Abstract: Aluminum alloys produced with Twin Roll Casting (TRC) technology still necessitate to be thoroughly investigated in some areas. Corrosion mechanisms operating under special conditions with the contribution of unique features of the microstructure are among those. Materials produced with TRC have unique features inherently generated due to the solidification path of the material during casting. Contrary to the very fine particles at the locations close to the free surface, centerline segregation (CLS) occurring at the mid-plane of the thickness have different morphological and compositional features than rest of the microstructure. While, unless directly exposed, it has almost no contribution to the general corrosion behavior of the alloy, some manufacturing processes generation new free surfaces in the material for CLS to be exposed to the corrosive media can be harmful for overall general corrosion performance of the alloy. It differentiates itself in corrosion behaviour with its compositional and morphological features than rest of the microstructure. Hence, influence of CLS on corrosion behaviour of two different alloys in 3000 and 8000 series employed for heat exchangers and packaging applications, respectively, are studied in the present study. Since CLS does not disappear with rolling passes but only changes its morphology, samples were taken at appropriate thickness of the downstream process that allow corrosion tests to be conducted at the cross section of the samples. Metallographical preparation techniques were used to reveal the CLS. Samples were dipped in to the HCl-NaCl containing test solution to observe the progress of corrosion in the matrix and heavily populated CLS areas during the course of test. Open-circuit potential (OCP) measurements were carried out on the rolled free surface, quarter plane and mid plane of the samples after milling the surface of the samples. Different samples produced with compositional and thermomechanical processing route variants were used. Results show that composition of the alloy and applied thermomechanical processes influence the corrosion characteristics of CLS and accordingly the overall corrosion performance. Other important finding is the contribution of manufacturing method to corrosion mechanism whether if it reveals the CLS by creating new free cross sectional surfaces.
Abstract: Due to their typically high hardness, excellent resistance against wear, and their low coefficient of friction, Physical Vapor Deposition (PVD) hard coatings are used on steels for a wide range of tools and components. Currently, however, the potential for wear protection of Al alloy components cannot be exploited. The thin PVD layers tend to collapse and disintegrate due to plastic deformation of the soft base material. Present research is focused on electron beam (EB) surface alloying, using Co-based additives to increase the surface hardness of the Al base material, producing an improved supporting effect for PVD coatings. The influence of different beam deflection techniques and EB parameters on the microstructure and hardness of alloyed layers was investigated. The properties of the duplex composite layers produced are strongly dependent on the thermal stability of the EB alloyed layers (type and amount of intermetallic compounds, coarsening effects) which are affected by the temperature-time cycle of the PVD process. This will be discussed by means of SEM and EDX investigations in correlation with XRD analysis. Measurements using scratch test with increasing load result in critical load values for the combined treatment that are 3 to 5 times higher when compared to only PVD-coated base material.
Abstract: This paper discusses two Al-Cu alloys for aerospace applications, one of which has an addition of between 1.6 and 2.0 wt.% of Li. The alloys are AA2024-T3 (Al-Cu) and AA2099-T8E77 (Al-Cu-Li). Microstructural analysis via Field Emission Gun Transmission Electron Microscope (FEGTEM) and Field Emission Gun Scanning Electron Microscope (FEGSEM) utilising Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Detector (EBSD) techniques have been used to characterise the two microstructures and phases contained within them. Anodic polarisation and immersion testing in a 3.5 wt.% NaCl solution have been carried out and a comparison of the corrosion mechanisms has been made. AA2024-T3 had a fine, equiaxed grain structure, whereas AA2099-T8E77 had a substantial amount of recrystallized grains. Finer grains were also observed on AA2099-T8E77, however, the vast majority were larger than the maximum detection limit of the EBSD technique. Intergranular and pitting corrosion were observed on both alloys following immersion testing, however, the intergranular corrosion (IGC) was more prominent on AA2099-T8E77. Anodic polarisation indicated that AA2024-T3 was more noble, highlighting that the Li-containing AA2099-T8E77 alloy was more susceptible to corrosion. The T1 (Al2CuLi) phase within AA2099-T8E77 was seen to be highly active following immersion and anodic polarisation tests. The corrosion pits on AA2099-T8E77 were seen to propagate to a depth of ~ 80 to 100 μm, with a maximum of 126 μm recorded. For AA2024-T3 the maximum depth recorded was 77 μm and the average depth was between 60 and 70 μm.
Abstract: The effect of heat treatment on the long-term corrosion resistance of Er containing 5083 aluminum alloy was studied using the micro-hardness test and mass loss test. The microstructure was analyzed by TEM. To maintain the strength, the annealing temperature was selected to be 100-230°C below the recrystallization onset temperature determined by the micro-hardness test. The plot of the annealing temperature versus the Intergranular Corrosion (IGC) initial time, which is determined by the Nitric Acid Mass Loss Test, showed a C-curve. The shortest IGC initial time (~1h) happened at 175°C, the nose temperature of the C-curve. When annealed at 125-200°C, the samples were IGC sensitive with the initial time less than 3h. The entirely IGC resistant (stabilized) samples were obtained when annealed at 220-230°C. The 175°C sensitized treatment was performed on the 220°C-stabilized samples, which showed that the 3-12h stabilization could significantly improve the resistance for long-term corrosion. TEM results showed that, for the IGC sensitive samples, β phases (Al3Mg2) grew along the grain boundary continuously, while for the stabilized samples, they were isolated on triangle grain boundary and phase boundary.
Abstract: Alternate immersion-emersion tests were performed for a 2050 aluminium alloy to characterize its corrosion resistance with exposure conditions representative of in service-conditions. Tests were performed for T34 samples and aged samples. After continuous immersion tests, T34 samples exhibited intergranular corrosion while intragranular corrosion was observed for aged samples. The alternate immersion-emersion tests led to a corrosion extension to the subgrain boundaries, for both T34 and aged samples, as shown by electron backscattered diffraction analysis.
Abstract: The influence of aging (T6 at 150°C and175°C, T8 at 150°C) and 0.72% Zn addition on the mechanical properties, microstructures and intergranular corrosion (IGC) behavior of Al-2.7Cu-1.7Li-0.3Mg alloys was investigated. With 0.72% Zn addition, the strength of the Al-Li alloy was increased. As the aging time was extended, the corrosion type of the studied Al-Li alloys was changed in the following order: pitting, local IGC, general IGC, local IGC and pitting again. For the T6 temper, as the aging temperature was elevated from 150°C to 175°C, the aging time period for IGC appearance was shortened. The pre-deformation before aging (T8 temper) also greatly shorten the aging time period for IGC. Meanwhile, the addition of 0.72% Zn decreased the IGC sensitivity and shortened the aging time period for IGC.
Abstract: In the present study, the corrosion behaviour of machined AA7150-T651 aluminium alloy has been investigated. It was revealed that a near-surface deformed layer with thickness of 500 nm, characterized by ultrafine grains, is present between the alloy surface and the bulk alloy. In the deformed layer, the MgZn2 precipitates were absent, while segregation bands developed at the grain boundaries. The presence of the segregation bands, rich in magnesium and zinc, promoted localized corrosion in the deformed layer. Potentiodynamic polarization of the alloy in deaerated 3.5 wt. % sodium chloride solution revealed current surges at potentials of-750 mV (SCE) and-670 mV (SCE) respectively. The second current surge was associated with the fast dissolution of the deformed layer. Immersion testing of the alloy in 3.5 wt. % sodium chloride solution showed that the deformed layer acted as anode while the bulk alloy served as cathode during corrosion process.
Abstract: Several accelerated tests for high-strength aluminum alloys, such as ASTM G34, ASTM G85 Annex 2, and ASTM G110 can produce significantly different results in third generation aluminum lithium (Al-Li) alloys. In this study, the reason for this inconsistency is investigated by comparing the electrochemical kinetics for AA2060 to those of AA2090 as well as legacy alloy AA7075-T6, and by comparing the performance of AA2060 in ASTM G110 and ASTM G85. The corrosion potential of AA2060 was found to be higher than that of AA7075-T6 and AA2060-T8E41. Also, it was found that although ASTM G110 does not produce exfoliation in AA 2060-T3 as in ASTM G85, the type of attack observed in cross-section is very similar in both tests.
Abstract: Galvanic deposition of Aluminium on steel substrate, starting from liquid ions, is considered. How the agitation of the liquid during deposition influences the homogeneity of the developed coating layer is investigated by a commercially available FLUENT® ANSYS® solver. Comparison of data coming from numerical simulation with experimentally obtained results are presented. The reported results demonstrate that the deposition process parameter influences the quality of the deposited aluminium.
Abstract: A KrF excimer laser was introduced for laser surface melting (LSM) of the aluminium alloys AA2124-T4 and AA6061-T4. The microstructural and compositional analysis was conducted using SEM, low-angle XRD, and TEM; the corrosion behaviour of as-received (AR) and laser-treated (LT) samples was evaluated by electrochemical techniques and immersion test in a 3.5% NaCl solution. A melted layer consisting of a re-solidified layer with refined microstructure and dissolution of intermetallic particles (IMPs), and a thin film of aluminium oxides at the top, was formed after LSM for both alloys. The corrosion resistance of both alloys was improved after LSM. The results of immersion test showed different corrosion behaviour for LT AA2124 and LT AA6061. The delamination of the melted layer was observed for AA2124 but was not observed for AA6061 after exposure to 3.5% NaCl solution for 24 h. This was attributed to the formation of copper-rich segregation bands in the melted layer of AA2124 due to higher content of copper in AA2124 than AA6061. A significant number of micro-pores were present in the melted layer for AA2124 treated with high number of laser pulses, leading to the decrease of the corrosion resistance.