Joining apparatus and method

Abstract

A joint is produced in at least two overlapping workpieces using a joining tool including a punch reciprocally disposed in a cylinder. The workpieces are disposed between the tool and a die. The tool applies a compressive force to deform the workpieces into a joint at a joining area between the tool and die. Continuous ultrasonic energy is applied to at least one of the workpieces in the joining area for at least part of the time during production of the joint to increase the ductility of at least one of the workpieces in the joining area. This induces an acousto-plastic effect in the material being formed which temporarily reduces its strength so as to exhibit increased ductility. The joining method may be used in self-piercing riveting and clinching techniques and enables viable joints to be formed in high strength materials or thick sheets of material.

Claims

1. A method for producing a joint in at least two overlapping workpieces using a joining tool including a punch to insert a self-piercing rivet into the workpieces to form the joint, the workpieces having a first surface that is nearest the tool, the method comprising: placing the workpieces between the tool and a die, positioning a rivet between the punch and the first surface, using the punch to insert the rivet into the at least two overlapping workpieces such that the first surface is pierced by the rivet, the punch applying a compressive force during rivet insertion to deform the workpieces into a joint at a joining area between the tool and the die, and applying continuous ultrasonic energy to at least one of the workpieces in the joining area around a perimeter of the rivet for at least part of the time during production of the joint so as to increase the ductility of at least one of the workpieces in the joining area due to the acousto-plastic effect, wherein the ultrasonic energy is applied by the die, or by a nose of the tool.

2. A method according to claim 1, wherein the at least two overlapping workpieces comprise a first workpiece that is nearest to the tool and a second workpiece that is nearest to the die, the rivet being inserted such that it does not penetrate through the second workpiece.

3. A method according to claim 2, wherein one or more intermediate workpieces are provided between the first workpiece and the second workpiece.

4. A method according to claim 1, wherein the die forms at least part of an ultrasonic horn.

5. A method according to claim 1, wherein the ultrasonic energy is applied directly to at least one of the workpieces.

6. A method according to claim 1, wherein the energy level imparted to the workpieces varies during the formation of the joint.

7. A method according to claim 1, wherein the ultrasonic energy is applied to only one of the workpieces.

8. A method according to claim 7, wherein the ultrasonic energy is applied to only the one workpiece nearest to the tool.

9. A method according to claim 7, wherein the ultrasonic energy is applied to only the one workpiece that is supported over the die.

10. A method according to claim 1, wherein the ultrasonic energy is applied in a direction that is substantially the same as the direction of the compressive force that deforms the workpieces.

11. A method according to claim 1, wherein the workpieces are clamped between the tool and the die by a clamping force in a region around the joining area for at least part of the time it takes to produce the joint.

12. A method according to claim 1, wherein substantially no clamping force is applied whilst the material is deformed by application of the compressive force and a clamping force is applied after production of the joint.

13. A method according to claim 12, wherein the clamping force applied after production of the joint is of such a magnitude that it reduces deformation in at least a first surface of the workpieces.

14. A method according to claim 1, wherein at least one of the workpieces is metal.

15. A method according to claim 1, wherein the ultrasonic energy is an acoustic vibration signal that is transmitted to the workpieces.

16. A method according to claim 15, wherein the acoustic vibration signal has a frequency in the range of 18 to 60 kHz.

17. A method according to claim 15, wherein the acoustic vibration signal has an amplitude of 2-50 m.

18. A joining apparatus for producing a riveted joint in at least two overlapping workpieces, the joining apparatus comprising: a joining tool having a reciprocal punch disposed over a die over which the workpieces can be supported, the punch being movable towards the die for inserting a rivet into the workpieces and applying a compressive force to the workpieces in a joining area, at least one of a nose of the joining tool and the die being coupled to a source of ultrasonic energy so that it can apply ultrasonic energy to at least one of the workpieces in the joining area around a perimeter of the rivet for at least part of the time during production of the joint so as to increase the ductility of at least one of the workpieces in the joining area due to the acousto-plastic effect.

19. A joining apparatus according to claim 18, wherein the ultrasonic source is coupled to at least one of the nose of the joining tool and the die via an ultrasonic horn.

20. A joining apparatus according to claim 19, wherein at least part of the die is provided by at least part of the ultrasonic horn.

21. A joining apparatus according to claim 20, wherein the ultrasonic horn has a first surface that faces the workpiece and has a die cavity defined therein.

22. A joining apparatus according to claim 18, wherein the die is biased towards the punch by a biasing member.

23. A joining apparatus according to claim 18, wherein the nose is configured for contacting workpieces, and the punch is reciprocally disposed such that it is movable between an extended position in which it extends from the nose and a retracted position in which it does not extend from the nose, and wherein the ultrasonic source is coupled to the nose.

24. A joining apparatus according to claim 23, wherein the nose is operable to apply a clamping force to the workpieces.

25. A joining apparatus according to claim 18, wherein the ultrasonic source is coupled to at least one of the nose of the joining tool and the die such that the ultrasonic energy is applied to the joining area in a direction that is substantially the same as the direction of the compressive force applied by the punch to deform the workpieces.

26. A method for producing a joint in at least two overlapping workpieces using a joining tool including a punch to insert a self-piercing rivet into the workpieces to form the joint, the workpieces having a first surface that is nearest the tool, the method comprising: placing the workpieces between the tool and a die; positioning a rivet between the punch and the first surface; using the punch to insert the rivet into the at least two overlapping workpieces such that the first surface is pierced by the rivet, the punch applying a compressive force during rivet insertion to deform the workpieces into a joint at a joining area between the tool and the die; and applying continuous ultrasonic energy to at least one of the workpieces in the joining area around a perimeter of the rivet for at least part of the time during production of the joint so as to increase the ductility of at least one of the workpieces in the joining area, wherein the ultrasonic energy is not applied by the punch.

27. The method of claim 26, wherein continuous ultrasonic energy is not applied to the rivet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

(2) FIG. 1 is a side view of joining apparatus including a rivet setting tool supported over a die in a C-frame, in accordance with the present invention;

(3) FIG. 2 is a schematic representation of part of the rivet setting tool in contact with two workpieces supported over a die and in which ultrasonic energy is applied via a die in accordance with the present invention; and

(4) FIG. 3 is a schematic representation of part of an alternative embodiment of the invention in which ultrasonic energy is applied directly to the workpieces.

DETAILED DESCRIPTION

(5) Referring now to FIGS. 1 and 2 of the drawings, a rivet setting tool 10 is mounted on an upper arm of conventional C-frame 11 above a rivet-upsetting die 12 supported in the lower arm. Self-piercing rivets R (one shown in FIG. 2 only) are inserted by the tool 10 into workpieces W (FIG. 2) supported over the die 12 as is well known in the art. It is to be appreciated that whilst the specific embodiment described herein relates to the feeding and insertion of rivets it has application to other fasteners that are inserted into workpieces using a die such as, for example, slugs.

(6) The C-frame 11 is mounted on a robot manipulator (not shown) such that it is movable with the tool 10 by the robot towards and away from the workpieces W as required. Feed apparatus (not shown) alongside the C-frame 11 is designed to supply rivets R to the setting tool 10 in a predetermined and controllable fashion from a bulk source. This may be achieved, by for example, using tape feed or a compressed gas (e.g. air) delivery system that propels the rivets along a tube or track. The control of the setting tool and the feed apparatus is normally via software. Such rivet supply systems and control systems are well known and will not therefore be described herein.

(7) The setting tool 10 is of conventional configuration and so is not described in detail. In general terms, it comprises a cylindrical housing 15 that houses a reciprocal plunger that is driven in translation relative to the housing by a hydraulic, pneumatic or electric drive. The housing has an end nose portion 14 with an annular surface for contact with the workpieces W in which the joint is to be formed and the plunger terminates in a punch 16 that reciprocates in a passage 17 extending through the nose. In order to insert a rivet R into the workpieces W the plunger is driven so that the punch descends in the passage 17 extends beyond the nose 14 and comes into contact with a rivet R that has been fed to end of the passage 17 in the nose 14. Continued application of the force drives the punch 16 through the nose 14 so that the rivet R is inserted into the workpieces W.

(8) In FIG. 2 the workpieces W are in the form of two sheet metal panels W.sub.1, W.sub.2 that lie one on top of the other over the die 12.

(9) The tool 10 is operable such that the nose 14 moves downwards from the position shown in FIG. 1 to engage and optionally clamp the workpieces W.sub.1, W.sub.2 together as shown in FIG. 2.

(10) The die 12 is generally cylindrical with a head 20 defining an open die cavity 21 for facing the setting tool punch 16 and a depending stem 22 that is of reduced diameter compared to the head such that an annular surface 23 extending radially relative to the central axis of the die 12 is defined on the underside of the head 20.

(11) One specific embodiment of the die is shown in detail in FIG. 2. In this particular arrangement, the die 12 is connected to an ultrasonic horn 25 (also referred to as a sonotrode). The horn 25 is coupled to a piezo-electric transducer (not shown) that is configured to vibrate at a suitable ultrasonic frequency such as, for example, 20 kHz. This acoustic vibration is transmitted from the transducer to the horn 25 by a booster (not shown) that is interposed between them and is configured to amplify the amplitude of the vibrations by a predetermined gain factor. In this manner acoustic vibrations (represented by arrow A) propagate along the length of the die stem 22 to the surface of the die head 20 so that they are imparted to the lowermost workpiece W.sub.2 by virtue of acoustic coupling between the die surface and the workpiece W.sub.2, the vibration propagating in substantially the same direction as the compressive force (represented by arrow F) applied by the punch 16.

(12) In operation the workpieces, which are represented in FIG. 2 by two sheets W.sub.1, W.sub.2 of overlying high strength steel, are introduced between the die 12 and the tool 10 such that they are supported over the die head 20 with a lower surface 30 of the lower sheet W.sub.2 in contact with the surface of the die head 20. The setting tool 10 is then operated so that the nose 14 descends and is brought into contact with an upper surface 31 of the upper sheet W.sub.1. The nose 14 applies a significant clamping force to compress the sheet material between it and the die 12. The force may be of such a magnitude that flow of material during the rivet insertion operation is affected. The longitudinal ultrasonic acoustic vibrations are transmitted to the lower sheet W.sub.2 in the same general direction as the compressive force F applied by the punch and die combination and serve to increase the ductility of that sheet by virtue of the acousto-plastic effect. The setting tool 10 is then operated to drive the punch 16 downwards to insert the rivet R into the workpieces W as described above. As the rivet R is inserted, the plastic form of the lower sheet W.sub.2 in the region of the joint enables the rivet shank to penetrate into the material and flare outwards sufficiently to provide a strong mechanical interlock without failure or cracking. It is inserted without full penetration of the lower sheet W.sub.2 such that the deformed end of the rivet R remains encapsulated by an upset annulus of the material. The softening of the material caused by the acousto-plastic effect ceases almost immediately after the transducer 26 is turned off and the original properties of the sheet material return.

(13) In the above described arrangement the ultrasonic acoustic energy is transmitted to the lower sheet W.sub.2 by the horn 25 both before the insertion of the rivet R commences and during its insertion. In practice, the ultrasonic acoustic energy may be applied by the transducer at any appropriate stage during the rivet insertion process as determined by the control system. For example, the energy may be applied before the rivet R comes into contact with the upper sheet W.sub.1, during contact, or at any stage between the rivet R starting to penetrate into the top sheet W.sub.1 and it piercing through the top sheet W.sub.1 so as to come into contact with the lower sheet W.sub.2. It may be also applied for a short time after the rivet R has been fully inserted.

(14) The ultrasonic acoustic energy may be applied at discrete time intervals during the rivet insertion for a given joint by turning the transducer on and off according to instructions programmed into the control system or according to feedback signals received during the insertion as a result of process monitoring. Moreover, the amplitude and/or frequency may vary during the insertion of a given rivet or may vary from joint to joint depending on its characteristics and the rivet type.

(15) It will be understood that any magnitude of clamping force may be used. For example it may be sufficient to apply a force that is designed only to hold the sheets steady during the riveting operation. Alternatively the force could be much more significant than this in that it could affect the flow of sheet material in and around the joint during the rivet insertion process. The magnitude of the clamping force may vary during the rivet insertion operation in any suitable manner and in accordance with a program being followed by the control system. For example, the clamping force may be relatively large before and during rivet insertion or may be almost negligible for a predetermined part of the rivet insertion process before increasing in magnitude significantly. Some examples of force clamping profiles are described in our patents U.S. Pat. No. 6,742,235 and EP 0675774, the content of which are hereby incorporated by reference.

(16) The die may be formed in its entirety by the ultrasonic horn or it may be defined only at the end of the horn. Moreover, it may simply be provided by a cavity defined in an end surface of the horn.

(17) It will be appreciated that other ultrasonic frequencies may be used as opposed to 20 kHz. Any frequency in the range 18 kHz to 60 kHz is thought to be suitable, but preferably the frequency is in the range 20 to 40 kHz. The amplitude of the acoustic vibrations emitted by the piezo-electric transducer may typically be in the region of 8-10 m but other amplitudes may be possible. The gain factor applied by the booster may be typically in the range 1.5 to 5 but other magnitudes are possible.

(18) In an alternative embodiment a cross-coupled booster is used so its input and output vibrations are out of alignment i.e. they may be disposed such that the transducer vibrations propagate in a direction that is 90 degrees to those propagating in the horn. In this arrangement the transducer and booster may lie along the lower arm of the C-frame 11 whilst the horn 25 extends upwardly towards the joint forming area as before.

(19) In a further alternative embodiment ultrasonic acoustic energy may be applied to the upper sheet W.sub.1 at the same time as it is applied to the lower sheet W.sub.2 by the die 12. For this purpose a second ultrasonic horn is fitted in the nose 14 and/or the punch 16 so that ultrasonic energy may be imparted to the upper sheet W.sub.1 at least. In the case of the nose 14, the energy is imparted into the region around the rivet. In the case of the punch the energy is transmitted through the rivet. In a yet further arrangement, only a single transducer is provided on the tool side so that the energy may be transmitted only via the nose and/or punch to the upper sheet W.sub.1.

(20) By suitable control of the frequency and ultrasonic amplitude under load using an ultrasonic generator with resonance tracking capability the acoustic energy may be transmitted to all sheets W in the stack either by applying the ultrasonic energy at the die, via the nose or via the punch and rivet.

(21) In FIG. 3, there is illustrated an alternative arrangement in which ultrasonic horns 25 are arranged to make direct contact with the upper sheet W.sub.1 and lower sheet W.sub.2 so that ultrasonic energy is imparted independently of the nose/punch or die. The horns in this instance are depicted as annular such that they surround the joining area but may take any suitable form. It is to be appreciated that only one of the horns may be used in practice.

(22) The above-described methods may be applied to clinching operations where the workpiece is deformed out of its plane into a die directly by a punch without using a rivet. Once the sheets are deformed into the button joint a rivet or slug may be inserted to strengthen the joint. Examples of clinching techniques to which the present invention is applicable are described in our patents stemming from WO93/10925, the content of which is incorporated herein by reference.

(23) The methods described above allow SPR and clinching to be used to form joints in higher strength materials (e.g. Ultra High Strength Steels or austenitic stainless steel 301, 316) or thicker materials than is currently possible. Joints can be made with lower compression (insertion) forces applied by the punch. This has several advantages, in particular C-frames can be made lighter thereby improving accessibility and cost. The rivet setting and clinching tools may be lighter and smaller, thereby reducing the size and cost of robot handling equipment. This in turn allows higher speed robot movements so that cycle times are reduced. Lower hardness rivets can be used and the improved ductility of the rivet reduces the frequency of failure by tearing, cracking or other fractures.

(24) Numerous modifications and variations to the embodiment described above may be made without departing from the scope of the invention as defined in the appended claims. For example, the source of ultrasonic energy may be provided by any suitable transducer besides a piezo-electric transducer such as, for example, a magnetostrictive transducer. Furthermore, the die may take any suitable form suitable for reacting the insertion forces, including a flat surface.

(25) The described and illustrated embodiments are to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the scope of the inventions as defined in the claims are desired to be protected. It should be understood that while the use of words such as preferable, preferably, preferred or more preferred in the description suggest that a feature so described may be desirable, it may nevertheless not be necessary and embodiments lacking such a feature may be contemplated as within the scope of the invention as defined in the appended claims. In relation to the claims, it is intended that when words such as a, an, at least one, or at least one portion are used to preface a feature there is no intention to limit the claim to only one such feature unless specifically stated to the contrary in the claim. When the language at least a portion and/or a portion is used the item can include a portion and/or the entire item unless specifically stated to the contrary.