Published on 21 January 2013
A successful programme to develop a unique 100kW electron beam gun, power supply and control system was completed by TWI for the French nuclear company Framatome.
The equipment was based on a combination of a computer designed beam generation system and solid state power supply technology.
Assembled at Abington and installed in TWI's 150m3 EB chamber, the machinery underwent an exhaustive equipment and welding performance test period - involving demonstrations on both 300mm aluminium alloy and 300mm steel - before it was shipped to France. Following interfacing on Framatome's 100m3 chamber at Le Creusot, the equipment was first used for welding procedure development in October 1990.
The main features of the chamber were its novel gun design and power source characteristics. Gun discharges that could lead to defective welds and reject components were detected very rapidly; the power supply responded in less than 0.3ms and the high voltage was switched off momentarily. This minimised the risk of interruption of the welding process by ingress of metal vapour in the gun column. Trials in the UK and France have shown that a welding beam of 75kW could be sustained for periods of over 2hr without power supply shutdown.
By using two focusing lenses in the gun column, the EB gun had a range of working distance at least 150 750mm. A beam convergence angle could be selected to suit the material thickness and type, independent of working distance. The beam quality was maintained throughout the power range, allowing good control of weld bead profile and fusion zone type for both thin gauge and thick section materials. Previous work conducted by Framatome and TWI emphasised the need for this high beam intensity and ability to adjust beam convergence angle over wide power and working distance ranges.
To meet demanding weld quality requirements, consistency of the welding beam over long periods and at high power were essential. It was achieved through the careful design of the beam generation optics for optimum precision and thermal stability. A large refractory metal cathode had been proved to last well over 20hr at 50kW without deterioration in profile. In addition, the repeatability for the machine was checked during the 2hr 75kW endurance tests by conducting repeated beam profile measurements and partially penetrating welds.
To give a clear image of the workpiece before welding, a miniature closed-circuit colour video camera had been built into the gun column to view the workpiece almost co axially with the welding beam. A clear TV image was produced allowing continuous monitoring during setting up and tack welding. There was also a cleaning mechanism built into the visual optics system. This minimised clouding of the image even at high power levels when the operator intermittently views the weld pool.
A real time seam tracking detector, invented by TWI, could also be fitted to the gun column. The detector provided a low noise backscattered electron signal which allows tracking in one of two orthogonal directions at power levels of up to 30kW.
TWI's EB Department had been pioneering development of high power equipment for many years. Work carried out in the 1970s extended the single pass penetration capability from a few tens of millimetres to 300mm in steel and 450mm in light alloys. Since that time there have been many new developments in equipment design, configuration and fabrication for EB welding to meet the exacting needs of industry.
For information about TWI's capabilities please contact us.