Electron beam welding of armoured tanks at TWI

By Olivier Nello, Colin Ribton and Chris Punshon

Electron beam welding is widely applied to the joining of aluminium alloys for military vehicles. High power EB welding has been performed successfully for 2000, 5000 and 7000 series aluminium alloys to produce single pass welds and takes advantage of high joining rate capability.

TWI has conducted extensive weld development studies using a 75kW EBW machine operated at a chamber vacuum of better than 6 x 10 ^-3 mbar (= 5 x 10 ^-3 torr). Some work has also been carried out on non-vacuum and reduced pressure (1mbar) electron beam welding of aluminium alloys, mainly to assess the potential of the processes.

Overview of electron beam welding aspects

The success of the application of electron beams for welding is based on the physical properties of the high energy density electron beam and can be summarised as follows:

Weld quality

Deep, narrow fusion zone - width of fusion zone controllable by beam deflection.
Autogenous process, no filler material necessary.
Low distortion welding process.

Beam generation and propagation

High efficiency conversion of electrical power into an electron beam.
High stiffness of beam axis for precise beam to seam alignment.
High reproducibility since beam parameters are electrical & numerically controlled.

Beam to workpiece interaction

High efficiency conversion of the beam's calorific energy into heat.
High capability for keyhole formation through energy density and radiance.
High process stability as power conversion is physically determined.

Process control

High capability for an extensive process and quality control with the help of the secondary effects generated by the beam at the impact point during welding like emission of light, X-rays or reflected electrons.

Electron beam welds are also purer and denser than fusion welds made by any other process. This is true for three basic reasons:

The welding atmosphere, a vacuum of 1.3 x 10 ^-4 mbar (= 1 x 10 ^-4 torr) is about ten times purer than is the purest inert welding gas.
The vacuum outgases the molten weld metal to a considerable degree, although porosity may also result.
Certain nonmetallics and other impurities may be preferentially ionised and expelled from the fusion zone.

Aluminium alloys welding aspects

Apart from metallurgical considerations a number of difficulties can be encountered in developing suitable EB welding conditions for all aluminium alloys.

The high conductivity of aluminium combined with relatively narrow fusion zone can cause problems related to lack of metal flow and high solidification rate. Thus, ragged weld beads, cold shut defects and penetration bead pinhole defects will appear.

A second problem is that the probability of gun discharging is greatly increased when welding aluminium alloys. It should be borne in mind that thick section aluminium alloys containing zinc and magnesium are generally recognised to be difficult materials to EB weld without gun discharge defects.

New developments in electron beam welding

Over the last few years good progress has been made towards solving these problems. More tolerant welding conditions have been established for aluminium alloys using the double lens column of TWI's EBW machine. The use of two lenses gives more versatility in choice of beam profile and hence fusion zone shape.

Electron beam welding at TWI in reduced pressure atmosphere (gun on the left)

In addition, the use of lower welding speed and defocusing and deflecting the beam to widen the weld pool, have been found beneficial. However at low welding speeds, the weld pool can become unstable leading to a stitching effect.

Finally gun discharge defects do not occur on the newer TWI's EBW machine because of improved gun design and HV supply features.