ABICOR Innovation Award 2004 (2nd Place)
Dr.-Ing. Pino Cordini (Alstom LHB GmbH)
Hybrid welding of zinc-coated fine steel.
The MSG welding method is used to a large extent in vehicle construction. It stands out thanks to low investment and operating costs as well as good automation and monitoring ability. Laser welding methods are also being increasingly used, the potential of which is based on a high process speed and a low component heat load. The ability of the laser method to bridge gaps is extremely limited, however, which leads to high demands on component precision.
An economical alternative method is laser MSG hybrid welding, which achieves a reduction of the laser power required by melting the auxiliary wire in the arc and allowing synergy effects of the combination of methods to be exploited. The hybrid welding method is already being used by industry for welding aluminium and thick steel materials. The zinc-coated steel materials used in the automobile industry are not yet being welded using the hybrid method due to their low process stability. Hybrid welding is considerably faster than conventional MSG welding. A consistently high weld seam quality can thus not be achieved by controlling the curve of modern welding current sources.
Within the context of development work, detailed understanding of the process of interaction light arc and laser beam particularly for the zinc-coated steel used in vehicle construction was drawn up. Since a stable process method cannot be achieved at economically relevant speeds without additional measures being taken, a process monitoring and control system was also built up and qualified.
Conclusions can be drawn about the welding depth by measuring the process radiation. This makes the detection of faulty root connection possible. The arc process is described using the current-time signal. The process monitoring system developed ensures the safe detection of instabilities using various methods of signal processing.
A control system was achieved on the basis of the detection of process faults, with the help of which weld seam quality and speed can be increased. By using this control system, statistically occurring process instabilities can be compensated before they influence weld properties. The ability of the method to bridge gaps was able to be increased by a good 10 %. Taking the welding speed at which no process faults occur as a basis, the use of the control system led to an increase in effective welding speed by approx. 15 %.
The results of the work were successfully implemented in cooperation with the steel industry for manufacturing samples of tailored tubes for interior high-pressure forming. Further fields of application are opened mainly in vehicle construction in the welding of non-linear tailored blanks and other components with gaps if complex clamping techniques cannot be justified due to the small quantities involved.