The welding of aluminium and its alloys

Electron beam welding

Electron beam welding is, like laser welding, a power beam process ideally suited to the welding of close square joints in a single pass. Unlike the laser beam, however, the electron beam process utilises a vacuum chamber in

8.6 Combined laser and MIG welding head. Courtesy of TPS-Fronius Ltd.

8.7 Principles of operation of the laser/MIG process. Courtesy of TPS-Fronius Ltd.

which is generated a high-energy density beam of electrons of the order of 0.25-2.5mm in diameter (Fig. 8.8).

The beam is generated by heating a tungsten filament to a high temper­ature, causing a stream of electrons that are accelerated and focused mag­netically to give a beam that gives up its energy when it impacts the target - the weld line. This enables very deep penetration to be achieved with a keyhole penetration mode at fast travel speeds (Fig. 8.9), providing low overall heat input.

The process may be used for the welding of material as thin as foil and up to 400 mm thick in a single pass. The keyhole penetration mode gives almost uniform shrinkage about the neutral axis of the component, leading to low levels of distortion. This enables finish machined components to be welded and maintained within tolerance. The transverse shrinkage also results in the solidifying weld metal being extruded from the joint to give some excess metal outside the joint (Fig. 8.10).

The major welding parameters are (a) the accelerating voltage, a 150kV unit being capable of penetrating 400 mm of aluminium; (b) the current applied to the electron gun filament, generally measured in milliamperes; and (c) the travel speed. The item to be welded is generally mounted on an NC manipulator, the gun being held stationary. The unwelded joint com­ponents are required to be closely fitting and are usually machined. Filler

metal is not normally added but if gaps are present this leads to concavity of the weld face.

The major drawback with this process is the need to carry out the welding in a vacuum chamber evacuated to around 10-3 to 10-2Pa. This requires expensive diffusion pumps and a hermetically sealed chamber large enough to accommodate the item to be welded. The cost of equipment, the accuracy with which components have to be machined to provide an accu­rate fit-up and the time taken to pump the chamber down can make the process non-competitive with more conventional fusion welding processes. For high-precision welding, perhaps of finished machined items where minimal distortion is required and for batch type applications where a number of items can be loaded into the chamber the process is capable of providing excellent results in a cost-effective manner.

Welding the aluminium alloys with the electron beam process presents one problem specific to the process, that of metal vapour from the weld pool causing arcing inside the electron beam gun. This is a particular problem with those alloys that contain low boiling point alloys such as mag­nesium and zinc. Arcing inside the gun interrupts the beam and causes cav­ities to be formed in the weld. This problem may be avoided by trapping the vapour by changing the beam path with a magnetic field or by shutting off the beam as soon as arcing is detected and re-establishing the beam

8.10 Single pass electron beam weld in 450mm thick A5083 alloy. Note the excess weld metal extruded on the weld face due to thermal contraction. Courtesy of TWI Ltd.

immediately the vapour has dispersed. This can be done extremely rapidly, the weld pool remains molten and cavity formation is avoided. Although some of the alloying elements, i. e. magnesium and zinc, are lost, this is gen­erally insufficient to cause a loss of strength. Elongated cavities in the fade - out region may be produced, particularly in circular components where a run-off tab cannot be used. These may be avoided by careful control of the travel speed and beam fade-out.

The non-heat-treatable alloys can be welded fairly readily without the addition of filler wire but hot cracking problems may be encountered in the more sensitive grades and in the heat-treatable alloys. As with laser welding, wire additions may help. Heat affected zones are small and strength losses are less than would be experienced in a similar thickness arc welded joint.