EB welding of large components without a vacuum chamber (April 2007)

TWI Ltd, Cambridge, UK

Paper presented at 7th International Conference on Beam Technology, Halle, Germany, 17-19 April 2007.

1 Introduction

Electron beam (EB) welding offers many advantages for thick-section fabrication, particularly when applied to large structures, where significant savings in both costs and time are anticipated because of the rapid joining rate achievable. Examples of this include the use of EB welding for the future fabrication of structures such as monopile foundations for offshore, wind-turbines, Figure 1. Typically, these are tubular structures of 3-6 metres diameter and >60m metres in length fabricated from rolled (see Figure 2) and welded constructional steel with thicknesses in excess of 80mm.

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 ^-3 mbar), 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 approaching 2000mm/min in a single pass and has recently been configured to permit pulsed operation.

This paper 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.

2 Reduced Pressure local vacuum EB welding

2.1 Reduced Pressure EB welding

Operation of high power EB welding systems at work-chamber pressures higher than 5x10 ^-2 mbar has been tested previously at low power, ^[1] 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 Figure 3. In this way it was possible to maintain a vacuum pressure in the gun electrode enclosure of ~10 ^-6 mbar 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.

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 was tested and demonstrated successfully in the laboratory at TWI for application to offshore pipelay of large-diameter, thick-walled pipes, Figure 4 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.

In particular, for the working distance range 50-500mm, welding performance was shown to be independent of working distance for a fixed-focus setting, Figure 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.

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.

2.2 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 described in reference ^[2] , all other attempts to apply EB welding using local vacuum have involved systems designed to operate at vacuum pressures lower than 5x10 ^-2 mbar. 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 and sophisticated pumping and sealing technology are eliminated. The TWI system employs steel brushes as the primary seal, Figure 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.

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, Figure 9.

2.3 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, Figure 10, or by use of a locally pumped vacuum chamber and sliding seals, Figure 11. In both cases operation is made simpler, cost effective and reliable by operation at Reduced Pressure (~1mbar). 

3 Non Vacuum EB welding

3.1 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. ^[6]

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. Attempts to weld thicker sections invokes excessive weld width and weld defects.

3.2 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 CO ₂ laser welding. The beam can be cleared of plasma, but only at relatively high gas flow rates which tend to adversely affect metal flow in the weld pool.

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.

At operating pressures of even several tens of millibar an electron beam undergoes minimal scattering, but as the pressure approaches one atmosphere substantial electron-atom collisions occur. For conventional NVEB equipment, operating at say 175kV, the useful working range for welding is only some 5 to 20mm. The degree of scattering can be reduced and working distance range extended by employing a higher accelerating voltages, see Figure 12 for an example of weld penetration at 200kV, however, this increases the size, weight and bulk of the equipment making it less favourable for mounting the gun, for example, on a Cartesian robot.

One new approach that promises to substantially increase penetration performance of NVEB is the use of pulsed beams. As in the case of high vacuum EBW, it has been found that penetration range can be increased for a given average power level by using high peak power levels. In a recent TWI research programme, it has been shown that penetration depth in steel can be increased by 50% combined with a corresponding reduction in weld width, particularly in the vicinity of the top bead. Pulsing also appears to alter the solidification mechanism that often leads to solidification cracks in deep NVEB welds. Figure 13 shows a compact 100kW NVEB gun column that incorporates a pulsing facility.

Figure 14 shows a melt run made in the flat position in low alloy steel using a pulsed non-vacuum electron beam. It will be noted that the fusion zone is almost parallel sided with a well-rounded tip. Apart from minor pores the fusion zone was sound. The run was made at an average power of 26.3kW at a welding speed of 480mm/min.

4 Concluding remarks

The ability to weld thick section material (25-150mm) in a single pass has been one of the driving forces behind the development of high power electron beam welding systems with potential for high productivity and high accuracy fabrication. The requirement to operate in high vacuum has precluded the use of the process in the fabrication of very large structures as the construction and operation of large vacuum chambers can be difficult to justify economically. The possibility of having portable local vacuum equipment which can be delivered to site and operated on a lease/hire basis improves practicality and economics of thick section EB welding. Operation at Reduced Pressure where process reliability and performance is, if anything, better than at high vacuum make this now a practical proposition and work is currently underway to manufacture an industrial system which will realise this opportunity.

Similarly the possibility to operate at atmospheric pressure with an non-vacuum system is equally attractive particularly where thinner materials are concerned (i.e. <50mm).With recent developments in beam pulsing and gun configuration it is likely that high speed welding of even thicker materials will be realised at atmospheric pressure in the near future.

5 Bibliography

1. Meyer E and Dorn L: 'Electron beam welding at different pressures'. Welding and Metal Fabrication, March 1970.
2. Punshon C S, Sanderson A and Belloni A: 'Reduced pressure electron beam welding for steel pipelines'. 6th International Conference on Welding and Melting by Electron and Laser Beams. Toulon, June 1998. Vol.1, pp.363-371.
3. Schumacher B W, Lowry J F and Smith R C: 'High power beams in the atmosphere' 4th International Seminar Long Island, NY, April 1976 by Universal Technology Corporation; 1976. ISBN 0 912426 055, 31pp.
4. Anderl, P, Kappelsberger and Steigerwald K. H: 'Electron beam welding of large size work pieces with mobile vacuum unit under nearly practical conditions'. Proc. Int. Technology conf. DVS 63 Essen, May 1980.
5. Yamamoto M et al: 'Study on local vacuum for low alloy steel'. IIW Doc. IV-581-92.
6. Sanderson A. 'Four Decades of Electron Beam Development at TWI' IIW Doc 1770-06, Welding in the World, IIW publication, Jan/Feb. 2007.