Page 74 - Welding of Aluminium and its Alloys

64 The welding of aluminium and its alloys

may be only a quarter the speed of a laser cut component, particularly in
thin sections.The other limitation is the bevel or taper of the cut face which
may be twice that of laser cutting, some 25% of the nozzle diameter or
around 0.2mm at the optimum cutting speed. The bevel can be reduced by
slowing the cutting speed with the penalty of an increased cost.

4.6 Mechanical cutting

Although the methods mentioned above can be applied to many fabricat-
ing activities,mechanical cutting techniques are used by most welding work-
shops as being the most cost-effective and versatile method. Cutting and
machining equipment is freely available in most fabrication shops and is
frequently less capital intensive than the sophisticated laser or plasma
cutting systems discussed above. Furthermore, the systems described in
Sections 4.2, 4.3 and 4.4 are capable of straight or simple bevel cuts only –
if double bevel preparations are required then two or more cuts are nec-
essary and J-preparations are not feasible. Edge preparations can be pro-
duced in a number of ways such as high-speed milling machines, edge
planers, routers and various types of saws. Where air-powered equipment
is used care needs to be taken to ensure that the air supply is clean, dry and
oil-free to prevent contamination of the surfaces, which would give rise to
porosity during welding.
Routers, planers and edge millers are capable of producing J- and U-
preparations when fitted with the correct shape of tools.The equipment for
these tasks can be hand-held and similar to that used for wood working,
the only requirement being the need for slightly greater power or floor
mounts for greater capacity. High cutting speeds can be used without the
need for lubricants or coolants, although this does not remove the need for
thorough cleaning. Hand-held rotary cutting machines are ideally suited to
back-gouging and for removing excess weld metal. The depth of cut can be
adjusted and various cutter forms are available, including V-blades for
bevelling and flat blades for weld cap removal.
The guillotine can be used to shear sheets of up to 6mm thickness without
the need for further preparation work. Over this thickness some dressing
of the sheared edges is necessary if the best weld quality is to be achieved.
Shearing of the edges of alloys containing more than 3.5% Mg is not
recommended if the edges are to enter service ‘as sheared’ because of the
risk of the work-hardened edges suffering from stress corrosion cracking.
Edges that are welded after shearing do not suffer from this problem.
Sawing is a very effective method of cutting and bevelling aluminium
using either portable or floor-mounted equipment. To achieve a good
quality cut high cutting speeds are necessary, around 2500 metres per
minute (mpm) peripheral surface speed for high-speed steel circular saw