Method and apparatus for joining metals

The present invention discloses methods and apparatus for joining of two workpieces. The present invention includes special preparation of the joining ends of the two workpieces to be joined. Methods of the present invention include delivering a high current, low voltage pulse to a pair of workpieces to be joined while applying and maintaining a constantly high load on said workpieces to thereby join the workpieces.

BACKGROUND OF THE INVENTION 
The present invention relates to the joining of two workpieces by a welding 
apparatus and technique. More specifically, the present invention relates 
to joining two workpieces via homopolar generator welding. 
Homopolar pulse welding (HPW) utilizes a high current, low voltage pulse 
produced by a homopolar generator to rapidly resistance heat an interface 
between two components to forging temperature. In typical HPW methods, 
flat ends of two workpieces are carefully aligned and held under a light 
initial load to focus heat generation at the interface. As the current 
pulse is discharged through the workpieces, the intense heat generated at 
this interface diffuses axially, softening the adjacent material. After a 
preset delay, an upset force is then applied to produce a forge weld at 
the interface. Only a few seconds are required from initiation of the 
pulse to completion of the weld. HPW may be used to rapidly join lengths 
of pipe in pipeline construction. It is particularly desirable for 
application in deep water offshore pipeline construction systems. HPW may 
also be used in joining rails in railroad construction. HPW may also be 
applicable for brazing, compaction, or forging. 
The homopolar generator used in HPW methods is based on the principle that 
a conductor moving normal to a magnetic field generates an electric 
potential difference between the conductor ends. In one such type of 
generator, the conductor is a disk or rotor rotating normal to the 
direction of a magnetic field generated by field coils surrounding the 
disk. A potential difference is generated between the rotor outside 
diameter and the rotor shaft. Sliding contacts on the shaft and rotor 
serve as current collection devices. A "disk-type" or "drum-type" 
generator may be used in connection with the present invention. 
FIG. 1 shows an exemplary embodiment of a "disk-type" homopolar generator 
10 that may be used in connection with the present invention. Further 
details of homopolar generators are disclosed in U.S. Pat. No. 4,544,874, 
the disclosure of which is incorporated herein by reference. 
Other solid state, forge welding methods differ from HPW in the nature of 
the energy source, joint preparation, and heating rates used to distribute 
heat to adjacent material. Such welding techniques include friction 
welding, flash butt welding, and shielded active gas (SAG) forge welding. 
For most electric resistance welding processes, welding heat primarily is 
generated at the interfaces and diffuses to adjacent material. Except for 
high frequency resistance welding techniques, intense interface heating 
results from contact resistance to the welding current flow. High 
frequency resistance welding and SAG forge welding use a high frequency 
current, flowing through the joint surface between the electrodes, to 
generate interface heat. Skin depth effects, controlled by the frequency 
of the alternating current, constrain heating to the near-surface 
material. 
Assorted current sources, including alternating and direct current types 
and stored energy machines using capacitor banks, provide the energy for 
other resistance welding methods. Except for capacitor discharge welding 
techniques, which have significantly more rapid heating rates than other 
techniques, heating rates for other resistance welding techniques are 
considerably slower than for homopolar welding. Friction and inertia 
welding use frictional heating from the relative motion between workpieces 
to introduce heat to workpieces. Intense heat generated at the interface 
diffuses to the adjacent material, heating it to its forging temperature. 
SUMMARY OF THE INVENTION 
The present invention in a broad aspect comprises a system for preparing 
opposing ends of two workpieces for joining by forming a step and bevel 
geometry on the opposing ends, contacting the formed ends under a forging 
load, and resistively heating the contacted ends under the forging load by 
discharging sufficient energy into the contacted ends. The system may also 
include applying and maintaining a substantially constant forging load 
between the workpieces during the joining process, and restricting the 
workpieces from transverse movement. By using this system, the joined 
workpiece creates a fin that may be removed. 
In exemplary embodiments, the workpieces may comprise HSLA steels. The 
substantially constant force may be applied by a hydraulic cylinder, and 
the sufficient energy is preferably delivered by a homopolar generator. 
The present invention in another broad aspect comprises a method for 
joining two workpieces having joining ends by contacting the joining ends 
to form a weld interface, mounting electrodes on each of the workpieces, 
applying and maintaining a substantially high force at a forging level of 
the workpieces, and resistively pulse heating the weld interface by 
discharging sufficient electrical energy through the electrodes to form a 
joined workpiece. This method may also include creating a step and bevel 
geometry on the joining ends of the workpieces. 
The present invention also resides in an apparatus for joining two 
workpieces, comprising a current source to provide a low voltage, pulsed 
DC current to resistively heat the workpieces, electrodes electrically 
connected between the current source and the workpieces, a welding fixture 
to support the workpieces, and a loading device, which may be controlled 
by a load controller, to apply a constantly high load at a forging level 
to the workpiece. 
The apparatus in one exemplary embodiment comprises a restricting means for 
limiting transverse movement of the workpieces during application of the 
current. The apparatus may further include a switch and busswork 
electrically connected between said current source and said electrodes. In 
an exemplary embodiment, the current source comprises a homopolar 
generator. The welding fixture may comprise a first platen and a second 
platen, with the workpieces clamped between the first platen and the 
second platen. Further, the joining ends may have a step and bevel 
geometry. In exemplary embodiments, the workpieces may be piping or rail. 
The present invention addresses the problems of the prior art by developing 
specific joint preparations of workpieces to be joined, which reduce the 
areas of contact between the workpieces. Additionally, the present 
invention provides a constantly high load to the workpieces during 
application of the resistance heating to permit joining of the workpieces. 
Localized heat is generated at and in the vicinity of the interface of two 
workpieces 20 from two separate phenomena: (1) interface heating resulting 
from incomplete contact at the mating surfaces or interface 22, which 
reduces the real area of contact to a multitude of disperse contact spots 
or asperities 24, thereby increasing the current path length and the 
current density 26 in the asperities 24, as shown in FIG. 2; and (2) 
increased current density 26 in the material adjacent to the interface 
resulting from the specific joint preparation 28, as shown in FIG. 3. With 
increasing temperature, the softened material displaces parallel to the 
interface 22 as the flow stress of the material is exceeded under the 
applied load. The workpiece shortens as the softened material is expelled. 
As the current discharges through the welding circuit, a metallurgical 
bond forms between the workpieces 20 and a smooth weld bulge 30 forms with 
an easily removable extension, fin 35, as shown in cross-section in FIG. 
4. The weld bulge 30 is free from notches or other stress raisers. Another 
benefit of the present invention is that impurities in the interface 
material are expelled into fin 35, and any inherent notches or stress 
raisers are discarded with the fin. 
In an exemplary embodiment, a low voltage, high amperage direct current 
pulse capable of delivering peak current densities of approximately 50 
kA/in..sup.2 to 100 kA/in..sup.2, and more preferably between 70 and 80 
kA/in..sup.2 for welding of steel workpieces, may be applied to the 
workpieces to be joined. This current source may be applied in a time 
duration of approximately 1 second to 10 seconds, and more preferably 
about 3 seconds to about 7 seconds. 
In an exemplary embodiment, the current source may be a homopolar 
generator. FIG. 5 is a block diagram of a HPW discharge circuit showing 
circuit resistances. In FIG. 5, homopolar generator 10 having an internal 
resistance 12 is connected via switch 56 through a circuit resistance 52 
and electrode resistance 54 to a pair of workpieces 20 to be joined at an 
interface 22, represented by bulk resistance 21 and interface resistance 
23. 
Other means of delivering the current densities required may be used. These 
means include capacitor banks and line voltage. However, capacitor banks 
deliver the energy in an amount of time on the order of microseconds, 
whereas the homopolar generator takes seconds to discharge its energy. For 
a given generator, variables relevant to weld properties include preset 
discharge speed and the magnitude of the field current. Discharge speed 
determines energy availability and field current controls the current 
pulse shape. 
The present invention preferably includes a joint preparation comprising a 
step and bevel geometry of a shaped end 60 of a workpiece 20, an example 
of which is shown in FIG. 6. As used herein the terms "step and bevel" or 
"end geometry" are not limited to that shown in FIG. 6, but may also 
include any shape or geometry having a narrowing width at the interface 
22, and a broadening width away from the interface 22 of the workpiece. 
The step and bevel preparations of the present invention are preferably 
centrally or symmetrically disposed relative to their respective 
workpieces. It may be desired, however, in a particular embodiment to 
prepare an eccentric positioning of a step and bevel preparation. 
The geometry of this joint preparation is used to control heat generation 
in and adjacent to the shaped end 60 and to expel the original interface 
surface. Changing the end geometry parameters shown in FIG. 6, such as 
step length 62, step width 64, bevel angle 66, and shoulder radius 68, for 
example, directly affects the local heating rates in the shaped end 60 and 
its resulting mechanical response under the applied load. 
Selection of a particular combination of end geometry parameters, in 
combination with other features of the present invention, may be used to 
provide a contaminant-free metallurgical bond across the entire workpiece 
area. 
In other welding methods, only projection welding, flash butt welding and 
shielded active gas forge welding use a shaped end. The shaped end in 
projection welds is designed to focus the heat generation and resulting 
deformation in the projection. Projection weld joint design uses 
"beveling" only and joins workpieces at discrete points rather than 
continuously over the entire workpiece area. Flash butt welding joint 
design uses "beveling" only to control arcing and to balance the heating 
between workpieces. 
In known HPW systems, no such end geometries or constant loading conditions 
are used. Instead, most HPW systems and methods comprise placing flat ends 
of two workpieces together so that they are carefully aligned in the axial 
direction. In known HPW systems, the workpieces are then held under a 
light initial load to focus heat generation at the interface between the 
two workpieces. As the current pulse discharges through the workpieces 
from the HPG, intense heat is generated at the interface. This heat 
diffuses axially, thereby softening the adjacent material. After a preset 
delay, an upset load is applied and maintained for a preset time. This 
load forges the softened material to form the metallurgical bond. The 
delay controls the extent of softening, and the duration of the upset load 
controls the extent of deformation. The process requires thermal diffusion 
and pulsed loading for success. Illustrations of this known process are 
shown in FIGS. 7 and 8, and are discussed further below. 
Other features of the present invention that lead to such a 
contaminant-free metallurgical bond include applied load parameters and 
generator parameters, as will be discussed below. As discussed herein, the 
present invention uses a high constant load that is applied to the 
workpieces to be joined. The constantly high load is applied at a forging 
level of the workpieces to be joined. This load will vary depending on the 
size and material of the workpieces. In other words, the constantly high 
load is applied as an upsetting force throughout the entire welding cycle. 
Combinations of selections of these parameters permit controlling the 
finished weld profile so that the bond may be characterized by a smooth 
weld bulge extending from the surface of the workpiece, and a thinner fin 
extending from the weld bulge. The weld bulge is preferably free from 
notches or other stress raisers, and in an exemplary embodiment, the fin 
contains much of the original interface material. 
The present invention is a marked improvement over prior art techniques, as 
the present invention reduces dependence on interface heating, and 
controls deformation by the end geometry of the workpieces to be joined, 
and minimizes the reliance on thermal diffusion, and simplifies load 
requirements. The reduced contact area and the high is constant load thus 
reduce contact resistance and interface heating. Using the geometry of the 
present invention with the high constant load has several benefits. During 
weld setup, the interface alignment is more tolerant to assorted 
misalignments, and acceptable surface finishes may range from 
approximately 30 .mu.m RMS to approximately 700 .mu.m RMS. During 
discharge, the characteristic discharge deformation of the interface 
material limits the peak temperatures and time at temperature, which 
therefore permits improved mechanical properties. 
Additionally, with prior methods, the interface experienced extremely high 
temperatures (estimated to be upwards of 50,0000.degree. F.), with 
resulting incipient melting of the high carbon/alloy material near the 
point of contact. This caused metallurgical discontinuities, producing 
poor mechanical properties in finished welds using prior methods. A lower 
peak temperature at the interface and a shallower heating gradient from 
interface to the parent metal assures a more coherent microstructure and 
better, more consistent mechanical properties in the finished weld of the 
present invention. 
The primary control of heat generation in the workpiece 20 of the present 
invention is control of the shape of the joint between the two workpieces 
20. The step and bevel joint preparation creates an axial cross-sectional 
area distribution that affects the local heating and the local stress 
magnitude. The reduced area increases the heating rates and produces an 
axial temperature gradient. The temperature gradient and the stress 
gradient coincide, resulting in rapid softening of the step to its flow 
stress. The softened step, containing the interface, rapidly displaces 
parallel to the interface and forms a fin 35. As current continues to 
discharge through the deforming joints, the bevel and radii soften and 
forge according to their cross-sectional area to form the weld bulge. 
Material yields as it softens to its flow stress, as shown in FIGS. 9 and 
10, discussed further below. 
The shaped end geometry of the present invention provides sufficient heat 
for welding without using the interface heat, as demonstrated by 
performing simulated welds. (It is to be noted that simulated welds use 
typical weld generators and fixture parameters; however, in simulated 
welds, a solid pipe replaces the pair of pipes. The solid pipe is machined 
to produce the equivalent cross-section of the pair of butted pipes.) The 
geometry of the present invention also permits current to pass through the 
shaped end to generate an in-place temperature gradient without primary 
reliance on thermal diffusion. 
In prior known HPW systems and methods, a light initial load was applied, 
followed by an upset load, as shown in FIG. 8. As shown in FIG. 8, a total 
current of approximately 215 kA was discharged into the system. An 
interface voltage of less than approximately 1 volt arose in the 
workpieces 20 to be joined. A light initial load of approximately 10 kip 
was applied to the workpieces 20. After approximately 1 to 4 seconds, an 
upset force of approximately 60 kip was applied for a brief period (i.e., 
0.5 to 2 seconds), thereby causing displacement and joining of the two 
workpieces 20. 
The present invention provides a constantly high load during the weld 
process, as shown in FIGS. 9 and 10. As shown in FIG. 10, a total current 
of approximately 216 kA is discharged into the system. An interface 
voltage of less than approximately 1 volt arises in the workpieces 20 to 
be joined. A substantially constant force of approximately 60 kip is 
applied to workpieces 20 throughout the joining process, thereby causing 
displacement and joining of the two workpieces 20. It is desired that the 
applied load remain substantially constant as the joint forges, thereby 
shortening the workpieces 20. Use of a constant load by the present 
invention simplifies the load control requirements. The higher load at 
initiation reduces resistance at the interface by increasing real contact 
area. Further benefits from constant loading of a high level include 
reduced interface resistance and lower peak temperature, improved 
metallurgy, and reduced time at temperature. 
Thus, the present invention as applied to the pipe discussed below in TABLE 
1 uses a load that is approximately 50 kip higher than the prior art upon 
initialization. As welding occurs, the prior art applied an upset load of 
approximately 60 kip, which occurred for approximately 0.5 to 2 seconds. 
The present invention thus constantly provides a much higher and constant 
load than that used in the prior art. 
The present invention thus combines a homopolar generator power source with 
constant loading and step and bevel geometry. This combination controls 
the heating and subsequent mechanical response of the workpiece, and 
provides a mechanism to expel the interface. It is envisioned that the 
present invention is also capable of joining extended surfaces, such as 
cylinders and bars. 
Further benefits of the present invention include the following advances. 
Lower homopolar generator discharge speed reduces system requirements. 
Reduced sensitivity to interface conditions, such as surface finish and 
waviness, create an improved weld profile with an easily removable fin. 
Because the time that the system operates at an elevated temperature is 
reduced, weld metallurgy is also improved. The metallurgy is also improved 
because dynamic recrystallization occurs after deformation commences when 
the material heats to flow stress, thereby increasing grain refinement in 
the weld microstructure. The present invention also provides improved 
impact toughness for certain high strength materials, such as QT (quenched 
and tempered) and CR (control rolled), without post-weld heat treatment. 
The geometry of the present invention also provides for reduction in 
heat-affected-zone (HAZ) softening and width. 
Exemplary ranges of welding parameters producing acceptable welds are shown 
in TABLE 1 below. It is to be understood that certain parameters may be 
greater or less than those shown in TABLE 1 and still fall within the 
scope of this invention. The values shown in TABLE 1 are for an exemplary 
welding process using nominal 3 inch Sch-80 API line pipe. Any size pipe 
will be welded in a similar manner, after adjusting these parameters to 
accommodate size and material differences. 
TABLE 1 
______________________________________ 
Range of Weld Parameters Producing Acceptable Welds 
For Nominal 3 Inch Sch-80 API Line Pipe 
Parameter Unit Low Setting 
High Setting 
______________________________________ 
Discharge Speed 
rpm 2000 2200 
Field Current 
A 300 400 
Applied Load kip 45 60 
Step Length in. 0.05 0.10 
Step Width in. 0.10 0.15 
Bevel Angle degree 30 45 
Shoulder Radius 
in. 0 0.15 
______________________________________ 
These ranges are within normal machining tolerances and existing HPW 
control system capabilities. Welds with acceptable mechanical properties, 
especially near parent metal impact toughness values have been made in 
high strength low alloy (HSLA) steels having carbon contents from 
approximately 0.08% to 0.13% by weight, and IIW.sub.EQ carbon 
equivalencies from approximately 0.23 to 0.33%. 
Examples of particular welding applications contemplated for the invention 
include butt joining of plates, landing gear joining, joining of jet 
turbine components, joining of tubing, and railroad rail joining. An 
application of particular interest is in joining of line pipe, especially 
where single station welding operation is required, as in J-lay systems. 
Exemplary materials that may be used in connection with the present 
invention include, for example, Inconel.TM., stainless steels, and 
titanium and its alloys.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
FIG. 11 shows an apparatus constructed according to the present invention. 
FIG. 11 includes a homopolar generator 10 connected via switches 56 and 
busswork 90 to electrodes 102, which are attached to workpieces 20. The 
workpieces in this instance are two sections of pipe. The hydraulic 
welding fixture 100 includes upper platen 110 and lower platen 112 to hold 
workpieces 20 in axial alignment during the welding operation. Below lower 
platen 112 is a load sensing device 114, which measures a load force from 
a hydraulic cylinder 106. Power to the hydraulic cylinder 106 is supplied 
from hydraulic lines 108. 
An exemplary method according to the present invention for welding 3" steel 
pipe is with a nominal cross-section of 3" will be discussed. It is to be 
noted that the choice of weld parameters, discharge speed, field current, 
etc., is determined by the specific material to be welded, including 
cross-sectional area and material type. For steels, peak current densities 
between approximately 70 to 80 kA/in..sup.2 produce acceptable welds. 
In operation, a homopolar generator 10 is accelerated until it reaches a 
speed of between approximately 1700 rpm and 2500 rpm, and more preferably 
between about 2000 rpm and 2200 rpm. It is to be noted that the speed for 
a given machine depends upon the current requirement. At the time the 
homopolar generator 10 reaches the desired speed, the homopolar generator 
10 is discharged and current is delivered through busswork 90 to the 
electrodes 102. The busswork 90 may be comprised of copper or aluminum, 
and is used to provide the desired current to workpieces 20 with minimal 
loss. The homopolar generator 10 is discharged via switches 56 into the 
busswork 90. In an exemplary embodiment the switches may be made of 
silicon-carbon. However, any switch capable of handling the large current 
may be used. Alternately, the circuit may be completed simply by dropping 
the brushes of the homopolar generator 10. 
The electrodes 102 are connected to workpieces 20 and, in an exemplary 
embodiment may be made of copper or copper alloy. In other embodiments, 
electrodes 102 may be made of other conductive material, such as aluminum. 
The electrodes 102 enable current to be introduced into workpieces 20 
without burning, arcing, or overheating the workpieces 20. The electrodes 
102 also act as a heat sink during cooling, after welding has occurred. In 
an exemplary embodiment, the electrodes 102 may be shaped so as to 
surround the workpieces 20 to which they are attached. 
In an exemplary embodiment, electrodes 102 may be configured as sleeves of 
contact fingers connected to a platen, serving as a base. Each sleeve may 
have slots cut so that the sleeve becomes a concentric ring of fingers. 
The ends of the fingers may be fastened to workpieces 20 via jackscrews 
mounted in a collar or a hydraulic cylinder may be used to push the 
fingers together. Also in an exemplary embodiment, the electrodes 102 may 
be placed such that they contact portions of the workpieces 20 near the 
joining is ends. The electrode gap 104 between electrodes 20 is shown in 
FIG. 11. In an exemplary embodiment, the electrode gap 104 may be between 
about 0.5 inch and 4 inches, and more preferably between about 1 inch and 
2 inches. 
Platens 110 and 112 may be comprised of any conductive material such as 
copper or copper alloy. In addition to the platens 110 and 112, 
restricting means (not shown in FIG. 11) may be provided to prevent 
transverse movement of the workpieces 20 during welding. In an exemplary 
embodiment, the restricting means may be linear bearings or radial clamps. 
Other means such as an internal mandrel may also be used. The restricting 
means acts to limit transverse movement of the workpieces 20 as the 
interface between workpieces 20 buckles under the applied load force. 
The load force is applied via hydraulic cylinder 106 to a load sensing 
device 114, which transmits the force to workpieces 20. In addition to 
hydraulic cylinder 106, other load devices such as lead screws or linear 
springs may be used. The constantly high load force is applied from a time 
prior to discharge of the homopolar generator 10 until a time the 
workpieces 20 have been joined. For the 3" pipe discussed, the load force 
applied may be between approximately 10 kip and 90 kip, and more 
preferably between about 45 kip and 60 kip. In an exemplary embodiment, 
the load force is substantially constant through the resistive heating and 
welding processes. 
Although desired to be substantially constant, the present invention 
contemplates that the applied load force may vary somewhat from a constant 
force. As shown in FIG. 10 for example, during initial application of the 
load force, a slight drop in force is seen approximately 0.2 second after 
application. This drop in force is caused by rapid softening and 
subsequent shortening of the workpieces that exceeds the displacement rate 
of the hydraulic system. However this drop does not affect the quality of 
the weld achieved. 
In operation, the homopolar generator 10 is accelerated to a rotational 
speed above the discharge speed via hydraulic motors, for example. 
Motoring is discontinued at this point and the rotor begins decelerating. 
In sequence, the field coils are energized, producing a uniform magnetic 
field across the rotor, and the brushes are lowered, further slowing the 
rotor. When the rotor speed decelerates to the set point, switches close, 
discharging the stored rotational kinetic of the rotor, as it is converted 
to a DC electrical pulse. The low voltage, large magnitude DC current 
pulse tracks through the electrodes 102 and into workpieces 20, providing 
both bulk heating and interface heating. Throughout this time, a constant 
load is applied to first platen 110 and second platen 112 to complete the 
weld. 
The workpieces 20 may be comprised of various metals, such as a nickel 
super alloy (Inconel.TM.), titanium, and stainless steel. In exemplary 
embodiments, the wall thickness of pipe workpieces 20 may be between 
approximately 0.2 inch and 0.6 inch, and more preferably between 0.3 
inches and 0.45 inch. However, the present invention may be used on 
workpieces having larger or small wall thickness than these. It is noted 
that the values discussed herein are listed in connection with welding of 
pipe having a wall thickness of 0.3 inches. 
The end geometry of the joining ends of the workpieces 20 creates contact 
between the two workpieces 20 to be joined. The workpieces 20 may be 
prepared with the desired end geometry by various methods. For example, 
the workpieces 20 may be lathed, machined, or otherwise prepared, such as 
with a pipe-shaping machine. As stated previously, the abutting ends of 
the workpieces are beveled to a reduced size. 
As shown in FIG. 6, parameters of the step and bevel geometry include 
contact point or step width 64, step length 62, bevel angle 66, and 
shoulder radius 68. In an exemplary embodiment, the step width 64 may be 
between approximately 0.1 inch and 0.3 inch, and more preferably between 
about 0.10 inch and 0.15 inch. The bevel angle 66 may be between 
approximately 25 degrees and 75 degrees and more preferably between about 
30 degrees and 45 degrees. The step length 62 may be between approximately 
0.01 inch and 0.2 inch, and more preferably between about 0.05 inch and 
0.10 inch. 
In an exemplary embodiment, the workpieces may be further prepared prior to 
the welding. For example, workpieces 20 may have their joining ends 
finished by sandblasting, wire brushing, or grinding. The purpose of 
finishing the workpieces is to provide smooth joining surfaces for 
improved weld quality and provide suitable contact for electrodes 102. 
Examples of welds performed by the methods of the present invention are 
shown in FIGS. 12A-12H. As seen in FIGS. 12A-12H, these welds have 
different shapes and qualities, as a result of different combinations of 
parameters used in making the welds. FIGS. 12A-12H show the initial 
cross-section of one of the workpieces to be joined, as 150A-150H, 
respectively, superimposed on the finished cross-section of the joined 
workpieces as 160A-160H, respectively. FIGS. 12A-12D show the results of 
welds in which the step length 62 is fixed at a short length of 
approximately 0.05 inch. FIGS. 12E-12G show the results of welds having a 
longer step length 62, of approximately 0.10 inch, thereby resulting in a 
longer fin 35. 
FIGS. 12A, 12C, 12E, and 12G show the results of a narrow step width 64, 
approximately 0.10 inch, whereas FIGS. 12B, 12D, 12F, and 12H show the 
results of a wide step width 64, approximately 0.15 inch. FIGS. 12A, 12B, 
12E, and 12F show the result of a bevel angle 66 of approximately 
30.degree., and FIGS. 12C, 12D, 12G, and 12H show the result of a bevel 
angle 66 of approximately 45.degree.. 
Further modification and alternative embodiments of this invention will be 
apparent to those skilled in the art in view of this description. 
Accordingly, this description is to be construed as illustrative only and 
is for the purpose of teaching those skilled in the art the manner of 
carrying out the invention. It is to be understood that the forms of the 
invention herein shown and described are to be taken as the presently 
preferred embodiments. Various changes may be made in the shape, size, and 
arrangement of parts. For example, equivalent elements or materials may be 
substituted for those illustrated and described herein, and certain 
features of the invention may be utilized independently of the use of 
other features, all as would be apparent to one skilled in the art after 
having benefit of this description of the invention.