TWI Industrial Member Report Summary 906/2008
By A Sanderson
When the phenomenon of keyhole welding, using an electron beam (EB), was discovered in the 1950s, it was a major step forward for thick section welding. Reduced distortion and elimination of costly joint preparations and even more expensive filler material were of great significance. But as beam powers and penetration levels increased, ambitions to free the process from the confines of a vacuum chamber grew. Sliding seals and local vacuum enclosures were tried with mixed success, mainly because of seal leakage problems. Partial vacuum machines became popular for small components and later Reduced Pressure EB welding minimised the risk of seal leakage. However, the tantalising possibility of projecting high power beams into the atmosphere offers even more advantages.
Many groups have researched non-vacuum electron beam welding (NVEBW). In the USA, Germany and the UK, it has been shown that penetration in steel of some 40 to 50mm can be achieved, albeit under laboratory conditions. A main objective of the current (CRP) 704-2 is to develop further NVEB technology to permit the reliable welding of materials in the thicknesses range of 3 to 75mm.
Previously, EB welding had a unique capability, but over the last few decades laser beam welding (LBW) has been developed which offers distinct advantages. In the meantime, NVEB developments have continued at TWI with several notable innovations aimed at further extending the beam power that can be usefully delivered to the workpiece. These have included beam pulsing, helium jet nozzles and vortex jet nozzles. In addition, an in-depth study of the gas dynamics for fine bore nozzles was undertaken. This also involved deriving a means of calculating the electron scattering effects, taking into account temperature and pressure gradients and gas species.
This NVEBW progress review and future strategy document does not attempt to make detailed process comparisons but rather aims to answer the specific question - what is the future for NVEBW? It also summarises and draws together all of the recent NVEB innovations in the context of TWI's 2007-2009 CRP. Discussion and comments are largely confined to the single pass, keyhole, deep welding processes, but it is recognised that there are other, thick section joining methods currently used, including a wide range of arc and friction processes.
- Review the present state of NVEBW developments at TWI.
- Update TWI Members on progress achieved in TWI's CRP.
- Compare the advantages and disadvantages of NVEBW with respect to laser welding.
- Identify application sectors and components that could benefit from the use of NVEBW.
- Outline TWI's future plans for NVEB exploitation.