Abstract: Aluminum alloys are commonly used in automotive and aerospace applications because of their good mechanical properties and low density. The aluminum-lithium (Al-Li) alloys are a class of Al-alloys which have some superior properties, such as lower densities than conventional ones and, for this reason, used as structural components in aircrafts. The addition of lithium aims to reduce density of the material with gains in the mechanical properties and in the specific modules, which is important for the aerostructures. However, the addition of this element usually increases localized corrosion susceptibility. The scanning electrochemical microscopy (SECM) technique can provide a good understanding about the corrosion behavior and the local reactions of this class of materials. In this work, different modes of SECM measurements were employed to characterize the AA2098-T351 surface in various conditions. The surface generation/tip collection (SG/TC) mode and competition mode were employed to study the hydrogen gas (H2) evolution and oxygen reduction, respectively, in the AA2098-T351 surface in the conditions: polished and unpolished. It was found that the main domains of H2 evolution and oxygen reduction are associated with the alloy surface in the polished condition.The feedback mode of SECM was employed to characterize the electrochemical activity of the surface after immersion in a solution containing ions chloride, in order to verify the influence of the oxide layer formation over the surface in the active/passive domains due the corrosion process. The previously attacked surface revealed a much lower electrochemical activity compared with the surface in the polished condition due to the protective character of the oxide formed during immersion test.

Palavras-Chave: electrochemistry; microscopy; electrodes; aluminium alloys; lithium alloys; corrosion

Abstract: The AA2198-T851 alloy is a new generation Al-Li alloy which finds application in the aerospace industry. Its preference over conventional Al alloys is premised on the fact that it exhibits better strength to weight ratio and improved fatigue resistance due to the addition of Li as a major alloying element. However, it is a relatively new alloy and its processing-structure-property-performance relationship is far from being established. This is very true for the relationship between its structure and corrosion resistance which is very scarce in the literature. Our recent findings have revealed that this alloy is highly susceptible to severe localized corrosion which is caused by the abundance of active hexagonal T1 (Al2CuLi) particles in its microstructure. The corrosion susceptibility is further promoted by the non-uniform precipitation of these particles which are often preferentially concentrated in bands aligned parallel to the {111}Al. Furthermore, bulk zonal heterogenities with pronounced non-uniform distribution of the active T1 particles are created when this alloy is welded using friction stir welding. The friction stir welding process is a solid state welding process that has been adjudged to be the best welding technique for Al alloys. Nonetheless, this process generates different zones in the weldments of Al alloys. Thus, zones exhibiting different electrochemical characteristics and severe galvanic interactions can occur when the weldment is exposed to a corrosive media. The galvanic interactions can lead to a faster propagation of attack in the most susceptible region of the weldment, which in this case is the parent material (with the highest volume fraction of the T1 particles). To establish this, the galvanic interactions between the zones of the weldment have been investigated by separately exposing the parent material and the stir zone of the weld and then by exposing the entire weldment using NaCl and EXCO solutions. Optical, scanning electron and transmission electron microscopes were also employed in the study. The results revealed that the parent material of the weldment was the most susceptible (as previously mentioned) and appeared to corrode at a faster rate when coupled to the more cathodic stir zone, thermomechanically affected zone and the heat affected zones of the weldment (with lower volume fraction of the T1 particles). The modes of corrosion in the zones of the weld were also observed to be different. However, the most susceptible region corroded intragranularly with the grain boundaries exhibiting more noble electrochemical characteristics.

Palavras-Chave: aluminium alloys; corrosion; corrosion resistant alloys; friction welding; lithium alloys; electrochemical corrosion; aerospace industry

Abstract: In this investigation, the intergranular corrosion (IGC) resistance of the AA2198-T8 alloy welded by friction stir welding (FSW) was investigated by ASTM G110-97 test. The different zones resulting from FSW showed distinct resistances to intergranular attack depending on the heating and/or mechanical effects produced by welding. The base metal (BM) was the most susceptible to IGC whereas the thermomechanically affected zone (TMAZ) and the stir zone (SZ), presented the highest resistances to IGC comparatively to the other zones. The results of IGC tests were correlated with those of thermal simulation, microhardness measurements, transmission electron microscopy (TEM) and optical microscopy (OM). The dissimilarities in corrosion resistance of the various zones were associated to differences in hardening phase quantities between the various zones. In the BM and HAZ, T1 phase, the main hardening phase, was found at grain boundaries and it resulted in IGC susceptibility. The slight IGC observed in the SZ and TMAZ was not associated with T1 phase which was rarely found but to another precipitate (T2 phase) which was found preferentially located at the grain boundaries in these zones.

Palavras-Chave: microstructure; intergranular corrosion; aluminium alloys; friction welding; coatings; welding; transmission electron microscopy

Abstract: The Al–Si is the most used cast Al alloy due to their wide application in commercial components. This element strongly reduces the fluidity and the overall mechanical properties through the formation of brittle intermetallic phases. The most common intermetallic is the β-Al5FeSi phase, which generally has a complex plate-like morphology that interrupts the continuity of the matrix, becoming preferential places for crack nucleation and growing. The α-Al8Fe2Si is another intermetallic phase which presents a Chinese script-like morphology. Generally, the α-Al8Fe2Si is less detrimental than β-Al5FeSi phase for the mechanical properties of the alloy owing to its compact morphology. Based on this, the aim of this work is to study the microstructure of the Al-9wt%Si alloy with Fe contents between 0.1 to 1.2 wt%. The chill casting process was chosen due to the higher supercooling of the material during solidification when compared with the sand casting. The material was melted in an induction furnace utilizing a crucible with 30 kg of capacity. The pouring temperature was 760°C. The micrographs were obtained with the aid of a PHILIPS EDAX XL 30 SEM. Figure 1(A) shows the microstructure of the Al-9wt%Si alloy with 0.1 wt% of Fe. It can be seen the Al dendrites and the distribution of the eutectic Si particles with needle shape morphology in the interdendritic regions. Despite the low amount of Fe present in this alloy and the high cooling rate of the chill casting process, it was possible to note the presence of the α-Al8Fe2Si intermetallic phase with Chinese script-like morphology near the eutectic Si particles. Figure 1(B) shows the microstructure of the Al-9wt%Si alloy with 0.4 wt% of Fe. It was possible to note the presence of α-Al8Fe2Si particles with higher amount and size than the alloy with 0.1 wt% of Fe. Figure 2(A) shows the microstructure of the Al-9wt%Si alloy with 0.8 wt% of Fe. It can be seen the Si and β-Al5FeSi particles. Figure 2(B) shows the microstructure of the Al-9wt%Si alloy with 1.2 wt% of Fe. It presents the eutectic Si particles between the arms of Al dendrites and the thick β-Al5FeSi particles formed near the eutectic regions. The high amount and size of β-Al5FeSi particles occurs due to high amount of Fe content. Note that the particles formed by the primary eutectic reaction are bigger that those formed by the secondary eutectic reaction. The sample with 1.2 wt% of Fe presented higher amount of β-Al5FeSi particles than the alloy with 0.8 wt% of Fe and, consequently, higher amount of micropores. Figure 3(A) shows the micropores limited by the β-Al5FeSi particles in the microstructure of the Al-9wt%Si alloy with 0.8 wt% of Fe. This micrograph also shows how β-Al5FeSi platelets constitute potential barriers for liquid metal feeding in a casting region subjected to shrinkage. The amount of β-Al5FeSi phase and, consequently, the amount of micropores is proportional to the Fe content. Figure 3(B) shows the β-Al5FeSi particles formed by the secondary eutectic reaction L→α+Si+ β-Al5FeSi inside a micropore of the Al-9wt%Si alloy with 0.8 wt% of Fe. These particles probably appeared when the vacancies were partially filled with the eutectic liquid and, after the solidification, nucleating and growing along the dendrite ramifications.

Palavras-Chave: microstructure; aluminium alloys; silicon alloys; mechanical properties; iron; scanning electron microscopy