Laser welding to make aircraft structures lighter

Citation

(2005), "Laser welding to make aircraft structures lighter", Aircraft Engineering and Aerospace Technology, Vol. 77 No. 2. https://doi.org/10.1108/aeat.2005.12777baf.001

Laser welding to make aircraft structures lighter

Laser welding to make aircraft structures lighter

Keywords: Aircraft, Welding, Lasers

At the recent EuroBLECH show in Hannover, Germany, Schuler Held revealed the development work it has been doing on the 3D, two-sided simultaneous laser-welding of Airbus 380 wing stringers.

Equipment supplied by Schuler Held Lasertechnik is to be found in a wide variety of industrial sectors, from car and special vehicle manufacturing to airbag production and shipbuilding. In addition to land and sea applications, however, Schuler Held Lasertechnik has now also entered the aviation sector.

The Dresden-based Fraunhofer Institut fur Werkstoff- und Strahltechnik (IWS) (Institute for Material and Beam Technology) develops and optimizes laser welding processes and tailored part designs for welding large-scale aircraft structures on behalf of its customer Airbus. The development of manufacturing and test technologies for the aerospace industry belongs to the core competencies of IWS.

In the field of joining technology, the IWS is concerned with the laser welding of fuselage and hydrofoil structures, the welding of titanium alloys and structural components as well as the joining of high-temperature materials.

Laser beam welding

A technology with great potential – the laser welding of aircraft parts is regarded as a pioneering technology for the efficient manufacturing of weight-reduced, metal fuselage structures. The technique has already been in use for three years at Airbus for the serial production of the A318.

Airbus has now commissioned IWS to research the possibilities of optimizing the manufacturing process and to develop welding technologies for XXL aircraft structures.

The main focus will be on the development and testing of new plant concepts, the further development of two-sided-simultaneous laser welding, as well as on welding in areas with restricted access and the testing of new laser beam sources.

The equipment supplied by Schuler Held Lasertechnik will also be used to carry out research into the optimization of properties (e.g. damage-tolerant construction methods), the reduction of distortion, the use of new, additional welding materials and more meaningful results from destructive test procedures.

The IWS also acts as a test bed for Airbus with regard to systems technology, and in particular the actual laser welding line, as well as optical 3D seam monitoring and online detection of the seams flank angle.

State-of-the-art

Until now, the overwhelming majority of all metal structural parts for aircraft were either riveted or glued. Some ten years ago, however, Airbus began developing alternative joining techniques for riveting so-called stringer panel connections.

In order to raise the aircrafts stability, such reinforcement elements are attached to various fuselage panels.

In the laser welding process, the stringer is simultaneously welded on both sides to the panel with the use of additional wire.

A roller gib system is used to ensure exact positioning of the stringer.

The advantages of laser welding

Laser welding is said to offer a number of benefits in comparison to riveting. The process allows a simpler stringer design, the omission of the rivet base, and a reduction in the amount of sealing material required.

This not only reduces weight significantly but also improves the parts corrosion behavior.

The increased automation rate, the improvement in working speed by a factor of 10 compared with riveting, and the reduction in material and logistic needs all result in a significant lowering of manufacturing costs.

Large work area for original parts

With the aid of the Schuler Held Lasertechnik line, which was first put into operation in June 2004, the Dresden-based IWS Institute is now working on optimizing its production- ready process. The equipment enables IWS to undertake three-dimensional, two-sided and simultaneous laser welding of stringers on extremely large and complex fuselage structures.

The work area measures 10 x 3 x 1m; allowing the production of full-scale components.

In order to achieve approximately the same beam parameters throughout the entire work area, the laser beam has to be widened by a factor of about 1.3.

To this end, the line also features specially adapted telescopes, which first widen the beam and then reduce it again before entering the Y-axis.

Machine set-up

In order to enable tests with both CO₂ and Nd:Y lasers, the line uses a Cartesian beam positioning system. The modular portal system of Schuler Held Lasertechnik consists of two side girders supported by columns. These girders bear the linear guides, the toothed racks and the external measurement systems for the X-axis. Both Y bridges travel along these side girders.

The Y bridges are made of high- strength aluminum and are positioned on both sides by rack-and-pinion drives (gantry-drive) in order to guarantee high positioning accuracy even with large bridge spans and heavy Z-axis constructions. On both Y bridges, the three Z-axes also use linear guides, external measurement systems and a rack-and-pinion drive. Two of the Z- axes each carry one laser welding head with the necessary positioning units for the A-, B- and C-axis.

The third Z-axis carries the clamping device for positioning and guiding the stringer.

Independent laser heads for improved weld seam – in contrast to the machines previously used by Airbus, the Schuler Held Lasertechnik line welds the stringer simultaneously along both sides with mechanically separated laser heads, which are mounted on the two independent Z-axes.

This allows the two welding heads and accompanying clamping device to be moved individually or in sync along any 3D paths within the working area – which also permits counter-balancing movements in six axes.

Besides guaranteeing perfect weld seam positioning, the time and effort required for setting up the equipment can also be reduced significantly. The decisive factor in ensuring seam quality is the ability to weld simultaneously and thus achieve an optimal stringer-panel connection. This minimizes material distortion and the formation of pores. At the same time, it ensures a homogeneous blending of the weld area with the additional material.

This technically sophisticated process, in which both laser beams must meet, requires a highly accurate guidance system.

Flexible and dynamic

The Schuler Held Lasertechnik line is said to be particularly flexible in its range of applications, as it is not only designed for two-sided and simultaneous welding of longitudinal stringers.

The manual replacement of laser head and clamping device will enable the machine in future to also weld on struts set in a transverse direction (clips or ribs). In order to guarantee maximum flexibility, movements along the machines longitudinal axis (X-axis) are carried out by two independent Y bridges (separate parallel axes). This enables relative movement between laser beam and workpiece either through the separate transport of the Y bridges along the X-axis or the separate transport of the sliding table along the X-axis.

However, it is also possible to move the Y bridges and sliding table in opposite directions along the X-axis simultaneously. This combination of gantry machine with flying optic and sliding table guarantees the machines extreme flexibility.

The movement of Y bridges and sliding table in opposite directions also results in greatly improved acceleration values and higher work speeds. Furthermore, it helps reduce the machines footprint. The use of a CO₂ laser also shortens the necessary beam lengths.

Laser sources

The choice of a suitable laser beam source for the particular process and material is of particular importance. As aluminum is highly reflective and heat conductive, laser sources which generate an extremely high beam quality (K 0.9) are best suited for welding lightweight structures. The line therefore incorporates two Rofin Sinar CO₂ lasers with 4.5kW each.

Research results so far

Comparison Nd:Y laser and CO₂ laser – both lasers produce low-pore welding seams, free of hot cracks, which meet all quality requirements. Owing to the lower energy of the CO₂ laser in comparison to the Nd:Y laser, the seams cross-sectional area is much smaller. However, the tendency for pores to form is greater with the CO₂ laser, while the tendency for hot cracks to form is lower.

In the case of CO₂ lasers and thin- walled stringers, the seam is generally convex, while Nd:Y lasers form mainly concave seams. With regard to longitudinal distortion, it was noticed that the bulging created by the Nd:Y laser was about twice as strong as that of the CO₂ laser.

The same applies to the angular distortion on the panel side – creating an enhanced Zeppelin effect. Overall, the CO₂ laser welding process runs much more smoothly and protecting the optics is therefore much easier.

Reduced distortion

IWS is also examining various methods for reducing distortion.

Airbus currently uses tacking and subsequent shot peening treatment.

The impact of hard particles on the surface creates residual stress and permanent deformation.

The resulting compressive stresses in the forming area generally improve the parts mechanical properties.

An alternative is to “over bend” the panel in order to reduce the angular distortion. In the case of such complex, spherically curved panels, however, this is extremely complicated.

Pre-heating the panel has produced encouraging results. This prevents the real cause of the deformation – the differing thermal expansion of panel and stringer. To this end, IWS has developed a heated vacuum clamping table.

Tape-shaped additional welding materials

Welding reinforcement elements in a transverse direction (clips or ribs) requires very short seams. Owing to the limited accessibility, however, conventional wire feeding is not possible. One solution developed by IWS for this problem is the prior depositing of tape material in the seam area.

For further details contact: Schuler Presses UK. Tel: +44 1827 311999; Fax: +44 1827 312999.