A remarkable spot welding process – Technology part X
New challenges call for new approaches. The materials used in production processes are continually refined and developed over the years. This also applies to the technologies employed. The latest requirement in a number of industries is for the capability of welding aluminum. A joining process that is difficult to implement using conventional resistance spot welding. KUKA Systems GmbH has the expertise to achieve a stable welding process for joining steel more efficiently and is thus in a position to overcome the obstacles posed by aluminum.
In manufacturing industries, a fundamental distinction is made between familiar materials such as steel with its variants on the one hand and the lesser known material aluminum on the other. As a system integrator, KUKA Systems already plays a leading role in the joining of steel. Where the automation of manufacturing lines is concerned, this means that the conventional spot welding process is not readily feasible for processing aluminum.
The new joining technology from the Augsburg-based system integrator is called KUKA RoboSpin and is a further development of classic resistance spot welding. The robot keeps moving continuously and thus succeeds in welding aluminum just as easily and quickly as steel. The reason is that aluminum melts at just 650 degrees Celsius whereas steel has a much higher melting point of 1550 degrees Celsius. This is the major difference between the resistance spot welding of aluminum and the welding of steels. The challenge encountered when joining aluminum parts using a spot weld gun is that the copper tips of the electrodes frequently stick to the metal and very often need to be dressed or replaced. For an enterprise striving for efficient and sustainable production, that is not cost-effective. Employing the KUKA RoboSpin technique, the joining specialist KUKA Systems achieves the same times as in steel welding processes while keeping the tip wear at the same low levels. An additional advantage is that the same gun geometries as for steel/steel welds can be used and the existing equipment retained with comparatively low adaptation requirements.
Use of the KUKA RoboSpin technique affords advantages in terms of cycle time and quality of the welding process. As technological leader, KUKA Systems has already implemented projects featuring this technique, which has now established itself in the production applications of the future. The process is mainly used in the automotive industry.
The KUKA RoboSpin technique
“The KUKA RoboSpin technique offers the major advantage that the uninterrupted motion of the robot saves a great deal of time. Rather than stopping during the welding operation, the robot continues moving towards to the next weld spot. This leads to a substantial improvement of the cycle time,” explains Christian Goldstein, Senior Manager in the Development and Process Technology department at KUKA Systems GmbH. At the moment of welding, the weld gun is rotated about the electrode axis. This transforms the traditional spot process into a continuous-path process with continuous robot motion. “We were thus able to reduce the repositioning times in the motion sequence and have made it possible to improve the welding process as a result,” continues Goldstein.
When welding steel, the fluid turning motion of the robot means that a higher number of spot welds can be achieved and the electrode tips have a longer service life. The rotation has a shaping effect on the copper electrode tips of the weld gun because the contact surfaces are only slowly enlarged by the tips being turned on the metal. Sheet thicknesses up to seven millimeters are welded using this joining technique. “This process also lends itself to welding special steels with coated surfaces, since the turning motion removes the surface coating at the weld spot and the material does not need to be prepared at these points,” explains Goldstein. One effect of large rotational angles is that coatings such as zinc, anti-corrosion paint and oxides are opened up, resulting in significantly more constant contact between the electrodes and the base material. Reproducibility of the weld quality is also better. In particular, the fast rotation of the weld gun at the end of each spot weld is instrumental in significantly decreasing the adhesive affinity of the tips to the metal.