Development of laser beam welding concepts for fuselage panels
To meet the future demands of the aerospace industry with respect to safety, productivity, weight and cost, new materials and joining concepts have been developed. The platform Green Regional Aircraft of the European Clean Sky Programme has set the objective to investigate lightweight metal components for lighter aircraft structures. Fuel saving and environmental advantages can be achieved through lighter airframe structures. Laser beam welding as an efficient joining technology for fuselage structures is already established in the aircraft industry for lower fuselage panels, because the welded panels provide a higher buckling strength and lower weight compared with the classical riveted designs [1,2]. Recent developments in the metallurgical field make it now possible to use laser-weldable Al-alloys of the 2xxx series such as 2198 and 2196 with a high structural efficiency index due to their high strength and low density [1,3]. In the present work the laser weldability of Al-Li alloys 2198 and 2196 was studied to determine the process parameters needed to obtain consistent laser welds, and to compare the mechanical behaviour with the conventional fuselage aluminium alloy combination 2024 and 7050. To optimise the dual laser welding process for skin-stringer structures (Fig. 1), a comprehensive finite element model has been established based on ABAQUS and Finite Element Analysis Toolbox using the Welding Modelling Toolbox module. The model is able to predict the thermal history and size of fusion zone for various welding conditions and also able to expand to more complicated welding configuration. The microstructures of the welds (Fig. 2) are investigated and the strength loss in the heat affected zone is also characterised. A noticeable equiaxed grain zone is found in all welding conditions which impose reliability risk. Porosities and microcracks are also spotted inside the fusion zone and, even along the fusion boundary when the laser power reaches to 2000W. A detailed EDX mapping has been done to reveal the inhomogeneity of composition within the fusion zone. In addition, a hot cracking susceptibility modelling has been carried out on the basis of the composition characterisation. The study also emphasizes the mechanical properties of the weld joint to gain an understanding of the underlying factors controlling the performance of the welds. During the demonstration phase the developed laser beam welding technology was applied for welding stiffened flat panels. The mechanical performance of the welded panels was investigated through the compression tests.