Achieving aerospace standard porosity levels when welding thin and thick-section aluminium using fibre-delivered lasers : executive summary

Environmental and commercial pressures have forced the aerospace industry to look at alternatives to riveting for the manufacture of aluminium aircraft structures. This resulted, at the end of last century, in an extensive study by Airbus into the possiblities of using CO2 lasers, which led to the process being implemented for a (small) number of stringer-to-skin fuselage panels in the newer Airbus models. Since this initial commercial success, new laser sources have become available that are more suitable for the welding of aluminium than CO2 lasers, in the form of Nd: YAG and Yb-fibre lasers. Both produce a wavelength that is absorbed more efficiently by aluminium alloys than the CO2 laser wavelength, resulting in an improved keyhole stability, as demonstrated in the late nineties for Nd: YAG lasers. In addition, Yb-fibre lasers have become available at output powers higher than available for Nd: YAG lasers, allowing thicker sections of aluminium to be welded in a single pass. However, despite their claimed advantages, no efforts were made to demonstrate the potential of these lasers for (aluminium) aircraft manufacture. For this reason, the author initiated a series of studies in 2001, with the overall aim to develop procedures to laser weld both thin (3.2mm) and thicksection (12.7mm) aerospace aluminium alloys using these fibre-delivered lasers to a weld quality, in particular related to weld metal porosity, suitable for aerospace service. The focus in this research was on weld metal porosity, because this is a particular problem when laser welding aluminium, either in the form of fine (hydrogen) porosity or larger porosity associated with an unstable keyhole behaviour. The benchmark weld metal porosity for this study was obtained from the stringent weld quality classes defined in BS EN 13919-2 and AWS D17.1. The approach to this research was in three parts, with work in the first aimed at demonstrating that a 3kW Nd: YAG laser was capable of producing low-porosity welds in 3.2mm thickness 2024 aluminium alloy, and thus can be considered for replacing the CO2 laser currently used for the stringer-to-skin fuselage application. Prior to the final part of the research, in which a 7kW Yb-fibre laser was used to demonstrate that these benchmark porosity levels could also be achieved in thicker section (aerospace-grade) aluminium, a comparison study was carried out to quantify the difference in welding performance between the Nd: YAG and the Yb-fibre laser. At an output power of 4kW focused in a 0.4mm diameter spot, the Yb-fibre laser was capable of a 30% higher welding speeds in 4mm (5083) aluminium alloy, or a 20% increase in depth of penetration for welding speeds between 1 and 15m/min, compared with the Nd: YAG laser. This improvement in welding performance, together with an output power of 7W, produced full penetration in 12.7mm thickness (aerospace-grade) AI-Zn-Mg-Cu aluminium alloy using the Yb-fibre laser autogenously, or in a hybrid configuration with a MIG arc. Both the autogenous laser and hybrid laser-MIG process were capable of producing welds with a weld metal porosity in line with the BS EN 13919-2 and AWS D17.1 benchmark conditions, at welding speeds of 0.55 and 0.75m/min, respectively. At these production rates, the 248 metres of stringer incorporated in a typical aluminium wing structure can be welded in 7.5 and 5.5 hours, in case of autogenous laser and hybrid laser-MIG, respectively, compared with 37.6 hours currently needed for the riveting process.