 GPa as discussed below.

The stagnation pressure of a free jet directed against a flat surface is: P. = = 2 PjVj (6) where v is the jet velocity and p is the density. Fluid jet pulses may be accelerated by forcing the pulses through a tapered nozzle, as noted above. When a high speed packet of liquid is extruded into a tapered nozzle, the leading edge of the pulse accelerates. This process has been used to generate hypervelocity jet pulses with stagnation pressures in excess of 4.5 GPa.

FIGURES 16A and 16B illustrate how an ultra high-pressure fluid pulse 12 can be accelerated by forcing the pulse through a tapered nozzle 10a that defines a tapered fluid channel 54a. The tapered fluid channel includes nozzle inlet 50 and nozzle outlet 46a. Due to the tapered configuration of the fluid

channel, nozzle inlet 50 has a larger diameter than nozzle outlet 46a. As fluid packet 12 advances through the nozzle, the leading edge of the pulse accelerates.

An analysis of the unsteady incompressible flow of a packet of water entering a tapered nozzle shows that the internal pressure developed inside an exponential or hyperbolic nozzle is much lower than the dynamic pressure of the jets. The relevant nozzle shape parameters are the area contraction ratio R and the ratio α of the nozzle length to the packet length: R=Ao/A and α=L/l (7) A, I where Ao is the entrance area,/1 is the exit area, L is the nozzle length, and 1 is the fluid packet length. The dynamic pressure amplification in an exponential nozzle is: <BR> <BR> <BR> <BR> P, Rln (R)<BR> <BR> <BR> Po « (8) while the peak internal pressure is only 25% of the exit dynamic pressure. (Even lower internal pressures can be obtained using hyperbolic nozzle designs).

Preferably, this method discharges ultra high-pressure fluid pulses through a tapered nozzle to generate free jet stagnation pressures that are high enough to cause plastic flow in a metal component, even when the nozzle outlet is located at a substantial standoff distance apart or away from the surface of the component.

Generating a free jet stagnation pressure of 2750 MPa, which is sufficient to deform a high strength steel sheet, requires a tapered nozzle area ratio of 5.5 (assuming P"= 300 MPa and a = 1). In order to produce a 0.5 mm diameter jet, the pulse generator must produce a 1.2 mm diameter pulse. The tapered nozzle is preferably designed to leave sufficient energy in the trailing edge of the fluid packet to clear the nozzle before the next pulse occurs.

Preferably the energy of the pulse matches or exceeds the kinetic energy of shot used for peening. The kinetic energy of a 1 mm diameter steel shot moving at 50 m/s is only 0.01 J. A 300 MPa jet pulse has a velocity of 775 m/s. The kinetic energy of a 1 mm diameter, 1 mm long cylindrical droplet of water moving at this velocity would be about 0.16 J or 16 times greater. The internal nozzle pressure of the ultra high-pressure fluid pulse generator would be about

680 MPa. At this pressure, nozzle erosion may be a concern. Experimental nozzles may be constructed from high strength, corrosion resistant steel. For production applications, it would be preferable to use pre-stressed carbide or diamond nozzles.

Although the present invention has been described in connection with several preferred forms of practicing it, those of ordinary skill in the art will understand that many other modifications can be made thereto within the scope of the claims that follow. Accordingly, it is not intended that the scope of the invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.