Note: Descriptions are shown in the official language in which they were submitted.
9998
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This invention relates to a process for contin-
uous seam welding of strips of sheet material at high
speed utilizing a laser beam as the welding source of
energy and to the weldment produced by such process.
In order to seam weld sheet material at high
rates of linear speed, two conditions must be fulfilled.
The weld energy must be delivered to the workpieces at
high density so that the heating is local, and the weld-
ment must be formed quickly before the heat diffuses away
into the bulk of the metal. Heretofore, the composition
of the material was particularly significant in controlling
the welding rate,especially where the material was a con-
ductive metal such as aluminum. In conventional gas and
electric welding processes, the welding speed is limited
to less than about 40 feet per minute even for light gage
metal material because the heating is not sufficiently
local, with a substantial amount of the heat being lost
to the metal bulk and the surroundings. High frequency
` resistance welding is able to accomplish high speed, in
some cases 300 to 400 feet per minute, but only in a
limited number of configurations where the contact area
; is narrow and the weld energy is concentrated in the con-
tact area. An electron beam provides a high energy density
source but requires a vacuum working environment to
r
provide a high density beam over a reasonable dis- ~
.
tance. Hence, all known welding processes to date are
either intrinsicelly incapable of welding workpieces at
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reasonably high travel speeds of at least 100 fpm, partic-
ularly for workpieces of sheet aluminum, or are otherwise
handicapped by specific conf:iguration limitations and
impractical fixturing requirements.
It has been discovered in accordance with the
present invention that a continuous welded seam can be
established between moving workpieces of sheet material
by utilizing a laser beam as the welding source of energy
provided certain critical requirements are met. Laser
beams have heretofore been successively employed as high
power, high energy density sources of energy to provide
deep penetrating welds and for spot welding. In all pre-
vious applications to which lasers have been applied in
the welding field, the direction has been to higher power
for deeper penetration. The process of the present inven-
tion is not limited to a specific minimum power density.
In fact, penetration through the cross-section of the
workpieces is undesirable to the process of this invention
and for certain applications detrimental. Stated other-
wise, the process of the present invention will produce a
welded seam between the strips of sheet material which is
not visible except at the ends of the seam. Any laser
source may be used although there will be a trade-off in
welding speed at reduced laser power. Using only a one
1 kw C02 continuous wave laser, welding speeds of up to
500 feet per minute have been achieved with excellent
weld quality.
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It has also been discovered that the weldment
produced by the process of the present invention is a
"Fusion Weld"g hereinafter defined as coalescence between
the base materials resulting from bringing them to a
molten state in the fusion zone; and which weldment i8
further characterized by the absence of a "Heat Affected
Zone (HAZ)" in the surrounding base material. HAZ is a
conventional term which is defined as that portion of the
base metal adjacent to the fusion zone which has not been
melted but whose mechanical properties or microstructure
have been altered by the heat from the formation of the
weld. The absence of a Heat Affected Zone (HAZ) surround-
ing the fusion zone is defined for purposes of the present
invention as the inability to detect microstructural al-
terations under a conventional optical microscope at up
to lOOx magnification. Under such circumstances the ex-
- tent of any microstructural alterationswould be less than
;0004 inches. All known welding processes to date produce
a weldment with a clearly discernible Heat Affected Zone
(HAZ) visible in most cases to the naked eye alone. Known
, . . . _
conventional laser and electron beam welds result in
weldments with a significant HAZ apparent in pho~omicro-
graphs taken with an optical microscope at 50x magnifica-
. .......... . ........ . . .......... ...... . ..
tion.
The method of continuous seam welding of moving
strips o sheet materia~ according to the present inven-
= ..... , . ............. _ ~ _ _ _ _ .. _._ _ _ . . __. _ . ~
tion comprises:
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directing at least one of the moving
strips toward the other to form a converging
Vee between the moving strips;
applying a force of more than zero pounds
! at a location contiguous to the point at which
said moving strips converge such that the
moving strips overlay one another in intimate
contact at the point of convergence; and
directing a laser beam of energy into
said converging Vee such that a continuous
` welded seam is established between the
moving oveelaid strips.
In addition, a continuous seal weldment is
j formed comprising a fusion weld nugget established
i between two base materials characterized by the absence
of a surrounding Heat Affected Zone (HAZ).
Accordingly, it is the principle object of the
, present invention to provide a process for welding moving
sheets of strip material at high speed using a laser beam
,~ 20 to establish a welded seam between the moving strips.
~` It is a further object of the present inventionto provide a weldment comprising a fusion weld nugget char-
acterized by the absence of a Heat Affected Zone (HAZ).
Further objects and advantages of the present in-
vention will become apparent from the following detailed
description when taken in connection with the accompanying
drswings in which:
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~85~64
- Figure 1 is a plan view of the preferred apparatus for practicing the process of the
present invention;
Figure 2 is a graphical representation
of weldment quality versus focal poin~ using
two different focal length lenses under an other-
wise given set of process parameters;
Figures 3a-3e are enlarged representa-
tional views of the converging Vee formed
between the pressure rolls for illustrating
the effects of the following parameters upon
welding performance: focal point position, focal
length and pressure roll diameters; and
Figures 4(a-b) and 5(a-b) are photo-micro-
graphs at lOOx magnification of the welded seam
between two strips of aluminu~ sheet at
400 and 500 feet per minute respectively.
Figure 1 illustrates apparatus for carrying out
the process of the present invention. Two strips of sheet
material 10 and 12 are drawn from storage reels 14 and 16
in a direction toward one another to form a converging Vee
x geometry with the strips 10 and 12 overlaying one another
at the point of convergence 18. The strips 10 and 12 are
driven into contact by pressure rolls A and B respectively,
such that the point of tangency between the pressure rolls
equals the point of convergence 18. Idler rollers 20 and
' 22 may be used to assist in manipulating the strips 10 and
12 and for maintaining tension in the strips as they are
being drawn. Although each sheet material 10 and 12 is
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lU~5 ~t;4
shown in the ~pparatus of Fi~lre 1 consisting of a wound
strip of continuous length, it is to be understood that the
strips of sheet material 10 and 12 are not limited to con-
tinuous length strips. Where the strips of sheet material
are of predetermined finite length an alternative dispens-
ing arrangement would be necess-ary to process the strips,
preferably consecutively, through the pressure rolls devices
A and B respectively. There are known dispensing arrange-
-- .
ments which can be employed with conventional equipment
to continuously or discontinuously, and at controlled
time intervals, feed strip material of finite length in a
manner conforming to the process of the present invention.
For practicing the process of the present inven-
tion, the strips 10 and 12 may be of any metal or plastic
composition although the composition of each shall be sub-
stantially compatible. Moreover, the properties of the
~- sheet material, such as its conductivity and thermal diffus-
ivity is not a limitation. Hence, the process is particular-
ly suited to welding conductive metals such as aluminum and
copper. Furthermore, the material thickness is limited
solely by practical handling and speed considerations. As
such, sheet material from very thin gage foil of only .001
inch in thickness to sheet thicknesses of up to 1/4 inch may
be readily welded by the process of the present invention.
` The strips 10 and 12 are drawn through the
pressure rolls A and B by traction devices 24 and 26
which draw the strips downstream of the point of con-
vergence 18 and along a predetermined and preferably
invariant path in the direction shown by the arrows in
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9998
i~ 85 ~
Figure 1. Although it is preferable to draw the strips
10 and 12 through the pressure rolls A and B from a point
` downstream thereof, the strips may be fed from upstream
of the pressure rolls or alternatively by driving the
pressure rolls themselves. The speed at which the strips
are driven through the rolls A and ~ is a process variable
which is influenced in a manner to be discussed at greater
length hereinafter.
A conventional source of laser energy 30 gen-
erates a laser beam 32 which is optically focused by
a lens 34, or other conventional optical medium, into the
' converging Vee formed between the moving strips 10 and 12
respectively. The power of the laser 32 is not a critical
factor in establishing a welded seam between the moving
strips; it is, however, one of the controlling variables
in determining the maximum travel speed at which a con-
tinuous weld can be made. For any laser of given power
there is an optimum relationship between focal length,
focal point position, beam diameter, beam orientation,
pressure roll diameter and welding speed which will pro~
~ duce a weld of acceptable quality. In fact, proper focus-
i ing of the laser beam 32 into the converging Vee is essen-
tial if one is to obtain a weld at all regardless of
laser power. Moreover, by appropriate focusing in accord-
ance with the present invention, optimum utilization of
the laser beam energy will be achieved. The focusing of
the laser beam will be discussed at greater length herein-
~ after in connection with Figures 2 and 3.
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lV~S9L64
The pressure rolls A and B perform a critical
function in combination with proper focusing of the laser
beam for practicing the process of the present invention.
It has been determined that the strips 10 and 12 must not
only abut each other in intimate relationship at the
point of convergence 18 but in addition there must exist
at least a nominal compressive force against the strips
at such location. A total absence of pressure will re-
sult in a total failure to achieve a continuous weld
between the moving strips even at substantially reduced
speeds with otherwise optimum process variables. The
magnitude of the compressive force does not appear sig-
nificant provided that at least some positive pressure
is being applied. Too much pressure is in fact a dis-
advantage and may cause physical deformation.
; It is to be understood that the weld to be
formed between the moving strips must exhibit continuity
~ as the strips advance. A lack of continuity in the seam
;, is equivalent for purposes of this disclosure to no weld
at all. Weld continuity can be established simply by
' visual inspection or by pressure testing the seam for the
existence of leaks. Obviously the quality of the weld
will be dependent upon meeting at least certain minimum
pressure requirements which will depend upon the applica-
tion of the welded strips.
The pressure rolls A and B are preferably con-
, ventional squeeze rollers having a circular periphery.
Other means may be employed provided such means assume a
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S9~64
curvilinear cnntour as e.lcll apyroaches the point ~f con-
ver~ncc. F`or bilaterial weld symmetry, the di~neters
of the l)ressure rolls A and }~ are equal.
Figures 2 and 3 indicate ~oth the importance of
focusing and ~he cli~le~er of the pressure rolls A and
to the quality of the weld.
To realize a weld the laser beam rnust be focused
into the converging Vee sub~tantially about the point of con
vergence. The latitude that may be taken in focusing
depends primarily upon the focal length, beam diameter, the
squeeze roll diameter and upon the speed to be attained.
Figures 2 and 3a-e are the result of a number of tests that
were conducted using a 1 kw C02 continuous wave 10.6 micron
laser, having a .5 inch diameter TEMoo mode output beam
which was focused through a 2.5 inch and a 3.75 inch focal
` length optical lens respectively to a focal spot diameter
of approximately .004 inches at focal points, fl, f2 and
f3. A number of additional focal point posltions relative
to the point of tangency were used to establish the outline
for the graphical representation of Figure 2. Extrapola-
tion from Figures 2 and 3 establish the importance of the
ollowing criteria for high speed continuous seam welding
of over at least 100 feet per minute:
- (a) The laser beam should be introduced substan-
tially along the ';plane of symmetry" which is hereinafter
de~ined as the plane which passes through the tangent point
18 between pressure rolls A and B and which lies parallel
to their longitudinal axes. When the laser beam is offset
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from the plane of symmetry but lies in a plane which is
parallel to the plane of symmetry a non-symmetrical weld
is formed between th~ strips. The extent of asymmetry is
directly proportional to the offset. However, the posi-
tion of the beam within the plane of symmetry is adjust-
able over a wide range of up to at least + 30 provided
the focal point is relatively accurately maintained as
will be explained hereafter;
(b) If optimum utilization of the laser beam
source is not required and the laser beam is of sufficient
j power then the focal point may be placed substantially about~
the point of convergence 18. If, however, optimum utili-
zation is desired then the focal point of the laser should
be maintained within a narrow focal point range from essen-
tially the point of convergence to a location downstream
thereof. It is to be understood that the expression
, "optimum utilization" for purposes of the present disclo-
sure means the ability to achieve a continuous weldment
¦ at the highest possible speed using the least amount of
, 20 laser beam energy. The focal position relative to the
point of tangency versus pressure is illustrated in Figure
2 for a 2.5 inch and a 3.75 inch focal length lens respec-
tively with a beam diameter of .5 inches. The focal point
range in which an acceptable continuous non-interrupted
welded seam is established between the moving strips will
vary with variations in the process parameters. For the
1 kw C02 laser as described heretofore and focused within
the plane of symmetry at two aluminum strips moving at a
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10~5464
speed of at least 400 feet per minute with 1-1/8 inch di-
ameter pressure rolls A and B, the acceptable focal point
range is only about .070 of an inch wide for the 2.5 inch
focal length lens and about .130 of an inch wide for the
3.75 inch focal length lens. Interestingly, and quite sur-
prisingly, the focal point range extends from about the
point of con~ergence in the downstream direction only. The
focal point range can be widened by reducing the diameter
of the pressure rolls A and B and/or the operational speed
f 10 and/or by either increasing the laser beam power or the
focal length or both. However, it is postulated that, for
high speed operation, an acceptable weld cannot be estab-
lished between the strips without focusing the beam to a
focal point essentially at the point of convergence or
' beyond it, i.e., downstream thereof even with a laser beam
of substantially higher power;
(c) The passage of a laser beam, which is of a
conical geometry, into a converging Vee geometry formed
between the advancing strips of material 10 and 12 re-
` 20 spectively may cause some clipping of the beam depending
on the size of the converging light cone, i.e., focal
. length, focal point position and pressure roll diameter.
For the focal point positions indicated hereinabove, clip-
~ ping of the laser beam by the pressure rolls was unavoid-
f, able. Under certain circumstances clipping may, in fact,
i be desirable. Cnce the laser beam strikes the pressure
roll, a portion of the laser beam energy will be reflected
into the converging Vee and hence into the active weld
.
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10~5~
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zone, a portion will be absorbed by the moving strips
and appear as heat, and a portion will be scattered dif-
fusely and lost. The farther the clipping occurs from
the point of convergence, the greater the fraction of
laser beam energy that will be lost.
; The relationship between clipping, if any, focal
point position, pressure roll diameter and focal length
;
are shown in Figures 3a-3e where the diameter of both
pressure rolls A and B was varied from a diameter of
1-1/8 inches to a diameter of two inches and the optical
lens 34 shifted along the optic axis and varied in focal
length from 2.5 to 3.75 inches to establish focal point
positions fl, f2, and f3 respectively. It should be under-
stood that reference to the diameter of the pressure rolls
A and B is intended to embrace the additional thickness
' provided by the strips 10 and 12. The laser beam diameter
in each case was 1/2 inch. For a focal point position fl
terminating downstream of the point of tangency 18 as is
' shown in Figures 3a and 3b clipping occurred at point C
with the 1-1/8 inch diameter pressure rolls A and B and a
focal length lens of 2.5 inches as shown in Figure 3a and
at point D with the two inch diameter pressure rolls A and
B and with the same focal length lens as is shown in Figure
3b. With a focal point position f2 terminating at the
point of tangency 18 as shown in Figure 3(c) using the
same 2.5 inch focal length lens and the two inch diameter
pressure rolls ~ and B clipping occurs at point E. With
a .5 inch diameter beam and a 3.75 inch focal length lens
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lV8~4~;~
focused at the focal point position fl, as is shown in
Figures 3d, using 1-1/8 inch cliameter pressure rolls A and
B, clipping occurred at point F which is closer to the
point of tangency than points C, D and E. This substan-
tiates the fact that the extent of beam clipping can be
reduced by increasing the focal length. Empirical evalua-
tion of the welds from Figures 3 (a-d) substantiates that
for a 2.5 inch focal length lens a better quality weld is
' achieved using the smaller diameter pressure rolls an~
for the 3.5 inch focal length lens a superior weld was
obtained over a broader range using the smaller diameter
pressure rolls. Hence, under otherwise given conditions
smaller diameter pressure rolls will result in greater
energy efficiency. If clipping is maintained sufficiently
close to the point of convergence, the Vee geometry will
effectively channel the laser beam energy into the weld
zone. The third focal point position f3 as is shown in
Figure 3(e) was established with a .5 inch diameter beam,
a 2.5 inch focal length lens and 1-1/8 inch diameter
pressure rolls A and B, and terminates at a location just
preceding the point of tangency 18, i.e., slightly upstream
of the point of tangency. Here, notwithstanding the fact
~; that no clipping occurs nor the closeness of the focal point
~ to the point of tangency 18, a continuous weld could not be
A achieved. Accordingly, the power of the laser beam is not
nearly as important in achieving a continuous weld as the
location of the focal point~ the size of the converging
light cone as determined by the focal length and beam
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diameter, and the pressure roll diameter 8S explained
hereinabove in paragraphs ~a), (b) and (c) respectively,
when optimum utilization of the laser beam source is re-
quired. Further, the properties of the converging Vee
geometry permit more effective absorption of the laser
beam energy resulting in higher welding speeds, and act
to inhibit balling of the welded material. The latter is
~' a problem commonly associated with edge weldment tech-
niques on thin section material.
Photomicrographs of the weldment produced by the
process of the present invention using the 1 kw C02 laser
` as defined heretofore and under conditions which fulfill
the criteria discussed hereinabove are shown in Figures
4(a-b) and 5(a-b) for sheet aluminum strips of .006 inches
` in thickness with Figures 4(a-b) showing the weldment
obtained at a welding speed of 400 feet per minute and
Figure 5(a-b) showing the weldment obtained at a welding
speed of 500 feet per minute respectively. The photo-
micrographs were taken using a conventional optical micro-
scope at lOOx magnification. The weldment in each case
has a micro-structure which is characteristic of all fusion
welds but shows no evidence of a Heat Affected Zone (HAZ)
at such magnification. A Heat Affected Zone, as stated
earlier, is normally visible to the naked eye. Both
Figures 4 and 5 show the weldment lengthwise, to illus-
trate the continuity of the weld along the length of the
seam, as well as in cross-section. The weld obtained at
400 feet per minute is more circular in cross-section than
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that obtained at 500 feet per minute as is evident from a
comparison of Figure 4b with Figure 5b. Both weldments
are symmetrical ~nd have a thlckness of only a fraction
of the strip thickness. In fclct, the thickness of the
weldment is essentially independent of the strip thickness.
, The examples referred to above relate to strip
material of aluminum. Other strip material compositions
were tested which substantiate the applicability of the
process to carbon steel, stainless steel, copper, brass
and dissirnilar materials represented by combinations of
the metals herein specified; all of which resulted in
equally successfully continuous welds. Accordingly, the
invention as disclosed and claimed herein should not be
construed as limited to any specific strip material com-
position. In addition, the weldment produced for each
case except stainless and carbon steel was characterized
by the absence of a Heat Affected Zone (HAZ).
It is to be understood that many variations are
possible in practicing the present invention. For example,
although Figure 1 describes the preferred system with the
laser beam directed substantially within the plane of sym-
metry and having its major vector component in the direction
of travel, an alternate embodiment would be to position the
strips to form a Vee configuration and then to move the
strips relative to the laser beam such that vectorially the
major component of the beam is perpendicular to the-direc-
tion of travel.
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~35~
SUPPLE~IENT ~ Y DISCLOSIn~E
In the foregoing description it was assumed
that the pressure rolls A and B were substantially non-
. compressible and therefore did not deform or flatten
out. In such instance the"point of tangency"l8 equals
or coincides with the"point o convergenc~'between the
pressure rolls. By "point of convergence" is meant the
point where the converging strips are first broughtinto
intimate contact with one another~ By "point of tangency"
is meant the unique point where two round incompressible
pressure rolls can ~ust be made to touch each other. In
the case of compressible pressure rolls, the point of
tangency can be defined as the point midway on the line
of contact formed by the rolls. Compressible pressure
rolls are squeeze rolls which will deform or flatten out
at or around the point where the rolls make contact with
strips 10 and 12. This teformation of flattening out of
the rolls causes the point at which the strips 10 and 12
. converge to actually shift or position ~tself away from
the point of tangency so that the point of convergence
between the strips is now actually in the upstream
direction.
. Reference is made to copending application 262884: filed October 6, 1976 by the same applicant on an invention
~ of Robert F. Heile.
