CAIO PALUMBO DE ABREU

CAIO PALUMBO DE ABREU

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The effect of heat treatment on the corrosion resistance of Ti and Ti-Nb alloys in fluoride solution
2012 - ABREU, CAIO P.; SILVA, LUCIANO M.; GRANDINI, CARLOS R.; ASSIS, SERGIO L.; COSTA, ISOLDA
Microstructural characteristics of the Al alloys
2020 - ARAUJO, JOAO V. de S.; MILAGRE, MARIANA X.; FERREIRA, RAPHAEL O.; MACHADO, CARULINE de S.C.; ABREU, CAIO P. de; COSTA, ISOLDA
Microstructure characteristics of two high-strength aluminum alloys, the 2024-T3 Al–Cu–Mg, and the new generation Al–Cu–Li alloy 2198, in the T8 and T851 tempers, were investigated in this study. For this purpose, microstructural and statistical analyses were carried out. The results showed equiaxed grains for the 2024-T3 and 2198-T851 alloys, whereas, elongated grains for the 2198-T8. Besides, the 2198-T851 alloy displayed slip bands in the grains due to the stretching stage, “51”. The 2024-T3 alloy showed at least two types of constituent particles, Al–Cu–Mg and Al–Cu–Mn–Fe–(Si); whereas Al–Cu–Li alloys showed only one type, Al–Cu–Fe, in their composition. Statistical analyses showed that the percentage of area covered by constituent particles was larger in the 2024-T3 alloy compared to the 2198 in both tempers, T8 and T851. On the other hand, the Al–Cu–Li alloys showed higher microhardness values relatively to the Al–Cu one. The differences among the nanometric phases present in Al–Cu and Al–Cu–Li alloys were analyzed by transmission electron microscopy. All the results were related to the different chemical composition and industrial thermomechanical processing of each alloy.
Effect of friction stir welding (FSW) on the electrochemical behavior and galvanic coupling of AA2024-T3 and AA7475-T651
2020 - BUGARIN, A.F.S.; ABREU, C.P. de; TERADA, M.; MELO, H.G. de; COSTA, I.
In this study, the local electrochemical activity of AA2024-T3 and AA7475-T761 joined by friction stir welding (FSW) was investigated as a function of time by electrochemical tests in 0.01 mol L−1 NaCl solution using a mini cell. The welding procedure resulted in increased electrochemical activity of the weld affected zones of both alloys, which electrochemical activities increased with immersion time, as demonstrated by the electrochemical behavior. In the heat affected zone (HAZ) and the thermomechanically affected zone (TMAZ) of AA7475-T651 the increased activity was due to precipitation of η phase at the grain boundaries stimulated by thermomechanical effects. On the other hand, the enhanced activity in the TMAZ/HAZ of the AA2024-T3 was associated to a large concentration of copper rich cathodic particles broken by tool rotation that were spread along this zone. However, the zone of highest electrochemical activity was the stir zone (SZ) and this was ascribed to galvanic coupling between the two alloys, AA2024-T3 and the AA7475-T651, where the former acted as cathodic and the latter as anodic area.
Corrosion resistance evaluation of the different zones on the AA2024-T3 and AA7475-T651 alloys welded by FSW
2017 - ABREU, C.P.; MELO, H.G. de; PEBERE, N.; MOGILI, N.V.V.; COSTA, I.; VIVIER, V.
The aeronautic industry has a great interest in joining dissimilar aluminium alloys used in different parts of an aircraft. The Friction Stir Welding (FSW) process has been considered an effective alternative to conventional techniques for the welding of high strength alumin-ium alloys. However, this procedure results in zones with different microstructures due to thermal and thermomechanical effects involved during the joining process, leading to the formation of the thermomechanically affected zone (TMAZ), the heat affected zone (HAZ), the stir zone (nugget), and the unaffected base metal (BM). As the corrosion resistance of materials depends on their microstructures, the aim of this study was to investigate the intrin-sic corrosion resistance of the different zones of the AA2024-T3 and AA7475-T761 alloys welded by FSW, without considering the coupling effects between the different zones. This was achieved by evaluating the corrosion resistance of each individual zone in 0.1 M Na2SO4 + 1 mM NaCl solution by electrochemical techniques, such as open circuit potential meas-urements, polarization curves, and electrochemical impedance spectroscopy, using an elec-trochemical cell with an exposed area of 0.78 mm2. The electrochemical results showed that the lowest corrosion resistance was associated to the nugget zone, on which a galvanic cou-pling between the 2024 and 7475 alloys takes place. The corrosion resistance was also low-ered on the TMAZ of both alloys. This behaviour was associated to the enhanced precipita-tion of the η phase (MgZn2) at the grain boundaries in the 7475 alloy and to the coarsening of S phase (AlCuMg) precipitates in the 2024 alloy, both processes favoured by the thermome-chanical effects in these specific zones as demonstrated by TEM analysis.
Macro and microgalvanic interactions in friction stir weldment of AA2198-T851 alloy
2019 - DONATUS, UYIME; SILVA, REJANE M.P. da; ARAUJO, JOAO V. de S.; MILAGRE, MARIANA X.; ABREU, CAIO P. de; MACHADO, CARULINE de S.C.; COSTA, ISOLDA
The galvanic interactions within and between the friction stir weld zones of the AA2198-T851alloy have been investigated using electrochemical and microscopy techniques. The parentmaterial (PM) was the most anodic region and exhibited pronounced severe localized corro-sion (SLC) both when coupled and isolated. The stir zone was the most resistant to corrosionand exhibited no SLC when coupled, but exhibited SLC when isolated. Profiles associatedwith dissolved oxygen consumption and hydrogen generation currents across the weldmentwere inversely related because the anodic (PM) region produced higher hydrogen bubblesand, interestingly, consumed more dissolved oxygen compared with the other regions.
Multiscale electrochemical study of welded al alloys joined by friction stir welding
2017 - ABREU, CAIO P. de; COSTA, ISOLDA; MELO, HERCILIO G. de; PEBERE, NADINE; TRIBOLLET, BERNARD; VIVIER, VINCENT
Friction stir welding (FSW) is an efficient way to join high strength aluminum alloys. However, FSW generates different microstructural areas in contact that may give rise to galvanic couplings, affecting the corrosion resistance of the assembly. In the present work, a multiscale electrochemical study of the 7475-T651 and 2024-T3 aluminum alloys butt-joined by FSW was carried out.Much lower impedances were associated with the FSW affected zones compared to the two aluminum base metals tested individually. Corrosion of the welded system resulted in the establishment of galvanic coupling, shown by local electrochemical impedance spectroscopy (LEIS) measurements, at which the AA7475 behaves anodically with respect to the AA2024. A Zn deposit was observed on the intermetallic particles of the AA2024 after 24 h of immersion in the electrolyte resulting from the galvanic coupling, which seems to reduce the galvanic coupling effects. Such a behavior in combination with LEIS results allowed a description of the galvanic coupling development between two different aluminum alloys (AA2024-T3 and AA7475-T761) butt-welded by FSW as a function of time from the early stage of immersion.
Influence of probe size for local electrochemical impedance measurements
2017 - ABREU, CAIO P. de; ASSIS, CAMILA M. de; SUEGAMA, PATRICIA H.; COSTA, ISOLDA; KEDDAM, MICHEL; MELO, HERCILIO G. de; VIVIER, VINCENT
Local electrochemical impedance spectroscopy (LEIS) is a promising technique for the characterization of heterogeneous surface reactivity. Significant development of LEIS relies on the improvement of the spatial resolution for reaching the micrometer scale. This work presents the influence of the probe size, especially in the high frequency domain where an inductive-like response ascribed to the stray dielectric capacitance between the two electrodes of the probe used for performing the local current measurement can be observed. A detailed analysis of the whole set-up (electrode geometry and measuring device) allows an analytical expression of the impedance for the whole system to be obtained. Based on the analysis of this expression and on experimental results, two different strategies have been proposed to cancel the HF time constant: a post treatment of the results via a preliminary electrical characterization of the local probe or the minimization of the electrolyte resistance by measuring the local potential in the close vicinity of the substrate. Both approaches were shown to be efficient.
Localized electrochemical impedance for characterizing welded areas of dissimilar Al alloys 2024-T3 and 7475-T761 joined by FSW
2016 - ABREU, C.P. de; COSTA, I.; MELO, H.G. de; PEBERE, N.; VIVIER, V.
Friction Stir Welding (FSW) is an efficient way to join high strength aluminum alloys avoiding de-fects that are usually found when conventional techniques are adopted. The aeronautic industry has shown great interest in this welding method, either for joining similar or dissimilar alloys. However, FSW generates different microstructural areas that could give rise to galvanic couplings on the weld-ing area. In the present study, FSW was used to join two similar and dissimilar aluminum alloys, the AA2024 and the AA7475 alloys. To evaluate the effect of the welding process on the corrosion re-sistance of the joined alloys, electrochemical impedance spectroscopy studies, either global or local-ized (LEIS), have been carried out in a low corrosive medium (0.1 M Na2SO4) or with a small addi-tion of chlorides (0.1 M Na2SO4 + 0.001 M NaCl). A gel visualization technique was also employed to detect possible galvanic couplings of the different zones formed during the FSW welding. OCP measurements were also performed on both individual aluminum alloys and on the zones affected by FSW. LEIS investigations were performed at different locations above the different microstructural zones induced by the FSW process on both alloys. Galvanic coupling was clearly evidenced at the interface between the two alloys, contrary to the results reported in literature for the welding of simi-lar alloys, for which no galvanic coupling was evidenced.
Investigação da corrosão localizada das ligas dissimilares (2024-T3 e 7475-T651) soldadas por FSW
2016 - ABREU, C.P.; MELO, H.G. de; PEBERE, N.; VIVIER, V.; COSTA, I.
The aeronautic industry has shown great interest in joining aluminum dissimilar alloys used in different parts of the aircraft. The Friction Stir Welding (FSW) process was developed has been considered as an effective alternative to con-ventional techniques for welding of aluminum alloys. This technique results in different microstructural areas affecting the corrosion resistance of the material. In this work, the corrosion resistance of the different zones formed in the 2024-T3 and 7475-T761 alloys welded by FSW has been investigated by electro-chemical techniques. An electrochemical cell with a small area exposed to the electrolyte (o-ring with 1.0 mm diameter) was used to allow testing the different zones generated, separately. Test solution consisted of 0.1M Na2SO4 + 1mM NaCl. The electrochemical results showed lower corrosion resistance associat-ed to the nugget zones. Besides, the TMAZ of the two alloys, mainly of the 7475-T651 presented corrosion resistance inferior to the BM and HAZ zones.
Caracterização da reatividade das ligas alumínio AA2024-T3 e AA7475-T651 soldadas por fricção (FSW)
2016 - ABREU, CAIO P. de
A soldagem por fricção (Friction Stir Welding - FSW) é um processo eficiente de unir ligas de alumínio de alta resistência evitando defeitos que são usualmente criados quando técnicas convencionais de soldagem são utilizadas. A indústria aeronáutica tem mostrado grande interesse neste método de soldagem, tanto para a união de ligas similares como dissimilares. Entretanto, este processo causa modificações microestruturais dependentes das condições de tratamento térmico ou termomecânico. Contato elétrico entre zonas de microestruturas diferentes, por sua vez, pode resultar em acoplamento galvânico. No presente estudo, a soldagem por FSW foi usada para unir duas ligas de alumínio dissimilares, AA2024-T3 e AA7475-T651 e o efeito desta soldagem na resistência à corrosão das juntas soldadas e na microestrutura das ligas foi avaliada. Na investigação da resistência à corrosão foram utilizados ensaios eletroquímicos, especificamente, medidas de potencial de circuito aberto (OCP) em função do tempo de exposição ao meio corrosivo, ensaios de polarização e de espectroscopia de impedância eletroquímica, global (EIS) ou local (LEIS), em duas soluções, seja 0,1 M Na2SO4 ou 0,1 M Na2SO4 + 1 mM NaCl. Os ensaios eletroquímicos evidenciaram efeito de acoplamento galvânico nas juntas soldadas. A caracterização microestrutural foi realizada por microscopia ótica, microscopia eletrônica de varredura, microscopia eletrônica de transmissão e por calorimetria diferencial. As zonas afetadas pela solda tiveram importantes modificações na microestrutura indicadas pela precipitação e dissolução de precipitados que afetam a resistência à corrosão localizada. A resistência à corrosão intergranular e a resistência à esfoliação das juntas soldadas também foram avaliadas e comparadas com as das ligas AA2024-T3 e AA7475-T651 não soldadas. Os resultados mostraram aumento da suscetibilidade das juntas soldadas a estas formas de corrosão em comparação com as ligas não soldadas sendo observado ataque mais severo na liga AA7475-T651. A identificação das áreas anódicas e catódicas resultantes do acoplamento galvânico nas juntas soldadas foi realizada por teste que consistiu na deposição de camada de gel (ágar-ágar) com indicador universal na superfície das ligas soldadas. A liga AA2024-T3 atuou como cátodo, enquanto a AA7475-T651, como ânodo no par galvânico. Além disso, evolução de hidrogênio foi observada na região de interface entre a zona termomecanicamente afetada e a termicamente afetada da liga AA7475-T651 mostrando que reações catódicas também ocorreram localmente nesta última liga. Resultados de LEIS obtidos nas diferentes zonas das duas ligas soldadas por FSW mostraram acoplamento galvânico na interface entre elas para tempos curtos de ensaio e deslocamento da região mais ativa com o tempo de ensaio para a liga AA7475-T651, mais precisamente para a interface entre a zona termomecanicamente afetada e a térmicamente afetada desta liga.