Method and apparatus for creep forming of and relieving stress in an elongated metal bar

Abstract

A hot creep stretch wrap forming method includes heating a metal bar to a forming temperature within a temperature range suitable for creep deformation thereof, applying a stretching force to the metal bar at a strain rate no greater than 0.05 inch/inch/second, and wrapping the metal bar around a die, preferably having a thermally and/or electrically insulative work surface. The stretching force is typically applied to a strain ranging from 0.5% to 15.0%. The metal bar most preferably is a titanium alloy with a forming temperature ranging from 0.45 to 0.60 of its melting temperature. The wrapped metal bar is held in position and its temperature maintained within the temperature range typically for 5 to 120 minutes for stress relief. Preferably, the metal bar is held substantially at the forming temperature throughout the process. Thermal insulation around the die and metal bar reduce heat loss from the metal bar.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to hot creep-stretch forming of metal parts, most particularly elongated metal bars. More particularly, the invention relates to hot stretch-wrap forming of titanium, titanium alloys and similar metals which are typically difficult to stretch-wrap form. In particular, the invention relates to hot stretch-wrap forming of a metal form using a die having a thermally and electrically insulated work surface.

2. Background Information

The present invention relates to the hot stretch-wrap forming of elongated metallic parts which are formed at high temperatures, and in particular parts made of titanium alloys which are manufactured by extrusion, forging, rolling, machining or a combination of these processes. Titanium alloys have been widely used as aerospace materials due to their excellent mechanical and corrosion properties in combination with being comparatively light weight. However, it is well-known that titanium alloys are difficult to form in general and require heating to a substantial temperature in order to properly form such parts. Titanium alloys are highly desirable for use in contoured structural members of an aircraft, but the formation of such structural members has been very limited due to the lack of a suitable and economically feasible method of forming such contoured members. The demand for such parts has increased with the desire for lightweight and high strength structural components such as chords in advanced airplanes.

One process currently available for forming elongated titanium parts is known as “bump forming”. This process involves the heating of an elongated part in the furnace to a predetermined temperature at which time the part is removed from the furnace and placed on forming blocks of a forming press. The press applies a bending force which results in a localized deformation of the part. The temperature of the part quickly decreases during formation and the resistance to forming thus significantly increases. Thus, bump forming requires repeated heating cycles to complete the forming process, which is time consuming and costly. In addition, the bending moment that results from bump forming causes tensile stresses in the section of the part above the neutral axis and compressive stresses below the neutral axis which lead respectively to cracks and wrinkles in the part. The considerable stress gradient within the part makes it difficult to control the geometry of the formed part. In addition, the localized deformation caused by the complex stress state of the part promotes the development of significant residual stresses therein, requiring an offline stress relief treatment with an expensive fixture. Bump forming also suffers from the lack of a guiding tool to achieve the required contour without resorting to a trial and error method. It is also difficult to maintain the structural integrity of the cross section, for example, along angles between flanges and the like. Post hot sizing has been suggested to improve the dimensional integrity of the formed part. Finally, bump forming is not amenable to computer simulation.

While the general concept of hot stretch-wrap forming has been known for some time, known prior art methods are not suitable for economically forming parts made of titanium alloys or other materials which are difficult to form. U.S. Pat. No. 2,952,767 granted to Maloney discloses an apparatus for stretch-wrap forming an elongated bar which is heated by resistance heating and wrapped around a metallic die heated by conventional heating elements within the die assembly. A major problem with this configuration is the electrical shunting effect that occurs between the heated die and the metal part as they contact one another, which leads to local overheating and necking of the part.

U.S. Pat. No. 4,011,429 granted to Morris et al. noted the above shunting effect and sought to overcome this problem by heating both the die and the elongated metal part via resistance by electrically connecting the die and the metal part in parallel and heating them with the same voltage. Unfortunately, this configuration is not practical because the parallel heating of the die and part requires a complex and prohibitively expensive configuration of the die. In addition, this method requires preheating of the part to a temperature substantially below its forming temperature while the die is heated to the forming temperature so that only the contacting portion of the part is brought up to the forming temperature upon contact with the die, which results in a non-uniform yield strength between the contacting and non-contacting portion of the part. Because the deformation process is not uniform, it is extremely difficult to maintain the structural integrity of the formed part and to minimize the development of residual stresses.

The present invention addresses these and other problems as will be evident from the subsequent description.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method comprising the steps of heating an elongated metal bar to a forming temperature within a temperature range suitable for creep deformation of the metal bar; applying a stretching force to the heated metal bar at a strain rate no greater than 0.05 inch/inch/second; and wrapping the heated metal bar around a die to form a wrapped metal bar.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagrammatic top plan view of the hot creep-stretch forming apparatus of the present invention showing thermally and electrically insulative material within the die cavity and a metal bar prior to wrapping around the die.

FIG. 2 is a sectional view taken on line 2-2 of FIG. 1 showing two layers of insulative material within the die cavity.

FIG. 3 is a sectional view similar to FIG. 2 showing the metal bar inserted into the die cavity with the insulative material separating the metal die and metal bar.

FIG. 4 is similar to FIG. 1 and shows the jaws having moved from the starting position of FIG. 1 to the completed position of FIG. 4 to wrap the elongated bar around the work surface of the die.

FIG. 5 is a view similar to FIG. 3 and shows the covered door in an open position and the stretch wrap formed metal bar being removed from the die cavity.

FIG. 6 is a diagrammatic stress/temperature map for commercially pure titanium where the numbers on the left represent normalized shear stress, the numbers on the right represent shear stress at 20° C., the top numbers represent temperature in ° C., and the bottom numbers represent homologous temperature.

FIG. 7 is a diagrammatic chart showing a region of plastic deformation, a region of uniform deformation, and a region of diffusional deformation.

FIG. 8 is a diagrammatic plan view of a prior art metal bar showing buckling and fractures caused by stresses caused during hot stretch-wrap formation.

FIG. 9 is similar to FIG. 8 and shows a metal bar formed by the hot stretch-wrap method of the present invention, showing that the metal bar is free of the buckling and fractures of the prior art metal bar of FIG. 8.

FIG. 10 is a sectional view of a metal bar having an L-shaped configuration.

Similar numbers refer to similar parts throughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The hot stretch-wrap forming apparatus of the present invention is indicated generally at 10 in FIG. 1. Apparatus 10 includes a die 12 and a pair of spaced jaws 14 which are configured to clamp a metal form shown as an elongated metal bar 16 adjacent respective ends thereof in order to stretch metal bar 16 when heated and wrap it around die 12. Apparatus 10 is particularly useful for hot creep-stretch forming of bars made of titanium alloys. Jaws 14 are attached to respective swing arms which are not shown but are well known in the art. Each of jaws 14 is in communication with an electrical power source 18 via conductors or wires 20 to form an electrical circuit for resistively heating metal bar 16. A plurality of heating elements 24 electrically connected with power source 18 via wires 22 may be inserted into die 12 for the heating thereof. Die 12 has a cavity-bounding surface 26 which defines a T-shaped die cavity 28 (FIG. 2). Surface 26 and cavity 28 have an arcuate configuration which extends from a first end 30 of die 12 to a second end 32 of die 12. First or inner and second or outer layers 34 and 36 of insulation are disposed within cavity 28 with first layer 34 abutting surface 26 of die 12 in a substantially continuous manner from first end 30 to second end 32 of die 12. Second layer 36 likewise abuts first layer 34 in a substantially continuous manner from first end 30 to second end 32. Layers 34 and 36 conform to surface 26 and thus are of a generally T-shaped configuration. Second layer 36 defines a working surface 38 which abuts metal bar 16 during the wrapping process. As shown in FIG. 2, metal bar 16 has a T-shaped cross section which is of a mating configuration with the T-shaped cavity 28 and working surface 38. Work surface 38 defines a T-shaped working space 40 in which bar 16 is disposed during the stretch-wrap process.

More particularly, each of first and second layers 34 and 36 is most preferably formed of a thermally and electrically insulative material. Alternately, one of layers 34 and 36 may be formed of a thermally insulative material and the other may be formed of an electrically insulated material if desired. While it is preferred to provide thermal and electrical insulation between die 12 and bar 16, it is contemplated that only a layer of thermal insulation or only a layer of electrical insulation may be used depending on the circumstances.

In the exemplary embodiment, layers 34 and 36 are formed of a flexible refractory material. This allows layers 34 and 36 to easily conform to the shape of the die cavity. In addition, the use of such flexible layers allows for versatility in positioning the layers prior to the wrapping process. For example, prior to insertion of the metal bar into the die cavity, the layers may be disposed within the die cavity (as shown), wrapped around a portion or all of the metal bar, or simply suspended between the cavity and the metal bar so that insertion of the metal bar into the cavity presses the insulation material into the desired shape. Layers 34 and 36 are typically refractory ceramic blankets. One such suitable ceramic blanket is sold under the name Kaowool. Such ceramic blankets typically provide both the thermal and electrical insulative properties previously described and are formed of woven ceramic fibers. These flexible blankets are also easily removed from the die cavity or the metal bar when degraded to a degree such that they are no longer useful for the present purpose. While such ceramic blankets are one form of a desirable insulative material, other suitable materials and/or coatings may be utilized which provide the thermal and/or electrical insulative properties needed for the present invention and which are capable of withstanding the heat and pressure utilized during the wrapping process.

Apparatus 10 further includes a thermal insulating cover 62 which generally surrounds die 12 and metal bar 16 when bar 16 is clamped between jaws 14 in its starting position. Cover 62 includes a trapezoidal top wall 64 which extends from a front wall 66 to a rear wall 68 and from a first side wall 70 to a second side wall 72. Cover 62 further includes a bottom wall 74 (FIG. 2) having substantially the same shape as top wall 64. Cover 62 includes a door 76 which includes a portion of top wall 64 and is hingedly connected via a hinge 78 to the remaining portion of top wall 64. The hinged design can also be replaced by retractable design. Door 76 also includes front wall 66 and a portion of bottom wall 74. Door 76 is movable between a closed position as shown in FIGS. 1-4 and an open position shown in FIG. 5. Each of walls 64, 66, 68 and 74 include an outer support wall 80 typically formed of a metal and an inner layer 82 of thermal insulation. Side walls 70 and 72 may also include respective support wall 80 and layer 82 of thermal insulation although it is noted that walls 70 and 72 are configured to allow metal bar 16 to move from the starting position to the completed position thereof. Thus, side walls 70 and 72 may either be completely open, partially open or may be moveable between open and closed positions so that in the open position metal bar 16 can move from the starting to the completed position. Thus, for example, side walls 70 and 72 may be hingedly connected to top wall 64 in order to move between such open and closed positions. The side walls can also be replaced by flexible curtains of ceramic fabric or blanket. In the exemplary embodiment, the insulation layers 82 of top wall 64 and bottom wall 74 respectively abut the upper and lower surfaces of die 12 while the insulation layer 82 of door 76 is spaced outwardly from die 12 to define a space therebetween which defines the starting position of metal bar 16. Cover 62 facilitates the ability to independently control the heating of die 12 and metal bar 16. The description and illustration of the cover is not limited to the hinged design. For example, other methods and designs using retractable sliding cover with or without flexible curtains of ceramic blanket or fabric for the side walls are not excluded from the scope of this invention.

The general operation of apparatus 10 is described with reference to FIGS. 1-4. With reference to FIG. 1, power source 18 is operated to cause an electrical current to flow through metal bar 16 to resistively heat metal bar 16 to a desired predetermined temperature. Once this temperature is reached, jaws 14 apply an outward stretching force as indicated at Arrows A, that is, a longitudinal tensile force or strain. Meanwhile, die 12 may or may not be heated depending on the particular circumstances. If die 12 is to be heated, power source 18 may be operated to resistively heat die 12 via wires 22 and heating elements 24. Whether or not die 12 is heated, jaws 14 are then moved toward die 12 to move bar 16 into working space 40 as indicated at Arrows B in FIGS. 3 and 4. Alternately or in combination, die 12 may be moved to facilitate the relative movement between die 12 and jaws 14. Jaws 14 are then moved as indicated at Arrows C in FIG. 4 to force bar 16 against work surface 38 of layer 36 to wrap bar 16 around die 12 to form the arcuate configuration of the formed part as shown in FIG. 4. The longitudinal stretching force continues to be applied to metal bar 16 during the wrapping of the bar around die 12. Thus, jaws 14 move from the pre-wrapping configuration of apparatus 10 shown in FIG. 1 to the post-wrapping configuration shown in FIG. 4.

The electrically insulative property of layer 34 and/or 36 prevents the electrical shunting between bar 16 and die 12 which was discussed in the Background section of the present invention. In addition, the thermal insulative property of layer 34 and/or 36 minimizes or eliminates the creation of hot spots in bar 16 which might otherwise be caused by die 12 when it is heated, and especially if not uniformly heated. The thermal insulative property also allows for the use of die 12 either without heating die 12 or heating die 12 at a substantially reduced level compared to known prior art configurations.

The method is more particularly illustrated with reference to FIGS. 6-7. FIG. 6 is a diagrammatic stress/temperature map for commercially pure titanium with a grain size of 0.1 mm, where the numbers on the left represent normalized shear stress (σs/μ), the numbers on the right represent shear stress at 20° C. (MN/m2), the top numbers represent temperature in ° C., and the bottom numbers represent homologous temperature (T/Tm) where T is the temperature of the metal in ° K. and Tm is the melting temperature of the metal in ° K. FIG. 6 is derived from FIG. 6.10 of the book entitled Deformation-Mechanism Maps: The Plasticity and Creep of Metals and Ceramics by Harold J. Frost and Michael F. Ashby, Pergamon Press, 1^st Ed. (October 1982) at p. 50. The map developed by Frost and Ashby indicates that there are specific conditions at which such titanium may be deformed by plastic deformation, by diffusion deformation or by creep formation without the involvement of plastic deformation or diffusion deformation. Such creep deformation primarily occurs in the region denoted at 84 in FIG. 6.

Applicants have determined that in order to produce a hot stretch-wrap formed titanium alloy bar of high quality which overcomes the stress problems of the prior art methods, the bar should be processed under specific conditions related to such creep deformation. As shown in FIG. 7, plasticity controlled deformation (block 86) involves poor metal flow and work hardening which leads to forming difficulty. On the other extreme, diffusional flow controlled deformation (block 88) involves fast creep deformation which leads to creep rupture. In addition, oxidation and surface contamination during diffusional deformation lead to α-case titanium products. By contrast, creep formation (block 90) within specific parameters eliminates the negative effects of plastic deformation and diffusion deformation and leads to the uniform deformation needed to produce the high-quality product desired.

More particularly, metal bar 16 is heated to a forming temperature ranging from 0.45 to 0.60 Tm (° K.) (about 650-925° C. or 1202-1690° F.) and pre-stretched at a controlled strain rate of less than 0.05 inch/inch/second to a strain of 0.5% to 3.0% while maintaining the temperature of metal bar 16 within said range. The strain rate preferably ranges from 0.00005 to 0.005 in/in/sec. Strain is defined as the difference between the stretched length and the original length of bar 16 (or portion thereof) divided by the original length of the bar or portion, respectively. For Ti-6Al-4V, a preferred titanium alloy, the forming temperature ranges from 1250-1450° F. Metal bar 16 is then creep stretch-wrap formed around layers 34, 36 and die face 26 under the same conditions. The above process minimizes residual stress within metal bar 16. However, once the step of stretch-wrapping produces the curved metal bar 16 of FIG. 4, jaws 14 hold metal bar 16 in this curved form against layers 34, 36 and die face 26 while the temperature of metal bar 16 is maintained continuously for a holding period ranging from 5 to 120 minutes to relieve any residual stress which may have developed during the stretching and wrapping process. The specific holding period depends upon the stress relaxation property of the metal of which bar 16 is formed. A longitudinal stretching force may or may not be applied during the holding period depending upon the specific circumstances.

Most preferably, metal bar 16 is maintained at a substantially uniform temperature (the forming temperature) throughout creep pre-stretching, creep stretch-wrap forming and the holding period. Once metal bar 16 is heated to the forming temperature, its temperature throughout these steps typically varies no more than 30° C. from the forming temperature, and preferably no more than 15° C. While the temperature of metal bar 16 may be maintained throughout each of these steps simply by heating metal bar 16, the use of thermal cover 62 greatly facilitates this process while reducing energy consumption. The use of one or more layers 34, 36 which have thermal insulation properties also helps prevent heat loss from metal bar 16 and thus assists in maintaining its temperature during the process, especially when layers 34, 36 are wrapped entirely around metal bar 16. In certain cases, maintaining a substantially uniform temperature for the duration of a given holding period may be achieved solely via heat retention provided by cover 62 and/or layers 34, 36 without additional heating of metal bar 16. The uniform temperature of bar 16 will typically be maintained even if die 12 is not independently heated or is heated to a temperature substantially below the forming temperature of bar 16.

The process thus produces a hot creep stretch-wrapped metal bar 16 in its final form (FIG. 9) substantially free of spring back and substantially without undesirable tensile or compressive stresses within the formed part which can respectively cause fractures 92 and buckling 94 as shown in an exaggerated manner in the prior art metal bar 96 of FIG. 8. Because the process virtually eliminates residual stress within metal bar 16, there is no need for an off-line post forming stress relieving treatment of metal bar 16, thus substantially reducing manufacturing time and costs. Additional aspects of the invention are described in greater detail in the copending patent application entitled Method And Apparatus For Hot Forming Elongated Metallic Bars, which is filed concurrently herewith and incorporated by reference herein.

Forming parameters and results of creep forming three extrusions are described with reference to FIG. 10 and the tables included hereafter. FIG. 10 shows an L-shaped extrusion or extruded bar 98 having a first leg 100 and a second leg 102 extending perpendicularly therefrom. Bar 98 represents each of the three tested extrusions which are discussed hereafter, more particularly extrusion 1, extrusion 2 and extrusion 3. Bar 98 along first leg 100 has a total width W1 and along leg 102 has a total thickness T1. Leg 100 has a thickness T2 and leg 102 has a width W2. Each of the tested extrusions were formed of Ti6AL-4V. For each of the tested extrusions, width W1 prior to forming was 1.610 inches, thickness T1 was 2.260 inches, width W2 was 0.350 inches and thickness T2 was 0.780 inches. The area of the cross section of bar 98 for each extrusion was 1.791 square inches. The scope of the invention is not limited to either the shape or the dimensions given above for illustration. The ratio of the total width to the total thickness typically ranges from 1:1 to 10:1.

TABLE 1
FORMING CONDITIONS
Parameters	Extrusion 1	Extrusion 2	Extrusion 3
Technique	Modified Prior Art	Current Invention	Current Invention
Die Temperature ° F.	1300	Ambient	Ambient
Part Temperature ° F.
Preheat	1300	1300	1350
Forming	1250	1300	1350
Die Insulation	Nil	Ceramic wool &	Ceramic wool &
		silica sheath	silica sheath
Forming
Tension	Negligible	25 T	18 T
Prestretch	Nil	1.5%	1.0%
Average Strain	<3 × 10−4	<3 × 10−4	<1 × 10−4
Rate (in/in/sec)
Post Forming Hold	No	15 Minute	30 Minutes
Metal Movement	Up to 0.5″	Minimalc <0.125″	Minimal <0.125″
	Near the ends	at the ends of the	at the ends of the
	of the formed portion	formed portion	formed portion

As shown in Table 1, extrusion 1 was formed with a modified prior art process using a die which was heated to approximately the forming temperature of extrusion 1. 10 A ceramic wool blanket was used as a barrier between the die and extrusion 1 during the heating to prevent the shunting effect for extrusion 1. The electric current passing through extrusion 1 for resistive heating was then cut off and the ceramic wool barrier was removed just before the wrapping process.

Extrusions 2 and 3 were formed using an unheated die faced with ceramic wool and silica sheath. Both were heated to the forming temperature by electric resistive heating before and during the wrapping process. After completion of the forming, these extrusions were held in position while continuing the resistance heating under the ceramic wool blanket. As may be seen from Table 1, extrusion 1 formed by the modified prior art showed substantial movement in the part after stress relieving while extrusions 2 and 3 showed minimal movement due to the pre-stretching and the post forming hold.

TABLE 2
PROPERTIES BEFORE AND AFTER FORMING
	After Forming
	Before	Extrusion 1	Extrusion 2
Properties	Forming	Sample 1	Sample 2	Sample 1	Sample 2
UTS (ksi)	136	135	135	134	134
YS (ksi)	122	121	121	122	121
%	14	13	14	12	13
Elongation
% Red.	33	30	28	32	30
Area
Alpha Case	Nil	.0005″	.00075″	.0005″	.0005″
Micro-	Normal	Normal	Normal	Normal	Normal
structure

As may be determined from Table 2, there is virtually no difference in the tensile properties at the formed and unformed portions of the extrusion. In addition, there is virtually no difference in the microstructure, alpha case and uniformity of structure between the formed and unformed portions of the extrusion. Thus, the creep forming has resulted in virtually no change in properties or microstructure compared to the original extruded and annealed bars.

In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed.

Moreover, the description and illustration of the invention is an example and the invention is not limited to the exact details shown or described.

Claims

1. A method comprising the steps of: heating an elongated metal bar to a forming temperature within a temperature range suitable for creep deformation of the metal bar; applying a stretching force to the heated metal bar at a strain rate no greater than 0.05 inch/inch/second; and wrapping the heated metal bar around a die to form a wrapped metal bar.

2. The method of claim 1 wherein the step of applying includes the step of applying a stretching force to the heated metal bar at a strain rate no greater than 0.05 inch/inch/second to a strain ranging from 0.5% to 15.0%.

3. The method of claim 2 wherein the step of heating includes the step of heating a metal bar formed of a titanium alloy to a forming temperature ranging from 0.45 to 0.60 Tm wherein Tm is the melting point of the titanium alloy.

4. The method of claim 2 wherein the step of applying includes, prior to the step of wrapping, the step of applying a stretching force to the heated metal bar at a strain rate no greater than 0.05 inch/inch/second to a strain ranging from 0.5% to 3.0%; and wherein the step of applying includes, during the step of wrapping, the step of applying a stretching force to the heated metal bar at a strain rate no greater than 0.05 inch/inch/second to a strain ranging from 0.5% to 15.0%.

5. The method of claim 1 wherein the step of heating includes the step of heating a metal bar formed of a titanium alloy to a forming temperature ranging from 0.45 to 0.60 Tm wherein Tm is the melting point of the titanium alloy.

6. The method of claim 1 wherein the step of heating includes the step of heating the metal bar to a temperature ranging from 650° C.-925° C. (1202° F.-1690° F.).

7. The method of claim 1 wherein the step of heating includes the step of heating a metal bar formed of a titanium alloy to a temperature ranging from 1250° F.-1450° F.

8. The method of claim 1 wherein the step of applying includes the step of applying a stretching force to the heated metal bar at a strain rate ranging from 0.00005 inch/inch/second to 0.005 inch/inch/second.

9. The method of claim 1 wherein the step of heating includes the step of passing an electrical current through the metal bar to heat the metal bar resistively.

10. The method of claim 9 further including the step of maintaining the metal bar at temperature which is substantially uniform throughout the metal bar and which is substantially constant throughout the steps of applying and wrapping.

11. The method of claim 9 wherein the step of wrapping includes the step of wrapping the heated metal bar around a die face of a metal die with a layer of electrically insulative material separating the die face from the heated metal bar to prevent electrical communication between the metal bar and the metal die.

12. The method of claim 1 further including the steps of holding the wrapped metal bar against the die after the step of wrapping and simultaneously maintaining the temperature of the wrapped metal bar within the temperature range for at least 5 minutes.

13. The method of claim 12 further including the step of maintaining the temperature of the wrapped metal bar within the temperature range for at least 10 minutes.

14. The method of claim 13 further including the step of maintaining the temperature of the wrapped metal bar within the temperature range for at least 20 minutes.

15. The method of claim 11 further including the step of maintaining the metal bar at a temperature which is within 30.0° C. of the forming temperature throughout the steps of applying, wrapping, holding and maintaining.

16. The method of claim 1 further including the step of positioning thermal insulation around the die and metal bar to reduce heat loss from the metal bar.

17. The method of claim 1 wherein the step of heating includes the step of heating the metal bar to a temperature which is substantially uniform throughout the metal bar.

18. The method of claim 17 further including the step of maintaining the substantially uniform temperature throughout the steps of applying and wrapping.

19. The method of claim 18 wherein the step of wrapping includes the step of wrapping the heated metal bar around a die face of the die and a layer of thermally insulative material disposed between the die face and the heated metal bar without independently heating the die face.

20. The method of claim 1 wherein the step of wrapping includes the step of wrapping the heated metal bar around a working surface which is formed of at least one of a thermally insulative material and an electrically insulative material.