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parts, thereby making available for off-line programming correct models for all the
welding cell components. The necessity of fixture design modifications due to joint
accessibility problems could be easily detected and performed by using graphical
simulation. The resulting CAD information should be used to manufacture the fixture.
Other possible sources of error due to fixturing are the presence of spatter or
other foreign matter in the locating surfaces of the fixture, fixture wear and thermal
expansion which may modify the position of the locating surfaces [ref. 111].
2.5.3 Dynamic variation
Thermal distortion during welding is another source of variation which can
affect the joint geometry during welding by modifying the weld volume (e. g.
increasing gap) and also by causing deviation from the initial joint line, which may
result in misplacement of the weld bead [ref. 111]. Therefore, component and fixture
design should take into account the thermal distortion in such a way as to minimise its
effects [ref. 105]. In general, the closer the fixture locating surfaces can be placed to
the joint the better [ref. 108]
2.5.4 Contact tip wear
Contact tip wear is another factor that affects the stability of the gas metal arc
welding process. The contact tip (usually made of a copper alloy) is responsible for
transferring the welding current to the electrode and for directing it towards the
workpiece. The inner surface of the contact tip should be smooth so that the electrode
slides smoothly and also makes good electrical contact [ref. 2].
Generally, the hole in the contact tube should be 0.13 to 0.25 mm larger than
the wire being used [ref. 2]. However, due to the friction with the sliding wire and
also to adhesion between wire and contact tip during welding [ref. 112], the hole
diameter increases gradually. If the electrode wears a particular area of the contact tip
for long enough (as in the case of localised wear induced by wire cast - see Figure
2.17) adhesion between the wire surface and the newly exposed contact tip material
will occur16 [ref. 113]. This introduces variations in the wire feed speed at the torch,
resulting in process instability [refs. 112,113]. Furthermore, the worn contact tip will
worsen the effect of wire cast, increasing the wire tip mislocation, relative to the joint
centreline.
The rate at which the contact tube wears depends on the temperature.
Therefore, any factor that might increase the contact tip temperature (e. g. higher
welding currents, smaller electrode extension, less efficient external cooling, etc) will
accelerate the wearing process [ref. 113]. Harder wires also accelerate the wearing
process but this can be minimised by choosing a contact tip made of a harder alloy
(e. g. copper/tungsten alloy) [ref. 103].
16 The mechanism by which adhesion between wire surface and contact tip occurs is well explained
in reference 113.
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