GB2040774A - Stirring weld metal - Google Patents

Abstract

Welding apparatus for electric arc welding of two workpieces of non- magnetic material spaced by a narrow gap has a welding head comprising a tungsten electrode 20 with gas shields 21 which extend substantially across the gap. The gas shields form, with the walls of the workpieces on the two sides of the gap, a gas-containing region providing the path for an inert gas to be fed around the arc. A first solenoid 27 around the welding head and adjacent the face of the workpieces 10, 11 co-operates with a second co-axial solenoid 30 adjacent the opposite face of the workpieces to produce a magnetic field interacting with the arc current flowing in the molten metal to cause stirring of this metal. The welding head is repetitively traversed along the gap 12 to form successive layers 13, 14 of weld metal joining the workpieces, the magnetic stirring causing the molten metal to wash over the surfaces of the workpiece thereby assisting in obtaining good heat distribution and a satisfactory weld. Permanent magnets may replace to solenoids to cause stirring. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electric arc welding apparatus and method This invention relates to electric arc welding and is concerned more particularly with a method of and apparatus for welding together two workpieces of non-magnetic material which are spaced by a narrow gap.

When two workpieces are spaced apart, it is necessary to introduce weld metal into the gap. If this gap is too deep to be filled during a single passage of the welding head along the length of the weld, then successive layers of weld metal must be put in the gap. It is an object of the present invention to provide an improved form of welding apparatus for use in narrow-gap welding with non-magnetic workpieces.

According to one aspect of the invention, a method of welding two workpieces of non-magnetic material separated by a narrow gap by electric arc welding and in which an electrode of a welding head is inserted into the gap and is repetitively traversed along the length thereof, whereby successive layers of weld metal are put into the gap is characterised in that the molten metal in the weldpool is stirred magnetically by electrically energised co-axial coils or annular permanent magnets moved along the length of the gap with the welding head, the coils or magnets being located adjacent opposite faces of the workpieces, each face being defined by portions of the workpieces extending away from the edges of the gap, the coils or magnets co-operatively providing a magnetic field in the region of the molten metal in the weldpool formed in the gap.

The magnetic field of the coils or magnets and the current, from the arc, flowing through the molten metal cause stirring of this metal. This distributes the heat more evenly and makes the molten metal wash over the parts of the workpieces defining the gap and so assists in melting this metal. The action of the magnetic stirring thus promotes the forming of a weld with good union to the workpieces.

Such magnetic stirring may be used, for example, with submerged arc welding or with TIG (tungsten inert gas) welding. The necessary weld metal is introduced into the gap in the known way.

According to another aspect of the invention, electric arc welding apparatus for welding together two workpieces of non-magnetic material separated by a gap of predetermined width, comprises a welding head insertable into the gap and having a non-consumable electrode, two shield members respectively on opposite sides of the electrode, each shield member having a width, measured in a direction normal to the plane defined by the axis of the electrode and the centres of the shield members, which is less than said predetermined width of the gap whereby the shield members, when the head is inserted in the gap, form, with the surfaces of the workpieces, a gas path around the electrode through which an inert gas can be fed for surrounding the arc, and wherein a first solenoid or annular permanent magnet is provided around the electrode and co-axial therewith for providing a magnetic field in the region of the molten metal in the weldpool formed in the gap, the first solenoid or magnet being of a diameter greater than the width of the gap and located adjacent to a face of the workpieces defined by portions of the workpieces extending away from the edges of the gap and a second co-axial solenoid or annular permanent magnet is provided adjacent to the opposite face of the workpieces.

The shield members preferably have a width only slightly less than the gap width. It is readily possible however, in the confined region of a narrow gap, to maintain an inert gas shield around the arc.

The electrode or shield members of this welding head can be inserted into the gap between two workpieces and can then be traversed along the length of the gap with weld metal being fed into the arc region so as to put down a layer of weld metal in the gap. Successive traverses enable successive layers of the gap to be filled in this way thereby enabling a deep narrow gap between two workpieces to be properly filled during the welding operation.

The arc, for the first run, extends between the electrode and the workpieces on the two sides of the gap thereby causing melting of metal of the workpieces, which metal mixes with the additional weld material fed into the arc to ensure a sound joint. As successive layers are deposited, the arc from the welding head also ensures melting of the weld metal deposited in previous layers to give a good union with newly fed-in weld material.

The solenoids or permanent magnets provide a magnetic field which, in conjunction with the welding current flowing from the electrode through the molten metal in the gap, causes stirring of this metal.

The movement of the molten metal causes the heat to be distributed more evenly and causes the molten metal to wash over the walls of the workpieces defining the sides of the gap so assisting in the melting of the metal on the surface of these walls.

Thus the magnetic stirring promotes the formation of a good weld between the two workpieces.

The solenoids may be formed by a helical coil wound for example on a cylindrical ceramic former.

The former for the first solenoid may be hollow so that the shielding gas, which is typically argon, can flow through the former into the region between the aforementioned shield members. The solenoid may be energised with a direct current. In this case, if it is desired periodically to reverse the direction of stirring, then a periodically reversed current, e.g. a low frequency alternating current, may be fed to the solenoids. For some materials, e.g. aluminium, it may be preferred to have an alternating current in the arc; in this case the solenoids may be energised with a supply synchronized with the arc current and a reversal of the direction of stirring can be obtained by reversing the phase of the supply to the solenoids with respect to that of the arc current.

The electrode may be a non-consumable electrode, e.g. a tungsten electrode, and, in this case, guide means may be provided for feeding weld metal through one of the shield members to the arc region beyond the tip of the electrode.

If the workpieces are of non-magnetic material, e.g. aluminium or austenitic steels or ESSHETE 1250, adequate magnetic field may be obtained in the molten metal in the weldpool, for effecting stirring of that metal, even if the solenoids or magnets are outside the gap.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawing which is a diagrammatic transverse section through a welding head and showing parts of two workpieces to be welded together.

Referring to the drawing, two workpieces 10, 11 of non-magnetic material with a narrow gap 12 between them, are to be welded together. The welding is effected by means of TIG (tungsten inert gas) welding using a series of filler runs in which weld material is deposited as shown for example at 13 and 14.

The welding apparatus has a head which is dimensioned so that it can fit within the gap. This welding head comprises a tungsten electrode 20 together with two gas shields 21 disposed on opposite sides of the electrode, only one of which is seen in elevation in the figure. These gas shields extend across the gap and, in this embodiment, are arcuate in transverse section. Other shapes may be employed for these shields. When the head is in use, the shields form, with the inwardly facing walls 23, 24 of the workpieces, a substantially enclosed gas region around the electrode for containing the inert gas, typically argon, which is fed into the welding head via a tuDe 25. This gas leaves the lower end of the region, forming an inert gas shield around the arc. For introducing weld metal into the arc region, there is provided a filler guide (not shown) extending through one of the gas shields 21, this filler guide permitting the feeding of filler wire into the arc region as is necessary to provide the additional material for the successive filler runs as the head is moved along the gap.

If the arc current is a direct current, then the solenoids 27, 30 may be energised with a direct current. This current may be periodically reversed or may be a low frequency alternating current so that the direction of the magnetic field and hence the direction of stirring is periodically reversed. If the arc current is an alternating current, the solenoids may be energised from a supply synchronised with the arc supply to give unidirectional stirring. Periodic reversal of the phase of the supply to the solenoids will give periodic reversal of the direction of the magnetic field and hence reversal of the direction of stirring.

The electrode 20 and shields 21 with the gas supply tube 25 are mounted, for movement in the axial direction of the electrode, within a gas shroud 26 forming part of the welding head. A first solenoid coil 27, co-axial with the electrode 20 is fixed around the gas shroud 26, which shroud, in use, lies closely adjacent to the surfaces 28, 29 of the workpieces extending away from the edges of the gap 12. A second coil 30, co-axial with coil 27, is arranged adjacent to the opposite face of the workpiece and the two coils are energisedin such senses that they co-operate in producing a magnetic field through the gap. Because the workpieces are of non-magnetic material, it is possible to use coils 26, 30 of a diameter greater than the width of the gap 12 and disposed outside the gap. Preferably the diameters of the coils are commensurate with the spacing between them so that there is fairly large volume over which the magnetic flux field is axial.

In operation the welding head is traversed along the gap so as to form one layer of filler material, such as layer 13. Successive traverses are made, depositing a layer of weld metal on each traverse. After each traverse the electrode 20 and shields 21 are withdrawn the appropriate distance through the gas shroud 26 whilst still leaving the coils 27,30 and the shroud closely adjacent to the faces of the workpieces 10. The magnetic field due to the solenoids, in conjunction with the arc current which flows through the molten metal in the weldpool underthe electrode, generates magnetic forces in the weldpool causing a stirring effect in the molten metal. The metal is stirred primarily in a circular direction around the axis of the electrode. As explained above, with a direct current arc, if the solenoid is energised with a direct current, the stirring is unidirectional. An alternating or other periodically reversed current will cause the direction of stirring to be periodically reversed; a low frequency is desirable in order to give adequate stirring movement in each direction as is required for good mixing. For an alternating arc, periodic reversal of the phase of the supply to the solenoids gives reversal of the direction of stirring.

The stirring gives a more even distribution of the heat in the molten metal but more particularly causes this molten metal to wash against the side walls of the workpiece defining the gap. This helds to ensure melting of the surface of the workpiece on these side walls and mixing of the molten metal from these walls with theadditional molten filler material thereby helping to ensure a good welded joint. The welding head is repeatedly traversed along the length of the gap to deposit successive layers, such as 14, 15, 16, of filler material until the whole of the gap has been filled.

The arrangement described above is used for welding non-magnetic materials e.g. aluminium or austenitic steels, and it is not necessary to have the coil in the gap closely adjacent the weldpool. It is possible to use solenoids outside the gap. The solenoids are of a diameter greater than the width of the gap to give a substantially axial magnetic field through the molten metal in the weldpool. Although a TIG welding process with a tungsten electrode 20 has been described, other techniques, e.g. submerged arc welding, may be employed.

Although the use of solenoids is preferable, as it facilitates easy reversal of the direction of stirring, in some cases, annular permanent magnets might be employed in place of solenoids 27,30.

Claims (7)

1. A method of welding two workpieces of non-magnetic material separated by a narrow gap by electric arc welding and in which an electrode of a welding head is inserted into the gap and repetitively traversed along the length thereof, whereby successive layers of weld metal are put into the gap, characterised in that molten metal in the weldpool is stirred magnetically by electrically energised coaxial coils or annular permanent magnets moved along the length of the gap with the welding head, the coils or magnets being located adjacent opposite faces of the workpieces, each face being defined by portions of the workpieces extending away from the edges of the gap, the coils or magnets co-operatively providing a magnetic field in the region of the molten metal in the weldpool formed in the gap.

2. A method as claimed in claim 1 wherein a submerged arc welding technique is employed.

3. A method as claimed in claim 1 wherein a TIG (tungsten inert gas) welding technique is employed.

4. Electric arc welding apparatus for welding together two workpieces separated by a gap of predetermined width, comprising a welding head insertable into the gap and having an electrode and two shield members respectively on opposite sides of the electrode, each shield member having a width, measured in a direction normal to the plane defined by the axis of the electrode and the centres of the shield members, which is less than said predetermined width of the gap whereby the shield members, when the head is inserted in the gap, form, with the surfaces of the workpieces, a gas path around the electrode through which an inert gas can be fed for surrounding the arc, and wherein a first solenoid or annular permanent magnet is provided around the electrode and co-axial therewith for providing a magnetic field in the region of the molten metal in the weldpool formed in the gap, the first solenoid or magnet being of a diameter greater than the width of the gap and located adjacent to a face of the workpieces defined by portions of the workpieces extending away from the edges of the gap and wherein a second co-axial solenoid or annular permanent magnet is provided adjacent to the opposite face of the workpieces.

5. Apparatus as claimed in claim 4wherein first and second solenoids are provided, each comprising a wire wound on an open-ended former.

6. Apparatus as claimed in either claim 4 or claim 5 wherein the first solenoid or permanent magnet is fixed with respect to a gas shroud around the electrode and wherein the electrode and shield members are mounted for movement relative to the shroud in the axial direction of the electrode whereby the electrode may be withdrawn gradually from the gap as successive layers of weld material are deposited whilst leaving the shroud closely adjacent to said one face.

7. Electric arc welding apparatus substantially as hereinbefore described with reference to the accompanying drawings.