GB2142208A - Arc welding method and electrode for narrow groove welding - Google Patents

Abstract

A narrow groove arc welding apparatus and method wherein a welding torch consisting of a non-consumable electrode provided with a radially displaced tip is oscillated angularly so as to provide for improved arc sweep. <IMAGE>

Description

SPECIFICATION Arc welding method and electrodefor narrow groove welding Tldisinvention relates to electrical arc welding and more particularlyto narrow groove welding.

In narrow groovewelding, side walls ofthe welded joint, usually extremely thick plates, act as massive chills frequently making it difficu Itforthe weld bead to fuse to the sides, causing defects related to a lack of fusion. Normally prescribed remedies such as increased heatinputfrom asingle welding torch oran a rray of weliding torches have limited effectiveness.

lnahotwiretungsten inert gas welding process, sidewall wetting can be improved if the arc established between a non-consumable tungsten electrode and the work-piece is oscillated to direct more of are energytothe sides of the groove. Several approaches to arc oscillation have been suggested or tried.

Magnetic pole piecesthat can be programmed to produce a magneticfield of varying intensity and direction to causeoscillation ofthewelding arc have been used for some time. However, the use of this approach in narrow groove welding presents a space problem. The necessity of the pole piece being near the arc causes crowding within the joint. Further problems associated with heating of the pole pieces, disturbing influences of the magnetic forces created during welding of materials with high magnetic permeability, blocking of visual access of the weld pool, and difficulties in automatic monitoring have limited the desirability of this approach.

Magnetic oscillation of an arc can be caused through use oftwin electrodes which carry out of phase alternating currents. The practicality of this approach is limited by the factthatthe frequency of oscillation will be relatively high, usually 60 Hz, and constant, and alternating current tends to increase the rate of deterioration of the tungsten electrode.

Conduction of the welding current away from the weld zone can be alternated between two or more groundstherebycausing the are to oscillate under the changing influence of the magnetic field formed around the current path. This approach requires that the relative positions ofthe grounds and the arc remain constant, leading to the necessity of having thetwo grounds slide along the weld. Although such a scheme may work for weldments of simple geometry,,it is mechanically too cumbersome for most welding jobs. Once again, crowding of the weld area maybe a problem.

Somewelding equipment utilizes an oscillating electrode assembly. Both lateral oscillation and oscillation about a pivot point have been used. An example ofthe pivot point oscillation technique is disctosed in U.S. Patent No. 3,396,263, issued August 6, 1968 to T. M. Evan metal.

Lateral oscillation and pivoting around a point pioduce the following arc sweeps respectively: SL=W-2X-D (1)

where (as shown in Figures 1A and 1B) S is the sweep distance for the hottest point under the arc, W is the distance between the plates being welded, D is the torch assembly diameter, X is the minimum distance between the torch assembly and the joint walls, h is the height of the tapered section of the electrode, 1. is the arc length, and b is the pivot point distance from the weld root.

Since the torch assembly usually serves to hold the electrode in place, effectively chill it and supply shielding gas around it, the diameter D can be substantial. Although X can be kept to a small value, it necessarily has a finite value to protect the torch assembly from damage. Equations (1) and (2) inherently implythatW-2X must be much larger than D for effective oscillation. No oscillation is possible if W-2X = D. Since W must be kept to a minimum, existing mechanical oscillation techniques are inadequate for some applications.

A number of welding processes and machines have been developed for oscillating an electrode in a consumable electrode welding system. Examples of these electrode oscillating techniques can be found in U.S. Patent No.4,074,105, issued February 14,1978to Minehisa et al.; U.S. Patent No.3,576,966, issued May 4,1971 toSullivan; U.S. Patent No. 2,163,657, issued June 27, 1939 to Beckman; U.S. Patent No.3,035,156, issued May 15,1962 to Staley; and U.S. Patent No.

3,609,292, issued September 28,1971 to Arnoldy.

However, these patents do not address a nonconsumable electrode welding system.

It is therefore the principal object of the present invention to provide a method and apparatus using non-consumable electrodes, such as a hot wire tungsten inert gas process, for use where space considerations limit electrode assembly movement.

With this object in view, the present invention resides in a non-consumable electrode assembly for electric arc welding comprising a highly conductive rod, an arc resistant rod connected to said highly conductive rod and having a tip and a central axis, characterized in that said tip is shaped to have a point which is radially displaced from said central axis.

With this arrangement arc sweep distance is increased by angular oscillation around the axis of a non-consumable electrode having a tip which is radially displaced from the axis ofthe electrode.

Rotation ofthis electrode will produce an arc sweep equal to twice the radial displacement of the tip.

The arc welding method ofthis invention is particularly useful for narrowgroovewelding and includes the creation of an arc between a metal workpiece and a non-consumable electrode, such as tungsten, having a radiallyoffsettipandtheangular oscillation of this electrode. In addition, !ateral or pivotal oscillation or both can be added to the angular oscillation to achieve a greater sweep distance. This lateral or pivotal oscillation adds movement which is perpendicularto the welding line.

Non-consumable electrodes in accordance with this invention are generally straight with a shaped tip at one end which is radially displaced from the central axis of the electrodes. The opposite ends ofthese electrodes are provided with means, such as a threaded stud, for attachment to another electrode made of a highly conductive material such as copper.

This highly conductive electrode can be of a smaller diameter due to its low resistivity, thereby allowing a smallertorch diameter.

The invention will become more readily appparent from the following description of a preferred embodi mentthereofdescribed, by way of example only, with reference to the accompanying drawings, wherein: Figures 1A and 1 B illustrate prior art methods of lateral and pivotal oscillation of non-consumable electrodes; Figure 2 shows a non-consumable electrode with a radially displacedtip which oscillates angularly in accordance with an embodiment of the present invention; Figure 3A and 3B showthe addition of lateral and pivotal oscillation to the electrode of Figure 2 in accordance with the present invention; Fig u res 4A th roug h 4H show alternate electrode geometries in accordance with the present invention; and Figure 5 showsthearcsweepsobtainedwith angular, lateral, and pivotal electrode oscillations and their combinations.

Referring tothe drawings in detail, Figure 1A shows a non-consumable electrode assembly having an electrode 10 having a conical tip 12 as used in a prior artwelding process employing lateral electrode oscillation.Thearcsweep distanceSLfor lateral electrode oscillation is given by: SL = W-2X-D (1) whereW is the distance between the plates being welded, D is the diamater oftorch assembly 14 and X is the minimum distance between the torch assembly 14 and thejointwalls 16.

Figure 1 B shows a non-consumable electrode assembly used in a priorartwelding process employing pivotal electrode oscillation. In this case, the arc sweep distance Sp is given by:

where W, D and X are as defined above and 10 is the length of arc 18, h is the height of tapered section of electrode 10 and b is the distance from pivot point 22 to the weld root Figure 2 shows a non-consumable electrode 20 with a radially displaced tip 24 which is subject to angular oscillations in accordance with the present invention. In this embodiment, the electrode tip has been displaced from the electrode axis 26 by a distance equal to the radius of the electrode 20.

Simple radial oscillation of this electrode by known oscillating means produces an arc sweep SA equal to: 5A = d where d is the diameter of electrode 20. Since the arc sweep is directly proportional to electrode diameter, increasing the diameter increases the arc sweeps attained by the present invention.

Arc sweep can also be increased by combining angular electrode oscillation with lateral or pivotal oscillation as shown in Figures 3A and 3B. The combination of angular and lateral oscillation produces an arc sweep SA+of: SA+L = W-2X-D+d (4) If angular and pivotal oscillation are combined, the arc sweep S#+p becomes:

Added sweep distance can be obtained if the electrode tip protrudes beyond the surface of the electrode. Figures4A, 4B, 4C and 4D show various embodiments of non-consumable electrodes 20,28, 30 and 32 which can be used in accordance with the present invention.These electrodes comprise a straight rod with a shaped tip and would be made of a non-consumable material such as tungsten.Figure 4D shows an electrode 32 provided with means for attachment to a highly conductive electrode 34 made of material such as copper. In this embodiment the means for attachment comprises a threaded section 36 of electrode 32 and a tapped slot 38 in electrode 34.

Figures 4E, 4F, 4G and 4H are end views of the electrodes shown in Figures 4A, 4B, 4C and 4D respectively.These end views illustrate the location of each electrode tip with respect to the electrode body. It should be apparent that simple angular oscillation ofthe electrodes of Figures4C and 4D would produce arc sweeps in excess of the electrode diameter.

Electrode tip geometries may be varied within the scope of this invention, with the preferred geometry having a tip oriented toward the joint corners to direct the arc energy where it is needed most. Various electrode tip geometries may be fashioned by simply grinding existing rods as in Figure 4A or by machining rods to provide means for attachmentto a highly conductive electrode such as copper. This would eliminate the need for long stemmed electrodes as well as reducethetorch size, since smaller diameter copper electrode holders can be used without danger of overheating due to the low electrical resistance of copper.

Figure 5 shows the arc sweeps obtained with angular, lateral, and pivotal electrode oscillations and theircombinations. In some cases, simple angular oscillation of an off-centertipped electrode alone may be an improvement over existing mechanical oscillation techniques. For example, using the offcenter tip geometry of Figure 4A, Figure 5 shows that fora torch diameter l)= 12.5 mm, with 1 0+h = 7.5 mm, b = 5 cm, and an electrode diameter of d = 4.76 mm, when the distance between the plates being welded W minus twice the minimum torch clearance Xis less than 17 mm, angular oscillation alone produces the greatest arc sweep.

The increased are sweeps provided by this invention result in quality improvements in narrowgroove welding processes. In some cases, the need for machined joint preparation may be eliminated, thereby providing substantial cost benefits.

Claims (6)

1. A non-consumable electrode assembly for electric arc welding comprising a highly conductive rod, an arc resistant rod connected to said highly conductive rod and having a tip and a central axis, characterized in that said tip is shaped to have a point which is radially displaced from said central axis.

2. A non-consumable electrode assembly as recited in claim 1, characterized in that said highly conductive rod and said arc resistant rod are interconnected by athreaded segment of said arc resistant rod at an end opposite said tip, and a tapped slot in said highly conductive rod which receives said threaded segmentofsaid arc resistant rod.

3. A non-consumable electrode assembly as recited in claim 1, characterized in that said highly conductive rod is made of copper, and said arc resistant rod is made oftungsten.

4. An electric arc welding method in which molten metal is deposited along a welding line by means of a non-consumable electrode as claimed in claims 1,2 or3, characterized in that an arc is formed between the non-consumable electrode having a tip radiallydisplacedfrom the central axis of said electrode and a metal workpiece, and said non constimable electrode is angularly oscillated, thereby causing a relatively wide arc sweep width.

5. An electricarcwelding method as recited in claim 4,characterized in that said non-consumable electrode is simultaneoulsy oscillated laterally along a linesubstantially perpendiculartosaid welding line.

6. An electricarcwelding method as recited in claim 5, characterized in that said non-consumable electrode is oscillated about a pivot point located along said non-consumable electrode.