Page 63 - Welding of Aluminium and its Alloys

54 The welding of aluminium and its alloys

high-quality and narrow cut. Plasma gases include air, argon, argon–
hydrogen, nitrogen and carbon dioxide. Cutting can be performed manu-
ally or mechanised with higher cutting speeds being achievable with mech-
anised and automated systems.
A plasma cut edge is generally not completely square. The top edge of
the cut may be rounded by some 1 or 2mm, particularly if the cutting energy
is high for the thickness of plate being cut or when high-speed cutting of
thin material is being carried out.The plasma jet also tends to remove more
metal from the upper part of the component than the lower part, resulting
in a cut wider at the top than the bottom with non-parallel sides.This ‘bevel’
angle may be between 3° and 6°. The cut surface may also be rough.The
quality of the cut is affected by gas type, gas flow rate, cutting speed and
operating voltage. High gas flow rates and high voltages will improve
the squareness of the cut and mechanised cutting will give an improved
appearance.
Arc cutting produces a HAZ and may cause melting at the grain bound-
aries. This results in micro-cracking, primarily of the heat-treatable alloys –
the 7000 series being particularly sensitive. As the thickness increases, the
likelihood of such cracking also increases. For this reason it is advisable
to machine back the plasma cut edges by about 3mm, particularly if the
component is to be used in a dynamic loading environment.
The composition of the gas for plasma cutting depends on the required
quality of the cut, the thickness of the metal to be cut and the cost of the
gas. Air is the cheapest option and single gas systems utilising air and a
hafnium electrode have been developed for the cutting of materials up to
approximately 6mm in thickness (Fig. 4.3).
Above this thickness nitrogen, carbon dioxide, argon–hydrogen or
mixtures of these gases may be used. For the thicker materials over, say,

Cooling Air Air Cooling
air air

4.3 Air plasma cutting. Courtesy of TWI Ltd.