COIL FOR THE MAGNETIC-PULSE WELDING OF TUBULAR PARTS AND RELATED WELDING METHOD

Abstract

A coil to magnetic-pulse weld tubular parts including an active portion having an active surface which is arrangeable to face an area of overlap between the tubular parts. The active surface defines a tubular opening oriented in an axial direction. The active surface has a predetermined axial length. The tubular opening has, in the axial direction, along the axial length of the peripheral surface, two sections having a constant cross-section. The two sections are connected therebetween by a section having a continuously increasing cross-section. A related magnetic-pulse welding method is also provided herewith.

Claims

1-6. (canceled)

7. A coil to magnetic-pulse weld tubular parts comprising an active portion of which a peripheral surface is positionable to face a working zone of a mutual overlap zone of the tubular parts, the peripheral surface defines a zone in a form of a tubular aperture elongated in an axial direction and having a predetermined axial length, the tubular aperture has, in the axial direction, over the predetermined axial length of the peripheral surface, two sections of constant cross section linked together by a section of monotonically increasing cross section.

8. The coil as claimed in claim 7, wherein the active portion comprises, on either side of the peripheral surface defining the tubular aperture, at least one of a chamfered and shelved portion.

9. The coil as claimed in claim 7, further comprising a magnetic field concentrator comprising the active portion.

10. A welding set comprising the coil as claimed in claim 7 and two tubular parts.

11. The welding set as claimed in claim 10, wherein the two tubular parts are arranged one inside other coaxially, in a position at a level of the coil.

12. The welding set as claimed in claim 10, wherein the two tubular parts are positioned in the tubular aperture such that all or part of the mutual overlap zone is facing the active portion.

13. A method for magnetic-pulse welding of two tubular parts, comprising the steps of: arranging the two tubular parts relative to one another to form a working zone, facing a peripheral surface of a coil such that an end of one of the tubular parts is positioned at a level of a smallest cross section of a tubular aperture, the peripheral surface defines a zone in a form of a tubular aperture elongated in an axial direction and having a predetermined axial length, wherein the tubular aperture has, in the axial direction, over the predetermined axial length of the peripheral surface, two sections of constant cross section linked together by a section of monotonically increasing cross section; and subjecting the working zone to a magnetic field such that a pressure is exerted on a wall of one of the tubular parts to press against a wall of other tubular part to provoke a permanent bonding thereof.

14. The magnetic-pulse welding method as claimed in claim 13, further comprising a step of arranging the two tubular parts into an inner and outer parts such that an end of the outer part is positioned at the level of the smallest cross section of the tubular aperture.

Description

DESCRIPTION OF THE FIGURES

[0042] The invention will be better understood on reading the following description given with reference to the attached drawings:

[0043] FIG. 1 schematically represents a perspective view of a coil for magnetic-pulse welding, according to an embodiment of the invention;

[0044] FIG. 2 schematically represents a front view of the coil of FIG. 1;

[0045] FIG. 3 represents a transverse cross section of the coil of FIG. 2 along the line AA, illustrating the profile of an active portion of said coil;

[0046] FIG. 4 represents an enlargement of a part of the profile of the active portion of the coil of FIG. 3;

[0047] FIG. 5 illustrates a comparison between the welding distances obtained by a coil of the prior art and a coil according to an embodiment of the invention, for a same pair of materials; and

[0048] FIGS. 6a and 6b illustrate a comparison between measurements of temperature generated in a coil of the prior art and in a coil according to an embodiment of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

[0049] FIGS. 1 to 4 illustrate a coil 10 for the magnetic-pulse welding of the two tubular parts 20, 30, according to a first embodiment. The two parts 20, 30 are produced in a metal material.

[0050] Such a coil 10 forms an integral part of a magnetic-pulse welding device which further comprises a storage unit 50 and one or more switches 51.

[0051] The storage unit 50 is configured and intended to store up a high energy, for example of the order of a few tens of kilojoules (kJ).

[0052] In a preferred embodiment, the storage unit is a battery of discharge capacitors.

[0053] The coil, for its part, is configured and intended to create a magnetic field concentrated in a delimited space, described later.

[0054] A first cylindrical tubular part, called outer part 20, has a diameter substantially greater than a diameter of a second cylindrical tubular part, called inner part 30, such that the inner part 30 penetrates into the outer part 20, with play.

[0055] The two cylindrical tubular parts 20, 30 are intended to be arranged one inside the other, coaxially, to form, at the level of their superposition, a so-called overlap zone 25, then to be welded at the level of all or part of said overlap zone by the coil 10.

[0056] Preferably, the overlap zone 25 is situated at the level of an end of at least one part, for example an end of the outer part 20.

[0057] The coil and the two tubular parts form, when said two tubular parts are in position at the level of the coil, a welding set.

[0058] In an embodiment that is not represented, when the outer part 20 is produced in a material exhibiting a very low electrical conductivity, such as, for example, a part produced in steel, an intermediate part, called pusher, preferentially cylindrical tubular, is positioned against an outer wall of the outer part. This intermediate part exhibits a good electrical conductivity.

[0059] A material exhibiting a very low electrical conductivity should be understood to be a material whose electrical conductivity is less than 10 MS/m.

[0060] The coil 10, generally called annular coil, comprises a body 11 in which is formed a tubular aperture 12 delimited by a so-called peripheral surface 121. Said tubular aperture is designed to receive the two parts 20, 30 arranged one inside the other with a view to them being welded.

[0061] The body 11 has a first lateral face 111 and a second lateral face 112, opposite said first lateral face.

[0062] The body 11 is produced in a material exhibiting specific characteristics in terms, on the one hand, of mechanical resistance to plastic deformation in order to have a current of very high intensity, of the order of a few hundreds of thousands of amperes, circulate therein, and, on the other hand, of resistance to high temperatures (that is to say a high melting point) so as not to melt during the welding.

[0063] In an example of embodiment, the material of the body is made of steel.

[0064] The body 11 further comprises a narrow slit 13 extending radially from the tubular aperture 12. Two symmetrically opposite contact plates 14a, 14b extend on either side of the slit 13.

[0065] The contact plates 14a, 14b comprise through orifices 15a, 15b for the passage of fixing means (not represented) configured to fix the coil to a base (not represented) linked to the energy storage unit 50 and to the switch or switches 51.

[0066] When the switch or switches 51 is/are closed, the contact plates 14a, 14b of the coil 10 are linked to the storage unit 50, and a current of high intensity circulates in the coil 10 producing a magnetic field.

[0067] The coil is designed for the current density in a zone of the coil to be sufficient to satisfy the welding conditions. This zone is called active portion 125. It is for example described in the document WO 2012/103873.

[0068] In the case of an annular coil as described in this embodiment, the current circulates through one of the contact plates, then into the coil 10 and emerges through the other contact plate. This current is concentrated, in the active portion 125, on a layer delimited by the peripheral surface 121 facing an outer wall of the outer part 20 and of a thickness corresponding to the skin thickness. The current generates, in the tubular aperture 12, a concentrated magnetic field.

[0069] In the nonlimiting example of a coil produced in steel, the skin thickness is of the order of a few millimeters for a frequency of a few tens of kHz.

[0070] The two parts 20, 30 are advantageously positioned in the tubular aperture such that all or part of the overlap zone 25 is facing the active portion 125.

[0071] The overlap zone 25 facing the active portion 125 is called working zone. Said working zone has a predefined length, called working length L. This working length L.sub.wz corresponds to a maximum welding length between the inner part and the outer part. In practice, the welding length is substantially less than this working length.

[0072] To produce a constant and comparable weld between the two parts, over all the peripheral circumference of said two cylindrical tubular parts 20 and 30, the tubular aperture 12 is preferentially cylindrical, like said two cylindrical tubular parts to be welded.

[0073] The outer part 20 has a diameter less than a minimum diameter of the tubular aperture 12.

[0074] The peripheral surface 121 has an axial length L.sub.b dimensioned so as to be at least equal to the working length L.sub.wz of the working zone.

[0075] The tubular aperture 12 advantageously has a decreasing cross section over the axial length L.sub.b of the peripheral surface 121 of the coil, in a direction starting from the second lateral face 112 toward the first lateral face 111.

[0076] In other words, the tubular aperture 12 has a diameter which decreases progressively, in a direction starting from the second lateral face 112 toward the first lateral face 111.

[0077] In a preferred embodiment, the tubular aperture 12 has, over the axial length L.sub.b of the peripheral surface 121, a succession of three sections, in a direction starting from the second lateral face 112 toward the first lateral face 111: [0078] a first section 122, of length L.sub.1, having a constant cross section, [0079] a second section 123, of length L.sub.2, having a monotonically decreasing cross section, [0080] a third section 124, of length L.sub.3, having a constant cross section.

[0081] In other words, the tubular aperture has a diameter d.sub.3, in the third section 124, less than a diameter di, in the first section 122.

[0082] The second section is defined by a slope of angle β.

[0083] Since the cross section of the tubular aperture 12 in the third section 124 is the smallest cross section, the level of the intensity of the current circulating in the coil will be higher in said third section. Furthermore, the magnetic field lines are closer together and the magnetic pressure is greater. Thus, the portion of the outer part 20 situated in this third section 124 will have a stronger acceleration in the welding method described later.

[0084] On the other hand, since the cross section of the tubular aperture 12 in the first section 122 is the largest cross section, the current density circulating in the coil will be lower in the first section, which will reduce the magnetic pressure in said first section. Furthermore, the coil is less stressed mechanically and thermally in this first section.

[0085] Such a tubular aperture profile advantageously makes it possible to use a storage unit delivering a lower energy to the coil, which improves the thermal and structural withstand strength of said coil. Such a storage unit delivering a lower energy also offers a financial benefit.

[0086] Such a tubular aperture profile also makes it possible to limit the stresses of the coil at the level of the third portion, which makes it possible to increase the life of the coil.

[0087] Such a tubular aperture profile also advantageously makes it possible to modify the gap between the coil 10 and the outer part 20, which has an impact on the fundamental parameters that are the collision point speed and the collision angle. Such a profile makes it possible, when the outer part 20 is introduced such that its end is situated at the level of the third section 124, in the smallest cross section, to keep the fundamental parameters within the window of weldability of the pair of materials forming the parts to be welded for a longer time. The quality and effectiveness of the weld between the outer part 20 and the inner part 30 are thus improved.

[0088] In a preferred embodiment, the length L.sub.3 of the third section 124 is less than the length L.sub.1 of the first section 122.

[0089] In a preferred example of embodiment, the length L.sub.3 is equivalent to 10% of the axial length L.sub.b of the peripheral surface 121, the length L.sub.1 is equivalent to 30% of the axial length L.sub.b of the peripheral surface 121 and the slope of the second section 123 has an angle β of 15°.

[0090] A reduced length L.sub.3 and a slope of pronounced angle β transfers the stresses to the third section 124 of the tubular aperture 12.

[0091] In another preferred embodiment, when a pusher is used, the length L.sub.3 of the third section 124 is equivalent to the length L.sub.1 of the first section 122.

[0092] In a preferred example of such an embodiment, for a coil produced in steel, the length L.sub.3 and the length L.sub.1 are equivalent to 20% of the axial length L.sub.b of the peripheral surface 121 and the slope of the second section 123 has an angle β of 10°.

[0093] In an embodiment illustrated in FIGS. 1 and 3, to withstand the current circulating in the coil and strengthen said coil, said body has a length L.sub.c, between the first 111 and the second 112 lateral faces, greater than the axial length L.sub.b of the peripheral surface 121.

[0094] In another embodiment, to further significantly reduce the plastic deformations of the coil during the welding, and consequently improve the mechanical hold, the active portion 125 comprises, on either side of said peripheral surface of the tubular aperture, a chamfered portion 126.

[0095] In another embodiment, to eliminate the effects of spiking and/or of pinching of the magnetic field lines, the tubular aperture 12 comprises, on either side of the peripheral surface 121, a rounded peripheral edge. Thus, the current density is better distributed, which avoids a concentration of stresses and also a temperature spike.

[0096] An example of a welding method using such a coil is now described.

[0097] To weld two parts together by magnetic-pulse welding, the method comprises a first step of positioning in the coil the two cylindrical tubular parts to be welded.

[0098] The two cylindrical tubular parts are positioned one inside the other to form the overlap zone.

[0099] The two cylindrical tubular parts are arranged in the coil 10 such that the working zone is placed in the tubular aperture of the coil, facing the active portion 125.

[0100] The two cylindrical tubular parts are held, in the tubular aperture, coaxially to one another, in an axial direction XX′ and with the tubular aperture of the coil, elongate in said axial direction XX′, by fixing means (not represented in the figures).

[0101] In a preferred example of implementation, the outer part 20 is positioned such that its end is placed in the smallest cross section of the tubular aperture 12, that is to say at the level of the third section 124.

[0102] The method then comprises a step of magnetic-pulse welding.

[0103] The working zone is subjected to a magnetic field originating from the active portion of the coil such that a pressure is exerted on the outer wall of the outer part, or on an outer wall of the pusher when said pusher is necessary, and presses it closely against an outer wall of the inner part to provoke the permanent bonding thereof.

[0104] FIG. 5 illustrates the welding distances obtained by a coil of the prior art and a coil according to an embodiment of the invention, for a same given pair of materials.

[0105] The coil of the prior art and the coil according to an embodiment of the invention exhibit the following identical characteristics: [0106] a tubular aperture of 50 mm diameter, [0107] the peripheral surface 121 has an axial length L.sub.b of 8 mm, [0108] the material is made of steel, [0109] the distance between the two parts to be welded is 2 mm, [0110] the frequency is a few tens of kiloHz.

[0111] The working length L.sub.WZ is identical to the axial length L.sub.b of the peripheral surface, in other words 7 mm.

[0112] The tubular aperture of the coil of the prior art has a constant cross section.

[0113] The tubular aperture of the coil according to an embodiment of the invention has: [0114] a first section, of length L.sub.1 equal to 20% of the axial length L.sub.b of the peripheral surface of the coil, of 80 mm diameter; [0115] a third section, of length L.sub.3 equal to 20% of the axial length L.sub.b of the peripheral surface of the coil, of 80 mm diameter; [0116] a second section, having a slope angle β of 10°.

[0117] For a given pair of materials for the tubular parts, whatever the form of the tubular aperture of the coil, the welding window is determined. This welding window is defined by the subsonic (curve S), hydrodynamic (curve H), fusion (curve F) and transition (curve T) curves. A collision angle limit, at 20°, is also indicated (curve A) in FIG. 5. More comprehensive explanations concerning the welding window can be found in the documents entitled “Explosive welding of aluminum to aluminum: analysis, computations and experiments”, Grignon et al., International Journal of Impact Engineering 30 (2004) p. 1333-1351.

[0118] In this welding window, the curve E represents the trend of the pair (collision angle, collision point speed) for a coil of the prior art. The bold portion E.sub.g of the curve E indicates the distance welded (almost five triangles representing 5 mm of welding). Over this welded distance, the collision angle varies enormously, between 15 and 20°, with possible repercussions on the quality of the weld.

[0119] The curve B represents the trend of the pair (collision angle, collision point speed) for a coil according to the chosen embodiment of the invention. Such a coil makes it possible to weld a zone over a distance of 6 mm (6 circles). Furthermore, it can be seen that, over most of this distance, the collision angle is kept almost constant, around 16°.

[0120] FIGS. 6a and 6b illustrates the results of simulation of the behavior in terms of temperature of a coil of the invention according to a first embodiment compared to a coil of the prior art.

[0121] The coil of the prior art has the following characteristics: [0122] a tubular aperture, of constant cross section over the axial length of the peripheral surface, of 50 mm diameter, [0123] the peripheral surface 121 has an axial length L.sub.b of 8 mm, [0124] the material is made of steel, [0125] the distance between the two parts to be welded is a few millimeters, [0126] the frequency is a few tens of kHz.

[0127] The working length L.sub.WZ is identical to the axial length L.sub.b of the active portion, in other words 8 mm.

[0128] The tubular parts are produced in a metal material, such as, for example, aluminum, copper or steel.

[0129] The coil of the invention according to a first embodiment has the following characteristics:

[0130] the peripheral surface 121 has an axial length L.sub.b of 8 mm, [0131] a tubular aperture having: [0132] a first section, of length L.sub.1 equal to 30% of the axial length L.sub.b of the peripheral surface of the coil, of 50 mm diameter; [0133] a third section, of length L.sub.3 equal to 10% of the axial length L.sub.b of the peripheral surface of the coil, of 50 mm diameter; [0134] a second section, having a slope angle β of 15°. [0135] the material of the coil is made of steel, [0136] the distance between the two parts to be welded is a few millimeters, [0137] the frequency is a few tens of kHz.

[0138] The tubular aperture of the coil is chamfered on either side of the peripheral surface.

[0139] The materials of the parts to be welded are made of metal, such as, for example, aluminum, copper or steel, which are identical or different.

[0140] FIG. 6a represents a transverse view of a radial cross section of the coil of the prior art. FIG. 6a illustrates the result of the simulation of the behavior in terms of temperature of the coil of the prior art.

[0141] FIG. 6b represents a transverse view of a radial cross section of the coil according to the first embodiment. FIG. 6b illustrates the result of the simulation of the behavior in terms of the temperature of the coil according to the first embodiment.

[0142] It can be seen that the maximum temperature generated in the coil of the prior art is 2700 K, whereas the maximum temperature generated in the coil of the invention according to the first embodiment is 2143 K, which represents a temperature differential of the order of 550 K.

[0143] It can also be seen that the temperature of the coil of the prior art is concentrated significantly on an edge of the peripheral surface of the coil, whereas the temperature of the coil of the invention according to the first embodiment is distributed over the two edges of the peripheral surface of the coil.

[0144] The above description clearly illustrates that, through its various characteristics and the advantages thereof, the present invention achieves the objectives that it set out to achieve. In particular, it provides a magnetic-pulse welding coil and an associative magnetic-pulse welding method that are suited to the welding of parts made of materials with low thermal conductivity. It advantageously exhibits a profile at the level of the peripheral surface 121 of the aperture such that the thermal and mechanical stresses applied to the coil during the welding are significantly reduced, improving the life of the coil. Such a form of coil also offers improved welding between the parts.