EP1830976A2

EP1830976A2 - Method and apparatus for performing a magnetic pulse forming process

Info

Publication number: EP1830976A2
Application number: EP05855345A
Authority: EP
Inventors: Boris A. Yablochnikov
Original assignee: Dana Inc
Current assignee: Dana Automotive Systems Group LLC
Priority date: 2004-12-27
Filing date: 2005-12-27
Publication date: 2007-09-12
Also published as: WO2006071766A3; JP2008525198A; EP1830976A4; US20060156776A1; WO2006071766A2; BRPI0519358A2

Abstract

A magnetic pulse forming process is performed by an apparatus (10) and a method that causes the plasticity of a workpiece (12) to be preliminarily increased to facilitate deformation to the desired shape. Initially, a mandrel (16) having a surface and an electrically conductive member (14) are provided. The workpiece (12) is oriented between the surface of the mandrel (16) and the electrically conductive member (14), and a first electrical current (36) is caused to flow through the workpiece (12) and the electrically conductive member (14) so as to increase the plasticity of the workpiece (12). Then, a second electrical current (40) is caused to flow through the electrically conductive member (14) and the workpiece (12) so as to cause to cause the workpiece (12) to be deformed into engagement with the surface of the mandrel (16).

Description

TITLE

METHOD AND APPARATUS FOR PERFORMING A MAGNETIC PULSE FORMING PROCESS

BACKGROUND OF THE INVENTION

[0001] This invention relates in general to magnetic pulse forming processes for deforming one or more metallic workpieces to a desired shape. In particular, this invention relates to an improved method and apparatus for performing such a magnetic pulse forming process wherein the plasticity of the workpiece to be deformed is preliminarily increased to facilitate deformation to the desired shape. [0002] Magnetic pulse forming is a well known process that can be used to deform one or more metallic workpieces to a desired shape. Typically, a magnetic pulse forming process is performed by initially disposing a portion of a workpiece either about or within a mandrel having the desired shape. Then, an electromagnetic field is generated either within or about the workpiece. When this occurs, a large pressure is exerted on the workpiece, causing it to move toward the mandrel. If the electromagnetic field is generated about the exterior of the workpiece, then the workpiece is deformed inwardly into engagement with the mandrel. If, on the other hand, the electromagnetic field is generated within the interior of the workpiece, then the workpiece is deformed outwardly into engagement with the mandrel. Magnetic pulse forming can also be used to deform two metallic workpieces to a desired shape by initially disposing portions of first and second workpieces in an overlapping relationship and generating the electromagnetic field either within or about the overlapping portions of the first and second workpieces.

[0003] Magnetic pulse forming can be used, for example, to form male and female members of a typical sliding spline type of slip joint. Such male and female members typically have respective pluralities of splines formed thereon. The male member is generally cylindrical in shape and has a plurality of outwardly extending splines formed on the outer surface thereof. The male member may be formed integrally with or secured to an end of a conventional driveshaft assembly, for example. The female member, on the other hand, is generally hollow and cylindrical in shape and has a plurality of inwardly extending splines formed, on the inner surface thereof. The female member may be formed integrally with or secured to a yoke that forms a portion of a conventional universal joint, for example. To assemble the slip joint, the male member is inserted within the female member such that the outwardly extending splines of the male member cooperate with the inwardly extending splines of the female member. As a result, the male and female members are connected together for concurrent rotational movement. However, the outwardly extending splines of the male member can slide axially relative to the inwardly extending splines of the female member to allow a limited amount of relative axial movement to occur therebetween. [0004] The male and female members of such a sliding spline type of slip joint can be formed from hollow cylindrical workpieces that are deformed to have the male and females splines by means of magnetic pulse forming techniques. Typically, however, a relatively large amount of deformation is required in order to deform the workpieces to form such male and female splines. In this specific application, as well in a variety of other applications, such a relatively large amount of deformation can require a relatively large amount of energy to perform the magnetic pulse forming process. Additionally, such a relatively large amount of deformation may weaken or cause damage to the workpiece. Thus, it would be desirable to provide an improved method and apparatus for performing a magnetic pulse forming process wherein the plasticity of the workpiece to be deformed is preliminarily increased to facilitate deformation to the desired shape.

SUMMARY OF THE INVENTION

[0005] This invention relates to an improved method and apparatus for performing a magnetic pulse forming process wherein the plasticity of the workpiece to be deformed is preliminarily increased to facilitate deformation to the desired shape. Initially, a mandrel having a surface and an electrically conductive member are provided. The workpiece is oriented between the surface of the mandrel and the electrically conductive member, and a first electrical current is caused to flow through the workpiece and the electrically conductive member so as to increase the plasticity of the workpiece. Then, a second electrical current is caused to flow through the electrically conductive member and the workpiece so as to cause to cause the workpiece to be deformed into engagement with the surface of the mandrel. [0006] Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Fig. 1 is a schematic side elevational view of a first embodiment of an apparatus for performing a magnetic pulse forming operation on a workpiece in accordance with this invention.

[0008] Fig. 2 is an enlarged sectional elevational view of a portion of the apparatus illustrated in Fig. 1 shown during the deformation of the workpiece. [0009] Fig. 3 is a further enlarged sectional elevational view of a portion of the apparatus illustrated in Figs. 1 and 2 shown after the deformation of the workpiece. [0010] Fig. 4 is a schematic side elevational view of a second embodiment of an apparatus for performing a magnetic pulse forming operation in accordance with this invention.

[0011 ] Fig. 5 is an enlarged sectional elevational view of a portion of the apparatus illustrated in Fig. 4 shown prior to the deformation of the workpiece. [0012] Fig. 6 is a schematic side elevational view of a third embodiment of an apparatus for performing a magnetic pulse forming operation on a workpiece in accordance with this invention.

[0013] Fig. 7 is an enlarged sectional elevational view showing four different versions of the apparatus illustrated in Fig. 6 shown prior to the deformation of the workpiece. [0014] Fig. 8 is a schematic side elevational view of a fourth embodiment of an apparatus for performing a magnetic pulse forming operation on a workpiece in accordance with this invention.

[0015] Fig. 9 is an enlarged sectional elevational view of a portion of the apparatus illustrated in Fig. 8 shown after to the deformation of the workpiece. [0016] Fig. 10 is an enlarged sectional elevational view of a portion of the apparatus illustrated in Figs. 8 and 9 shown after the deformation of the workpiece. [0017] Fig. 11 is a schematic side elevational view of a fifth embodiment of an apparatus for performing a magnetic pulse forming operation on a workpiece in accordance with this invention.

[0018] Fig. 12 is an enlarged sectional elevational view of a portion of the apparatus illustrated in Fig. 11 shown after the deformation of the workpiece.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0019] Referring now to the drawings, there is illustrated in Fig. 1 a first embodiment of a system, indicated generally at 10, for performing a magnetic pulse forming process in accordance with this invention. A tubular workpiece 12 is located co-axially relative to an axis L defined by an outer electrically conductive member 14 and an internal mandrel 16. The mandrel 16 includes a first end 18, a second end 20, and an elongated intermediate portion, indicated generally at 22. An outer surface of the intermediate portion 22 has a shape the corresponds to a desired final shape of the outer tubular member 12 after the magnetic pulse forming process has been performed. In the illustrated embodiment, the outer surface of the intermediate portion 22 of the mandrel 16 has a plurality of longitudinally extending splines 24 formed thereon. However, it will be appreciated that the outer surface of the intermediate portion 22 of the mandrel 16 may have any desired shape. [0020] The first end 18 of the mandrel 16 is surrounded by the tubular workpiece 12 and is supported by a first electrically conductive insert 26 and an electrically conductive end member 28. The electrically conductive end member 28 is spaced from and electrically isolated from the outer member 14. The first insert 26 is preferably formed from a material that is a good electrical conductor, such as copper, for example. The second end 20 of the mandrel 16 is surrounded by a dielectric bushing 30. A bushing 32 surrounds the dielectric bushing 30 and abuts an inner wall of the tubular workpiece 12. The dielectric bushing 30 electrically insulates the bushing 32 from the mandrel 16 and prevents a short circuit from occurring with the tubular workpiece 12 during preheating, as will be explained in detail below. The dielectric bushing 32 also co-axially centers the mandrel 16 within the tubular workpiece 12. A second electrically conductive insert 34 is disposed between an outer wall of the tubular workpiece 12 and the outer electrically conductive member 14 to provide reliable and substantially uniform annular electrical contact between the outer electrically conductive member 14 and the tubular workpiece 12. The second insert 34 is also preferably formed from a material that is a good electrical conductor, such as copper, for example.

[0021 ] A preheating current source 36 is electrically connected to the outer electrically conductive member 14 and to the electrically conductive end member 28. A switch 38 is provided to selectively interrupt the flow of electrical current from the preheating current source 36 to the outer electrically conductive member 14 and to the electrically conductive end member 28. The amplitude of the preheating current generated by the preheating current source 36 can be set as desired, but is preferably less than one hundred kiloamps. A pulse forming current source 40 is also electrically connected to the outer electrically conductive member 14 and to the electrically conductive end member 28. The amplitude of the pulse forming current generated by the pulse forming current source 40 can be set as desired, but is preferably at least five hundred kiloamps. It should be understood that larger or smaller current amplitudes could be used without departing from the scope and spirit of the invention. A switch 42 is provided to selectively interrupt the flow of electrical current from the pulse forming current source 40 to the outer electrically conductive member 14 and to the electrically conductive end member 28. The energy needed for producing a pulse forming current in the pulse forming current source 40 can be stored in one or more energy storing devices or capacitors 44, which can be charged by a high-voltage charging supply (not shown). Other energy storing devices, such as a motor/ generator set or some other suitable pulse source, for example, can be used without departing from the scope and spirit of the invention.

[0022] In operation, it is desirable to provide a circumferentially uniform distribution of the pulse forming current with the outer electrically conductive member 14 and the electrically conductive end member 28, especially surrounding the first insert 26 and the second insert 34. Before beginning the forming cycle, the tubular workpiece 12 has a shape and position shown by dashed lines and depicted as A in Fig. 1. A gap G thus initially exists between the tubular workpiece 12 and the mandrel 16. To perform the pulse forming process, the capacitors 44 of the pulse forming current source 40 are charged to the operating voltage. Air trapped in the gap G is preferably evacuated through special passages (not shown) formed in the mandrel 16 or the bushing 32. The air is evacuated to militate against the captured air acting as a damper and reducing the effectiveness of the deformation process. The switch 38 of the preheating current source 36 is then moved to the closed position to supply current to preheat the tubular workpiece 12. This preheating is accomplished by causing an electrical current to be passed through the electrically conductive end member 28, the first insert 26, the tubular workpiece 12, the second insert 34, and the outer electrically conductive member 14. It is desirable to maintain the power and the current of the preheating current source 36 as high as possible to provide the fastest preheating time to a desired plasticity of the tubular workpiece 12 without causing deformation. [0023] After the tubular workpiece 12 has reached the desired plasticity, the switch 38 of the preheating current source 36 is moved to the open position, and the switch 42 of the pulse forming current source 36 is moved to the closed position. The capacitors 44 are thus caused to discharge the stored energy through the electrically conductive end member 28, the first insert 26, the preheated tubular workpiece 12, the second insert 34, and the outer electrically conductive member 14. The arrows in Fig. 1 show the direction of the electric current along the tubular workpiece 12 and through the outer electrically conductive member 14. The electrodynamic pressure of the magnetic field accompanying the current compresses a desired portion of the tubular workpiece 12 until an internal surface thereof contacts an outer surface of the mandrel 16. The shape of the outer surface of the mandrel 16 determines the final shape of the tubular workpiece 12, which in the embodiment shown forms the splines 24. [0024] Fig. 2 shows the tubular workpiece 12 of Fig. 1 during the magnetic pulse forming operation, illustrating a magnetic field M that is induced between the tubular workpiece 12 and the outer electrically conductive member 14. As shown therein, the splines or teeth of the outer electrically conductive member 14 are located opposite the splines or teeth 24 formed on the mandrel 16. The configuration of the magnetic field M in the gap between the tubular workpiece 12 and the outer electrically conductive member 14 is shown for the last stage of the forming process, i.e., when the pulse current is being caused to flow. The frequency of the pulse current has been found to be desirable when between ten and twenty kilohertz, although it is understood that different current frequencies may be used as desired. Due to a proximity effect, the current is concentrated very close to the surface of the tubular workpiece 12. As a result, the discharge current is concentrated in thin surface layers of the outer electrically conductive member 14 and the tubular workpiece 12 that are faced one to the other, and magnetic field M has a wave form. In the embodiment shown in Fig. 2, the inner wall of the outer electrically conductive member 14 includes splines formed therein. It is understood that the inner wall could have a different cross-sectional shape such as circular, for example, without departing from the scope and spirit of the invention.

[0025] Fig. 3 illustrates the tubular workpiece 12 of Fig. 1 as being attached to the mandrel 16 after the magnetic pulse forming operation has been performed. After the forming operation, the first insert 26 and the second insert 34 are removed, and the tubular workpiece 12, the dielectric bushings 30, the bushings 32, and the mandrel 16 are then disassembled.

[0026] The method as described in Figs. 1, 2, and 3 relate to the use of an external inductor for deforming the tubular workpiece 12 inwardly onto the mandrel 16 disposed therein. However, the method of this invention can also be used in conjunction with an external inductor (not shown) for deforming the tubular workpiece 12 outwardly within a mandrel (not shown) disposed thereabout. To accomplish this, the inner surface of the mandrel would be provided with the desired splined cross sectional shape for the tubular workpiece 12, which is expanded outwardly into engagement therewith. After forming the splines, the shorter, unformed end of the inner tubular member could be removed to permit the splined section of the formed tubular member to be inserted into the splined portion of the formed tubular member 12 described above to provide a sliding spline type of slip joint. Thus, the formed tubular members would slide axially relative to one another, while allowing a limited amount of relative axial movement to occur therebetween. [0027] Figs. 4 and 5 illustrate a second embodiment of a system, indicated generally at 50, for performing a magnetic pulse forming process in accordance with this invention. In this embodiment, the magnetic pulse forming operation causes a radial expansion of an inner tubular member 52 into an external die or mandrel 54. In this second embodiment, the external mandrel 54 includes an annular array of axially extending teeth or splines 56 formed thereon. A cylindrical inner electrically conductive member 58 includes a first end 60 that is supported by a first electrically conductive end member 62 by means of a first insert 64. A second end 66 of the inner electrically conductive member 58 is supported by a second insert 68. [0028] To militate against a supported portion of the inner tubular member 52 adjacent the second end 66 of the inner electrically conductive member 60 from being radially deformed, a dielectric bushing 70 can be disposed adjacent the supported portion of the inner tubular member 52 and be embedded into the external mandrel 54. A supported portion of the inner tubular member 52 adjacent the first end 60 of the inner electrically conductive member 58 is seated on a second dielectric bushing 72. The supported portion of the inner tubular member 52 adjacent the first end 60 of the inner electrically conductive member 58 is supported by a second electrically conductive end member 74 and a third insert 76, which contacts the external mandrel 54. In this embodiment, the first electrically conductive end member 62, the second electrically conductive end member 74, the inner tubular member 52, and the external mandrel 54 are concentrically disposed about longitudinal axis L. The second dielectric bushing 72 militates against radial deformation of the supported portion of the inner tubular member 52 adjacent the first end 60 of the inner electrically conductive member 58. The remainder of the structure and the forming process is the same as described above for Fig. 1. The direction of flow of the current is indicated by the arrows and before the forming cycle, the inner tubular member 52 has a shape and position shown by dashed lines and depicted as B in Fig. 4. [0029] Fig. 5 shows a sectional elevational view of the inner tubular member 52 of Fig. 4 during the magnetic pulse forming operation illustrating the magnetic field M induced between the inner tubular member 52 and the inner electrically conductive member 58. As shown therein, the splines or teeth of the inner electrically conductive member 58 are located opposite the splines or teeth 56 formed on the mandrel 54. The configuration of the magnetic field M in the gap between the inner tubular member 52 and the inner electrically conductive member 58 is shown for the last stage of the forming process, i.e., when the pulse current is being caused to flow. The frequency of the pulse current has been found to be desirable when between ten and twenty kilohertz, although it is understood that different current frequencies may be used as desired. Due to a proximity effect, the current is concentrated very close to the surface of the inner tubular member 52. As a result, the discharge current is concentrated in thin surface layers of the inner electrically conductive member 58 and the inner tubular member 52 which are faced one to the other, and magnetic field M has a wave form. In the second embodiment shown in Fig. 5, the outer wall of the inner electrically conductive member 58 includes splines formed therein. It is understood that the outer wall could have a different cross sectional shape, such as circular, for example, without departing from the scope and spirit of the invention. [0030] Fig. 6 illustrates a third embodiment of a pulse forming system 80 including a magnetic field concentrator or electrically conductive bushing 82. The field concentrator 82 is inserted between an outer electrically conductive member 84 and an outer tubular member 86 and is electrically insulated from both the outer electrically conductive member 84 and the outer tubular member 86. The arrows in Fig. 6 show the current path when proximity effects are taken into account. The field concentrator 82 facilitates a method of concentrating and shaping the magnetic field and is useful when it becomes necessary to frequently readjust the pulse forming system 80 to form a tubular member having different diameters and shapes. The field concentrator 82 works by an induction principle, i.e., electric contact is not required with any elements of a discharge circuit, and it is much easier to change than any element of the circuit. The field concentrator 82 permits the number of splines to be formed to be easily changed. During the forming of tubular members, to create different diameters and numbers of splines, only the field concentrator 82 and first and second inserts 88, 90 need to be changed. An electrically conductive disc 92 is added to the outer electrically conductive member 84 to facilitate the changing of the field concentrator 82. The remainder of the structure and the forming process are the same as described above for Fig. 1, and like reference numbers are used to indicate similar structures. [0031 ] Fig. 7 is a sectional elevational drawing of the magnetic field concentrator 82 of Fig. 6 illustrating the effect on the magnetic field with the magnetic field; concentrator 82 having different cross sectional shapes. Sectors a) and b) are shown such that the smooth, cylindrical surfaces of the outer electrically conductive member 84 and the field concentrator 82 provide the shortest magnetic line O in the gap between those surfaces. In sectors c) and d), a method of providing a desirable distribution of a current by means of using axially extending splines or teeth 94 on the external surface of the field concentrator 82 and axially extending splines or teeth 96 on the internal surface of outer electrically conductive member 84. The splines 94 are disposed between the splines 96 and are insulated from one another (the insulation is not shown to make clear the changing configuration of magnetic field). The deeper and the larger the quantity the splines 94 and the splines 96, the lower the magnetic loss.

[0032] Fig. 8 illustrates a fourth embodiment of a magnetic pulse forming system 100, wherein the magnetic pulse forming system 100 causes a radial compression of an inner tubular member 102 and an outer tubular member 104. Prior to the forming operation, the inner tubular member 102 and the outer tubular member 104 can be press fit together such that a desired amount of overlap exists. A mandrel 106 is co- axially disposed along axis L within the inner tubular member 102 and the outer tubular member 104. The mandrel 106 includes a first end 108, a second end 110, and an elongated intermediate portion, indicated generally at 112. An outer surface of the intermediate portion 112 has a shape in accordance with a desired final shape of the inner tubular member 102 and the outer tubular member 104 after the pulse forming operation has been conducted. In the fourth embodiment, a plurality of teeth or splines 114 is formed on the outer surface of the intermediate portion 112, as illustrated in Fig. 9. The first end 108 of the mandrel 106 is surrounded by the inner tubular member 102, and the outer tubular member 104 and is supported by a first electrically conductive insert 116 and an electrically conductive end member 118. The electrically conductive end member 118 is spaced from and electrically isolated from an outer electrically conductive member 120. The first insert 116 is preferably formed from a material that is a good electrical conductor, such as copper, for example. The second end 110 of the mandrel 106 is surrounded by a dielectric bushing 122. A bushing 124 surrounds the dielectric bushing 122 and abuts an inner wall of the outer tubular member 104. The dielectric bushing 122 electrically insulates the bushing 124 from the mandrel 106 and prevents a short circuit from occurring with the outer tubular member 104 during preheating, as discussed above. The bushing 124 also axially centers the mandrel 106 within the outer tubular member 104. A second electrically conductive insert 126 is disposed between an outer wall of the outer tubular member 104 and the outer electrically conductive member 120 to provide reliable and substantially uniform annular electric contact between the outer electrically conductive member 120 and the outer tubular member 104. The second insert 126 is also preferably formed from a material that is a good electrical conductor, such as copper, for example. The remainder of the structure and the forming process is the same as described above for Fig. 1. The direction of flow of the current is indicated by the arrows. It is understood that the inner tubular member 102 and the outer tubular member 104 could be disposed within the mandrel 106 and the outer electrically conductive member 120 disposed within the inner tubular member 102 and the outer tubular member 104 to deform the inner tubular member 102 and the outer tubular member 104 outwardly into engagement with the mandrel 106 without departing from the scope and spirit of the invention.

[0033] Fig. 9 illustrates the fourth embodiment of the invention of Fig. 8 showing the inner tubular member 102 and the outer tubular member 104 on the mandrel 106. Fig. 10 illustrates of the inner tubular member 102 and the outer tubular member 104 of Fig. 8 removed from the mandrel 106 after the magnetic pulse forming operation. The fourth embodiment illustrated in Figs. 8, 9, and 10 is particularly useful in forming a collapsible telescopic driveshaft. The forming according to the method militates against any relative axial or rotational movement between the inner tubular member 102 and the outer tubular member 104 under normal operating conditions of the driveshaft. However, when an unusually large axial force is applied to the ends of the driveshaft assembly, such as during an automobile accident, the inner tubular member 102 and the outer tubular member 104 will move telescopically relative to one another. The amount of force required to cause such telescopic movement can be adjusted by varying the length of the overlapping section of the inner tubular member 102 and the outer tubular member 104.

[0034] Fig. 11 illustrates a fifth embodiment of a magnetic pulse forming system 130, wherein the magnetic pulse forming system 130 causes a radial compression of an inner tubular member 132 with an annular recess 134 formed therein by an annular recess 136 formed in a mandrel, indicated generally at 138. The recess 134 facilitates cutting, chamfering, and other machining operations that may be necessary for joining the inner tubular member 132 with an outer tubular member (not shown). In cases where only an axial current is applied as illustrated in Fig. 11, to provide the relatively deeper deformation in the area of the annular recess 134 of the inner tubular member 132 a greater amplitude of current may be required. One solution has been found to add an azimuthal component to the current in the area of the recess 134 of the inner tubular member 132 by means of an inductor 142 which is connected in series with an outer electrically conductive member 144 and an electrically conductive outer end portion 146. It is desirable that the inductor 142 be mechanically strong, have a very low inductance, and provide a uniform distribution of current in the places of contact with the outer electrically conductive member 144 and the electrically conductive outer end portion 146. One such inductor 142 which has been found to satisfy these requirements is described in U.S. Pat. No. 4,129,846 to Yablochnikov, the disclosure of which is incorporated herein by reference. The remainder of the structure and the forming process are the same as described above for Fig. 1, with the same structure having the same reference numerals. Fig. 12 is an involuted partial sectional view of the inner tubular member 132 of Fig. 11 with the inner tubular member 132 attached to the mandrel 138 after the magnetic pulse forming operation. [0035] In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.

Claims

What is claimed is:

1. A method of deforming a workpiece comprising the steps of:

(a) providing a mandrel having a surface;

(b) providing an electrically conductive member;

(c) orienting a workpiece between the surface of the mandrel and the electrically conductive member;

(d) causing a first electrical current to flow through the workpiece and the electrically conductive member so as to increase the plasticity of the workpiece; and

(e) causing a second electrical current to flow through the electrically conductive member and the workpiece so as to cause to cause the workpiece to be deformed into engagement with the surface of the mandrel.

2. The method defined in Claim 1 wherein said step (a) is performed by providing the mandrel having an outer surface, said step (b) is performed by providing the electrically conductive member having an inner surface, and said step (c) is performed by orienting the workpiece between the outer surface of the mandrel and the inner surface of the electrically conductive member.

3. The method defined in Claim 1 wherein said step (a) is performed by providing the mandrel having an inner surface, said step (b) is performed by providing the electrically conductive member having an outer surface, and said step (c) is performed by orienting the workpiece between the inner surface of the mandrel and the outer surface of the electrically conductive member.

4. The method defined in Claim 1 wherein said step (c) is performed by connecting a preheating current source so as to cause a first electrical current to flow through the workpiece and the electrically conductive member.

5. The method defined in Claim 4 wherein said step (c) is performed by connecting the preheating current source through an electrically conductive end member and a first electrically conductive insert to a first portion of the workpiece and by connecting a second portion of the workpiece through a second electrically conductive insert to the preheating current source.

6. The method defined in Claim 1 wherein said step (d) is performed by connecting a pulse current source so as to cause a first electrical current to flow through the workpiece and the electrically conductive member.

7. The method defined in Claim 6 wherein said step (d) is performed by connecting the pulse current source through an electrically conductive end member and a first electrically conductive insert to a first portion of the workpiece and by connecting a second portion of the workpiece through a second electrically conductive insert to the pulse current source.

8. The method defined in Claim 1 wherein said step (c) is performed by connecting a preheating current source so as to cause a first electrical current to flow through the workpiece and the electrically conductive member, and wherein said step (d) is performed by connecting a pulse current source so as to cause a first electrical current to flow through the workpiece and the electrically conductive member.

9. The method defined in Claim 8 wherein said step (c) is performed by connecting the preheating current source through an electrically conductive end member and a first electrically conductive insert to a first portion of the workpiece and by connecting a second portion of the workpiece through a second electrically conductive insert to the preheating current source.

10. The method defined in Claim 9 wherein said step (d) is performed by connecting the pulse current source through the electrically conductive end member and the first electrically conductive insert to the first portion of the workpiece and by connecting the second portion of the workpiece through the second electrically conductive insert to the pulse current source.