WO2010046937A1

WO2010046937A1 - Method for butt welding sheet metal plates, and welding machine for the application of this method

Info

Publication number: WO2010046937A1
Application number: PCT/IT2008/000656
Authority: WO
Inventors: Eugenio Rodolfo Mattioni
Original assignee: Me-Ca S.R.L.
Priority date: 2008-10-21
Filing date: 2008-10-21
Publication date: 2010-04-29
Also published as: EP2349629A1

Abstract

A butt welding method for sheet metal plates comprises the following steps: providing a first and a second sheet metal plate edge (Pl, P2) arranged to butt against each other in such a way that a gap (G) is left between them; and forming a through weld between the first and second sheet metal plate edges in the gap. The formation of the through weld comprises the following operations: forming a first weld bead (DW) on one side of the plate edges by continuous wire arc welding using a pair of wire electrodes (21a, 21b) converging towards the gap and extending transversely with respect to the direction of extension of the gap, this first weld bead penetrating into the gap by an amount equal to a fraction of the thickness of the plate edges; and forming a, second weld bead (UW) on the opposite side of the plate edges, using continuous wire arc welding, this second weld bead penetrating into the gap until it meets the first weld bead.

Description

Method for butt welding sheet metal plates, and welding machine for the application of this method

The present invention relates to a butt welding method for sheet metal plates which may have different thicknesses, comprising the following steps: providing a first and a second sheet metal plate edge arranged to butt against each other in such a way that a gap is left between them; and forming a through weld between said first and second sheet metal plate edges in said gap-

As is known, sheet metal plate butt welding processes are used in various technical fields, for example in shipyards, boiler works and structural steel workshops in general.

The problem encountered in sheet metal plate welding is that of providing a perfect weld with full penetration on both sides with low deformation, without turning the plates over.

In a widely used welding technique, the weld is carried out on the upper side of the plate, and an underlying backing made from copper, ceramic or protective flux is used. In this process, welding is carried out on the top side, according to parameters such that complete penetration of the thickness is achieved. The shape of the lower part of the weld is determined by the profile of the backing used. A drawback of this process is that it is necessary to use high currents, resulting in high temperatures, high shrinkage and other deformation. Another drawback is that the underside of the joint is concealed, and therefore the weld cannot be monitored in real time, but only after completion of the process, and it is therefore necessary to carry out repairs and adjustments on various joints until the parameters are optimized. Moreover, since the space between the plates and the backing is not in a protective atmosphere (the protection is provided on the opposite side), point defects often occur.

The most common form of welding with an underlying backing is the submerged arc process. This system is optimal for thicknesses of more than 10 mm, but in mechanized production it has the limitation of requiring an idle movement on the backing bar for the distribution of the flux in the case of an underlying backing with flux. The method reaches a critical state at thicknesses of 10 mm or less, with a loss of resilience of the joint and significant deformation of the plates; consequently, the welding speeds cannot exceed 600 mm/min.

Another technique of welding with backing is MIG welding with an underlying ceramic backing. This is demanding in terms of the welding parameters, and the ceramic backing has to be replaced for each weld, while the speeds are similar to, if not lower than, those for submerged arc welding.

Another technique which is used is the hybrid laser process. With this technique, it is possible to create fully penetrating welds at high speed (more than 3 m/min. in some cases), with very few deformations, a pleasing joint appearance and low filler rod consumption. The drawbacks are the reduced thicknesses (up to 8 mm) that can be handled, and the very high joining accuracy required (the butting of the plate edges to be welded must be accurate to decimal measurements, with a gap of 0.1 mm), making it necessary to carry out machining before the welding. This machining has a considerable effect on welding time and the cost of plant; it is also difficult to include it in the normal industrial environments in which plate welding takes place.

The object of the present invention is, in general terms, to eliminate, or at least reduce, the aforementioned problems of the prior art, and more specifically to provide a sheet metal plate welding method which requires no preparation of the plate edges and which enables high operating speeds and low deformation of the plates to be achieved.

The invention therefore proposes a method of the type specified above, in which the production of the through weld comprises the following operations: forming a first weld bead on one side of the plate edges by continuous wire arc welding using a pair of wire electrodes converging towards the gap and extending transversely with respect to the direction of extension of the gap, said first weld bead penetrating into the gap by an amount equal to a fraction of the thickness of the plate edges; and forming a second weld bead on the opposite side of the plate edges, using continuous wire arc welding, said second weld bead penetrating into the gap until it meets the first weld bead.

The applicant has discovered that the double wire electrode welding head can be used to fill relatively wide gaps with good penetration, leaving an excellent joint for the weld from the opposite direction. The through weld produced in this way has excellent properties and can be made at high speed.

The invention also proposes a welding machine for the application of a butt welding method for sheet metal plates, comprising a support on which a first and a second plate edge can be positioned so that they butt against each other; and a first welding head for continuous wire arc welding, which can be positioned on one side of the plate edges, and which comprises a wire electrode, characterized in that it comprises a second welding head for continuous wire arc welding, which can be positioned on the opposite side of the plate edges and which comprises a pair of converging wire electrodes extending in a transverse plane with respect to an operating direction of the welding machine.

Preferred embodiments of the invention are described in the dependent claims.

These and other objects and advantages will be made clearer by the following detailed description of a preferred, but non-limiting, embodiment of the invention, provided with reference to the attached drawings, in which:

Figures 1 to 3 show a set of welding heads of a welding machine for the application of a method according to the invention, in a perspective, front and rear view respectively;

Figure 4 shows the set of welding heads of Figures 1 to 3 in a cross-sectional view taken along the line A-A in Figure 2; and

Figures 5 to 7 show a lower head of the welding set of Figures 1 to 3 associated with a carriage, in a perspective, plan and front view respectively. Figures 1 to 4 show a set of welding heads of a welding machine for the application of a method according to the invention. This welding machine is suitable for the butt welding of sheet metal plates, and therefore comprises a support, indicated in a general way by S in Figures 2 and 3, which is designed to position correctly with respect to each other a first and a second plate edge Pl, P2 to be welded. The arrangement of the remainder of the welding machine is not essential for the purposes of understanding the invention, and therefore will not be described.

The set of welding heads of the welding machine comprises an upper welding head 10 which can be positioned above the plate edges. This upper welding head 10 is conventionally designed to carry out arc welding under gas protection by a continuous wire method according to the MIG system. It therefore comprises, in a conventional way, a wire electrode 11 which generates the electric arc and the weld material required for welding, together with a nozzle 12 connected to a gas supply system (not shown) to create a protective atmosphere in the welding area. Wire feed means (not shown) are provided to compensate continuously for the consumption of the wire electrode 11 during the welding process. The wire electrode 11 for creating the electric arc emerges from the welding head 10 perpendicularly to the plane defined by the plate edges Pl and P2. The welding head 10 is movable, by means of a movement system (not shown), in the direction of extension of the gap G formed by the plate edges Pl and P2 which are butted against each other.

According to the invention, the set of welding heads of the welding machine also comprises a lower welding head 20 which can be positioned under the plate edges. This lower welding head 20 is conventionally designed to carry out arc welding under gas protection by a continuous wire method according to the MIG system. According to the invention, the lower head 20 comprises a pair of wire electrodes 21a, 21b positioned so as to converge towards the gap G. These wire electrodes 21a, 21b extend in a transverse plane with respect to the direction of extension of the gap G. Each wire 21a, 21b is associated with a corresponding nozzle 22a, 22b connected to a gas supply system (not shown) to create a protective atmosphere in the welding area. Preferably, the wire electrodes 21a, 21b are positioned so as to form an acute angle θ between them, this angle preferably being in the range from 40° to 80°. The lower welding head 20 is movable in the direction of extension of the gap G, and is fitted on a motorized carriage 25, shown in Figures 5 to 7. The motorized carriage 25 is provided with a motor 26, and can move along longitudinal guides R substantially parallel to the orientation in which the gap G is positioned between the edges Pl and P2 (in other words, the direction of operation of the machine). The carriage 25 is also provided with a following device 25a, shown in the diagrams as a mechanical feeler with a wheel, for following the welding joint.

The carriage 25 is also provided with means for adjusting the angles of inclination of the lower welding head 20 with respect to the plane defined by the plate edges Pl, P2. More specifically, it has a curvilinear transverse guide 27 formed by a set of rollers positioned in a curvilinear configuration, which extends in a transverse plane perpendicular to the direction of the gap G. A curvilinear traveller 28 is mounted so that it can travel along this curvilinear transverse guide 27. A welding head support 29 is fixed to this curvilinear traveller 28. Curvilinear longitudinal channels 31 are formed on this welding head support 29, these channels extending in planes parallel to the direction of extension of the gap G. The lower welding head 20 is provided with rollers 32 which interact with the curvilinear longitudinal channels 31 and with a curvilinear part of the welding head support 29 to enable the inclination of the lower welding head 20 to be adjusted in the vertical longitudinal plane of the gap G. The curvilinear traveller 28" also interacts with the curvilinear transverse guide 27 to enable the inclination of the lower welding head 20 to be adjusted in a transverse plane, perpendicular to the direction of extension of the gap G.

Wire feed means are provided to compensate continuously for the consumption of the wire electrodes 21a, 21b during the welding process. These wire feed means comprise winders for reels 33 fitted on the carriage 25, together with a wire advance and control mechanism 34 fitted on the lower welding head 20.

In the welding process according to the invention carried out with the machine described above, a first weld bead DW is made on the underside of the plate edges, using the pair of wire electrodes 21a, 21b of the lower welding head 20. This first weld bead is made so as to penetrate into the gap G by an amount equal to a fraction of the thickness of the plate edges Pl, P2 (it is to be understood that the thicknesses of these edges can differ from each other). A second weld bead UW is made on the upper side of the plate edges, using the wire electrode 11 of the upper head 10, this bead being made so as to penetrate into the gap until it meets the first weld bead DW.

The welding cycle can be carried out in many different ways and can be controlled in different ways according to production requirements. It is possible to start the lower weld and then start the upper weld after a short time interval, completing the welds at the same speed; it is also possible to begin with the lower weld and start the upper weld in such a way that both welds are completed at the same time. Clearly, the variables, in the form of the welding parameters and speed, will have an effect on the waiting time of the upper head.

In the example described above, the double wire electrode head is the head which is positioned underneath. However, this is not essential for the purposes of the invention. It is also possible to have a configuration in which the upper welding head is provided with a double wire electrode with the arrangement of the wire electrodes described above, and in which the lower welding head is provided with a single wire electrode. This configuration could be particularly suitable for thin plates. In all cases, the first welding pass is always executed with the double wire electrode head.

The applicant has applied the method according to the invention to plates with thicknesses in the range from 3 mm to 15 mm, using a welding machine configuration as shown in the diagrams. The welding edges were those produced by the cutting system conventionally used on construction sites, such as oxyacetylene or plasma cutting. When these systems are used on 15 m sheet metal plates, the resulting parallelism error is approximately 1.5 mm. The edges were not prepared in any way.

It was found that the double wire electrode welding head was able to fill gaps of 3 ÷ 4 mm between plates with a thicknesses of 5 ÷ 6 mm with a penetration of 2 ÷ 3 mm, leaving an optimal joint for welding from the opposite side. The speeds achieved in the trials varied from 600 to 1000 mm/min. Essentially, it was found that joints with a pleasing appearance could be produced at high welding speeds, with low deformation and small quantities of ^'weld material, by using a relatively simple machine without preparation of the joints.

Claims

1. Butt welding method for sheet metal plates, comprising the following steps: providing a first and a second sheet metal plate edge (Pl, P2) arranged to butt against each other in such a way that a gap (G) is left between them; and forming a through weld between said first and second sheet metal plate edges in said gap, characterized in that the forming of the through weld comprises the following operations: forming a first weld bead (DW) on one side of the plate edges by continuous wire arc welding using a pair of wire electrodes (21a, 21b) converging towards the gap and extending transversely with respect to the direction of extension of the gap, said first weld bead penetrating into the gap by an amount equal to a fraction of the thickness of the plate edges; and forming a second weld bead (UW) on the opposite side of the plate edges, using continuous wire arc welding, said second weld bead penetrating into the gap until it meets the first weld bead.

2. Welding machine for the application of a butt welding method for sheet metal plates, comprising a support (S) on which a first and a second plate edge (Pl, P2) can be positioned so that they butt against each other; and a first welding head (10) for continuous wire arc welding, which can be positioned on one side of the plate edges, and which comprises a wire electrode (11), characterized in that it comprises a second welding head (20) for continuous wire arc welding, which can be positioned on the opposite side of the plate edges and which comprises a pair of converging wire electrodes (21a, 21b) extending in a transverse plane with respect to an operating direction of the welding machine.

3. Welding machine according to Claim 2, in which the wire electrodes (21a, 21b) of the second welding head form an acute angle (θ) between them.

4. Welding machine according to Claim 3, in which the wire electrodes (21a, 21b) of the second welding head form an angle (θ) in the range from 40° to 80° between them.