The invention relates to a method for welding rails at spaced facing frontal ends of the rails, comprising supplying welding material into a gap between said frontal ends .

During the construction and maintenance/renewal of railways often rails have to be connected. Because of the specific characteristics of the rail (e.g. material proper ^¬ ties and profile) and conditions, the connection between rails has to fulfil severe requirements, the most common method for connecting rails encompasses welding, for which at present three main welding techniques are used.

A first state of the art welding technique is flash butt welding. It uses a specially designed welding head with dedicated electric and hydraulic power pack. After the frontal ends have been aligned (vertically and horizon ^¬ tally) the power pack directs a large electrical current through the rails while the frontal ends of the rails are moved closer and apart by the hydraulic system. This move ^¬ ment results in an arc heating the frontal ends to a temper- ature at which they soften. When softened sufficiently, the frontal ends are pushed against each other with a large force for creating the weld which thereafter is further finished (by removing superfluous material and by grinding) .

Although this know technique produces an optimal weld in terms of strength (bending, tensile and fatigue) and may be carried out in a fully automated and controlled man ^¬ ner, a major disadvantage is posed by the large dimensions of the welding head which does not allow to weld in confined spaces. Furthermore a length of about 30-35 mm of rail mate- rial is consumed during this procedure, such that in the field a substantial amount of the rails should be loosened to allow a supply of such length through a tensile force performed by the welding head. This makes this technique la ^¬ bour and time consuming and difficult, or even impossible, to use when welding switches.

Another state of the art welding technique com- prises thermite welding in which a mixture of iron and alu ^¬ minium powder (thermite) is ignited for causing a highly exothermic reaction for melting these components and the frontal ends of the rails and fusing the frontal ends with the molten thermite. Previous to this procedure a welding gap of about 30 mm has to be made between the facing frontal ends (for example by cutting the rail by means of grinding or burning) , after which the frontal ends have to be aligned (vertically and horizontally) for compensating shrinkage during the welding process. Then a ceramic mould is posi- tioned around the welding gap and the frontal ends are pre ^¬ heated .

Although this known technique is globally used in a widespread manner as it may be used for a large variety of rail profiles and materials, is applicable in confined spaces and also may be used in switches, it has a number of disadvantages. It is time consuming and labour intensive, requires a manual control of parameters (among which the pressure of the propane and oxygen during preheating and the preheating time) , is dangerous in that the chemical reaction once started cannot be stopped in case of an emergency and is prone to mistakes of the operators.

A third welding technique used today for welding rails is puddle arc welding, which mainly is used in the field of Light Rail. It starts with grinding or burning a gap of about 15 mm between the frontal end of the rails, followed by aligning the frontal ends (vertically and hori ^¬ zontally) for compensating shrinkage during the welding process. Next a casing of ceramic or copper elements is erected around the welding gap and finally the actual welding occurs using welding electrodes. After the weld has been completed, finishing occurs.

This welding technique is rather cheap and may be used in confined spaces and is applicable both to old and new worn-out rails. However, it is also time consuming and very labour intensive, and prone to mistakes because it mainly is carried out manually and as such the skills of the operator are very important.

In view of the above it is an object of the pre ^¬ sent invention to provide an alternative method for welding rails at spaced facing frontal ends of the rails.

In accordance with the present invention the method is characterized in that part of welding said frontal ends is carried out using a laser beam for performing a laser welding process and that another part of welding said frontal ends and supplying welding material is carried out by an arc welding process.

The combined (hybrid) welding process of laser welding and arc welding yields a method which can be highly automated with very reproducible and high quality results in which the risk of human errors is reduced as much as possi ^¬ ble. A welding head (or assembly) which is capable of per ^¬ forming both welding processes can be dimensioned very small such as to be applicable in very confined spaces. The method is applicable to switches too and there is no need to loosen a substantial length of the rails. The method is less labour intensive and can be stopped at any time.

In accordance with one embodiment of the method, the laser beam for per-forming the laser welding process, as seen in a direction of welding, precedes the welding arc for performing the arc welding process. The laser welding process yields a sufficient depth penetration through the so- called keyhole effect, according to which, once a certain power level is reached, molten rail material evaporates and is heated resulting in ionised metal vapour (plasm or plasm arc) which expands in such a manner that surrounding molten material is expelled. This results in a small and deep open ^¬ ing (keyhole) which remains open as long as the travel speed is not too high and the power level is sufficient for evapo ^¬ rating the metal. The arc welding is performed in the same process zone as the laser welding and in combination a high quality weld is obtained. The elevated temperature in the process zone yields a welding electrode which melts in an ionising manner which then functions both as electrode and as source of welding material.

In one embodiment the method is such that during welding a distance between the centre of the laser spot and the welding arc is maintained at 1-10 mm, preferably at 2-6 mm, and most preferably at 4 mm.

In another embodiment the method is such that at the start of welding the frontal ends of the rails are posi- tioned at a spacing of 3-75 mm, preferably at 4-8 mm, and most preferably at 6 mm. Such a spacing may be realised by cutting -such as through grinding or burning- the respective frontal ends to the desired length (generally during repairs or maintenance/renewal) or by positioning the rails in a corresponding manner (generally during construction) . Too small a spacing may result in less depth penetration of the hybrid welding process while further the risk increases of the welding arc directly contacting a frontal end. Too large a spacing may cause problems with respect to the welding ma- terial flowing into contact with the frontal ends of the rails and with respect to obtaining a gas protection of the welding zone. In one embodiment the welding is started at the foot of the rails and successively moves upward through the web and the head of the rails. The molten welding material is very liquid and will tend to flow downwards. Thus working from the foot upward seems optimal.

It is possible, then, that the welding is per ^¬ formed in successive horizontal or vertical layers. This means that the welding process in a reciprocating manner yields successive layers of welding material one on top of the other or aside each other.

When, in one embodiment, below the feet of the rails and bridging the gap between the frontal ends thereof, a backing strip is positioned, a good starting point may be obtained for the ignition of the welding arc and for the deposition of the welding material and for avoiding that it flows away in an uncontrolled manner. Such a backing strip generally may be removed when the welding has been com ^¬ pleted. It is conceivable that also at other locations around the gap additional backing strips (or casing members) will be used for stabilising the molten weld material and for improving/stabilising the protective gas atmosphere at the welding zone.

In a special embodiment welding is performed in two successive stages, a first one from one side, as seen in a transverse direction, of the gap between the frontal ends and a second one from the opposite side of said gap, each for defining a weld in a corresponding part of the gap, wherein after completion of the welding said welds together define a single continuous weld. This may eliminate problems with respect to reaching every location within the gap.

For performing the laser welding process the laser may having a wavelength in a range of 700-1500 nm, prefera ^¬ bly of 1030-1070 nm, and most preferably of 1064 nm, may have a focussing length of 50-400 mm, and preferably of 200 mm, may have a focus diameter of 0,1-50 mm, and preferably of 0,6 mm, may have a power in a range up to 20 kW, and preferably between 2-4 kW (for ionising metal vapour to a plasm for opening a keyhole) and may have optics for achiev ^¬ ing a laser spot diameter (out of focus) of 0,1-75 mm, pref ^¬ erably of 4-8 mm, and most preferably of 6 mm. These charac ^¬ teristics may be applied in any convenient combination.

In one embodiment the arc welding process com- prises one of MIG arc welding (process no. 131/132/133 ac ^¬ cording to EN ISO 4063), MAG arc welding (process no.

135/136/138 according to EN ISO 4063) and self-shielded tub ^¬ ular-cored arc welding (process no. 114 according to EN ISO 4063), and preferably the arc welding process is performed in a spray arc mode and/or a short circuit welding mode. In such a spray arc mode it may be prevented that the electrode yields coarse droplets of welding material which could in ^¬ terfere with the laser beam while absorbing laser energy (which then would not reach the rails) .

The arc welding process may be performed with any of the following parameters (alone or in combination) : an electrode supply speed of 1-10 m/min, ; a welding current of 50-500 A, preferably 260 A; a welding voltage of 3-38 V, preferably 26 V. These parameters may depend from the type of electrode used and from the welding mode.

In one embodiment the rails previously to the la ^¬ ser welding process and the arc welding process are pre ^¬ heated at the frontal ends. During welding the temperature of the rails increases. Directly after welding cooling will start along a cooling curve. When the duration of such a cooling is too short or too long, the resulting weld will not comply with the requirements. Preheating is a manner to feed additional heat in the material for influencing the cooling curve.

Preferably the frontal ends are preheated to at least 250°C, preferably to at least 500°C, and most prefera- bly to at least 600°C, which up to now has proven to yield good results.

Pre-heating may be carried out in different ways, for example by using any of the following means: a propane- oxygen burner; inductive heating means.

A further manner for influencing the quality of the final weld is the manner in which the electrode is ori ^¬ ented. In one embodiment good results are obtained when the arc welding process is carried out in a push mode with an electrode angle of 10-50°, preferably of 20-40°, and most preferably of 30°.

Also the travel speed (the speed with which the laser beam and electrode, thus the welding assembly, are moved with respect to the rails) can play a role and in one embodiment the travel speed during welding is 1-10 mm/sec, such as to keep the keyhole open.

The welding may be carried out under a protective gas atmosphere, preferably under an inert gas atmosphere, preferably argon. The gas may originate from the arc welding torch, the electrode or from any other appropriate source.

Moreover, it is conceivable that the laser welding process is carried out using two laser beams, each for in ^¬ teraction with a different one of the frontal ends of the rails. As a result a larger gap between the frontal ends of the rail could be allowed while still obtaining a good weld- ing result.

Finally it is possible that at least one welding characteristic in at least one of the foot, web and head of the rail is different from said parameter at the remaining one(s) of the foot, web and head of the rail. For example the welding may be carried out in such a manner that spray arc welding is used in the regions of the foot and head of the rail, whereas short circuit welding is used in the re- gion of the web. Basically any of the variables (such as, for example, laser power, laser spot diameter, welding current and voltage, travel speed) mentioned in the above may be varied over the rail, if desired.

Hereinafter the invention will be elucidated while referring to the drawings, in which:

Figure 1 in a perspective view illustrates two rails to be connected by the method according to the present invention;

Figure 2 shows a view according to II-II in figure 1 while carrying out the method according to the present in ^¬ vention;

Figure 3 shows a view according to III in figure 1 while carrying out the method according to the present in ^¬ vention, and

Figure 4 is used to illustrate two embodiments of the method in which the welding occurs from one or from two opposite sides.

Firstly referring to figure 1, two rails or rail sections 1 and 2 are illustrated which have to be welded to- gether using a method in accordance with the present inven ^¬ tion. The rails 1,2 comprise facing frontal ends 3,4 (best illustrated in figure 3) which are positioned at a spacing s, for example 6 mm, for defining a gap 5 between the frontal ends.

The spacing s between the frontal ends 3,4 of the rails 1,2 for example may result from positioning the rails correspondingly (such as during construction of a new rail- way section) , but also for example may be obtained by remov ^¬ ing part of an existing rail section or rail sections (such as by cutting through grinding or burning) .

The rails 1,2 define a longitudinal direction hav- ing two opposite senses li and I2 and a transverse direction having two opposite senses ti and t2.

Once the gap 5 between the frontal ends 3,4 has been established the frontal ends will be preheated, for ex ^¬ ample to a temperature of at least 500°C, such that after the welding is completed a cooling curve of the rail mate ^¬ rial may be obtained which is favourable for achieving the required characteristics (such as strength). Among others such a preheating may be carried out by a propane-oxygen burner or by inductive heating means.

Figure 2 schematically shows a view in accordance with II-II (in the longitudinal direction li) in figure 1, thus a frontal view of the frontal end 4 of rail 2 (of which in figure 2 only a part has been illustrated) . As will fol ^¬ low from figure 2, a part of welding the frontal ends 3,4 is carried out using a laser beam 6 (which originates from a laser source 7) for performing a laser welding process and another part of welding said frontal ends and supplying welding material is carried out by an arc welding process (such as, for example MIG welding) using a welding torch 8 with electrode 9 that in a manner known per se create a welding arc 10 and a protective gas atmosphere 11 (for exam ^¬ ple argon) .

As seen in a direction of welding (represented by the travel speed V in figure 2), the laser beam 6 for per- forming the laser welding process precedes the welding arc

10 for performing the arc welding process. The laser welding process yields a sufficient depth penetration through the so-called keyhole effect, according to which, once a certain power level is reached, part of molten rail material (repre ^¬ sented by the molten pool 13) evaporates and is heated, re ^¬ sulting in ionised metal vapour (plasm or plasm arc) which expands in such a manner that surrounding molten material is expelled. This results in a small and deep opening (keyhole 12 schematically indicated in figure 2) which remains open as long as the travel speed V is not too high and the power level is sufficient for evaporating the metal. It follows that the arc welding is performed in the same process zone as the laser welding and in combination a high quality weld is obtained. The elevated temperature in the process zone yields a welding electrode 9 which melts in an ionising manner which then functions both as electrode and as source of welding material.

As follows clearly from figure 2, in the illus ^¬ trated embodiment the arc welding process is carried out in a push mode with an electrode angle a of 10-50°, preferably 30°. The travel speed V during welding preferably is kept at 1-10 mm/ sec.

Figure 3 shows a view in accordance with III (thus in the transverse direction ti) . In this view the welding torch 8 and electrode 9 lie in front of the laser beam 6 (only indicated schematically in broken lines; as known the laser beam 6 over a focussing length firstly converges -to a focal point, preferably with a focus diameter of 0,6 mm- and then again diverges to a spot diameter at the process zone where also the arc 10 occurs; said spot diameter generally will be in a range of 0,1-75 mm, and optics -see figure 2- may be used for obtaining it) . The movement of the welding assembly occurs in a direction into the plane of the draw ^¬ ing .

Below the gap 5 and bridging it a backing strip 15 is positioned which may define the starting point for the weld to be made and which can prevent molten rail and weld ^¬ ing material from flowing away in an uncontrolled manner. Other backing strips or casing members (not illustrated) may be provided too at other locations. The backing strip(s) 15 may be removed when the weld has been completed.

Whereas in figure 3 the use of a single laser beam 6 has been illustrated, it also is conceivable that the la ^¬ ser welding process is carried out using two laser beams, each for interaction with a different one of the frontal ends 3,4 of the rails 1,2, for example when a larger gap 5 is used which makes it difficult to have a single laser beam 6 interacting efficiently with both frontal ends 3,4.

Finally reference is made to figure 4 for explain ^¬ ing some different manners in which the welding may occur. In figure 4 the main constitutive parts of a rail are the foot 16, web 17 and head 18. In one embodiment the welding is started at the foot 16 of the rail and successively moves upward through the web 17 and the head 18. In such an embod ^¬ iment the welding preferably is performed in successive hor- izontal layers (represented by the horizontal broken lines) . For example a welding assembly 19 (generally comprising a laser and welding torch) may travel in direction t2 for generating the lowermost weld layer (over the entire horizontal extension of the respective part of the rail), then moves up a step and idles back (in direction t∑) to a new starting po ^¬ sition for generating the next weld layer, and so on in a reciprocating manner until the entire weld has been completed .

In an alternative embodiment welding is performed in two successive stages, a first one from one transverse side of the gap 5 and a second one from the opposite trans ^¬ verse side of said gap 5, each for defining a partial weld in a corresponding part of the gap 5, wherein after completion of the welding said welds together define a single con ^¬ tinuous weld. Thus, as illustrated in figure 4, the process as described in the previous paragraph now in part is car- ried out by welding assembly 19 which welds in direction t2, not over the entire extension of the rail parts 16-18 but only until reaching a dividing line 20 (which may have any arbitrary position, but generally will have a rather symmetrical position) . An equivalent partial welding process, but with a mirror image, will be carried out by the welding as ^¬ sembly 19' from the other side which welds in direction ti.

Although welding assemblies 19 and 19' may be sep ^¬ arate welding assemblies (for example functioning at the same time from opposite sides in opposite directions), they may be defined by a single welding assembly which can be ro ^¬ tated over 180° and which is displaced from one side to the other .

The welds made by welding assemblies 19 and 19' together define a full weld over the entire extension of the rail parts 16-18.

When the weld has been completed, cooling will oc ^¬ cur, followed by a final finishing (for example grinding) .

The invention is not limited to the embodiments described before which may be varied widely within the scope of the invention as defined by the appending claims.