Electron beam welding large parts...without a vacuum chamber?
TWI Bulletin, September - October 2008
EB offers many advantages for thick-section fabrication
Chris Punshon. After graduating with an honours degree in metallurgy from Sheffield University, Chris Punshon joined TWI in 1983 and his current responsibilities include the development of Electron Beam Applications in the Electron Beam, Friction and Forge Processes Department.
Allan Sanderson is the Technology Chief of the Electron Beam Group. He started work at TWI (then BWRA) in 1966after qualifying as a metallurgist in Manchester. His pioneering work in the 1970s on the generation and physics of high power electron beams, formed the core of his PhD carried out in collaboration with City of London Polytechnic.
This is particularly apposite when applied to large structures, where significant savings in both costs and time are anticipated because of the rapid joining rate achievable. As Chris Punshon and Allan Sanderson report, examples of this include the use of EB welding for the future fabrication of structures such as monopile foundations for offshore wind-turbines (Fig.1).
Fig.1. Offshore wind turbines on monopile support foundations
Typically, these are tubular structures of 3-6 metres diameter and 30 to 60m metres in length fabricated from rolled (Fig.2) and welded constructional steel with thicknesses in excess of 80mm.
Fig.2. Section of wind turbine foundation being rolled from thick steel plate (courtesy Dillinger)
Appropriate application of EB welding in a single pass is anticipated to lead to a cost and time saving in excess of 50% when compared with more conventional fabrication practice making use of submerged-arc-welding (SAW). Similar savings can be shown for structures fabricated in thick section austenitic stainless steel and nickel alloys where the cost of welding consumables and filler wire also become significant. To date, however, the full potential of the EB process has not been realised commercially for thick section welding and large structures because of restrictions associated with working at high-vacuum, with the entire structure to be welded enclosed in a vacuum envelope.
TWI has demonstrated that operating the EB process in the pressure range 0.1-10mbar, so-called 'Reduced-Pressure', in preference to high-vacuum (~10-3mbar), offers the possibility of eliminating the need for a vacuum chamber by permitting the practical use of local sealing and pumping on a large structure. In adopting the Reduced Pressure Electron Beam (RPEB) process variant, when compared to traditional high-vacuum operation, problems of achieving adequate sealing on the component are much reduced and the effect of weld pool emissions and out-gassing of the component on the gun performance are eliminated.
To date, however, RPEB welding has only been applied industrially in a few specific cases. It is envisaged that many more industrial applications of the process could be promoted, and the true viability demonstrated, by the further development of practical local sealing devices. This is currently the subject of a development project at TWI.
In addition a system has been developed at TWI which allows transmission of high power beams into air at atmospheric pressure. This system is capable of welding steel and copper of thicknesses greater than 25mm at speeds approaching2000mm/min in a single pass and has recently been configured to permit pulsed operation.
This article will describe the development of local vacuum systems for field deployment of Reduced Pressure EB welding and the optimisation of the Non-Vacuum EB welding process illustrating the potential for using both methods in cost-effective fabrication of large structures.
Reduced pressure local vacuum EB
RPEB welding
Operation of high power EB welding systems at work-chamber pressures higher than 5x10-2mbar has been tested previously at low power, but not considered practical for higher powers because of gas ingress to the gun envelope causing beam scattering and increased risk of high voltage breakdown. In the 1990s, in pursuit of improved performance in Non-Vacuum EB welding, TWI developed a system which allowed operation of a high-power electron gun with the work piece at a pressure in the range 0.1-10mbar or so-called 'Reduced Pressure'.
In this system, the electron gun electrode geometry was carefully designed to permit a beam of 0-100kW power to be transmitted through a differentially-pumped, beam transfer column in which each pumped stage was separated by a small diameter orifice allowing passage of the beam whilst restricting the flow of gas up the column, see Fig.3. In this way it was possible to maintain a vacuum pressure in the gun electrode enclosure of ~10-6mbar whilst the beam was delivered into air at atmospheric pressure or a reduced pressure of ~1mbar. Where the beam exits the column an overpressure helium gas feed can be used as an option which reduces scattering of the beam and provides a background welding atmosphere of helium which assists in prevention of weld pool oxidation.
Fig.3. Schematic representation of reduced pressure electron gun column
With this development came the possibility of working either with big chambers pumped to a coarse vacuum pressure, thus minimising the pump-down time, system cost and operating sensitivity, or as in the work reported here, with local seals and pumping applied to weld joints on work pieces too large to be contained entirely in a vacuum envelope. This concept (Fig.4) was tested and demonstrated successfully in the laboratory at TWI for application to offshore pipelay of large-diameter, thick-walled pipes, and illustrated that welds could be made in this pressure regime with consistent high quality and significantly improved process tolerance when compared to conventional high-vacuum EB welding.
Fig.4. Local vacuum reduced pressure EB welding system manufactured for offshore pipelay
In particular, for the working distance range 50-500mm, welding performance was shown to be independent of working distance for a fixed-focus setting, (Fig.5) and a target pressure of 1mbar was selected as the best compromise for Reduced Pressure operation in terms of simplification of vacuum engineering and reliable welding performance. The system was shown to operate well in the pressure range 0.1-10mbar.
Fig.5. Transverse sections from RPEB welds in API 5L x 65 C-Mn steel pipe of 25mm (10.75" diameter) and 41mm wall thickness (28" diameter) made with gun to work distance of 270mm and 50mm, respectively
To date, the process has been applied successfully to steels, stainless steels, nickel alloys, copper alloys, as well as aluminium and titanium alloys with similar results to those achieved with the high-vacuum process variant, and has exhibited significantly greater process tolerance in terms of material preparation details and system reliability. This is illustrated by the example in Figure 6 below in which 304L type stainless steel of 80mm thickness was welded at ~1mbar pressure with a 5mm mismatch and joint gap of ~1mm. The welding speed was 200mm/min with a beam power of 35kW.
Fig.6. Transverse section from an EB weld in 80mm thick 304L type stainless steel produced at ~1mbar pressure and deliberate mismatch of 5mm
Local vacuum system development
Examination of the literature and previous work has illustrated that almost since the first industrial use of electron beams for welding there has been a desire to apply the process to large components using local pumping and sealing. With the exception of the TWI/Saipem work all other attempts to apply EB welding using local vacuum have involved systems designed to operate at vacuum pressures lower than 5x10-2mbar. The attempts to work at high-vacuum, although reasonably successful in the short term, were eventually derailed by inconsistent sealing and pumping performance. The ability to work at so-called Reduced Pressure greatly improves the potential reliability of local seals and local vacuum pumping as the need for high levels of cleanliness, sophisticated pumping and sealing technology are eliminated. The TWI system uses steel brushes as the primary seal, (Fig.7-8), and with two differential pumping stages a pressure level of less than 1mbar can be achieved reliably on plate with a typical hot rolled surface finish.
Fig.7. Brush seals on reduced pressure local vacuum head
Fig.8. Reduced pressure local vacuum head and 45mm thick plate
With this arrangement and a single stage roughing pump pumping each stage it was established that a pressure of less than 1mbar could be achieved reliably in less than 10 seconds pumping time and could be maintained whilst traversing a stainless steel plate with a weld bead on the surface. Notably the pressure level was observed to improve when traversing the surface of the plate, (Fig.9).
Fig.9. Pumping curves for two stage reduced pressure local vacuum head showing static behaviour improvement in pressure with motion of the head
Potential applications
With the possibility of using high power EB welding at Reduced Pressure with a local, mobile vacuum head a number of industrial applications become feasible which hitherto would have required the construction of very large vacuum chambers. In addition, the field welding of large non-portable structures is made possible. This can be achieved either by means of the local mobile seal and Reduced Pressure EB gun, (Fig.10), or by use of a locally pumped vacuum chamber and sliding seals, (Fig.11). In both cases operation is made simpler, cost effective and reliable by operation at Reduced Pressure (~1mbar).
Fig.10. Schematic representation of local, mobile vacuum seal for RPEB welding of thick walled tubular component on-site
Fig.11. Schematic representation of locally sealed vacuum chamber for welding of thick walled tubular component on-site
Non vacuum EB welding
System description
Early attempts to use non-vacuum welding, for thick materials, were held back by the impractical requirement for very short stand-off distances (~10mm) and reduced weld quality consistency that accompanies the greater degree of beam scattering that occurs at atmospheric pressure. However, the developments described below have significantly improved this situation.
As with RPEB welding, NVEB welding potentially offers many benefits over and above those of in-chamber EBW and laser beam welding, notably elimination of the need for a vacuum chamber and a single pass thick section welding capability. Unfortunately beam spreading occurs, as the electrons collide with gas at atmospheric pressure and particularly with metal ions from the weld pool; this causes severe scattering of the beam. This limits not only the weld depth achievable, but also the viable gun column to work piece distance. Typically the working distance has to be restricted to 30mm or less, and the maximum penetration achievable in steel is less than 50mm.
Effect of pulsing
Plasma control devices such as helium gas jets have been tried with NVEB, but these appear to have a limited effect compared with the improvements achievable using such devices when CO2 laser welding. The beam can be cleared of plasma, but only at relatively high gas flow rates which tend to affect metal flow in the weld pool adversely.
In the case of Reduced Pressure welding operating at above 1mbar, similar plasma and electron scattering effects have been experienced, albeit at longer working distances. Reducing the pressure level can extend working distance, but this increases work chamber pump down time and places greater emphasis on chamber seal integrity.
One promising line of research is to pulse the electron beam, so that the plasma and gas level in the vicinity of the weld are allowed to decay when the beam is off, but with a sufficiently high duty cycle and power level that the weld pool does not collapse or solidify. From a literature search on laser and EB pulsing it appears that a pulse frequency of up to 10 kHz could be required to gain good control of the plasma.
