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LASER WELDING OF ALUMINIUM FOR THE AUTOMOTIVE INDUSTRY

Objective


Inside this project an application technology for high speed welding of car body relevant aluminium alloys, thicknesses and geometries was successfully developed.
Adaptive optics were studies inside the laser source and in the welding head as well. Opportunities of an active control of the beam parameters by adaptive mirrors were detected by numerical calculations and by an experimental setup. Changing the curvature of the rear resonator mirror enables to control the beam propagation factor Q*.
A focussing head with adaptive mirror, wire feeder, cross jet to protect the optics, shielding gas jet and welding depth sensor was set up successfully in a prototype system. This system includes a high precision laser robot and a prototype laser with very high beam quality and controllability in the range of 3 to 6 kW. A polarizing optics was successfully adapted and installed for the COMAU laser robot at the IFSW of University of Stuttgart and later at the Centro Ricerche FIAT (CRF). This additional device allows a free choice of the desired angle between polarization plan and working direction.
The final qualification of this prototype system was shown for the application on a car body subgroup made of aluminium.
The development of the system included the improvement of the laser robot performed by COMAU.
In order to increase the dynamic performances, the limitations of the existing controller of the movement, based on traditional simple PID loop for velocity and position, were overcome developing a new approach based on a digital independent joint regulator (modified pole placement technique). This activity improved by a factor of 2 the static and dynamic repeatability and the straight line accuracy at constant speed of 10m/min. The constant speed precision was improved to 5%. The circular roundness could not be improved. Welding tests demonstrated the good quality of motion reached.
The beam delivery system was tested with different kinds of mirrors. Copper mirrors without coating were chosen showing the lowest distortion under power load.
The process monitoring and control was accomplished developing the hardware and software module which were installed inside the robot controller. Two sensors were managed: one to read the direct beam power and the other to read the power reflected from the welding pool. The ratio of both signals was found to be proportional to the weld penetration.
Trumpf developed a laser source which was based on a standard 2600W system. Improvements of the gas flow conditions and installation of a bigger turbo blower enabled twice as much rf-input. By these modifications a laser output power of 5700W was achieved. The other aspect of the output beam, the propagation factor was optimized by combination of curvatures of the rear mirror and the output window, material of bending mirrors and beam cross section. This laser source suitable for aluminium welding will have a more extended market of welding applications where fast and deep welding is demanded. Another field is cutting of thick material.
Different kinds of feed back controls were developed to enable constant laser output power. During these investigations several more or less dynamic power sensors were studied.
The automotive industry has strong interest in a wider use of aluminium in order to further reduce weight and fuel consumption of cars and due to future development like e.g. the "city car". A major obstacle preventing wider use of aluminium in the automotive industry is its poor weldability. Laser welding has the potential to solve the technological and economical problems associated with aluminium welding. The prime object of the proposed project is to develop laser aluminium welding for automotive applications to a stage, where it maches the speed, quality and reproducibility requirements of car body assembly. This will be achieved by investigating and optimizing process parameters and by using adaptive process control. The project includes the development of a prototype robotic system. The project tasks will be accomplished by an interdisciplinary team of project partners with expertise in the areas of the laser source, robotics, welding head and process sensors, adaptive optics, process parametes, aluminium and aluminium metallurgy as well as automotive applications.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Trumpf Lasertechnik GmbH
Address
Johann-maus-straße 2
71252 Ditzingen
Germany

Participants (4)

AMAG-FORSCHUNGS & ENTWICKLUNGES GMBH
Austria
Address
Postfach 3
5282 Ranshofen
Centro Ricerche Fiat SCpA
Italy
Address
Strada Torino 50
10043 Orbassano Torino
Comau SpA
Italy
Address
Strada Orbassano 20-22
10092 Beinasco
Universität Stuttgart
Germany
Address
Pfaffenwaldring 43
70569 Stuttgart