This work deals with the effect of a new “bolt-head” pin profile on the friction stir welding performance of the aluminum alloy 6061-T6, compared to traditional pin profiles. Friction stir welding parameters such as the tool rotation speed and the welding speed were investigated together with the different pin profiles; the results show that the new “bolt-head” pin profile leads to better mechanical properties of welded specimens. The pin profiles used in this work were the straight square (SS), straight hexagon (SH), taper cylindrical (TC), and the straight hexagon “bolt-head” (SHBH). It was found that the last pin profile improves the material flow behavior and the uniform distribution of plastic deformation and reduces the formation of macroscopic defects on the welded zone. Mechanical tensile tests on welded specimens were performed to determine the tensile strength: the specimens welded with the SHBH pin profile have shown the highest mechanical properties. An approach is presented for material flow on this aluminum alloy using the SHBH pin profile, which is related to the improvement on the resulting mechanical properties.

Ultrasonic fatigue tests at the frequency of 20 kHz were carried out on the hot rolled low alloy steel grade (known as R5), which according to the International Classification Society of offshore systems, it is a steel with the highest ultimate tensile strength to manufacture mooring chains and accessories intended to position mooring applications. Tests were carried out at constant load ratio R = −1 on pre-corroded and no-corroded hourglass shape specimens; pre-corrosion was implemented according to ASTM standard G85. Stress concentration between two and three close hemispherical pitting holes on hourglass shape specimen undergoing uniaxial loading were obtained by finite element method. Experimental results show that fatigue life decreases with the proximity of two close pitting holes on the fracture surface. An exponential law is proposed for the stress concentration factor evolution with the proximity of two close pitting holes of identical radius; the trend of this law is confirmed by experimental results. Finally, fracture surfaces were analyzed and conclusions were listed concerning fatigue life, proximity of pitting holes on fracture surface, and the associated stress concentration factors.

Ultrasonic and conventional fatigue tests were carried out on the AISI-SAE AA7075-T6 aluminum alloy, in order to evaluate the effect of artificial and induced pre-corrosion. Artificial pre-corrosion was obtained by two hemispherical pitting holes of 500-μm diameter at the specimen neck section, machined following the longitudinal or transverse direction of the testing specimen. Induced pre-corrosion was achieved using the international standard ESA ECSS-Q-ST-70-37C of the European Space Agency. Specimens were tested under ultrasonic fatigue technique at frequency of 20 kHz and under conventional fatigue at frequency of 20 Hz. The two applied load ratios were: R = −1 in ultrasonic fatigue tests and R = 0.1 in conventional fatigue tests. The main results were the effects of artificial and induced pre-corrosion on the fatigue endurance, together with the surface roughness modification after the conventional fatigue tests. Crack initiation and propagation were analyzed and numeric models were constructed to investigate the stress concentration associated with pre-corrosion pits, together with the evaluation of the stress intensity factor in mode I from crack initiation to fracture. Finally, the stress intensity factor range threshold ΔK_TH was obtained for the base material and specimens with two hemispherical pits in transverse direction.