Power - New and renewable

Electron beam welding

Electron beam welding (EBW) is a fusion process with the capacity to produce single pass high integrity welds in a wide range of materials. Known as a power beam process, EBW uses a highly focused beam of electrons as a heat source. When it strikes the workpiece the high power density causes almost instantaneous local melting and some vaporisation of the workpiece material. The electron beam is thus able to establish a 'keyhole' delivering heat deep into, or through, the material being welded. The 'keyhole' is characteristic of electron beam welding. To prevent the beam dissipating and so losing power, EBW is carried out in a vacuum chamber.

EBW equipment typically comprises an electron gun, high voltage power source, vacuum chamber or enclosure, pumping equipment and a workpiece or gun manipulator and control system.

Production attributes

The EB process can produce extremely narrow, parallel sided, deep penetration welds in a vacuum. EBW exhibits the following desirable characteristics:

low heat input
low distortion
high joining rate
small heat affected zone
single pass
no consumables
no contamination or oxidation
high process efficiency

Very thick materials can be joined rapidly by high power EBW. With a 100kW system, reliable single pass welding of thick section materials such as 100mm in copper, 250mm in steel and 450mm in aluminium alloys can be achieved.

Materials

EBW can be used to join a variety of metallic materials including steels, aluminium, copper, nickel, titanium and magnesium alloys and refractory metals. Generally filler additions and pre-heat are not required. Although, in some cases, the addition of filler material or post weld heat treatment may be used to tailor the mechanical properties of the weld.

EBW also allows joining of dissimilar metals, i.e. those with different melting points and thermal conductivities. Some combinations which are not weldable by other processes are thus readily welded by electron beam.

Reduced pressure electron beam welding

The difference between reduced pressure and conventional EBW is in the design of the electron gun. Novel gun design allows the process to be carried out at a rough vacuum level, or reduced atmospheric pressure, (hence the name reduced pressure) of around 1mbar compared with 10 ^-5 - 10 ^-2 mbar for conventional EBW. The technology was initially developed at TWI in 1992 and is has undergone continued development since this time.

A major benefit of the reduced pressure requirement is the ability to function with local seals or vacuum arrangements rather than a traditional vacuum chamber. Thus, large structures are now EB weldable.

Setup and pump down times for RPEBW are up to 3 times quicker than conventional EBW so allowing better utilisation of equipment and higher throughput. A further economic benefit is the lower capital purchase cost when compared to conventional EBW as a large chamber and high performance vacuum pumps are not required.

The use of reduced pressure electron beam welding can provide substantial productivity improvements over conventional arc welding processes. This has already been demonstrated in cases such as pipeline laying and nuclear waste canisters. The benefits increase as the thickness to be welded increases. Electron beam processes can weld components in excess of 100mm thick in a single pass.

Reduced pressure electron beam welding (RPEBW) is a manufacturing technology, which is seen as having potential to provide enhancement to the heavy fabrication industry. This would be achieved by providing a step change in welding and fabrication methodology, resulting from the ability to weld thick section material in a single pass at rapid speeds.

Additional significant savings could also be realised on the basis of: -

Absence of attendant requirements for procurement, storage and drying and QA of welding consumables
Reduction in repairs and costs for weld rectification
Reduction in number of welding stations and attendant footprint and maintenance costs
Re-visiting the fabrication sequence and optimising for application of the RPEB process
Benefits will be greater for higher thicknesses