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What are the benefits of TWI's electron beam real time seam tracking?


Frequently Asked Questions

Electron beam welding produces a deep and narrow fusion zone which must be accurately aligned with the seam. In many cases, the machine operator positions the beam at the beginning of the seam and fixture jigging is relied upon to give consistent alignment. However, misalignment of the workpiece and thermal distortion or expansion of the component during welding can cause missed joint defects to occur. The real-time seam tracker corrects for mismatch between the beam and joint line as the weld is taking place.

The seam tracker operates by detecting the joint line position just ahead of the weld pool. This is achieved by rapidly sweeping the welding beam across the seam at a speed sufficient to avoid melting. The particles backscattered from the surface are collected using a specialised detector which filters out the high energy electrons from the charged vapour and ions. The scan takes typically 50µsec and has no effect on welding performance.

The collected signal gives information on topographic details which is used by the control unit to identify the joint line position and correct for any misalignment with the beam by either moving the workpiece or offsetting the beam deflection. Up to ten readings are taken per second - consequently correction is very rapid. With most high voltage electron beam welding machines only a minimal joint preparation is necessary to emphasise the seam position. At 10kW beam power, typically a 0.2 x 1mm 'V' preparation is adequate. For welding conditions using a beam defocused at the workpiece surface, a high speed lens is used to raise the focal position for the duration of the deflection sweep in order to obtain high resolution of the joint position. If there is excessive run-out of the workpiece, a buzzer sounds which indicates that the tracker is operating at 80% of its maximum corrective capability. Other status messages are displayed on a front panel LCD. An important quality of the tracker is that it will ignore weld tacks and other features which obscure the joint line. Applications for the real-time seam tracker range from thick section (120mm) circumferential welding of steam turbine diaphragms for power stations to long thin section (6mm) material for the nuclear industry.

For further information see Joining Technologies or please contact us.

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