TWI Industrial Member Report Summary 487/1994
By C N Ribton
Electron beam welding can be prone to defects formed by process interruption due to an electrical breakdown between the gun and the anode or the column wall. The breakdown reduces the accelerating potential to zero and the weld pool keyhole collapses as the beam power is removed. This can form void type defects in the fusion zone. Power supplies are required that can react quickly to avoid large surges of current or extended welding interruption when a discharge occurs. Although there are many different configurations of high voltage power supply it is believed that switch-mode inverters give the greatest benefits to industrial users. The inherently low stored energy and high speed of response allows the power supply potentially to control the output voltage at a sufficient rate when a discharge occurs such that collapse of the weld pool does not occur.
This report describes the work carried out at TWI on the control electronics for 5kHz switchmode power supplies for electron beam welding. The supplies (TWI have 3 operational on site and one in Framatome, Le Creusot) were built by a sub-contractor but have been improved extensively in order to meet operational requirements. The physical characteristics of high voltage vacuum discharges are discussed in relation to electron beam welding. A brief review is given of strategies to control discharges.
Computer modelling of the high voltage monitoring circuit and measurements made during operation are reported. Details are given of the new control functions and the electronic implementation of them. Some initial welding trials using the discharge control system are described.