CA1085464A

CA1085464A - Method for laser seam welding of moving workpieces

Info

Publication number: CA1085464A
Application number: CA263,798A
Authority: CA
Inventors: Joseph D. Adlam
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1975-10-24
Filing date: 1976-10-20
Publication date: 1980-09-09
Also published as: AU1892476A; AU502774B2; BE847586A

Abstract

LASER WELDING

ABSTRACT

A continuous seam weldment is established between two strips of sheet material while the strips are moving by forming a converging Vee geometry between the moving strips, applying a pressure at the point of convergence and focusing a laser beam into the converg-ing Vee.

S P E C I F I C A T I O N

Description

iL085~64 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- ~
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tance. Hence, all known welding processes to date are either intrinsicelly incapable of welding workpieces at '' ` 7 ~ '?

., ~; ' 1~)8~4t;4 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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'~08~ 4 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:
, 1~5~64 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 .~ .

,,, ,, 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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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.

;j 8 s , 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 'J'J9 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 .~ .

i 10 :, ~o~ 64 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 .. . .

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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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 9~98 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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~ S ~4 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 , ~5 4~ 4 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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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.

Claims

WHAT IS CLAIMED IS:

1. A method for continuous seam welding of flexible strips of metallic sheet material having reflec-tive surfaces while the strips are moving, comprising the steps of:
(a) directing the moving strips toward one another to form a converging Vee between the moving strips with the reflective surfaces facing one another;
(b) applying a force of above zero pounds at a location contiguous to the point at which the moving strips converge such that the moving strips overlay one another in intimate contact at the point of convergence;
(c) generating a laser beam of energy;
(d) providing an optical medium for focusing said laser beam;
(e) focusing said laser beam with said optical medium to produce a converging beam of laser energy;
and (f) directing said converging beam of laser energy into said converging Vee with the focal point located substantially about said point of convergence, the geometry of said converging Vee being such that portions of said converging beam of laser energy that are incident upon the reflective surfaces of said moving strips at a location ahead of said point of convergence are reflected at least in part by the surfaces of said moving strips in a direction toward said point of con-vergence so that a continuous welded seam is established between the overlaid strips.

2. A method as defined in claim 1 wherein the moving strips are passed between two pressure rollers with the rollers arranged such that the strips form a converging Vee.

3. A method as defined in claim 2 wherein the point of convergence of the moving strips substantially equals the point of tangency between said rollers.

4. A method as defined in claim 3 wherein said laser beam is focused substantially within the plane of symmetry passing through the point of tangency between said pressure rollers.

5. A method as defined in claim 2 wherein the strips are aluminum.

6. A method as defined in claim 2 wherein the strips are stainless steel.

7. A method as defined in claim 2 wherein the strips are copper.

8. A method as defined in claim 2 wherein the strips are brass.

9. A method as defined in claim 2 wherein the strips are of carbon steel.

10. A method as defined in claim 2 wherein the strips are of dissimilar metal selected from the group consisting of: aluminum, copper, brass, carbon steel and stainless steel.

11. A method as defined in claim 1 wherein the power of said laser beam is about one kilowatt.

12. A method as defined in claim 4 wherein said laser beam lies substantially within the plane of symmetry between the moving strips with its major component in the direction of travel.

13. A method as defined in claim 4 wherein said laser beam lies substantially within the plane of symmetry between the moving strips with its major component perpendicular to the direction of travel.

14. A method of fusion welding at least two thin flexible strips of sheet material composed of a high-ly conductive metal selected from the group consisting of aluminum, copper and brass, having a thickness in the range of from about 0.001 to about 1/4 inch and having reflective surfaces, while the strips are moving at a relatively high rate of speed of at least about 100 feet per minute, comprising the steps of:
(a) directing the moving strips toward one another to form a converging Vee between the moving strips with the reflective surfaces facing one another;
(b) providing a pair of pressure rollers;
(c) passing the moving strips between the pair of pressure rollers;

(d) applying a force above zero pounds against the moving strips with the pair of pressure rollers at a Location contiguous to the point at which the moving strips converge such that the moving strips overlay one another in intimate contact at the point of convergence;
(e) generating a low power laser beam of energy;
(f) providing an optical medium for focusing said laser beam;
(g) focusing said laser beam with said optical medium to produce a converging beam of laser energy;
and (h) directing said converging beam of laser energy into said converging Vee with the focal point located substantially about said point of convergence, the geometry of said converging Vee being such that portions of said converging beam of laser energy that are incident upon the reflective surfaces of said moving strips at a location ahead of said point of convergence are reflected at least in part by the surfaces of said moving strips in a direction toward said point of convergence so that a fusion weld is established between the overlaid strips.

15. A method as defined in claim 14 wherein said laser beam is focused substantially within the plane of symmetry passing through the point of tangency between said pressure rollers.

16. A method as defined in claim 1 wherein the moving strips travel at a speed of at least about 100 feet per minute.

17. A method as defined in claim 14 wherein the power of said laser beam is about one kilowatt.

18. A method for continuous seam welding of flexible strips of metallic sheet material having re-flective surfaces while the strips are moving, comprising the steps of:
(a) directing the moving strips toward one another to form a converging Vee between the moving strips with the reflective surfaces facing one another;
(b) providing a pair of pressure rollers of substantially equal diameter;
(c) passing the moving strips between the pair of pressure rollers;
(d) applying a force of above zero pounds against the moving strips with the pair of pressure rollers at a location contiguous to the point at which the moving strips converge such that the moving strips overlay one another in intimate contact at the point of convergence;
(e) maintaining the speed of the moving strips at least at about 100 feet per minute;
(f) generating a laser beam of energy;
(g) providing an optical medium for focusing said laser beam;

(h) focusing said laser beam with said optical medium to produce a converging beam of laser energy;
and (i) directing said converging beam of laser energy into said converging Vee substantially along the plane of symmetry passing through the point of tangency between said pressure rollers while main-taining the focal point of said converging beam of laser energy at a location substantially about said point of convergence, the geometry of said converging Vee being such that portions of said converging beam of laser energy that are incident upon the reflective surfaces of the moving strips at a location ahead of said point of convergence are reflected by the surfaces on the moving strips in a direction toward said point of convergence so that a continuous welded seam is established between the overlaid strips.