Page 187 - Welding of Aluminium and its Alloys

170 The welding of aluminium and its alloys

9.3 Modern robotic resistance spot welding cell. The robot welds on
one jig while the operator unloads/loads the second jig. Courtesy of
British Federal.

seam welds.Variations in the thickness of the oxide film will affect the resis-
tance between both the electrodes and the plates and at the plate interface.
The resistance may be measured by clamping a single plate between two
electrodes and applying a set current of some 10–15 A, measuring the
voltage and calculating the resistance by the use of Ohm’s Law. Material as
delivered may have a resistance of between 300 and several million
microhms, cleaned and prepared plate should have a resistance of 10–100
microhms. For the highest and most consistent quality this range should be
tightened to some 10–30 microhms.
To achieve this low level of resistance the cleaning and storage recom-
mendations in Section 4.7 should be followed. Mechanical abraded surfaces
generally provide longer electrode lives than chemically cleaned surfaces,
with mill finished sheets giving the shortest lives. Special lubricating oils
such as polybutene have been used to extend electrode life by reducing the