Page 190 - Welding of Aluminium and its Alloys

Resistance welding processes 173

viding the highest power output, with capacities ranging from 5 to
400kVA. The portable guns such as those used on robots in the automo-
tive industry generally range in capacity from 10 to 150kVA.The design of
the welding head is important in reducing electrode tip wear, assisting in
reducing porosity and cracking and enabling high production rates to be
achieved. The head characteristics that affect electrode wear comprise the
speed at which the head approaches the job – larger equipment may have
a two-speed head that applies the full force only after the initial electrode
contact is made. Although tip life is extended the slow approach speed will
increase the weld cycle time and reduce production rates.The inertia of the
head affects the speed of acceleration and deceleration and ideally the head
should be designed to be as light as possible consistent with rigidity. Too
much flexure of the arms will result in accelerated electrode wear due to
movement between the electrode and the workpiece and unacceptable
electrode alignment.
Low inertia is also required during weld pool solidification.As the molten
weld metal cools and solidifies, the weld nugget contracts. The electrode
must be able to respond rapidly and be capable of following this slight
deformation if sound and high-quality welds are to be produced. A
‘squeeze’ is therefore often applied, which assists in consolidating the weld,
reducing shrinkage porosity and hot cracking.

9.4.3 Welding electrodes

The bulk of the cost of a spot weld is the cost of dressing or replacing the
electrode, the life being defined as the number of spot welds that can be
made with a pair of electrodes while maintaining a minimum weld nugget
diameter. Pick-up of aluminium on to the tip and rapid wear are the two
main reasons for the short life of spot welding electrodes. High welding
currents, surface finish and electrode forces further assist in shortening the
electrode life. It is not uncommon in very high-quality applications such as
aerospace for the electrode to require cleaning after as few as 20 spot welds.
Electrode life may be extended by the use of replaceable caps on the
electrode tips or, it is claimed, by the use of copper alloys with increased
hardness which reduces mushrooming of the tip. Increases in hardness can
be achieved by alloying with zirconium or cadmium–chromium and dis-
persion hardened with aluminium oxide. Of these the 1% Cd-Cu are used
for the softer alloys with the harder 1% Cr-Cu or 21% Cr-Zr-Cu alloys for
the welding of the cold-worked or age-hardened alloys.
The profile of the electrode tip is important with respect to both the tip
life and weld quality. Tips may be conical, truncated conical, flat, domed or
cylindrical. Of these types the truncated cone and the dome predominate.
The most commonly recommended shape is the domed tip, the shape of