Friction stir welded joints of Al–Li Alloys for aeronautical applications: butt-joints and tailor welded blanks

Abstract

Structural design for safety critical components, as airframes, requires comprehensive characterization of the materials and joints properties. This characterization must take into account a large amount of variables required for accurate design to ensure structural integrity during the different phases of the product life cycle of an aircraft. One of the most important variables in this characterization is fatigue behavior due to the cyclic loads during the life of an airframe.The application of welding processes to replace riveted joints in aeronautical structures is an attractive option, since it allows joints with less stress concentration points and might be applied efficiently without overlapping the two joining parts (with a butt-joint configuration), reducing the joint weight. This weight reduction can have a small impact in production costs, but has a huge impact in the life cycle costs. Friction stir is the most appealing welding process in aeronautical structures since it has been shown to produce joints with excellent properties when applied to aluminum alloys and can deal with hardened precipitated aluminum alloys, since it is a semi-solid state process. Furthermore, this joining process allows to join dissimilar materials and plates with different thicknesses, creating in this way tailor welded blanks, which support the manufacturing of optimized panels by tailoring their strength in function of their needs.This article presents a mechanical characterization of friction stir welds of last generation aluminum–lithium alloys, including tailor welded blanks with different thicknesses, with potential applications in aeronautical and aerospace components. Mechanical characterization with crack initiation and fatigue crack growth tests were included in this study for an enhanced assessment of these alloys behavior, and was preceded by welding parameter calibration.