CA1226336A - Laser beam - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A 2-25 mm ferrous metal plate is welded by a heavy-duty laser (5-25kw) which melts through the plate and into a support member below to give a unitary weld. A gas supply to the laser beam focus is provided such that on the one hand the plasma is retained at least partially near the weld pool to give good energy transfer and on the other the plasma does not all escape.

Description

This invention relates to welding and more especially to welding of metal usually but not invariably ferrous metal in the form of plate or structural members.

A common welding requirement, with which the present invention is particularly concerned is the welding of one plate to another, at, or almost at, a right-angle configuration, i.e. a " T " weld.

Typically, this is effected by creating a weld "fillet"
along each internal corner, which necessitates two runs. The need to angle or bevel the attached plate corners to improve weld penetration also arises if heavy gauge plate is used. Similar techniques are used when a rolled or fabricated section is to be attached by welding to a face of a plate.

The deep penetration characteristics of high-powered laser beams have led them to be considered as a source of energy in welding processes. Our copending Canadian Application No.
452,512 filed April 19, 1984 entitled "Laser Welding" describes such a process particularly adapted to use in workshops or site conditions. The process involves the use of an impingement mem-ber such as a wire so that any gaps in the fit-up between the plates to be welded do not lead to a loss of energy due to escape of the focussed laser beam through the gap, i.e. without energy transfer and plasma formation. The method of the prior invention can be applied both to butt welding and to T-welding.

The present invention sets out to provide technique of welding utilizing the deep penetration of a finely focussed laser beam to achieve melting and welding of a T-section, or like, weld.

According to the present invention there is provided a method for welding a ferrous metal plate from 2 to 25 mm thick to an underlying attachment member, contacting or in close proximity 3~

to the plate; in which a 5 to 25 kw laser beam is focussed upon the plate and is moved along relative thereto at the other side of the plate prom the said contact area, characterized in that a filler wire is fed to the beam focus and a supply of gas is directed to at leas-t partiall.y confine the plasma formed by impingement on the metal and filler wire, whereby the focussed beam also melts the metal both of the plate and of the underlying member together with the filler wire so that the molten metal in total may be controlled to fill any gaps arising between the plate and the attachment member, and to either side of the weld line, whereby upon solidification a weld line of uniform nature and appearance is formed.

Thus the present inve.ntion provides a method for weld-ing a metal plate to an underlying attachment member, contactingor in close proximity to the plate: in whlch a high energy laser beam is focussed upon the plate and is moved along relative thereto at the other side of the plate from the said contact area, a supply of gas being directed to at least partially con-fine the plasma formed, whereby the focussed beam melts the metalboth of the plate and of the underlying member so that upon solidification a weld line is fsrmed.

Usually, both the plate and the underlying member are ferrous. By "plate" is generally understood material at leasc 2 mm thick. We have found that from 2 to 25 mm is preferred and that a laser of 5 to 25kw intensity can be utilized.

When the underlying member is spaced from the plate the method can still be used as because of the filler wire molten material will flow to attempt to fill any such space. The filler wire can make up this deficiency and improve weld characteris-tics.

In particular when two relatively thick plates or like members are placed together in actual work conditions (as dis-~Z1~33~

tinct from laboratory or experimental conditions) there will inevitably be overlapping patches in which the plates are spaced apart rather than contiguous as all points. secause the members are relatively thick, moreover, this cannot be cured by pressure upon the assembly. If a through weld is made with a laser, through both thicknesses, and if the two plates are in face-to-face contact, then the molten metal, the formation of which is facilitated by the retention of a heat-transferring plasma, is of uniform shape and external appearance throughout the weld. If, however, a gap arises between the two plates then molten metal as well as filling in the weld line will attempt to run between the plates to fill the gap. Indeed, it is preferable that it should do so to prevent corrosion arising between the adjacent but not contiguous surfaces. However, the loss of molten metal means that the internal keyhole which transverses the weld line as the laser head is moved, and which is deflned by a dynamic equi-librium of the moving beams and the adjacent molten metal, changes its geometry. ThlS ls because molten metal dralns away down into the gap between the plates. secause it changes its 20 - geometry, the impingement of the beam can be altered in detail and the generation of the plasma can vary, whereby the heat transfer is altered, and the whole process is varied. In the present invention control of thls variability is affected by feeding in a filler wire. The filler not only provides extra material to enter the gap, but also provides a uniformity of con-ditlons as the laser beam is traversed along the weld line.
Thus, typically part of the filler is melted Rand indeed most of the filler may be melted where necessary to fill in a larger gap) while part of the filler is impinged upon by the laser beam and helps to form the necessary vapour and plasma for heat transfer purposes. When there is no gap, as will be the case from time to time along the weld line, there need be very little feed of filler wire, and such wire that is fed is mostly vapourized or converted to plasma rather than going to provide extra material in the weld. In practice, this variation of the feed of filler wire in relation to the passage along the weld line can be done either by varying the speed at which the wire is fed or by vary-ing the speed of traverse of the weld or bothO Thus the present invention provides a process in which both the uniformity of heat transfer conditions and uniformity of filling of the weld line are provided by judicious relative feed of a filler material.

The relationship between the plate and the underlying attachment member can be that of a "T" weld, i.e. a weld where the underlying plate is placed at, or nearly at, right-angles to the upper plate. The weld then passes into the end face of the underlying plate, in a single deep penetration pass, giving a "through-welded" joint of unique form. Suitably three weld lines are used, with the central llne defining a weld passing into the end face and the lines located to either side of the central line defining welds passing only partially into the end face and being partially visible at the internal corners.

An alternative relationship is that where the underly~
ing attachment member has been formed to contact as a lip or mar-gin parallel to the underside of the upper plate. The width ofthe formed lip may permit two or more parallel through welds according to the invention to be formed.

Another alternative relationship is, of course, that of two overlapping but generally parallel plates. Other relation-ships can also be envlsaged, as shown in more detail below.

- 3a -,33~

The upper plate utilized for a T-weld is more preferably from 2 15 mm in thickness; the underlying plate may be of any thickness.

We are aware that GB Patent 1 268 044 describes a laser welding process which cuts through an upper sheet of material, usually as spot welding. The process therein described utilises the technology then available, i.eO
lasers of lower power, and carries out the process on thin sheet. It recognises an inherent limitation in the process, namely that the power density of the radiation must he kept below a ceiling value to avoid vaporisation to such an extent that there is insufficient material left to form a weld. Moreover, it is concerned with thin material, 0.38 mm sheet being given by way of example, in which thermal conduction problems, and weld characteristics, are different from those obtainable with thicker material. Thus, application of the process of this prior Patent to thicker grades of material is contraindicated.

We have now established, by analogy with the techniques C~7aa/~a /o~ ~s~
I- of our copending~Application~referred to above, that gas supply to the weld zone is of crucial importance for thicker plate material. When this form of maximum penetration is needed, according to the present invention, high-powered lasers are being used to the limits of their performance. A balance of welding speed

2~33@~ , and control of plasma escape is essential, on a considerably larger scale than hither~o9 and this is provided by gas supply by means of which a trade-off between speed and depth can be achieved.

The invention will be further described with reference to the accompanying drawings, in which:_ Figure l is a perspective view of theoretical laser welding of two plate portions in a single pass, operating from above;

Figure 2 is a section through equipment which can be used in accordance with the invention, taken along the weld line;

Figure 3 shows the structure of a "through-weld" in accordance with the invention, embodied as a T-weld;

Figure 4 shows the structure of a similar weld attaching the lip of a strengthening bar or girder to the underside of a plate, involving more than one such "through weld";

Figure 5 shows a hybrid weld structure utilising welding in accordance with the present invention together with a weld of a type described in our copending Application referred to above;

33~

Figure 6 shows a further variant weld form in according with the present invention, and Figure 7 shows a practical use for the weld technique of the present invention.

Figure l shows a plate l along which, in the line shown, there impinges a laser beam 4, focussing at or near the surface. Energy in the beam vapourises a "key-hole"
through the material with the vapour pressure in the "key-hole" sustaining the molten walls 6.In practice, the vapour is prevented at least in part from escaping out of the top of the "key-hole" (where it could form a laser-blocking plasma) by a controlled jet of helium (see Figure 2~. As the beam is moved along the plate the "key-hole" moves. The beam belt materials ahead of the orifice, as material solidifies behind the orifice. A
deep penetration can be formed usually inone pass.

Figure 2 shows a diagrammatic section along a weld line of essential features of equipment which can by way of example be used to carry out the method of the invention.

2~ The laser beam 7 impinges 0l1 the plate 2 with its focus at or near the top metal surface. If de3ircd, //
consumable wire lo rt~ fed to, or near the focus I, either to control the weld profile e.g. when the members to be welded are slightly spaced apart, or anyway to 6~36 alter weld composition. Impingement of the beam upon the metal ox the plate causes metal vapour to be produced and a plasma to be formed. Gas hood 8 supplies a shielding gas such as helium gas or a helium-based mixture through base channel 14, central duct 15/ and surrounding duct 16~ in a gas-flow configuration such as to confine the plasma. British Patent No. 1 591 793 describes a typical such hood in more detail.
Impingement of the beam on this plasma at focus absorbs energy which is transferred to the surrounding metal and continuously forms molten walls 19 defining "key-hole" 20 extending down through the plate 2 ancl into the underlying member 3 The molten walls progressively solidify at weld 21,joining the two members 2 and 3.

Figure 3 shows that the weld material 25 is of a characteristic shape, being broader at the top, slightly bulbous in the centre, and tapering to a point. It is surrounded by a narrow zone of heat-afEected metal 23.
The weld firmly attaches the plate 24 to the underside of the plate 22, even though only a single pass from above has been used.

For those instances where the strength of a weld configuration as shown in Figure 3 may be inadequate, it is possible to operate as shown in Figure 4. In this Figure a plate 27 is attached to an underlying strut or girder 28 by three parallel welds 29 as shown. Each weld extends through the top plate and into a lip 31 of the girder 28. The welds are collectively surrounded by areas of heat-affected metal 30 as shown. If desired, welds 29 can csmpletely penetrate not only the top plate 27 but also the lip 31, and be visible upon the back of the lip.

Figure 5 shows three welds 33 in accordance with the present invention between a top plate 34 and a underlying plate 35. ThiS underlying plate is in turn welded with a T-weld 36 to a plate 37 at right angles, using the laser techniques described and claimed in our f s s-/,~
copending~Application~referred to above and involving the use of a beam-interceptor material.

Figure 6 shows a further variant of weld in accordance with the invention. In this, using the references of Figure 3, the upper plate 22 is secured to plate 24 not only by central weld 25 but also by two side welds 25a themselves visible along the internal corners of the T, and utilising a filler wire to improve weld profile.

Figure 7 shows assembly of a frequently encountered sub-unit utilised in ship-building. It comprises a plate 38, itself built up by butt-welding of strips, those welds being omitted for clarity. On the plate are welded parallel stiffeners 39. Over the stiffeners are welded ~2~33~ii deep webs 40, each apertured to receive stiffeners and each with an upper formed lip 40a; some webs cross the stiffeners 39 and some run arallel to them.

Such a unit may be made by handwelding or by the techniques, and on the assembly line, described in our e " , ~.S~, 5-~
co-pending//Application,~referred to above.

The unit is to be further assembled to another sub-assembly of plate 38 and stiffeners 39 as shown.
Hitherto, this has involved difficult techniques of "overhand" welding in inconvenient locations. However, using the techniques of the present invention "throughwelds" can be made through the upper plate with minimum handling and inconvenience.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for welding a ferrous metal plate from 2 to 25 mm thick to an underlying attachment member, contacting or in close proximity to the plate; in which a 5 to 25 kw laser beam is focussed upon the plate and is moved along relative thereto at the other side of the plate from the said contact area, charac-terized in that a filler wire is fed to the beam focus and a sup-ply of gas is directed to at least partially confine the plasma formed by impingement on the metal and filler wire, whereby the focussed beam also melts the metal both of the plate and of the underlying member together with the filler wire so that the molten metal in total may be controlled to fill any gaps arising between the plate and the attachment member, and to either side of the weld line, whereby upon solidification a weld line of uni-form nature and appearance is formed.

2. A method as claimed in claim 1 in which the rela-tionship between the plate and the underlying attachment member is that of a T-weld in which the weld passes into the end face of the underlying plate.

3. A method as claimed in claim 2 in which three weld lines are used, with the central line defining a weld passing into the end face and the lines located to either side of the central line defining welds passing only partially into the end face and being partially visible at the internal corners.

4. A method as claimed in claim 1 in which the plate is welded in one or more passes to a lip formed on the underlying attachment member.

5. A member as claimed in claim 1 in which two over-lapping plates are welded in one or more passes.

6. method as claimed in claim 1, 2 or 3 in which the said plate is from 2 to 15 mm in thickness.