Single-Sided Laser Beam Welding of Dissimilar T-Joints for the Aircraft Industry

The double-sided laser beam welding of skin-stringer joints is an approved method for producing defect-free welds. But in case of limited accessibility - as for the welding of skin-clip joints - this method could often not be applied. For this reason the single-sided laser welding of T-joints becomes necessary. Another advantage is the reduced manufacturing effort for the welding with just one laser beam in a single welding pass. However, the main obstacle for the use of single-sided laser welding of T-joints is the occurrence of weld defects as porosity and insufficient or excessive penetration which could not be avoided just by the variation of the welding parameter. An additional complexity represents the combination of dissimilar and hard-to-weld aluminium alloys. Al-Cu and Al-Zn alloys - like AA2024 and AA7050 - are widely used in the aircraft industry due to their high strength-to-density ratio and easy availability. But these alloys are also often associated with distinct weldability problems especially for fusion welding techniques like laser beam welding. The porosity mainly arises from the vaporisation of volatile alloying elements and the resulting instabilities of the narrow keyhole.

The objective of the present study is to enable single-sided laser beam welding of dissimilar T-joints using the example of AA2024 (skin) and AA7050 (stringer). For this purpose a high-power fibre laser with an enlarged beam diameter - in comparison to the commonly used CO₂ and Nd:YAG lasers - is utilized in order to improve the degassing and the temperature distribution in the weld. The crucial welding parameters are identified and adjusted for achieving defect-free weld seams. The obtained joints are compared to double-sided welded T-joints made of typical aircraft aluminium alloys. In this regard the single-sided welded joints showed a differing weld seam appearance but comparable mechanical properties.