Method for attaching a metal ring in a frame and induction coil obtained by said method

Abstract

A method for attaching a metal ring (4) in a bore (3) of a frame (2) made of a different metal, wherein: the ring (4) is set in the bore (3) by way of high-energy forming suitable for urging an outer surface of the ring against the bore (3) with a speed capable of driving out any impurity from an interface between the ring and the frame; the interface between the ring (4) and the frame (2) is heated to a temperature and for a length of time that are determined in accordance with the respective materials of the ring and the frame such as to obtain diffusion welding between the outer surface of the ring and the bore. A single-turn coil for magnetic forming carried out by the method is also described.

Claims

1. Method applied to the production of an induction coil for electromagnetic forming for attaching a metal ring (4) in a frame (2) made of a different metal, wherein: a bore (3) is produced at a distance from the edges of the frame (2), which bore passes through the thickness of said frame and has a predetermined internal diameter; a metal ring (4) having an external diameter which corresponds to the internal diameter of the bore and having a width which is less than or equal to the thickness of the frame (2) is inserted into the bore (3); the metal ring (4) is set in the bore by means of high-energy forming suitable for pressing an outer surface of the ring against the bore (3) at a deformation speed of greater than 15 m/s, which is capable of driving out any impurities from an interface between the ring and the frame; the interface between the metal ring (4) and the frame (2) is heated to a temperature and for a length of time that are determined in accordance with the respective materials of the metal ring and the frame so as to obtain diffusion welding between the outer surface of the metal ring and the bore, and the metal ring has an electric conductivity of greater than 30.10.sup.6 S.Math.m.sup.1 and the frame is made of rigid material which has a Young's modulus of greater than 200 GPa.

2. Method according to claim 1, wherein the high-energy forming is selected from the processes of electromagnetic forming, electrohydraulic forming or explosive forming.

3. Method according to claim 1, wherein the interface between the ring (4) and the frame (2) is heated by induction.

4. Method according to claim 1, wherein a copper ring having a thickness of between 2 and 10 mm and preferably between 2 and 5 mm is used.

5. Method according to claim 1, wherein a steel frame is used.

6. Method according to claim 1, wherein a radial slot (10) is cut out which passes through the ring (4) and the frame (2) so as to define an electrical path which goes around the ring from one side of the slot to the other.

7. Single-turn induction coil (1), in particular for electromagnetic forming, comprising: a rigid frame (2) made of a material which has a Young's modulus of greater than 200 GPa, said frame comprising a central bore (3); a turn (5) made of conductive material having a conductivity of greater than 30.10.sup.6 S.Math.m.sup.1 which is set and diffusion-welded inside said central bore; a radial slot (10) which passes through the frame (2) and defines the ends of the turn (5).

8. Coil according to claim 7, wherein the frame comprises, on each side of the slot (10), an attachment foot (6) which is electrically connected to the frame and is suitable for being connected to a terminal of a current pulse generator.

9. Coil according to claim 8, wherein each attachment foot (6) is respectively welded (12) to a portion of the frame, on either side of the slot (10).

10. Method according to claim 2, wherein the interface between the ring (4) and the frame (2) is heated by induction.

11. Method according to claim 2, which is applied to the production of an induction coil for electromagnetic forming, wherein a metal ring which has an electric conductivity of greater than 30.106 S.Math.m1 and a frame made of rigid material which has a Young's modulus of greater than 200 GPa are used.

12. Method according to claim 3, which is applied to the production of an induction coil for electromagnetic forming, wherein a metal ring which has an electric conductivity of greater than 30.106 S.Math.m1 and a frame made of rigid material which has a Young's modulus of greater than 200 GPa are used.

13. Method according to claim 4, wherein a steel frame is used.

14. Method according to claim 4, wherein a radial slot (10) is cut out which passes through the ring (4) and the frame (2) so as to define an electrical path which goes around the ring from one side of the slot to the other.

15. Method according to claim 5, wherein a radial slot (10) is cut out which passes through the ring (4) and the frame (2) so as to define an electrical path which goes around the ring from one side of the slot to the other.

Description

(1) Other aims, features and advantages of the invention will become apparent upon reading the following description and the accompanying drawings, in which:

(2) FIG. 1 shows the steps of the method according to the invention;

(3) FIG. 2 is a perspective view of an electromagnetic forming coil which is obtained by the method according to the invention; and

(4) FIG. 3 is a sectional view of an electromagnetic forming coil which is obtained by the method according to the invention.

(5) In the following description, reference is made to FIG. 1 to describe the steps of the method and to FIGS. 2 and 3 to identify the parts.

(6) In step 101 (FIG. 1), a metal frame 2 is prepared, which is preferably made of steel, which in the example shown in FIGS. 2 and 3 is a steel plate having a height of approximately 150 mm, a length of approximately 200 mm and a thickness of approximately 20 mm. Of course, the frame 2 does not necessarily have a rectangular shape but can also assume a circular, semi-circular or other shape. In this step, a bore 3 is produced having an axis which is orthogonal to the main face of the frame 2, substantially in the centre of the plate. The sizes of the frame 2 and the bore 3 are selected in accordance with the size of the ring which is to be installed in the frame to subsequently form an induction coil. In the example shown, the bore produced has a diameter of approximately 60 mm.

(7) In parallel with the preparation of the frame, in step 102, a cylindrical ring 4 made of copper or another material having a high conductivity, for example of greater than 30.10.sup.6 S.Math.m.sup.1, such as gold, silver, aluminium, beryllium etc., is prepared. The ring 4 has an external diameter which corresponds to the diameter of the bore 3, providing a positive clearance, for example of a few tenths or hundredths of a millimeter, making it possible to insert the ring in the bore without having to use a press or other tools which may negatively affect the surface condition of the bore or of the outer face of the ring. The ring 4 has a width which is of the same order of magnitude as the thickness of the frame 2. It is possible to provide for the width of the ring to be less than the thickness of the frame so as to provide a recess between the side of the ring and the edge of the bore, for example to centre any tools subsequently or else to define a forming region which is narrower than the thickness of the frame.

(8) The ring 4 has a wall thickness of approximately 2 to 10 mm and preferably of 2 to 5 mm, depending on the intensity of the electric currents which are likely to pass therethrough, as will be seen below.

(9) In step 103, the ring 4 is inserted into the bore 3. At this stage, the ring 4 is held in the bore by friction. The frame and the ring are then mounted in a high-energy forming apparatus, such as an apparatus for electromagnetic forming by expansion, by mounting an electromagnetic forming coil inside the ring 4 in order to set the ring in the bore. A current pulse is then applied to this coil, the properties of which pulse are established in accordance with the nature of the materials and the sizes of the parts.

(10) By way of example, in order to set a copper ring having a thickness of 5 mm and an external diameter of 60 mm in a bore of 60 mm which is made in a steel frame, a coil is used which is powered by an electric pulse of 20 kV, which develops an energy of approximately 40 kJ for a duration of approximately 50 s. In this case, the copper ring deforms at a speed of up to 200 m/s and develops a pressure of approximately 40 MPa at the interface between the ring and the bore.

(11) Alternatively, other high-energy forming techniques can be used such as electrohydraulic forming, in which the frame and the ring are arranged in a tank which is filled with water, and a very high-energy electric discharge between electrodes placed in the vicinity of the centre of the ring generates a shock wave which extends radially and presses the outer face of the ring against the wall of the bore. Likewise, the technique of explosive forming can also be used.

(12) In any case, during the forming, the ring 4 is deformed radially, and the outer face thereof is pressed against the wall of the bore at a speed which is greater than or equal to 15 m/s. At this speed, the air remaining between the ring and the bore is driven out in the form of a jet, carrying away therewith any impurities or contaminants (pollution, oxides, etc.) which would have been present between the two surfaces. Furthermore, the two opposing surfaces of the ring and the frame undergo intense plastic deformation, which creates a series of micro-ripples on the contact surface, thus allowing close contact between the two surfaces.

(13) In step 104, diffusion welding is carried out between the outer face of the ring and the inner wall of the bore. Due to the close contact and the residual compressive stress between the outer face of the ring and the inner wall of the bore obtained during the previous step, an initial interface was produced between the two materials of the ring and of the frame, which is suitable for producing a weld by atomic diffusion of one of the two materials in the other. For this purpose, this interface is heated to a high temperature, of approximately 50% to 90% of the melting point of the materials present. For example, in the case of a copper ring in a steel frame, the aim is a temperature of approximately 800 C. in the region of the interface for a duration of a few tens of seconds to a few minutes.

(14) To do this, induction heating of the interface between the copper and the steel is provided by introducing inside the ring 4 an electromagnetic coil which is suitable for generating a variable electromagnetic field. The eddy currents generated in the metal of the ring and more particularly in the frame in the vicinity of the bore create an increase in temperature in the region of the joint, allowing the diffusion of the atoms from the ring into the bore and vice versa. The parameters of the operating conditions of this step can be checked and adjusted by observing the diffusion depth, which must be of approximately a few microns.

(15) At the end of step 104, a ring is thus obtained which is doubly attached by setting and welding in the frame.

(16) However, in the particular case of producing a coil for electromagnetic forming, a current path should be produced inside the coil and, for this purpose, the ring attached in the frame should be transformed into a turn 5 of the coil.

(17) For this purpose, in step 105, a radial slot 10 is produced, for example by sawing, from the inside of the ring 4 and passing completely through the frame 2 so as to create an electrical discontinuity on either side of the slot. As a result, an electric current entering through one side of the slot 10 and leaving through the other side must necessarily follow a path which goes around the bore and the ring 4 which thus forms a turn 5. In the above-mentioned example, for a ring having an external diameter of 60 mm, a slot is provided which has a width of approximately 2 mm.

(18) It should be noted that the production of such a slot 10 is possible only as a result of the diffusion welding carried out in step 104, which in turn is made possible by the setting in step 103. Without this welding, the imbalance introduced by the slot 10 in the radial stresses which are exerted between the ring 4 and the bore could lead to a disconnection of the ring and the bore. Such a disconnection (even a partial disconnection) would then generate one or more gaps between the frame 2 and the turn 5, resulting in a loss of efficiency and reliability of the coil.

(19) In order to complete the production of the electromagnetic forming coil 1, in step 106, a series of machining processes are carried out, for example a counterbore 11 in the frame 2, which is concentric with the ring 4 and with the bore 3, as well as two attachment holes 9 which make it possible to centre and attach a tool for guiding parts to be shaped in relation to the coil 1.

(20) Likewise, in step 107, feet 6 are attached to the frame 2 on either side of the slot 10. Said feet can be attached by any means, but preferably they are welded to the frame by an autogeneous weld 12 in order to ensure electrical conductivity between the feet and each side of the turn 5.

(21) The feet 6 further comprise clamping holes 7 which make it possible to attach the coil 1 to an electromagnetic forming plate (not shown) so as to limit the transmission of forces which seek to move apart the feet and enlarge the slot 10 during electromagnetic forming operations in which the coil 1 is used. Likewise, the feet 6 comprise, on either side of the slot 10 and on the face thereof which is opposite the frame 2, grooves 8 which are suitable for receiving terminals for connection of the coil 1 to the electrodes of the pulse generator.

(22) The coil 1 is lastly completed in step 108, during which an insulating sheet is mounted between the walls of the slot 10 to prevent any arcs between the two ends of the electric circuit. The insulating sheet is preferably a sheet of synthetic material, in particular made of polypropylene, having dielectric properties which are suitable for the current pulses used. Furthermore, it is not impossible for the insulating sheet to be produced by overmoulding of the coil 1.

(23) Of course, the sequence of the operations 105 to 108 is not necessarily carried out in this order. Thus, it may be preferable to weld a single foot to the frame before producing the slot 10 which will then extend through this foot or else to carry out the machining processes in step 106 before producing the slot 10.

(24) Of course, the operations 101 to 104 which define the method for attaching a ring in a frame can be used for the purposes of producing objects other than an electromagnetic forming coil. For example, the method for attaching a metal ring in a frame made of a different metal can be used to produce rotating bearings such as connecting rod bearings or other applications.