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Steps for the improvement of damage tolerance behaviour of laser beam welded skin-skin-joints for aircraft structures

: Dittrich, D.; Winderlich, B.; Brenner, B.; Hackius, J.

Vollertsen, F. ; Wissenschaftliche Gesellschaft Lasertechnik -WLT-:
Lasers in manufacturing 2007 : Proceedings of the Fourth International WLT-Conference Lasers in Manufacturing, LIM 2007, Munich, Germany, June 18th - 22nd, 2007
Stuttgart: AT-Fachverlag, 2007
ISBN: 978-3-00-021449-3
International Conference on Lasers in Manufacturing (LIM) <4, 2007, München>
Fraunhofer IWS ()
damage tolerance; Al-butt joint; crack propagation; socket geometry; wave seam; Laserstrahlschweißen; Nietkonstruktion; Gewichtseinsparung; Herstellkosten; Stirnnaht; Aluminiumlegierung; Fehlertoleranz; Rißausbreitung; Qualitätsprüfung; Flugzeugbau

In the competition to lower weight and production cost of aircraft structures currently two main approaches are competing: the metallic and the CFRP-layout. On the metallic site it could be shown that Laser beam welding (LBW) is a very promising way to overcome current limitations of a conventional riveted fuselage regarding reducing weight and production costs. LBW as a thermal joining process affects metal properties in difference to previous joining technologies. During the welding process the well designed microstructure in the parent sheet metal will be irreversible damaged and replaced by a cast microstructure. That affects the static strength of the weld joint, which will reach about 70-80 % of the base material. It will be shown in the experimental results that a laser beam welded butt joint has a higher crack propagation rate. To reach new damage tolerance requirements of the fusion zone in a butt welded joint condition, two ways are possible: 1. Reducing the stress intensity at the crack tip by using a socket around the fusion zone, to reduce the nominal stress at the cross section of the socket. 2. Changing the weld seam design with the aim of reducing the crack length inside the fusion zone. The goal of the work is to increase the damage tolerance of welded skin-skin joints. The paper presents the latest results of LBW butt joints of the alloy AlMgSiCu for sheets using modern lasers of very high beam quality (K>0,9). Special attention will be paid on the damage tolerance behaviour. To improve this, several approaches like influencing the macroscopic crack path, reducing the stress at the crack tip, the use of local working crack stoppers are used. These methods are compared concerning their ability the increase the crack propagation resistance of a welded structure to the high level of a non welded one.