The objectives of the proposed project are to:
- Extend the power range of the plasma gun through;
- improved understanding of the factors effecting RF plasma emissivity
- revised electron gun and bench tests
- Optimise the electron gun to produce the best beam for welding, texturing and cutting
- Design and build an electron gun column incorporating the plasma source and integrate this onto a processing system
- Demonstrate the RF plasma electron gun capability and develop processing parameters
Most electron guns used for welding equipment and similar processing use a thermionic emitter as their source of electrons. In this type electron source a refractory metal (usually a tungsten ribbon) or compound (eg lanthanum hexaboride) is heated to a high temperature where the electrons can escape the surface and they are then accelerated by an applied electric field. There are a number of inherent problems with this design:
- The cathode dimensions are very critical, and as it runs hot it distorts and evaporates such that these dimensions change
- The cathode lifetime can be short eg 6 hours weld time on industrially applied equipment, maintenance costs can be high, and with such frequent servicing there is increased risk of incorrect gun set up leading to beam inconsistency
- During the lifetime of the cathode the beam quality can vary, changing welding performance and requiring re-adjustment of welding parameters to maintain weld quality
- Cathode lifetime is shortened if the gun vacuum is compromised, as the residual gas chemically reacts with the cathode, degrading its electron emissivity or even causing it to fail
- Cathode wear can be accelerated by ion bombardment – these ions originating from the weld pool
The idea of this proposed project is to enable the utilisation of plasma cathodes for electron beam (EB) welding equipment. This type of cathode uses an ionised low pressure gas as the source of electrons as an alternative to heated refractory material, and thereby avoids the disadvantages listed above. Through the advantages below, they offer high integrity beam generation with low maintenance:
- No wear – gun ‘sealed for life’ and low maintenance
- No variation throughout lifetime giving consistent welding performance
- Ability to operate in coarse vacuum – robust design for harsh production environments
- Rapid pulsing (switching on/off of beam in times similar to lasers
The fundamental design of a plasma cathode electron gun has been developed within an exploratory project at TWI. The design uses a radio frequency (RF) excited plasma to generate beams of up to 3.2kW power at 60kV. This has been the subject of a patent application, and has been presented in several journal publications and conference papers during 2014. A report on this work also has been submitted to the Research Board (Report No. 22029.1/2013/1543.1). The proposed project will take this development forward to extend the power range of the gun, integrate it with electron optics and apply the gun to materials processing – primarily welding, texturing and cutting, to develop and demonstrate its capability.
TWI’s Electron Beam group have supported many member companies to improve EB weld quality and consistency, and this has been an important technology theme. This has been pursued through parameter development, electron gun design, electron optics, system improvements and the introduction of beam monitoring equipment to production environments. We anticipate that the plasma cathode electron source will overcome some of the fundamental operating problems encountered in EB welding and will further improve weld quality and productivity.
Relevant Industry Sectors
Technical and Economic Benefits
Titanium gas turbine drums and zircalloy nuclear components are high value parts commonly EB welded. Maintaining high quality beam is a priority to avoid rework or scrap. The thermionic cathode used is the major cause of beam variation and cathodes will be changed after a short duration, 6 to 20 hours, to give the most consistent performance.
Automotive and sensor applications eg turbochargers, gears and pressure switches are also common - productivity is the priority. The EB gun is run for 10s of hours between cathode change. The plasma cathode EB gun would avoid the need for frequent service.