Aircraft panels stiffened by friction stir welded extruded parts: Mechanical characterization

Friction StirWelding is a young solid state joining technology that, invented and patented at TheWelding Institute (TWI) of Cambridge (UK) in 1991, has been showing many potentialities. Due to the interesting features of FSW, lots of research activities have been carried out all over the world, on different materials (aluminium alloys first of all, but also steel, titanium, magnesium, copper, polymers, etc.) and for different geometries, even if the most investigated one is, without any doubt, the butt joint configuration. Other geometries, such as lap joint, even if potentially very interesting, have been less studied. In the present paper, attention has been focused on joints simulating a geometrical configuration used in aircraft structures to manufacture bulkhead panels. The aim is to exploit the feasibility of substituting rivets with a friction stir weld. An AA 2024 T351 T shape extruded stringer was overlap friction stir welded to an AA 2056 T3 clad rolled sheet (see Table 1 for chemical compositions) by means of two parallel beads, as illustrated in Figure 1. The principal welding process parameters adopted are: rotational speed of 400 RPM, welding speed of 100 mm/min and tilt angle of 3°. It is important to put in evidence that the two beads were one by one welded with the same rotational verse and welding direction. Static and dynamic tests were carried out in two different loading conditions. The first loading condition, named "hoop stress", simulates the pressurization of the cylindrical fuselage with a load applied to the skin, perpendicularly to the stringer; the second loading condition, named "T pull", simulates the pulling action of stringer on the skin. On the basis of the experimental characterization carried out, the following conclusion can be drawn: · FSWappears to be a suitable welding technology to make sound lap joints of extruded parts on rolled sheets. The obtained overlap friction stir welds resulted free of distortions and appreciable defects. · Static mechanical properties in hoop stress loading way are very high compared to the skin parent alloy. The joint mechanical efficiency resulted close to one. · Static mechanical properties in T-pull loading appear very high if compared with those provided by conventional riveted joints. · Fatigue properties are fully governed by the hooking material in the advancing side of beads: this defect plays a significant role in the crack initiation both upon T-pull and hoop stress loading. Its significant role is present at every fatigue stress level, even if the failure modes can be significantly different. As a consequence, much care should be put in setting the process parameters and in designing the tool geometry in order to minimize the hook defect occurrence. Probably an mportant role, in hook defect occurrence, is played by skin cladding layer too.