Page 183 - Welding of Aluminium and its Alloys

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Resistance welding processes

9.1 Introduction
Resistance welding is a fusion welding process that requires the application
of both heat and pressure to achieve a sound joint. The simplest form
of the process is spot welding where the pressure is provided by clamping
two or more overlapping sheets between two electrodes (Fig. 9.1).A current
is then passed between the electrodes, sufficient heat being generated at
the interface by resistance to the flow of the current that melting occurs,
a weld nugget is formed and an autogenous fusion weld is made between
the plates. The heat generated depends upon the current, the time the
current is passed and the resistance at the interface.The resistance is a func-
tion of the resistivity and surface condition of the parent material, the size,
shape and material of the electrodes and the pressure applied by the
electrodes.
There are a number of variants of the resistance welding process includ-
ing spot, seam, projection and butt welding. It is an economical process
ideally suited to producing large numbers of joints on a mass production
basis. Spot welding in particular has been used extensively in the automo-
tive industry, albeit mostly for the joining of steel and in the aerospace
industry for airframe components in aluminium alloys. Seam welding is
used in the production of thin sheet, leak-tight containers such as fuel tanks.
Projection welding is generally used for welding items such as captive nuts
onto plate. This variation is not normally used on aluminium and is not
covered in this chapter. Flash welding, unlike spot and seam welding that
require a lap joint, is capable of making butt welds.This is achieved by resis-
tance heating the abutting faces and then forging them together.
There are a couple of characteristics of aluminium that make it more dif-
ficult to resistance weld than steel. The most significant is its high electrical
conductivity, requiring high welding currents and large capacity equip-
ment. Secondly, the electrodes are made from copper which alloys with
aluminium, resulting in rapid wear and a short electrode life.

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