GB1562922A

GB1562922A - Laser welding

Info

Publication number: GB1562922A
Application number: GB43872/76A
Authority: GB
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1975-10-24
Filing date: 1976-10-22
Publication date: 1980-03-19
Also published as: JPS5260256A; DE2647082A1; SE413998B; ES452604A1; NL7611748A; IL50752A0; FR2328543B1; FR2328543A1; DD127457A5; SE7610675L; ES462440A1; BR7607073A

Description

(54) LASER WELDING (71) We, UNION CARBIDE CORPORA TION, a Corporation organised and existing under the laws of the State of New York, United States of America, whose registered office is, 270 Park Avenue, New York, State of New York 10017, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for continuous 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 weldment 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 conductive 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 contact area. An electron beam provides a high energy density source but requires a vacuum working environment to provide a high density beam over a reasonable distance. Hence, all known welding processes to date are either intrinsically incapable of welding workpieces at reasonably high travel speeds of at least 100 fpm, particularly for workpieces of sheet aluminum. or are otherwise handicapped by specific configuration limitations and impractical fixturing requirements.

It has been discovered 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 previous 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 invention 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 otherwise, 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 CO2 continuous wave laser, welding speeds of up to 500 feet per minute have been achieved with excellent weld quality.

It has also been discovered that the weldment produced by the process of the present invention is a "Fusion Weld", hereinafter defined as coalescence between the base materials resulting from bringing them to a molten state in the fusion zone; and which weldment is 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 < .roper- ties or microstructure have been altered by the heat from the formation of the weld.

The absence of a Heat Affected Zone (HAZ) surrounding the fusion zone is defined for purposes of the present invention as the inability to detect microstructural alterations under a conventional optical microscope at up to 100x magnification.

Under such circumstances the extent of any microstructural alterations would 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 weidments with a significant HAZ apparent in photomicrographs taken with an optical microscope at 50x magnification.

There is provided by the present invention a method for continuous seam fusionwelding together of two flexible strips of metallic sheet material 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 to provide facing reflective surfaces; (b) applying a force at a location contiguous to the point at which the moving strips converge to bring the moving strips into 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) direccting said converging beam of laser energy into said converging Vee with the focal point located about said point of convergence, so that any portions of said converging beam of laser energy that are incident upon the reflective surfaces of said moving strips at a location in front 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, whereby a continuous welded seam is established between the strips.

In the method of the invention, a continuous seal weldment is formed comprising a fusion weld nugget established be tween two base materials characterized by the absence of a surrounding Heat Affected Zone (HAZ).

It has also been discovered that when optimum utilization of the laser beam source is required the laser beam must be focused substantially within the plane of symmetry and within a narrow focal point range extending only in the downstream direction from essentially at the point of convergence to a point further downstream thereof. For purposes of the present disclosure "optimum utilization of the laser beam source" means the ability to achieve a continuous weldment at the highest possible speed using the least amount of laser beam energy. The ability to minimize laser power and still weld at high speed should not be underestimated for it provides a substantial economic advantage over a system which must otherwise depend upon significantly higher laser power to achieve a continuous weldment at high speed.

The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which: IFigure 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 point using two different focal length lenses under an otherwise given set of process paramecers; Figures 3a-3e are enlarged representational 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 diameter; and Figures 4(ab) and 5(a-b) are photomicrographs at lOOx magnification of the welded seam between two strips of aluminum 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 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. In connection with the following discussion, it should be noted that the pressure rolls A and B would, without the intervention of the strips and assuming the rolls are just in contact, provide a point of tangency between them. However, since strip intervention exists, it is a national point of tangency which coincides with the point of convergency of the strips. By the latter is meant the point where the strips are first brought into contact with one another.

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 shown in the apparatus of Figure 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 continuous length strips. Where the strips of sheet material are of predetermined finite length an alter native dispensing arrangemenr would be necessary to process the strips, preferably consecutively, through the pressure rolls devices A and B respectively. There are known dispensing arrangements 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 invention, the strips 10 and 12 may be of any metal composition although the composition of each shall be substantially compatible. Moreover, the properties of the sheet material, such as its conductivity and thermal diffusivity is not a limitation.

Hence, the process is particularly 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 to i 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 convergence 18 and along a predetermined and preferably invariant path in the direction shown by the arrows in 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 B is a process variable which is influenced in a manner to be discussed at greater length hereinafter.

A conventional source of laser energy 30 generates 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 continuous weld can be made. For any laser of given power there is an optimum relationship between focal length, focal point position relative to the point of convergence, beam diameter, beam orientation, pressure roll diameter and welding speed which will produce a weld of acceptable quality. In fact, proper focusing of the laser beam 32 into the converging Vee is essential if one is to obtain a weld at all regardless of laser power. Moreover, by appropriate focusing, optimum utilization of the laser beam energy will be achieved. The focusing of the laser beam will be discussed at greater length hereinafter in connection with Figures 2 and 3.

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 result 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 significant provided that at least some positive pressure is being applied. Too much pressure is in fact a disadvantage and may cause physical deformation.

It is to be understood that the weld to Ibe 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 application of the welded strips.

The pressure rolls A and B are preferably conventional squeeze rollers having a circular periphery. Other means may be employed provided such means assume a curvilinear contour as each approaches the point of convergence. For bilaterial weld symmetry, the diameters of the pressure rolls A and B are equal.

Figures 2 and 3 indicate both the importance of focusing and the diameter of the pressure rolls A and B to the quality of the weld.

To realize a weld the laser beam must be focused into the converging Vee substantially about the point of convergence. 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 CO2 continuous wave 10.6 micron laser, having a .5 inch diameter TEMPO 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. dia meter of approximately .004 inches at focal points, fl, 12 and f3. A number of additional focal point positions relative to the point of tangency were used to establish the outline for the graphical representation of Figure 2. Extrapolation from Figures 2 and 3 establish the importance of the following criteria for high speed continuous seam welding of over at least 100 feet per minute: (a) The laser beam should be introduced substantially along the "plane of symmetry which is hereinafter defined as the plane which passes through the tangent point 18 between pressure rolls A and B and which lies normal to and bisects the shortest line which can be drawn between the centres of the axis of the two rolls. When the laser beam is offset from the plane of symmetry but lies in a plane which is parallel to the plane of symmetry a non-symnietrical weld is formed between the strips. The extent of asymmetry is directly proportional to the offset. However, the position of the beam within the plane of symmetry is adjustable over a wide range of up to at least + 300 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 power then the focal point may be placed substantially about the point of convergence 18. If, however, optimum utilization is desired then the focal point of the laser should be maintained within a narrow focal point range from essentially the point of convergence to a location downstream thereof. As pointed out above, the expression "optimum utilization" for puproses of the present disclosure means the ability to achieve a continuous weldment at the highest possible speed using the least amount of 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 respectively 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 CO laser as described heretofore and focused within the plane of symmetry at two aluminum strips moving at a speed of at least 400 feet per minute with 19 inch diameter 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 surprisingly, the focal point range extends from about the point of convergence 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 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 established between die 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 respectively 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, clipping of the laser beam by the sheets at the location of the pressure rolls was unavoidable. Under certain circumstances clipping may, in fact, be desirable.

Once the laser beam strikes a sheet at said location, a portion of the laser beam energy will be reflected from the surface of the sheet into the converging Vee and hence into the active weld zone, a portion will be adsorbed by the moving strips and appear as heat, and a portion will be scattered diffusely and lost. The farther the clipping occurs from the point of convergence, the greater the fraction of laser beam energy that will be lest.

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 1k inches to a diameter lof two inches and the optical lens 34 was shifted along the optic axis and varied from one having a focal length of 2.5 to one having a focal length of 3.75 inches to establish focal point positions fl, 12, and f3 respectively. It should be understood 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 i 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# 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 annd B and with the same focal length lens as is shown in Figure 3b. With a focal point position 12 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 A and B clipping occurs at point E. With a .5 inch diameter beam and a 3.75 inch focal length lens focused at the focal point position fl, as is shown in Figures 3d, using 1k inch diameter 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 substantiates the fact that the extent of beam clipping can be reduced by increasing the focal length. Empirical evaluation 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 and 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 1k 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 and the closeness of the focal point to the point of tangency 18, a continuous weld could not be 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 diameter, and the pressure roll diameter as explained'here- inabove in paragraphs (a), (b) and (c) respectively, when optimum utilization of the laser beam source is required. 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 techniques on thin section material.

Photomicrographs of the weldment produced by the process of the present invention using the 1 kw CO2 laser as defined heretofore and under conditions which fulfill the criteria discussed hereinabove are shown in Figures 4(a-b) and Sfa-b) fbr 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 photomicrographs were taken using a conventional optical microscope at 100x 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 illustrate 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 that obtained at 500 feet per minute as is evident from a comparison of Figure 4b with Figure 5b. Both weldments are symmetrical and have a thickness of only a fraction of the strip thickness. In fact, 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 dissimilar 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 composition. In addition, the weldment produced for each case except stainless and carbon steel was characterized by the absence of a Heat Affected Zone (EiA'Z).

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 symmetry 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 the beam is perpendicular to the direction of travel.

WHAT WE CLAIM IS:- 1. A method for continuous seam fusion-welding together of two flexible strips of metallic sheet material 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 to provide facing reflective surfaces: (b) applying a force at a location contiguous to the point at which the moving strips converge to bring the moving strips into intimate contact at the point of convergence; (c) generating a laser beam of energy;

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. inch focal length lens and the two inch diameter pressure rolls A and B clipping occurs at point E. With a .5 inch diameter beam and a 3.75 inch focal length lens focused at the focal point position fl, as is shown in Figures 3d, using 1k inch diameter 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 substantiates the fact that the extent of beam clipping can be reduced by increasing the focal length. Empirical evaluation 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 and 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 1k 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 and the closeness of the focal point to the point of tangency 18, a continuous weld could not be 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 diameter, and the pressure roll diameter as explained'here- inabove in paragraphs (a), (b) and (c) respectively, when optimum utilization of the laser beam source is required. 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 techniques on thin section material. Photomicrographs of the weldment produced by the process of the present invention using the 1 kw CO2 laser as defined heretofore and under conditions which fulfill the criteria discussed hereinabove are shown in Figures 4(a-b) and Sfa-b) fbr 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 photomicrographs were taken using a conventional optical microscope at 100x 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 illustrate 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 that obtained at 500 feet per minute as is evident from a comparison of Figure 4b with Figure 5b. Both weldments are symmetrical and have a thickness of only a fraction of the strip thickness. In fact, 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 dissimilar 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 composition. In addition, the weldment produced for each case except stainless and carbon steel was characterized by the absence of a Heat Affected Zone (EiA'Z). 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 symmetry 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 the beam is perpendicular to the direction of travel. WHAT WE CLAIM IS:-

1. A method for continuous seam fusion-welding together of two flexible strips of metallic sheet material 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 to provide facing reflective surfaces: (b) applying a force at a location contiguous to the point at which the moving strips converge to bring the moving strips into 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 about said point or convergence, so that any portions of said converging beam of laser energy that are incident upon the reflective surfaces d said moving strips at a location in front 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, whereby a continuous welded seam is established between the strips.

2. A method as defined in claim 1 wherein the focal length of said converging beam is chosen in relation to the radius of curvature of the strips at the convergence thereof such that when said converging team is directed into said converging Vee said beam strikes the moving strips in the vicinity of the converging Vee closest to said point of convergence.

3. A method as defined in claim 1 or 2 wherein the moving strips are passed between two pressure rollers with the rollers arranged so as to form the strips into the converging Vee.

4. A method as defined in claim 1, 2 or 3 wherein the point of convergence of the moving strips substantially equals the point of tangency between said rollers.

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

6. A method as defined in any of the preceding claims, wherein the strips are aluminum.

7. A method as defined in any of preceding claims 1 to 5 wherein the strips are stainless steel.

3. A method as defined in any of preceding claims 1 to 5 wherein the strips are copper.

9. A method as defined in any of the preceding claims 1 to 5 wherein the strips are brass.

10. A method as defined in any of preceding claims 1 to 5 wherein the strips are of carbon steel.

11. A method as defined in any of preceding claims 1 to 5 wherein the strips are of dissimilar metal selected from the group consising of: aluminum, copper, brass, carbon steel and stainless steel.

12. A method as defined in any of the preceding claims wherein the power of said laser beam is substantially one kilowatt.

13. A method as defined in any of the preceding claims wherein said laser beam lies substantially within the plane of symmetry between the moving strips and is directed in the direction of travel of the strips.

14. A method as defined in any of the preceding claims 1 to 12 wherein said laser beam lies substantially within the plane of symmetry between the moving strips and is directed perpendicularly to the direction of travel.

15. A method as defined in any of the preceding claims wherein the metal sheets each have a thickness in the range of from 0.001 to 0.25 inch.

16. A method as defined in any of the preceding claims wherein the strips are moved at a speed of at least 100 feet per minute.

17. A method as defined in claim 16 wherein the strips are moved at a speed of from 400 to 500 feet per minute.

118. A method as defined in any of the preceding claims wherein the strips are moved at the same speed.

19. A method as defined in claim 3 or any of the preceding claims 4 to 18 as dependent thereon, wherein the rollers are of equal diameter (as hereinbefore defined).

20. A method as defined in claims 3 to 13 or any of preceding claims 14 to 19 as dependent thereon, wherein the roller diameter (as hereinbefore defined) is 2" or less.

21. A method as defined in claim 20 wherein the roller diameter is from 1+ inch to 2".

22. A as defined in any of the preceding claims wherein the power of the laser beam is 1 kilowatt.

23. A method of fusion-welding substantially as hereinbefore described with reference to the accompanying drawings.

24. A weldment whenever produced by the method of any of the preceding claims.