Autogenous Yb:YAG laser disk welding domain of AA6061-T4 aluminium alloy

Since the beginning of the 21st century, the share of organic matrix composite materials has increased considerably in aircraft structures [1]. In order to remain competitive for the manufacture of new single-aisle aircraft, new metal alloys or new solutions for using these alloys will have to be developed. Concerning aluminium alloys, which remain key alloys for the aeronautical field, two solutions for joining by friction stir welding or laser welding seem promising to replace the riveting assemblies developed in the 1920s. These complementary processes result in weight savings by eliminating excess thickness or sealant and productivity gains [2–4].

Concerning more specifically laser welding, there are nowadays several laser sources such as CO2 lasers or solid-state lasers (fiber or disc). CO2 sources have been implemented faster in the industry because they were more powerful and cheaper. The advent of the new disc sources allows for higher quality, higher power and lower cost solid state laser beams [5,6]. In addition, their shorter wavelength compared to CO2 lasers reduces reflection problems when welding alloys such as aluminium alloys [7]. Indeed, the physical and chemical characteristics of high-strength aluminium alloys lead to welding difficulties that must be considered in the welding procedures. The most frequently encountered problems are the difficulty to obtain sufficient penetration depths, correct bead geometries, weld beads without porosity and cracks. Insufficient penetration of the fusion zone is due to insufficient laser beam energy during welding. Geometrical defects such as undercut or incomplete fusion can be generated when welding speeds are too high [8]. Hot cracking occurs at the end of the solidification of the alloy when the dendritic skeleton is not sufficiently formed to resist deformation and the permeability of the liquid medium is very low. At this stage, the material has low strength and low ductility. In order to limit this susceptibility to hot cracking, Hu et al. propose to optimize welding parameters, to reduce thermal contraction stresses by preheating or by modifying the chemical composition of the molten bath with a filler metal [9]. The small porosities are due to the low miscibility of hydrogen in the solid state of aluminium alloys [10]. The presence of hydrogen in the liquid bath during welding is due to poor surface preparation [11]. Large porosities can also exist in welds. They result from instability of the keyhole during the welding phase [12]. The extent of these weld defects depends, among other things, on the welding parameters, hence the need to study the influence of each parameter on the characteristics of the weld [13]. In this study, the weldability range of AA6061-T4 alloy, without filler metal, was investigated using an Yb: YAG source.