Page 68 - Welding of Aluminium and its Alloys

Preparation for welding 59

a steady output, continuous wave (CW) laser light or in a pulsed output
mode. In this latter case the power output on the peak pulse may be as
much as 20 times the average power.
The wavelength of light from the CO 2 laser is 10.6 microns (micro-
metres) and at this wavelength is easily absorbed by most solids, enabling
the CO 2 laser to be used on a wide variety of materials. This long wave-
length has a disadvantage, however, in that it cannot be transmitted by glass
or fibre optics but requires reflecting metal mirrors for manipulating the
beam and materials such as zinc selenide or gallium arsenide for focusing
lenses.The Nd-YAG laser light is an order of magnitude less at 1.06 microns,
allowing the use of glass lenses for focusing and fibre optic cable for beam
transmission. This offers a clear advantage over the CO 2 laser, since it
permits the marriage of commercially available manipulating equipment
such as NC (numerically controlled) gantries and robots with the laser.
The power output of currently available Nd-YAG lasers is limited to around
6 kilowatts, however, restricting the thickness of materials that can be
cut.
The laser cutting process consists of focusing the beam through a cutting
nozzle onto the surface to be cut, the concentration of energy being suffi-
cient to vaporise the material, creating a ‘keyhole’. With continuous wave
lasers there is generally more melting than vaporisation and an assist gas is
used to blow away the vapour and any molten metal, creating a narrow
clean cut as the beam is traversed along the item. The pulsed lasers gener-
ally provide enough energy that the laser beam imparts sufficient force to
the vapour that the vapour itself removes any molten metal. The assist gas,
introduced either through the cutting nozzle or co-axially with it, is used
not only to blow away any molten metal but also to protect the lens from
spatter or debris ejected from the cut.
The assist gas for cutting aluminium may be oxygen, nitrogen or air.
Oxygen is a reactive gas with aluminium and will give higher cutting speeds
than nitrogen. Nitrogen, however, will give a better quality cut in terms of
squareness and roughness than will oxygen. Air is a compromise but is the
cheapest of the gases. Gas pressure is an important variable that needs to
be controlled to give the best quality of cut – high gas pressures give the
most effective metal removal but too high a pressure may damage the focus-
ing lens, since this forms part of the pressure system. As the assist gas pres-
sure is increased the lens also needs to be thickened in order to carry the
increased pressure. The pressure of gas in the cut is also influenced by the
distance between the nozzle and the workpiece. For example, high-pressure
cutting may require a stand-off distance of only some 2.5mm. The rela-
tionship between stand-off and pressure in the kerf is not simple, however,
as most laser cutting is done with supersonic gas velocities. It is essential
that the nozzle stand-off distance and nozzle condition are closely