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

Wind turbine application

As the capacity of wind turbines grows up to 5MW the structural requirements of the foundations is already leading to substantial increases in steel thickness. To meet future mass productivity needs the use of cheaper and faster fabrication processes such as RPEB will be driven by the growing competitiveness within the industry. Preliminary studies have shown that for foundations around 50mm thick and 3m diameter the fabrication labour cost alone using RPEB is a factor of 2 lower than the case of fabrication using submerged arc welds and production time reduces by close to an order of magnitude. A summary of the benefits of RPEB over conventional submerged arc welding is given below:

Increased speed, reduced cost

The labour time for RPEB is approximately half of that for SAW. Elimination of waiting periods for preheat, cooling period to inspection, statutory delay for final inspection and repairs amounts to a further time saving The result is that the use of RPEB could enable the rate of production to be increased by a factor of 5 to 10.

Owing to the increased rate of fabrication, RPEB would enable increased production capacity of existing fabrication plants and will avoid the need to build new production plant or extend production plant as demand grows.