Magnetically impelled welding offers an appealing alternative to more conventional techniques 

One of latest methods of welding tube to tube joints and tube to flange joints is still little known in industry. Yet the technique offers the advantage of very high welding rates which can provide great commercial advantage when used in production. The method is called magnetically impelled arc butt (MIAB) welding and it uses the fact that a magnetic field can be used to rapidly scan a welding arc along a joint. The result is a very fast welding process which offers large cost savings for a range of joint configurations.

In order to make a MIAB weld, the two components are clamped in line, with a gap between the butting faces of about 5mm. The crucial step is to introduce a magnetic field into the joint region between the tube ends. An arc welding power supply is then used to produce an arc between the two tube faces and the arc begins to rotate around the tubes due to the presence of the magnetic field.

Velocities

With the correct field profile, acceleration of the arc is rapid until the arc speed stabilises at velocities in excess of 50m/sec. To an observer, it appears as if there is a continuous glowing arc completely filling the space between the two tubes. The rotating arc produces a continuous heating effect on the butting faces of the tubes almost instantaneously and the ends of the tubes heat up rapidly until they are white hot. At this moment, the control system extinguishes the arc and forges the components together to make a weld.

The total time for the complete weld depends upon the diameter and wall thickness involved . Welding time is about one second for 20mm diameter tube rising to about six seconds for 50mm tube. In addition to the welding time, the loading and unloading time for MIAB welding machines is about 10 to 20 seconds per item. It is possible to make two welds simultaneously on each end of a central tube thus offering a further reduction in welding time.

This process has been used most recently to replace flash butt welding in the manufacture of the Ford Transit rear axle. Both ends of the square axle are welded simultaneously with a floor-to-floor time of less than one minute.

MIAB welding offers an alternative technique to resistance butt, flash and friction processes and has several advantages. In particular, welding times are about ten percent of those required for resistance welding, arc welding and friction welding. The components are not rotated or displaced as with friction welding so that alignment of the components is maintained and the process can be used to weld non-circular components which are difficult to complete using friction welding.

Limitation

The main limitation to the technique is that wall thicknesses about 6mm can be difficult to weld since the rotating arc may not cause the entire butting surfaces to be heated uniformly. There are, however, techniques which enable the thicker sections to be welded and wall thicknesses up to 20mm have been welded using MIAB.

The MIAB process uses a forging action to weld together the component surfaces which have been heated by the rotating arc. As with the other forge processes, this results in the production of a flash on both the inner and outer surfaces of the components. For many purposes, this welding flash can be left in place. If however, it is necessary to maintain the bore of a welded tube assembly or restore the external profile, then the flash should be removed.

MIAB welding machines, generally speaking are mechanically uncomplicated. The machine comprises a rigid base unit upon which are mounted two work holding clamping devices: one fixed, one sliding. These are normally hydraulically actuated from a free-standing power pack.

Impulsion

The electromagnet coils and field profile control which provides the impulsion for the arc are mounted between the clamps and may be retractable to allow access for loading/unloading of the workpieces. The electrical /electronic controls and are power supply are housed in a free standing cabinet together with a process monitoring system if this is also required. Machines are usually manually loaded although automated work handling can be included to achieve higher production rates.

Mobile MIAB machines have also been built which can be used on-site and allow transmission pipelines to be welded with potential savings due to improved productivity over other arc welding processes. Trials have taken place in Canada using MIAB welding to join 12" diameter cross country gas transmission pipelines in extremely rugged terrain.

Tel. +44 (0) 1223 84 44 44