Electrodes and electrode maintenance
The copper electrodes used for aluminium resistance spot welding are designed in different shapes and sizes
depending on the application. Radius style electrodes are used for high heat applications, electrodes with a truncated tip for high pressure, eccentric electrodes for welding corners, offset eccentric tips for reaching into corners and small spaces, and finally offset truncated for reaching into the work piece itself.
A new development for aluminium resistance spot welding is GM’s patented multi-ring domed electrode. The new welding technique works on sheet, extruded and cast aluminium and enables stronger welds because the multi-ring domed electrode head disrupts the oxide on the aluminium surface.
GM’s proprietary multi-ring domed electrode head
(Source: GM)
Recommended by the Resistance Welders Manufacturing Association are electrodes made from group A class 1 alloys. Group A class 1 alloys have the highest electrical conductivity.For the standard Al-Mg and Al-Mg-Si car body sheet alloys, electrodes A class 2 can be used too. Efficient cooling of the electrodes must be ensured; the coolant flow rate should be 5 - 10 litres per minute (more than in case of steel). It must be also noted that for aluminium welding, the electrode tip diameter and dome radius are bigger than for steel.
Schematic of an electrode for aluminium resistance spot welding
Newer studies showed that the face diameter D should be smaller than indicated in the figure above. It was found that for aluminium sheet thicknesses between 0.8 and 4.0 mm, a face diameter D between 5.0 and 10.0 mm promotes more uniform contact distribution at the electrode and interfacial surfaces, reduces undersized welds that occur intermittently and significantly increases electrode life.
The main problem connected with resistance spot welding of aluminium and its alloys is the short life time of the electrodes. The rapid deterioration of the tip surface of the copper electrode is the result of the high pressure, the high temperature and in particular alloying processes during welding. The accumulation of aluminium on the electrode face causes increased resistance heating at the electrode-aluminium interface and therefore even more aluminium melts and sticks to the electrode. Once significant aluminium accumulation has occurred, deterioration of the weld consistency and quality is rapid. Unless some sort of electrode maintenance is employed, typically only between 300 and 3000 aluminium spot welds can be achieved on a set of electrodes before the weld quality drops below a minimum threshold.
Numerous process and material strategies to increase the electrode life performance have been proposed in the past. Most concepts focused on reducing electrode erosion through a variety of surface coatings and treatments. Such techniques can enhance spot welding performance, but are often difficult to implement and expensive.
More efficient proved to be the introduction of a regular electrode cleaning step, ideally before electrode wear contributes to poor weld quality. Dressing allows the user to restore a worn electrode to a desired geometry, thereby eliminating changes in electrode topography and diameter due to pitting and erosion. The actual type and frequency of electrode cleaning, process times and tool designs are dependent upon the overall cleaning strategy. The process takes up to several seconds and is typically completed during the part transfer operation.
In many automotive resistance spot welding lines, electrode dressing has been introduced to extend electrode life time and improve weld consistency both for steel and aluminium. A wide range of commercially available electrode dressing equipment has been developed; including hand-held dressers for manual guns and stand-mounted systems for robots.
Hand-held dresser (left) and stand-mounted robot dresser with swarf tray (right)
(Source: University of Warwick)
A common feature of cutters designed for steel is the emphasis on reshaping the sides of the electrodes to remove mushrooming and thereby maintain welding current density. In aluminium resistance spot welding, mushrooming of the electrode does not occur and the main requirement of the cutter would be to take light cuts from the face to remove aluminium,oxides and pits. However, this is not the optimum solution because the required frequent electrode dressing may lead to substantial geometrical changes, i.e. the electrodes must be replaced early.
A more efficient solution for aluminium resistance spot welding proved to be the use of less aggressive electrode maintenance methods and to employ a dressing cutter only when the tip is badly damaged. The use of suitable abrasive wheels was found to be extremely effective at removing aluminium from the electrode face. An additional advantage is that the adaptable buffing wheels maintain the profile on domed electrodes.
Ring of aluminium on electrode removed by a short buffing operation
(Source: University of Warwick)
Using a polishing wheel enables the electrodes to be buffed clean within the component cycle time, with only a minimal change in electrode geometry even after hundreds of buffing cycles.Restoration of the original electrode geometry is less necessary for aluminium than for steels since there is little mechanical deformation during electrode wear. The effect of periodic buffing after a relatively short number of spot welds is a significantly improved consistency of the aluminium resistance spot welding process, reduced process cost (less electrode replacement) and increased productivity (less down-time for electrode changes).
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