The invention relates to a method for performing laser welding, in which method the a lens and/or a mirror used for laser welding is divided into at least two parts or a laser beam is conducted in at least two parts into the welding head in order to create at least two focal points so that the distribution of the laser beam is used at least partly in a treatment for inaccurate preparation surfaces before the proper welding occasion.

When cutting material the created cutting surfaces are too inaccurate for an effective laser welding. The inaccuracy is based on that in the cutting surfaces there are bevellings that make the cutting surface uneven particularly in the longitudinal direction of the object but also in the lateral direction of the object. When two these kinds of cutting surfaces are aiming to be welded in order to join to each other, the cutting surfaces are placed side by side, in which case the width of the gap there between is variable. It is difficult to weld the gap having a variable width with great velocity, because a welding medium is needed in different amounts depending on the width of the gap.

It is known from the JP patent application 60199585 a laser welding machine where the width of the gap between the surfaces to be welded is controlled by a laser beam which is focused to both surfaces to be welded. On the basis of the received width information the welding velocity or the amount of an additive can be changed in order that it is created a welding seam having desired properties.

In the US patent 5841097 it is described a method and an apparatus for using two or more laser beams in welding of objects so that the laser beams oscillate along the welding direction. In the US patent 5841097 it is presented as one embodiment a solution, where two laser beams are used so that it is achieved with the first laser beam the preheating of the zone to be welded and with the second laser beam it is performed the proper welding. The JP patent application 1143785 relates to a laser beam device where it is used multi-focused laser beams, but having separate axes. Also in this JP patent application 1143785 separate laser beams are used for different purposes in order to perform the same action; one laser beam is used for preheating, another is used for the proper action.

In the preceding publications, the US patent 5841097 and the JP patent application 1143785, it is used two laser beams for different actions, but these publications are not at all concentrated to the control and the treatment of a gap between surfaces to be combined. Instead, respectively in the JP patent application 60199585 it is measured the mutual position of surfaces to be welded by controlling the gap, but in this publication it is not mentioned any treatment for surfaces before the proper welding action.

The object of the present invention is to eliminate some drawbacks of the prior art and to achieve an improved method for welding of two objects to each other by laser welding so that the inaccuracy between the surfaces to be welded can be decreased already before welding. The essential features of the invention are enlisted in the appended claims.

In the method according to the invention a laser beam to be used in welding is conducted through at least a two-piece optical element. The two-piece optical element contains at least two parts connected to each other or at least two separate optical elements or also so that one part is separate and the other part contains at least two parts connected to each other. Thus it is achieved for the laser beam focal points composed of different parts. As an optical element it can be used also a combination of a lens and a mirror. The different parts of an optical element or the separate optical elements are different from each other from the view of a shape or from the view of properties, in which case the power of the laser beam conducted through the optical element is controlled on the basis of the fact which part or which point in the optical element is faced by the laser beam either going through the optical element or reflecting from the optical element. Changing thus the power of the laser beam it is advantageously performed before the proper welding a preheating of the surfaces to be welded forming the inaccurate gap. Further pressing the preheated surfaces to be welded with hydraulic or mechanical pressing devices the weldability of the inaccurate gap is improved without needing in the proper welding essential amounts of welding additives. In accordance with the invention the laser beam going through the optical element can further be advantageously utilized in the adjustment of the height of the welding seam or by heat treating the properties of the weld.

The optical element used in the method of the invention is advantageously an optical lens or an optical mirror, which is divided into two parts. When the optical element is a mirror, it can be installed immovably at its position or movably, in which case the movability is reached by moving the parts of the mirror to each other or the mirror itself is moved towards the object to be welded. The adjacent parts of the optical element are different from each other either so that two adjacent parts have a different refraction index or that two adjacent parts are different in their shapes. The adjacent parts of the optical element can also be different from each other so that the adjacent parts have a different refraction index and the adjacent parts are different in their shapes. When using in the adjacent parts of the optical element different refraction indexes and/or shapes the power distribution in the laser beam flux formed of separate laser beams used in welding is adjusted desired in order to realize the separate stages in the method according to the invention.

According to the invention the different parts of the optical element are positioned towards the welding direction of the surfaces to be welded so that the part having a large refraction index is in that point where the proper welding is carried out. So the optical element refracts the laser beam more than the part having a small refraction index, in which case more power conducted by the laser beam is made to be focused to the proper welding. The method according to the invention can be applied to laser welding, when the surfaces to be welded are moved towards the laser welding head, or so that the laser welding head is moved towards the surfaces to be welded. When the surfaces to be welded are moved towards the laser welding head, the method according to the invention can be used essentially in continuous operation. When the laser welding head is moved towards the surfaces to be welded, the method according to the invention can be used periodically in continuous operation.

The invention is described in more details in the following referring to the drawings, wherein

Fig. 1 illustrates a preferred embodiment of the invention schematically in a side view,

Fig. 2 illustrates the surfaces to be welded in accordance with the embodiment of Fig. 1 schematically from above seen,

Fig. 3 shows the power distribution in accordance with the embodiment of Fig. 1 as a schematical figure,

Fig. 4 illustrates another preferred embodiment of the invention schematically in a side view, Fig. 5 illustrates the surfaces to be welded in accordance with the embodiment of Fig. 4 schematically from above seen.

According to Figs. 1 and 2 a laser welding head 1 is kept during welding in its position, while the surfaces 2 and 3 to be welded to each other are moved in the direction 4 shown as an arrow. The surfaces 2 and 3 to be welded form to each other a gap 5 before welding. According to the invention a laser beam flux 6 is conducted through a lens 9, composed of the parts 7 and 8, to the surfaces 2 and 3 to be welded. The refraction index of the first lens part 7 in the welding direction is smaller than the index of the second lens part 8 in the welding direction. The proportion of the laser beam flux 6 going through the first lens part 7 in the welding direction thus refracts less than the proportion of the laser beam flux 6 going through the second lens part 8 in the welding direction. The proportion of the power in the laser beam flux 6 going through the first lens part 7 in the welding direction is used for preheating of the surfaces 2 and 3 to be welded in a zone 10 illustrated in Figs. 1 and 2 with broken lines. In order to reduce possible inaccuracies in the gap 5 formed by the surfaces 2 and 3 to be welded, the surfaces 2 and 3 to be preheated are pressed with the pressing devices 11 and 12 advantageously essentially simultaneously with the preheating. The preheated and pressed surfaces 2 and 3 to be welded are welded to each other by means of the power received from the proportion of the laser beam flux 6 going through the second lens part 8 in the welding direction in a zone 13 illustrated with broken lines.

In Figs. 1 and 2 it is also illustrated with broken lines a zone 14 where a finishing of the welding seam 15 between the welded surfaces 2 and 3 so that the height of the welding seam 15 is adjusted desired, if needed. The finishing of the welding seam 15 is performed with that proportion of the laser beam flux 6, which goes through the final end of the lens 9 in the welding direction of the second lens part 8 in the welding direction having a different radius of curvature. Thanks to the change in the radius of curvature the power of the laser beam flux 6 going through the final end of the lens 9 is smaller than in the proper welding in the zone 13, and thus the height of the welding seam 15 can be adjusted by heating the welding seam 15.

In Fig. 3 the power distribution is illustrated in the preceding zones 10, 13 and 14.

In the embodiment according to Figs. 4 and 5 a laser welding head 21 is positioned movably in the welding direction 24 so that using the laser welding head 21 it can be welded surfaces 22 and 23 to be welded positioned in their positions in the welding direction 24. It is installed before the laser beam flux 25 a mirror 40 which reflects the laser beam flux 25 to a lens 26. The laser beam flux 25 is conducted through the lens 26 to the surfaces 22 and 23 to be welded. The lens 26 is manufactured of homogenous material, but the lens 26 is formed of different parts 38 and 39 from the shape of view, by means of the parts 38 and 39 the power of the laser beam flux 25 conducted through the lens 26 can be adjusted.

5 In the Figs. 4 and 5 it is separated with broken lines zones 27, 28 and 29. The first zone 27 in the welding direction 24 illustrates the preheating and pressing of the surfaces 22 and 23 to be welded, the second zone 28 in the welding direction 24 illustrates the proper welding of the surfaces 22 and 23 and the third zone 29 in the welding direction 24 illustrates the finishing of the created 10 welding seam 30. In the zone 27 the laser beam flux 25 conducted through the lens 26 is adjusted by means of the design of the lens 26 to have smaller power than the laser beam flux 26 conducted through the zone 28.

According to the invention the objects 31 and 32 containing the surfaces 22 and

15 23 to be welded are first positioned in the welding position, in which case the surfaces 22 and 23 to be welded form to each other a gap 33 having an inaccurate width. Further, it is fastened to the objects 31 and 32 the pressing devices 34 and 35, which press the objects towards each other and which are during the welding occasion movable in the welding direction 24. The pressing 0 devices 34 and 35 are provided with hydraulic devices 36 and 37, by means of which the pressing of the objects can be adjusted in accordance with the movement of the laser welding head 21 at least in the zones 27 and 28. When the laser welding head comes to the zone 27, the preheating of the surfaces 22 and 23 to be welded is started by means of the laser beam flux 25 conducted 5 through the lens 26. Essentially simultaneously it is started the pressing of the objects 31 and 32 to each other by the pressing devices 34 and 35 in order to reduce the inaccuracies in the width of the gap 33. The pressing of the objects 31 and 32 is carried out so that every point of the object to be pressed follows the laser welding head 21. When the laser welding head 21 is in the welding 0 direction 24 transferred in every pressed point of the object into the zone 28 of the proper welding, the pressing of the objects 31 and 32 is progressively reduced, until when the laser welding head 21 reaches the zone 29, the pressing of the objects 31 and 32 with the pressing devices 34 and 35 is stopped in the area already welded. Pressing is continued in the preheated and unwelded part in accordance with the movement of the laser welding head 21 , until the surfaces 22 and 23 are completely weided. For the part of the objects 31 and 32 in the zone 28 it is carried out the welding of the surfaces 22 and 23 to each other, in which case it is created the welding seam 30. For the welding seam of the part for the objects 31 and 32 in the zone 29 it is carried out the finishing treatment by the laser beam flux 25, which power is smaller than in the zone 28 in connection with the welding.