Surface condition and preparation
The surface condition of the aluminium sheets is one of the most important deciding factors in achieving consistent quality of resistance spot and
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 resistance 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 recommendations 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
Table 9.1 Typical welding parameters for 50Hz equipment single phase AC units. Valid for 1XXX, 3XXX, 5XXX and 6XXX alloys
friction between electrode tip and the sheet but care needs to be taken to ensure that there is not excessive oil present.