Page 170 - Welding of Aluminium and its Alloys

Other welding processes 153

8.4 Laser weld of dissimilar thickness of automotive panelling.
Courtesy of TWI Ltd.

weld pool at an angle of around 45° will improve the bead shape on both
root and cap.
The majority of welding has been carried out on butt welded sheets (Fig.
8.4).The joint between the sheets results in an increase in beam absorption
and improved keyhole stability compared with bead-on-plate welds. Good
fit-up is necessary for autogenous welding if undercut is to be avoided.This
requires either square machined or high-quality guillotined edges. Where
lower-quality edges are produced, wire additions can be used to cope with
any gaps. Where extrusions are welded, a small ‘V’ on the weld face aids in
penetration by reflecting laser light within the preparation.

8.3.2 Nd-YAG solid state welding
The wavelength of the light from solid state lasers is only a tenth of the
wavelength of light from a gas laser. It is believed that this permits better
coupling of the beam with the parent metal. The short wavelength also
enables the laser light to be transmitted via fibre optics, rather than by the
use of the copper mirrors that are used to manipulate the light from the
CO 2 laser (Fig. 8.5). This gives greatly improved flexibility, allowing the use
of a robot to move and position the beam. Most of the techniques used for
CO 2 welding apply to the solid state laser and, as with the CO 2 laser, there
is a critical power density required to achieve keyhole penetration. An
average laser power less than 1kW is regarded as being the lower accept-
able limit for the avoidance of lack of penetration or porosity.