Aim

Reducing the number of riveted joints is one way to reduce the weight of metallic structures within the aeronautical industry.

Laser welding is one alternative. Yet, in order to achieve stressfree joints, stringers need to be welded from both sides simultaneously with high precision. This means that both weld heads need to follow the seam within  ±50 µm throughout the entire workspace of 10.000 x 3.000 x 1.000 mm. At the same time, the stringer needs to be pressed against the main structure in order to avoid the formation of a gap.

Filler material and gas needs to be added to the process for a near pore free seam result. This again requires highly accurate positioning within the entire work space.

Challenges

To achieve the aims described above, a laser welding machine was designed with two portals, three Z-axis and a moving working table. Two laser welding heads and one pressing unit for the positioning of the stringers form the basis for the welding process.

The challenge for ibs Automation was to develop the complete control system for more then 30 axis, allowing welding speeds above 10m/min while still maintaining a welding accuracy of ±50 µm. In addition, a postprocessor was to be developed allowing for easy programming with CAD-files.

Solution

Two Sinumerik 840D control units are the basis for the control system. Both are connected through the fast-IPO-Link on the interpolation level.

One control unit is responsible for the laser welding heads. Within each  of the two channels there is a customized 6-axis transformation developed by ibs Automation as compile cycle software. This allows for the integration of the linear axis X, Y, and Z and the orientation axis A, B and C for the programming of the TCP.

The second control unit is required for the stringer positioning unit. Another customized kinematic transformation and the inclusion of a force control solution enable accurate positioning of the stringer and constant pressure on the stringer.

Both laser welding heads are equiped with profile sensors and controlled by seam tracking functions developed by ibs Automation, in order to achieve the required path accuracies.

Conclusion: The control solution developed by ibs Automation enables two sided simultaneous laser welding of 3D components with speeds of up to 13m/min and a maximum path deviation of ±50 µm.

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