Novel ultra-high-strength aluminum alloys provide enormous lightweighting potential for modern car body design. However, joining such alloys can be challenging. Refill friction stir spot welding is a solid state joining process that provides fundamental advantages compared to conventional joining technologies when welding aluminum alloys. This work presents refill friction stirspot welding for joining3-mm-thick Al-Mg-Si alloys. The welded joints have been optimized for shear load condition by the design of experiment and analysis of variance. The results show that it is possible to obtain welds of relatively thick Al-Mg-Si alloys with good mechanical properties. Microstructure analyses show that rotational speed and plunge depth play important rolesin the bonded width and hook height, which affect the mechanical performance of the joint. KeywordsRefill friction stir spot welding; Aluminum alloy; Design of experiments; Response surface methodology; hook; shear layer Highlights • Refill friction stir spot welding of 3-mm-thick Al-Mg-Si alloy single-lap joints was demonstrated • The influence of joining parameters on joint mechanical performance was determined.• A mathematical model for estimating lap shear strength was successfully established.

In general, tensile armour wires of flexible pipes that are designed for sour applications have their strength limited to 850 MPa due to the possibility of embrittlement phenomena to occur. A Thermally Sprayed Aluminum (TSA) coating 250 μm thick was applied to high strength steels with UTS of 1470 MPa and YS of 1280 MPa. Three specimens conditions were evaluated: full coating, no coating and coating with a designed defect. The load was applied using a four point bending fixture, maintaining a constant stress of 90% of material’s yield strength. All tests were performed in accordance with recommendations of NACE TM 0177 method B. The test solution was distilled water with NaCl 5.0% saturated with a gas mixture of 10,000 ppm of H2S in balance with CO2 during 720 hours. It was observed that samples without coating were more susceptible to the effect of the environment presenting higher degradation and failure. The fractures presented typical characteristic of the Sulfide Stress Corrosion Cracking (SSCC). Furthermore, it was detected parallel cracks to the surface of the wires indicating the embrittlement phenomenon of Hydrogen-Induced Cracking (HIC). On the other hand, coated samples with and without defects did not fail during the 720 hours of testing. A posterior non-destructive testing and a metallographic analysis did not identify the presence of cracks. These results were attributed to the physical barrier of the aluminum coating and the cathodic protection generated by the preferential aluminum corrosion. This preliminary study shows that TSA coatings can be a good alternative to increase the corrosion resistance of armour wires in sour environments allowing the application of higher strength steels.

This work evaluated the hydrogen embrittlement resistance of the nickel-based UNS N08830 alloy through a hydrogen charging study, slow strain rate (SSR), and fracture toughness tests. The microstructure evaluation showed large-size austenitic grains with no evidence of second phase precipitation, a high fraction of low-angle grain boundaries, and texture of {100} and {111} planes in the rolling direction. From SSR tests results, strain to fracture and reduction of area embrittlement indexes of 25.3 and 42.1% were found, respectively. A modest drop in fracture toughness of approximately 20% was observed. From fractography, it was observed a prevailing mixed micromechanism of fracture comprised of microvoids coalescence and quasi-cleavage flat facets in secondary cracks aligned with the rolling direction. The quasi-cleavage flat facets showed nanovoids at the slip line intersections, which in turn confirmed Hydrogen-Enhanced Localized Plasticity (HELP) as the prominent embrittlement mechanism. Because of the material's crystallographic texture, a good part of hydrogen was not transported to a direction normal to the applied stress as it would happen for pure diffusion but instead followed the dislocations in the rolling direction. That effect caused that less hydrogen was concentrated in the main crack tip, which inevitably increased the overall energy for fracture.