Resistance welding of non-ferrous alloys
Reason for the project
Resistance welding of non-ferrous alloys (Al and, in addition, also Cu and Ni alloys) involves a number of obstacles that limit its application considerably. The oxide film of aluminium is, for example, an electrical isolator that varies with the background of the material. Another problem is the (too) good conductivity (both electrical and heat conductivity) of the metal itself, in particular, for Al and Cu.
The life of the laser electrodes is therefore also usually a lot lower than for steel. There is also little data available about the resistance weldability of non-ferrous allows (in particular, for projection welding and for Ni).
New developments in current sources based on higher frequencies or condenser discharge to supply more energy in a shorter time, however, offer an attractive perspective to also resistance weld materials that are difficult to weld to produce high quality. Adjusted electrode materials and lost wire can increase the electrode life.
The use of software packages was also an interesting issue with regard to simulation and the approach to the resistance welding process. This may give the user the option to obtain a good picture in a simple manner of the resistance welding process.
Two aspects were also closely studied that is often underestimated unjustly. On the one hand, we have the influence of the local geometric extension with regard to projection welding. It was determined to which degree an adjustment of the projection form leads to an improvement of the quality/joint efficiency of the resistance weld joint. On the other hand, we have the locking systems that ensure that the weld nugget is supported optimally during and after welding. Developments were achieved in relation to this that allow the locking force to be varied during the entire welding cycle and can therefore ensure that, for example, cavities are reduced considerably with regard to aluminium alloys.
Resistance welding of aluminium alloys has to deal with a number of problems that influences strongly the quality of the welds. The oxide film, for example, not only has an impact on the contact resistance but also the spread of the contact resistance. In addition, the better ductility of the aluminium alloys (when compared to steel and certainly in relation to higher temperatures) ensure that the joining process becomes more critical (especially in relation to projection welding). After all, if the ductility is too extensive because of a too high welding force in combination or not with the welding heat, the contact force between the parts to be joined will diminish at this location. The contact resistance at these location is too great to form a weld nugget. The weld nugget will be created where the contact resistance/current density combination is the most favourable. There is therefore no certainty about the exact location, size of the weld nugget diameter and whether a weld nugget will or will not be formed. The issue in these cases is therefore to transfer the heat as soon as possible to the parts to be welded and to ensure the welding force is as small as possible so that ductility remains limited. We must also ensure in this context that the welding time is not too short so that the weld nugget has the time to grow sufficiently and that the welding force is sufficiently large to prevent splashing. The scope of application for resistance welding of non-ferrous and aluminium alloys, in particular, is therefore noticeably smaller than with regard to steel grades. For some (projection welding) applications, it is even the case that the implementation of a hardware-based system (spring system) on the machine table or in the welding head is required for the realisation of a weld and/or the improvement of the existing weld quality.
Where multiple internal imperfections were determined when spot welding different aluminium alloys, tests were carried out where the influence of such imperfections on the weld strength was investigated. Internal imperfections in the weld nugget were simulated by drilling holes in the centre of the weld nugget . The diameter of these holes varied from 1.5 to 3 mm. The welded test pieces were subsequently subjected to shear tension tests and compared with tension tests on undrilled test pieces. It could be established that internal imperfections have little or no adverse effect on the mechanical properties of the weld joints.
Research was also carried out into the electrode life within the framework of this project. Modified electrodes (CuCrZr electrodes with a TiC coating), for example, were compared to reference electrodes (CuCrZr). In addition, the influence of the oxide film on the life of the electrodes was also studied. A remarkable conclusion within this context was that the use of modified electrodes extends their life by 20-25%.
The influence of the oxide film on the life of the electrodes is comparable as the one related to the use of modified electrodes. An extension of the life of approximately 20-25% could be achieved when the oxide film was (chemically) removed. When both the oxide film is removed (current practice) and modified electrodes are used, the life of welding electrodes can be approximately doubled when compared with reference electrodes where the oxide film is not removed from the pieces.
A lost wire system was also investigated. A tape electrode is used in such a system that is positioned between the actual welding electrode and the surface to be welded. After every spot weld, the tape is displaced slightly. This means that the actual welding electrodes are protected. An additional benefit of such a system is that it introduces additional resistance and therefore extra heat in the weld set-up. This means that welding can take place with a lower welding current.